Enviro Wiki - User contributions [en]

Articles

2026-08-19T11:42:42Z

Admin:

{| class="wikitable sortable"
|-
!Title!!First Author!!Linking Phrases
|-
|[[Groundwater Sampling - No-Purge/Passive]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||passive sampling, no purge sampling, grab samplers, diffusion samplers, sorptive samplers
|-
|[[ Long-Term Monitoring (LTM)]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||long-term monitoring, LTM, LTM objectives, LTM programs, LTM challenges
|-
|[[PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)]]
|[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]
|PFAS, MNA, natural attenuation
|-
|[[Sorption of Organic Contaminants]]||[[Richelle Allen-King|Allen-King, Richelle]]||
|-
|[[PFAS Transport and Fate]]
|[[Dr. Richard Anderson|Anderson, Richard, Ph.D.]]
|PFAS, fate and transport
|-
|[[Mass Flux and Mass Discharge]]||[[Dr. Michael Annable, P.E. |Annable, Michael, Ph.D., P.E.]]||source reduction
|-
|[[PFAS Toxicology and Risk Assessment]]
|[[Jennifer Arblaster|Arblaster, Jennifer]]
|PFAS, toxicology, risk assessment
|-
|[[Metal(loid)s - Small Arms Ranges]]|| Dr. Amanda Barker |[[Dr. Amanda Barker|Barker, Amanda, Ph.D.]]
|
|-
|[[Munitions Constituents – Photolysis|Munitions Constituents - Photolysis]]
|[[Dr. Warren Kadoya|Kadoya, Warren, Ph.D.]]
|
|-
|[[Munitions Constituents - Soil Sampling]]
|[[Dr. Samuel Beal|Beal, Samuel, Ph.D.]]
|
|-
|[[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways|Vapor Intrusion - Sewers and Utility Tunnels as Preferential Pathways]]
|[[Lila Beckley|Beckley, Lila]]
|vapor intrusion
|-
|[[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
|[[Dr. Barbara Bekins|Bekins, Barbara, Ph.D.]]||
|-
|[[Bioremediation - Anaerobic Secondary Water Quality Impacts]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||secondary impacts, water quality (in regards to anaerobic conditions)
|-
|[[Design Tool - Base Addition for ERD]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||aquifer acidity, base addition
|-
|[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
|[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||
|-
|[[Low pH Inhibition of Reductive Dechlorination]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||low pH inhibition
|-
|[[In Situ Toxicity Identification Evaluation (iTIE)]]||[[Dr. G. Allen Burton |Burton, Allen, P.E.]]||toxicity evaluation
|-
|[[OPTically-based In-situ Characterization System (OPTICS)]]
|[[Dr. Grace Chang|Chang, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents - Electrochemical Treatment]]
|[[Dr. Brian P. Chaplin|Chaplin, Brian, Ph.D.]]
|munitions constituents remediation
|-
|[[PFAS Sources]]
|[[Dr. Dora Chiang|Chiang, Dora, Ph.D.]]
|
|-
|[[Biodegradation - Cometabolic]]||[[Dr. Kung-Hui (Bella) Chu |Chu, Kung-Hui (Bella), Ph.D]]||cometabolic biodegradation
|-
|[[Munitions Constituents - Composting]]
|[[Harry Craig|Craig, Harry]]
|
|-
|[[Chemical Oxidation (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||ISCO, chemical oxidation
|-
|[[Chemical Oxidation Oxidant Selection (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||chemical oxidant, oxidant (in regards to ISCO)
|-
|[[Chemical Oxidation Design Considerations(In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||screening, design, implementation, oxidant delivery (in regards to ISCO)
|-
|[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]||[[Dr. Rula Deeb |Deeb, Rula, Ph.D.]]||PFAS, perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS)
|-
|[[Metal and Metalloid Contaminants]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal contaminant(s), metalloid contaminant(s), metal(s), metalloid(s),
|-
|[[Metals and Metalloids - Mobility in Groundwater]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal mobility, aqueous speciation, adsorption, precipitation, colloidal transport (in regards to metals)
|-
|[[Monitored Natural Attenuation (MNA) of Metal and Metalloids]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||MNA, attenuation of metal(s), natural attenuation processes, attenuation (in regards to metals)
|-
|[[Metal and Metalloids - Remediation]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||remediation, in situ technologies, contaminant removal (in regards to metals)
|-
|[[pH Buffering in Aquifers]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||ph buffer, natural pH buffer, engineered pH buffer
|-
|[[Munitions Constituents - Sorption]]||[[Dr. Katerina Dontsova |Dontsova, Katerina, Ph.D.]]||energetics, sorption
|-
|[[Biodegradation - Hydrocarbons]]||[[Dr. Elizabeth Edwards |Edwards, Elizabeth, Ph.D.]]||hydrocarbon, biodegradation
|-
|[[Source Zone Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||source zone modeling
|-
|[[Plume Response Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||plume response modeling
|-
|[[REMChlor - MD]]
|[[Dr. Ron Falta|Falta, Ron, Ph.D.]]
|
|-
|[[In Situ Groundwater Treatment with Activated Carbon]]
|[[Dr. Dimin Fan|Fan, Dimin, Ph.D.]]
|
|-
|[[LNAPL Remediation Technologies]]||[[Dr. Shahla Farhat |Farhat, Shahla, Ph.D.]]||
|-
|[[Sustainable Remediation]]||[[Paul Favara |Favara, Paul]]||sustainable remediation, social, economic and environmental impacts
|-
|[[Munitions Constituents]]||[[Dr. Kevin Finneran |Finneran, Kevin, Ph.D.]]||Explosives, energetics, insensitive munitions
|-
|[[Biodegradation - Reductive Processes]]||[[Dr. David Freedman |Freedman, David, Ph.D.]]||biotic reduction, biotic reductive processes, hydrogenolysis, dihaloelimination, coupling, organohalide respiration
|-
|[[Remediation of Stormwater Runoff Contaminated by Munition Constituents|Munitions Constituents - Remediation of Stormwater Runoff]]||Fuller, Mark, Ph.D.||energetics, insensitive munitions, stormwater runoff
|-
|[[Subgrade Biogeochemical Reactor (SBGR)]]||[[Jeff Gamlin |Gamlin, Jeff]]||SBGR, subgrade biogeochemical reactor, bioreactor
|-
|[[Thermal Remediation - Smoldering]]||[[Dr. Jason Gerhard |Gerhard, Jason, Ph.D.]]||smouldering remediation, self-sustaining treatment for active remediation, STAR
|-
|[[Contaminated Sediments - Introduction]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[Contaminated Sediment Risk Assessment]]
|[[Richard Wenning|Wenning, Richard]]
|
|-
|[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[PFAS Ex Situ Water Treatment]]
|[[Dr. Scott Grieco |Grieco, Scott, Ph.D.]]
|
|-
|[[Stream Restoration]]
|[[Dr. Natalie Griffiths|Griffiths, Natalie, Ph.D.]]
|
|-
|[[Passive Sampling of Sediments]]
|[[Dr. Philip M. Gschwend|Gschwend, Philip]]
|
|-
|[[Phytoplankton (Algae) Blooms]]
|[[Dr. Nathan Hall|Hall, Nathan]]
|
|-
|[[PFAS Soil Remediation Technologies]]||[[James_Hatton |Hatton, Jim]]||PFAS, Soil source zones
|-
|[[Proteomics and Proteogenomics]]
|[[Dr. Kate Kucharzyk|Kucharzyk, Kate, Ph.D.]]
|
|-
|[[N-nitrosodimethylamine (NDMA)]]
|[[Paul Hatzinger|Hatzinger, Paul, Ph.D.]]
|
|-
|[[Alternative Endpoints]]||[[Elisabeth Hawley |Hawley, Elisabeth]]||management of complex sites
|-
|[[Thermal Remediation]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, in situ thermal
|-
|[[Thermal Remediation - Steam]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Steam Enhanced Extraction
|-
|[[Thermal Remediation - Electrical Resistance Heating]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Electrical Resistance Heating
|-
|[[Thermal Conduction Heating (TCH)]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal desorption
|-
|[[Thermal Remediation - Combined Remedies]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation
|-
|[[Thermal Conduction Heating for Treatment of PFAS-Impacted Soil]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, PFAS
|-
|[[Munitions Constituents- TREECS™ Fate and Risk Modeling|Munitions Constituents - TREECS™ Fate and Risk Modeling]]||[[Dr. Billy E. Johnson |Johnson, Billy, Ph.D.]]||munitions constituents fate and transport modeling, TREECS
|-
|[[Munitions Constituents - Alkaline Degradation]]
|[[Jared Johnson|Johnson, Jared]]
|
|-
|[[Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)|Vapor Intrusion - Assessing VI Impacts in Neighborhoods with Groundwater Contaminated CVOCs]]
|[[Dr. Paul C. Johnson|Johnson, Paul, Ph.D.]]
|vapor intrusion
|-
|[[Munitions Constituents - IM Toxicology]]||-----||insensitive explosives, insensitive munitions, IMX-101, IMX
|-
|[[Landfarming]]
|[[Dr. Roopa Kamath|Kamath, Roopa, Ph.D.]]
|
|-
|[[NAPL Mobility]]
|[[Andrew Kirkman|Kirkman, Andrew]]
|
|-
|[[Perchlorate]]||[[Thomas Krug |Krug, Thomas]]||perchlorate
|-
|[[Injection Techniques for Liquid Amendments]]||[[Thomas Krug |Krug, Thomas]]||amendment injection
|-
|[[Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)]]
|[[Dr. Johnsie Ray Lang|Lang, Johnsie Ray, Ph.D.]]||PFAS
|-
|[[Characterization Methods – Hydraulic Conductivity]]
|[[Dr. Gaisheng Liu|Liu, Gaisheng, Ph.D.]]
|
|-
|[[Compound Specific Isotope Analysis (CSIA)]]||[[Dr. Barbara Sherwood Lollar, F.R.S.C. |Lollar, Barbara S., FRSC]]||Compound Specific Isotope Analysis (CSIA)
|-
|[[Passive Sampling of Munitions Constituents|Munitions Constituents - Passive Sampling]]
|[[Dr. Guilherme Lotufo|Lotufo, Guilerme, Ph.D.]]
|
|-
|[[Vapor Intrusion (VI)]]||[[Chris Lutes |Lutes, Chris]]||vapor intrusion
|-
|[[Bioremediation - Anaerobic]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||anaerobic bioremediation
|-
|[[Bioremediation - Anaerobic Design Considerations]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||
|-
|[[Biodegradation - 1,4-Dioxane]]
|[[Dr. Shaily Mahendra|Mahendra, Shaily, Ph.D.]]
|
|-
|[[Direct Push (DP) Technology]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push, DP, DP machines, DP technology
|-
|[[Direct Push Sampling]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push sampling, soil sampling, groundwater sampling, well installation, soil vapor sampling (in regards to DP)
|-
|[[Direct Push Logging]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push logging, Cone Penetration Testing, CPT, Electrical Conductivity, EC, Hydraulic Profiling Tool, HPT, Membrane Interface Probe, MIP, Optical Imaging Profiler, OIP
|-
|[[Remediation Performance Assessment at Chlorinated Solvent Sites]]||[[Travis McGuire|McGuire, Travis]]||multi-site studies
|-
|[[LNAPL Conceptual Site Models]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Long-Term Monitoring (LTM) - Data Analysis]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data analysis, analysis methods (in regards to LTM)
|-
|[[Long-Term Monitoring (LTM) - Data Variability]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data variability (in regards to LTM), LTM evaluation
|-
|[[Munitions Constituents - Abiotic Reduction]]||[[Dr. Jimmy Murillo-Gelvez |Murillo-Gelvez, Jimmy, Ph.D.]] ||
|-
|[[Supercritical Water Oxidation (SCWO)]]
|Nagar, Kobe
|
|-
|[[Matrix Diffusion]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Groundwater Flow and Solute Transport]]||[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]||groundwater flow, advection, dispersion, diffusion, molecular diffusion, mechanical dispersion
|-
|[[Molecular Biological Tools - MBTs]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||MBT, Molecular Biological Tool(s)
|-
|[[Quantitative Polymerase Chain Reaction (qPCR)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||qPCR, Polymerase Chain Reaction
|-
|[[Sediment Capping]]
|[[Dr. Danny Reible|Reible, Danny]]
|
|-
|[[Stable Isotope Probing (SIP)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||SIP, Stable Isotope Probing
|-
|[[Metagenomics]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||metagenomics
|-
|[[Natural Source Zone Depletion (NSZD)]]||[[Tom Palaia |Palaia, Tom]]||natural source zone depletion, NSZD
|-
|[[Amendment Distribution in Low Conductivity Materials]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||
|-
|[[Polycyclic Aromatic Hydrocarbons (PAHs)]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||polycyclic aromatic hydrocarbons, PAH(s)
|-
|[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]
|[[Florent Risacher|Risacher, Florent, M.Sc.]]
|
|-
|[[1,2,3-Trichloropropane]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||TCP, trichloropropane
|-
|[[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR)]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||ZVI
|-
|[[Mercury in Sediments]]
|[[Dr. Grace Schwartz|Schwartz, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents – Sample Extraction and Analytical Techniques|Munitions Constituents - Sample Extraction and Analytical Techniques]]
|[[Dr. Austin Scircle|Scircle, Austin]]
|
|-
|[[Geophysical Methods]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[Geophysical Methods - Case Studies]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[PFAS Treatment by Anion Exchange]]
|[[Dr. Timothy J. Strathmann|Strathmann, Timothy, Ph.D.]]
|PFAS
|-
|[[Munitions Constituents - Dissolution]]||[[Dr. Susan Taylor |Taylor, Susan, Ph.D.]]||explosive(s), dissolution
|-
|[[PFAS Treatment by Electrical Discharge Plasma]]
|[[Dr. Selma Mededovic Thagard|Thagard, Selma Mededovic, Ph.D.]]
|PFAS
|-
|[[Chemical Reduction (In Situ - ISCR)]]||[[Dr. Paul Tratnyek |Tratnyek, Paul, Ph.D.]]||In Situ Chemical Reduction, ISCR
|-
|[[Injection Techniques - Viscosity Modification]]||[[Michael Truex |Truex, Michael]]||viscosity, viscosity modifiers, viscosity modification
|-
|[[Soil Vapor Extraction (SVE)]]||[[Michael Truex |Truex, Michael]]||soil vapor extraction, SVE
|-
|[[Munitions Constituents - Deposition]]||[[Michael R. Walsh, P.E., M.E.|Walsh, Michael, P.E.]]||explosive deposition, energetics deposition
|-
|[[Vapor Intrusion - Separation Distances from Petroleum Sources]]
|[[Dr. James Weaver|Weaver, James, Ph.D.]]
|vapor intrusion
|-
|[[Zerovalent Iron Permeable Reactive Barriers]]
|[[Dr. Richard Wilkin|Wilkin, Rick, Ph.D.]]
|
|-
|[[Monitored Natural Attenuation (MNA)]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, In Situ MNA, natural attenuation, natural attenuation processes
|-
|[[Monitored Natural Attenuation (MNA) of Fuels]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to petroleum hydrocarbons and fuel components)
|-
|[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to chlorinated solvents)
|-
|[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
|[[Dr. John Wilson|Wilson, John, Ph.D.]]
|MNA, natural attenuation
|-
|[[Chlorinated Solvents]]||[[Dr. Bilgen Yuncu, P.E. |Yuncu, Bilgen, Ph.D., P.E.]]||chlorinated solvents
|-
|[[Petroleum Hydrocarbons (PHCs)]]
|[[Dr. Bilgen Yuncu, P.E.|Yuncu, Bilgen, Ph.D., P.E.]]
|Petroleum Hydrocarbons (PHCs)
|-
|[[Photoactivated Reductive Defluorination - PFAS Destruction]]
|[[Dr. Suzanne Witt|Witt, Suzanne, Ph.D.]]
|PFAS destruction
|-
|[[Hydrogeophysical Methods for Characterization and Monitoring of Groundwater-Surface Water Exchanges]]
|[[Dr. Lee Slater|Slater, Lee, Ph.D.]]
|geophysics, hydrogeophysical methods
|-
|[[Hydrothermal Alkaline Treatment (HALT)]]
|[[Dr. Brian Pinkard|Pinkard, Brian]]
|
|-
|[[1,4-Dioxane]]
|[[Matthew Zenker|Zenker, Matthew]]
|
|-
|[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
|[[Dr. John F. Stults|Stults, Dr. John]]
|PFAS, vadose zone, lysimeter, field investigation
|-
|[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]||[[Dr. Yida Fang |Fang, Yida, Ph.D.]]||PFAS destruction
|
|-
|[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
|[[Dani Tran|Tran, Dani]]||MNA, natural attenuation, natural attenuation processes, chlorinated solvents
|-
|[[PFAS Leaching Characterization with the Leaching Environmental Assessment Framework (LEAF)]]||[[Dr. Jennifer Guelfo |Guelfo, Jennifer, Ph.D.]]
|PFAS, PFAS leaching
|}

Articles

2026-08-19T11:40:43Z

Admin:

PFAS Transport and Fate

2026-08-19T11:39:31Z

Admin:

The transport and fate of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]] in the environment is controlled by the nature of the PFAS source, characteristics of the individual PFAS, and environmental conditions where the PFAS are present. Transport, partitioning, and transformation are the primary processes controlling PFAS fate in the environment. PFAS compounds can also be taken up by both plants and animals, and in some cases, bioaccumulate through the food chain.
Understanding PFAS transport and fate is necessary for evaluating the potential risk from a PFAS release and for predictions about PFAS occurrence, migration, and persistence, and about the potential vectors for exposure. This knowledge is important for site characterization, identification of potential sources of PFAS to the site, development of an appropriate conceptual site model (CSM), and selection and predicted performance of remediation strategies.

<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s): '''

*[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]
*[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
*[[PFAS Ex Situ Water Treatment]]
*[[PFAS Leaching Characterization with the Leaching Environmental Assessment Framework (LEAF)]]
*[[PFAS Soil Remediation Technologies]]
*[[PFAS Sources]]
*[[PFAS Treatment by Electrical Discharge Plasma]]
*[[Supercritical Water Oxidation (SCWO)]]

'''Contributor(s): '''
[[Dr. Richard Anderson]] and [[Dr. Mark Brusseau]]

'''Key Resource(s): '''

*[https://pfas-1.itrcweb.org/ Per- and Polyfluoroalkyl Substances (PFAS), PFAS-1. ITRC 2020]<ref name="ITRC2020">Interstate Technology and Regulatory Council (ITRC), 2020. Technical/Regulatory Guidance: Per- and Polyfluoroalkyl Substances (PFAS), PFAS-1. ITRC, PFAS Team, Washington DC. [//www.enviro.wiki/images/2/2e/ITRC_PFAS-1.pdf Report.pdf]</ref>.

*[//www.enviro.wiki/images/d/de/Brusseau2018manuscript.pdf Assessing the Potential Contributions of Additional Retention Processes to PFAS Retardation in the Subsurface]<ref name="Brusseau2018">Brusseau, M.L., 2018. Assessing the Potential Contributions of Additional Retention Processes to PFAS Retardation in the Subsurface. Science of the Total Environment, 613-614, pp. 176-185. [https://doi.org/10.1016/j.scitotenv.2017.09.065 DOI: 10.1016/j.scitotenv.2017.09.065]  [//www.enviro.wiki/images/d/de/Brusseau2018manuscript.pdf Article pdf]</ref>.

==Introduction==
The transport and fate of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]] is a rapidly evolving field of science, with many questions that are not yet resolved. Much of the currently available information is based on a few well-studied PFAS compounds. However, there is a large number and variety of PFAS with a wide range of physical and chemical characteristics that affect their behavior in the environment. The transport and fate of some PFAS could differ significantly from the compounds studied to date. Nevertheless, information about the behavior of some PFAS in the environment can be ascertained from the results of currently available research.

PFAS transport and fate in the environment is controlled by the nature of the PFAS source, characteristics of the individual PFAS, and environmental conditions where the PFAS are present. Perfluoroalkyl acids (PFAAs) (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]] for nomenclature) are strong acids and are anionic in the environmentally-relevant pH range. They are extremely persistent in the environment and do not degrade or transform under typical environmental conditions. Polyfluoroalkyl substances (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]] for nomenclature) include compounds that have the potential to degrade to PFAAs. These compounds are commonly referred to as PFAA precursors or just ‘precursors’. Because some polyfluoroalkyl substances can degrade into PFAA via biotic or abiotic degradation pathways, PFAAs are sometimes referred to as “terminal PFAS” or “terminal degradation products”.
The most important molecular properties controlling PFAA transport are the carbon chain length and functional moieties of the headgroups (e.g., sulfonate, carboxylate). The molecular properties of PFAA precursors are more varied, with different carbon chain lengths, headgroups and ionic states<ref name="Buck2011">Buck, R.C., Franklin, J., Berger, U., Conder, J.M., Cousins, I.T., de Voogt, P., Jensen, A.A., Kannan, K., Mabury, S.A., and van Leeuwen, S.P.J., 2011. Perfluoroalkyl and Polyfluoroalkyl Substances in the Environment: Terminology, Classification, and Origins. Integrated Environmental Assessment and Management, 7(4): pp. 513-541. [https://doi.org/10.1002/ieam.258 DOI: 10.1002/ieam.258]   [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/ieam.258 Open Access Article]</ref><ref name="Wang2017">Wang, Z., DeWitt, J.C., Higgins, C.P., and Cousins, I.T., 2017. A Never-Ending Story of Per- and Polyfluoroalkyl Substances (PFASs)? Environmental Science and Technology, 51(5), pp. 2508-2518. American Chemical Society. [https://doi.org/10.1021/acs.est.6b04806 DOI: 10.1021/acs.est.6b04806]   [https://pubs.acs.org/doi/pdf/10.1021/acs.est.6b04806 Free Download from ACS]</ref> (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]]). All of these properties can influence transport and fate of PFAA precursors in the environment.

Important environmental characteristics include the nature of the source (mode of input into the environment), the length of time that the source was active, and the magnitude of the input, as well as precipitation and infiltration rates, depth to groundwater, surface water and groundwater flow rates and interactions, prevailing atmospheric conditions, the properties of the porous-media (e.g., soil and sediment) and aqueous solution, microbiological factors, and the presence of additional fluid phases such as air and non-aqueous phase liquids [[Wikipedia: Non-aqueous phase liquid | (NAPLs)]] in the vadose zone and water-saturated source. In the subsurface, soil characteristics (texture, organic carbon content, clay mineralogy, metal-oxide content, solid surface area, surface charge, and exchange capacity) and solution characteristics (pH, redox potential, major ion chemistry, and co-contaminants) can influence PFAS transport and fate.

==PFAS Transport and Fate Processes==
[[File:AndersonBrusseau1w2Fig1.png | thumb | 600px | Figure 1. Illustration of PFAS partitioning and transformation processes. Source: D. Adamson, GSI, used with permission.]]
Transport, partitioning, and transformation are the primary processes controlling PFAS fate in the environment (Figure 1). PFAS compounds can also be taken up by both plants and animals, and in some cases, bioaccumulate through the food chain. However, PFAS uptake and bioaccumulation is not discussed in this article (see “Environmental Concern” section of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]).

*'''Transport:''' PFAS can be transported substantial distances in the atmosphere<ref name="Ahrens2012">Ahrens, L., Harner, T., Shoeib, M., Lane, D.A. and Murphy, J.G., 2012. Improved Characterization of Gas–Particle Partitioning for Per- and Polyfluoroalkyl Substances in the Atmosphere Using Annular Diffusion Denuder Samplers. Environmental Science and Technology, 46(13), pp. 7199-7206. [https://doi.org/10.1021/es300898s DOI: 10.1021/es300898s]   Free download available from [https://www.researchgate.net/profile/Tom_Harner/publication/225046057_Improved_Characterization_of_Gas-Particle_Partitioning_for_Per-_and_Polyfluoroalkyl_Substances_in_the_Atmosphere_Using_Annular_Diffusion_Denuder_Samplers/links/5cc730c4299bf12097893fdc/Improved-Characterization-of-Gas-Particle-Partitioning-for-Per-and-Polyfluoroalkyl-Substances-in-the-Atmosphere-Using-Annular-Diffusion-Denuder-Samplers.pdf ResearchGate].</ref>, surface water<ref name="Taniyasu2013">Taniyasu, S., Yamashita, N., Moon, H.B., Kwok, K.Y., Lam, P.K., Horii, Y., Petrick, G. and Kannan, K., 2013. Does wet precipitation represent local and regional atmospheric transportation by perfluorinated alkyl substances? Environment International, 55, pp. 25-32. [https://doi.org/10.1016/j.envint.2013.02.005 DOI: 10.1016/j.envint.2013.02.005]</ref>, soil<ref name="Braunig2017">Bräunig, J., Baduel, C., Heffernan, A., Rotander, A., Donaldson, E. and Mueller, J.F., 2017. Fate and redistribution of perfluoroalkyl acids through AFFF-impacted groundwater. Science of the Total Environment, 596, pp. 360-368. [https://doi.org/10.1016/j.scitotenv.2017.04.095 DOI: 10.1016/j.scitotenv.2017.04.095]</ref>, and groundwater<ref name="Weber2017">Weber, A.K., Barber, L.B., LeBlanc, D.R., Sunderland, E.M. and Vecitis, C.D., 2017. Geochemical and Hydrologic Factors Controlling Subsurface Transport of Poly- and Perfluoroalkyl Substances, Cape Cod, Massachusetts. Environmental Science and Technology, 51(8), pp. 4269-4279. [https://doi.org/10.1021/acs.est.6b05573 DOI: 10.1021/acs.est.6b05573]   [https://bgc.seas.harvard.edu/assets/weber2017_final.pdf Free Download]</ref>. The primary mechanisms controlling PFAS transport are [[Wikipedia:Advection | advection]] and [[Wikipedia:Dispersive_mass_transfer | dispersion]], similar to other dissolved compounds. For additional information on transport in groundwater, see [[Advection and Groundwater Flow]] and [[Dispersion and Diffusion]].

*'''Partitioning:''' Partitioning of PFAS between the mobile and immobile phases is one of the most important processes controlling the rate of migration in the environment. The primary mobile phases are typically air and water. Relatively immobile phases include stream sediments, soils, aquifer material, NAPLs, and interfaces between different phases (air-water, NAPL-water). Partitioning of a significant portion of the PFAS mass into an immobile phase increases the amount of material stored in the system and slows the apparent rate of migration in the mobile phase – a phenomenon that has been observed in field metadata<ref name="Anderson2019">Anderson, R.H., Adamson, D.T. and Stroo, H.F., 2019. Partitioning of poly-and perfluoroalkyl substances from soil to groundwater within aqueous film-forming foam source zones. Journal of Contaminant Hydrology, 220, pp. 59-65. [https://doi.org/10.1016/j.jconhyd.2018.11.011 DOI: 10.1016/j.jconhyd.2018.11.011]   Manuscript available from [https://www.researchgate.net/profile/Hans_Stroo3/publication/329227107_Partitioning_of_poly-_and_perfluoroalkyl_substances_from_soil_to_groundwater_WITHIN_aqueous_film-forming_foam_source_zones/links/5e56996b299bf1bdb83e2f69/Partitioning-of-poly-and-perfluoroalkyl-substances-from-soil-to-groundwater-WITHIN-aqueous-film-forming-foam-source-zones.pdf ResearchGate]</ref>.

*'''Transformation:''' Transformation of PFAS is controlled by the molecular structure of the individual compounds. Perfluorinated compounds, including PFAAs, are resistant to abiotic and biotic transformation reactions under typical conditions and highly persistent in the environment. In contrast, precursors can be transformed by both abiotic and biotic processes, often resulting in the production of so-called “terminal” PFAA daughter products.

==Transport and Partitioning in the Atmosphere==
Air serves as a transport media for PFAS, particularly for uncharged polyfluorinated PFAS. Airborne PFAS transport contributes to global distribution and can lead to localized deposition to soils and surface water in the vicinity of emission sources<ref name="Simcik2005">Simcik, M.F. and Dorweiler, K.J., 2005. Ratio of Perfluorochemical Concentrations as a Tracer of Atmospheric Deposition to Surface Waters. Environmental Science and Technology, 39(22), pp. 8678-8683. [https://doi.org/10.1021/es0511218 DOI: 10.1021/es0511218]   Free download available from [https://www.researchgate.net/profile/Matt_Simcik/publication/7444956_Ratio_of_Perfluorochemical_Concentrations_as_a_Tracer_of_Atmospheric_Deposition_to_Surface_Waters/links/5f035861299bf1881603c3be/Ratio-of-Perfluorochemical-Concentrations-as-a-Tracer-of-Atmospheric-Deposition-to-Surface-Waters.pdf ResearchGate]</ref><ref name="Prevedouros2006">Prevedouros, K., Cousins, I.T., Buck, R.C. and Korzeniowski, S.H., 2006. Sources, Fate and Transport of Perfluorocarboxylates. Environmental Science and Technology, 40(1), pp. 32-44. [https://doi.org/10.1021/es0512475 DOI: 10.1021/es0512475]   Free download available from [https://d1wqtxts1xzle7.cloudfront.net/39945519/Sources_Fate_and_Transport_of_Perfluoroc20151112-1647-19vcvbf.pdf?1447365456=&response-content-disposition=inline%3B+filename%3DSources_Fate_and_Transport_of_Perfluoroc.pdf&Expires=1605023809&Signature=Z6KqgaDN6lKdAazoe6qoASoCtVystG5i~5EnrTcb~qMg3xZPz4O49Kghh62WmMzqEKE788~6EwrnlBVo9o6cM0hjf2vymFYxg4mx-eSIOEonfFjk6RonSaWp5gRbA6m~SNjwsjaKXID3OQyWIlLVpUd2LzAdI5rLGFA~gIXXtNPyCArLuGn-kbPYUIcBUg5TIkTZ6TDLXF~ujmzK9tNv~55UYabsJL4pmwIGC2sNGkEyJrYMfU577fbactdrmQXTJH7XbgpfDSfd4-xWkDZTdvVf~TypDDqUCZdtCkY8wINdpqtfe1KEzLrAj7rxxALAHUYxlVbPB45XTkLAGe5qww__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA Academia]</ref><ref name="Ahrens2011">Ahrens, L., Shoeib, M., Harner, T., Lane, D.A., Guo, R. and Reiner, E.J., 2011. Comparison of Annular Diffusion Denuder and High Volume Air Samplers for Measuring Per- and Polyfluoroalkyl Substances in the Atmosphere." Analytical Chemistry, 83(24), pp. 9622-9628. [https://doi.org/10.1021/ac202414w DOI: 10.1021/ac202414w]   Free download available from [https://www.informea.org/sites/default/files/imported-documents/UNEP-POPS-POPRC11FU-SUBM-PFOA-Canada-2-20151211.En.pdf Informea].</ref><ref name="Rauert2018">Rauert, C., Shoieb, M., Schuster, J.K., Eng, A. and Harner, T., 2018. Atmospheric concentrations and trends of poly-and perfluoroalkyl substances (PFAS) and volatile methyl siloxanes (VMS) over 7 years of sampling in the Global Atmospheric Passive Sampling (GAPS) network. Environmental Pollution, 238, pp. 94-102. [https://doi.org/10.1016/j.envpol.2018.03.017 DOI: 10.1016/j.envpol.2018.03.017]   Open access article available from [https://reader.elsevier.com/reader/sd/pii/S0269749117352521?token=4C770E6E8AEDB0B3BA6A1D5B2C20ED5385F81823612551FA3380AAA1DA7A978F9CB36834AF6B7F91F35FF57E32013252 ScienceDirect]   [//www.enviro.wiki/images/e/e6/Rauert2018.pdf Report.pdf]</ref>.

PFAAs, which are ionic and possess a negative charge under ambient environmental conditions, are far less volatile than many other groundwater contaminants. An online database of vapor pressures and Henry’s Law constants for different PFAS, including PFAAs, is maintained by the Interstate Technology Regulatory Council<ref name="ITRC2020" />. In general, vapor pressures of PFAS are low and water solubilities are high, limiting partitioning from water to air<ref name="ITRC2020" />. However, under certain conditions, particularly within industrial stack emissions, PFAS can be transported through the atmosphere in both the gas phase and associated with fugitive particulates. In particular, volatile compounds including fluorotelomer alcohols (FTOHs) may be present in the gas phase, whereas, PFAAs can aerosolize and be transported as particulates<ref name="Ahrens2012" />. In addition, precursors can be transformed to PFAAs in the atmosphere, which can result in PFAA deposition.
Short-range atmospheric transport and deposition can result in PFAS contamination in terrestrial and aquatic systems near points of significant emissions, impacting soil, groundwater, and other media of concern<ref name="Fang2018">Fang, X., Wang, Q., Zhao, Z., Tang, J., Tian, C., Yao, Y., Yu, J. and Sun, H., 2018. Distribution and dry deposition of alternative and legacy perfluoroalkyl and polyfluoroalkyl substances in the air above the Bohai and Yellow Seas, China. Atmospheric Environment, 192, pp. 128-135. [https://doi.org/10.1016/j.atmosenv.2018.08.052 DOI: 10.1016/j.atmosenv.2018.08.052]</ref><ref name="Brandsma2019">Brandsma, S.H., Koekkoek, J.C., van Velzen, M.J.M. and de Boer, J., 2019. The PFOA substitute GenX detected in the environment near a fluoropolymer manufacturing plant in the Netherlands. Chemosphere, 220, pp. 493-500. [https://doi.org/10.1016/j.chemosphere.2018.12.135 DOI: 10.1016/j.chemosphere.2018.12.135]   Open access article available from [https://reader.elsevier.com/reader/sd/pii/S0045653518324706?token=E541D5C4B200C8626A86F41049FE9DCA92652BC9A8BA7D9E47832C08070AB5AF256F4872474C50B5C4908F5CA4C24947 ScienceDirect].   [//www.enviro.wiki/images/4/4a/Brandsma2019.pdf Report.pdf]</ref>. Releases of ionic PFAS from factories are likely tied to particulate matter, which settle to the ground in dry weather and are also wet-scavenged by precipitation<ref name="Barton2006">Barton, C.A., Butler, L.E., Zarzecki, C.J., Flaherty, J. and Kaiser, M., 2006. Characterizing Perfluorooctanoate in Ambient Air near the Fence Line of a Manufacturing Facility: Comparing Modeled and Monitored Values. Journal of the Air and Waste Management Association, 56(1), pp. 48-55. [https://doi.org/10.1080/10473289.2006.10464429 DOI: 10.1080/10473289.2006.10464429]   Free access article available from [https://www.tandfonline.com/doi/pdf/10.1080/10473289.2006.10464429?needAccess=true Taylor and Francis Online]   [//www.enviro.wiki/images/b/b2/Barton2006.pdf Report.pdf]</ref>. The impact of other potential sources, such as combustion emissions or wind-blown fire-fighting foam from fire training and fire response sites, on the fate and transport of PFAS in air may need to be assessed.

Long-range transport processes are responsible for the wide distribution of neutral and ionic PFAS across the Earth as evidenced by their occurrence in biota, surface snow, ice cores, seawater, and other environmental media in regions as remote as the Arctic and Antarctic<ref name="Bossi2016">Bossi, R., Vorkamp, K. and Skov, H., 2016. Concentrations of organochlorine pesticides, polybrominated diphenyl ethers and perfluorinated compounds in the atmosphere of North Greenland. Environmental Pollution, 217, pp. 4-10. [https://doi.org/10.1016/j.envpol.2015.12.026 DOI: 10.1016/j.envpol.2015.12.026]</ref><ref name="Ahrens2010">Ahrens, L., Gerwinski, W., Theobald, N. and Ebinghaus, R., 2010. Sources of polyfluoroalkyl compounds in the North Sea, Baltic Sea and Norwegian Sea: Evidence from their spatial distribution in surface water. Marine Pollution Bulletin, 60(2), pp. 255-260. [https://doi.org/10.1016/j.marpolbul.2009.09.013 DOI: 10.1016/j.marpolbul.2009.09.013]</ref>.
Distribution of PFAS to remote regions far removed from direct industrial input is believed to occur from both: a) long-range atmospheric transport and subsequent degradation of volatile precursors; and b) transport via ocean currents and release into the air as marine aerosols (sea spray)<ref name="DeSilva2009">De Silva, A.O., Muir, D.C. and Mabury, S.A., 2009. Distribution of perfluorocarboxylate isomers in select samples from the North American environment. Environmental Toxicology and Chemistry: An International Journal 28(9), pp. 1801-1814. [https://doi.org/10.1897/08-500.1 DOI: 10.1897/08-500.1]</ref><ref name="Armitage2009">Armitage, J.M., 2009. Modeling the global fate and transport of perfluoroalkylated substances (PFAS). Doctoral Dissertation, Institutionen för tillämpad miljövetenskap (ITM), Stockholm University. [//www.enviro.wiki/images/e/ef/Armitage2009.pdf Report.pdf]</ref>.

==Transport and Partitioning in Aqueous Systems==
PFAS adsorb from water to a variety of solid materials including organic materials, clay minerals, metal oxides, and granular activated carbon<ref name="Du2014">Du, Z., Deng, S., Bei, Y., Huang, Q., Wang, B., Huang, J. and Yu, G., 2014. Adsorption behavior and mechanism of perfluorinated compounds on various adsorbents – A review. Journal of Hazardous Materials, 274, pp. 443-454. [https://doi.org/10.1016/j.jhazmat.2014.04.038 DOI: 10.1016/j.jhazmat.2014.04.038]</ref>. This process is thought to occur through two primary mechanisms: 1) sorption to organic-carbon components of the solids; and 2) electrostatic (and other) interactions with inorganic constituents of the solids, including clay minerals and metal-oxides<ref name="Guelfo2013">Guelfo, J.L. and Higgins, C.P., 2013. Subsurface Transport Potential of Perfluoroalkyl Acids at Aqueous Film-Forming Foam (AFFF)-Impacted Sites. Environmental Science and Technology, 47(9), pp. 4164-4171. [https://doi.org/10.1021/es3048043 DOI: 10.1021/es3048043]   [https://mountainscholar.org/bitstream/handle/11124/80055/Guelfo_mines_0052E_10298.pdf?sequence=1#page=64 Doctoral Dissertation]</ref><ref name="Zhao2014">Zhao, L., Bian, J., Zhang, Y., Zhu, L. and Liu, Z., 2014. Comparison of the sorption behaviors and mechanisms of perfluorosulfonates and perfluorocarboxylic acids on three kinds of clay minerals. Chemosphere, 114, pp. 51-58. [https://doi.org/10.1016/j.chemosphere.2014.03.098 DOI: 10.1016/j.chemosphere.2014.03.098]   Free download available from [https://www.researchgate.net/profile/Lixia_Zhao8/publication/262148355_Comparison_of_the_sorption_behaviors_and_mechanisms_of_perfluorosulfonates_and_perfluorocarboxylic_acids_on_three_kinds_of_clay_minerals/links/5b1be5dca6fdcca67b681a4f/Comparison-of-the-sorption-behaviors-and-mechanisms-of-perfluorosulfonates-and-perfluorocarboxylic-acids-on-three-kinds-of-clay-minerals.pdf ResearchGate].</ref>. The relative contribution of each mechanism varies depending on surface chemistry and other geochemical factors, as well as the molecular properties of the PFAS. In general, the impact of electrostatic interactions with charged soil constituents is more important for PFAS than non-polar, hydrophobic organic contaminants (e.g. hydrocarbons, chlorinated solvents). Adsorption of PFAS by solids is often nonlinear, with greater sorption at lower solute concentrations. The impacts of adsorption kinetics and their potential reversibility on PFAS transport have not yet been examined for most PFAS compounds.

Sorption of hydrocarbons, chlorinated solvents and other hydrophobic organics is often controlled the by organic-carbon components of the solid phase (see [[Sorption of Organic Contaminants]]). However, studies of PFAS sorption to solid phase organic carbon have reported conflicting results. In a study of field sites with aqueous film-forming foam (AFFF, a type of fire-fighting foam) releases, solid phase organic carbon content was found to significantly influence PFAS soil-to-groundwater concentration ratios. Statistical modeling was then used to derive apparent organic carbon partition coefficients for 18 different PFAS<ref name="Anderson2019" />. A recent compilation of published organic carbon partition coefficients found a good correspondence to PFAS molecular structure<ref name="Brusseau2019a">Brusseau, M.L., 2019. Estimating the relative magnitudes of adsorption to solid-water and air/oil-water interfaces for per-and poly-fluoroalkyl substances. Environmental Pollution, 254B, p. 113102. [https://doi.org/10.1016/j.envpol.2019.113102 DOI: 10.1016/j.envpol.2019.113102]</ref>. However, other studies have shown a general lack of correlation between solid phase partition coefficients and organic carbon<ref name="Li2018">Li, Y., Oliver, D.P. and Kookana, R.S., 2018. A critical analysis of published data to discern the role of soil and sediment properties in determining sorption of per and polyfluoroalkyl substances (PFASs). Science of the Total Environment, 628, pp. 110-120. [https://doi.org/10.1016/j.scitotenv.2018.01.167 DOI: 10.1016/j.scitotenv.2018.01.167]</ref>. It is possible that greater variability may be observed for broader data sets that incorporate different ranges of PFAS concentrations, different solution conditions, different measurement methods, and field-based data which often have less well-defined conditions and may also be influenced by other retention processes<ref name="Anderson2019" /><ref name="Brusseau2019a" />.

[[File:AndersonBrusseau1w2Fig2.png | thumb | 500px | Figure 2. Example of expected orientation and accumulation of PFAS at air-water interface. Source: D. Adamson, GSI, used with permission.]]
Most solids present in the environment contain both fixed-charged (negative) and variably charged surfaces. At neutral to high pH, variably charged clay minerals have a net-negative charge. As a result, negatively charged PFAAs do not strongly interact electrostatically in most soils, although as the soil pH decreases electrostatic sorption would be expected to increase in soils with variably charged clay minerals. Cationic and zwitterionic precursors are expected to be more strongly sorbed than anionic PFAAs in most environments due to well-established cation exchange reactions. Other factors, including ionic strength, composition, and the presence of co-solutes, can affect adsorption of PFAS<ref name="Higgins2006">Higgins, C.P. and Luthy, R.G., 2006. Sorption of Perfluorinated Surfactants on Sediments. Environmental Science and Technology, 40(23), pp. 7251-7256. [https://doi.org/10.1021/es061000n DOI: 10.1021/es061000n]</ref><ref name="Chen2009">Chen, H., Chen, S., Quan, X., Zhao, Y. and Zhao, H., 2009. Sorption of perfluorooctane sulfonate (PFOS) on oil and oil-derived black carbon: Influence of solution pH and [Ca2+]. Chemosphere, 77(10), pp. 1406-1411. [https://doi.org/10.1016/j.chemosphere.2009.09.008 DOI: 10.1016/j.chemosphere.2009.09.008]</ref><ref name="Pan2009">Pan, G., Jia, C., Zhao, D., You, C., Chen, H. and Jiang, G., 2009. Effect of cationic and anionic surfactants on the sorption and desorption of perfluorooctane sulfonate (PFOS) on natural sediments. Environmental Pollution, 157(1), pp.325-330. [https://doi.org/10.1016/j.envpol.2008.06.035 DOI: 10.1016/j.envpol.2008.06.035]   Free download available from [https://www.researchgate.net/profile/Gang_Pan2/publication/23189567_Effect_of_cationic_and_anionic_surfactants_on_the_sorption_and_desorption_of_perfluorooctane_sulfonate_PFOS_on_natural_sediments/links/5be19d23a6fdcc3a8dc2550d/Effect-of-cationic-and-anionic-surfactants-on-the-sorption-and-desorption-of-perfluorooctane-sulfonate-PFOS-on-natural-sediments.pdf ResearchGate]</ref><ref name="Guelfo2013" /><ref name="Zhao2014" />.

Most PFAS compounds act as surface-active agents (or [[Wikipedia:Surfactant | surfactants]]) due to the presence of a hydrophilic headgroup and a hydrophobic tail. The hydrophilic headgroup will preferentially partition to the aqueous phase and the hydrophobic tail will preferentially partition to the non-aqueous phase (air or organic material). As a result, PFAS tend to accumulate at interfaces (air-water, water-NAPL, water-solid) (Figure 2). This tendency to accumulate at interfaces can influence transport in the atmosphere (on water droplets and hydrated aerosols), in the vadose or unsaturated zone at air-water interfaces, in the presence of NAPLs, and in wastewater treatment systems<ref name="Brusseau2018" /><ref name="Brusseau2019b">Brusseau, M.L., 2019. The Influence of Molecular Structure on the Adsorption of PFAS to Fluid-Fluid Interfaces: Using QSPR to Predict Interfacial Adsorption Coefficients. Water Research, 152, pp. 148-158. [https://doi.org/10.1016/j.watres.2018.12.057 DOI: 10.1016/j.watres.2018.12.057]   [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6374777/ Author’s Manuscript]</ref>.

In theoretical and experimental studies of transport in unsaturated porous media, adsorption at the air-water interface increased PFOS and PFOA retention<ref name="Brusseau2018" /><ref name="Lyu2018">Lyu, Y., Brusseau, M.L., Chen, W., Yan, N., Fu, X., and Lin, X., 2018. Adsorption of PFOA at the Air-Water Interface during Transport in Unsaturated Porous Media. Environmental Science and Technology, 52(14), pp. 7745-7753. [https://doi.org/10.1021/acs.est.8b02348 DOI: 10.1021/acs.est.8b02348]   [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6312111/ Author’s Manuscript]</ref><ref name="BrusseauEtAl2019">Brusseau, M.L., Yan, N., Van Glubt, S., Wang, Y., Chen, W., Lyu, Y., Dungan, B., Carroll, K.C., and Holguin, F.O., 2019. Comprehensive Retention Model for PFAS Transport in Subsurface Systems. Water Research, 148, pp. 41-50. [https://doi.org/10.1016/j.watres.2018.10.035 DOI: 10.1016/j.watres.2018.10.035]   [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6294326/ Author’s Manuscript]</ref>, contributing approximately 20% to 80% of total retention in sands and soil. The impact of oil-water interfacial adsorption on PFAS transport was also quantitatively characterized in recent studies and shown to contribute to total retention on a similar scale as air-water interfacial adsorption<ref name="Brusseau2018" /><ref name="BrusseauEtAl2019" />. These processes may result in increased PFAS mass retained in NAPL source zones, increased PFAS sorption with the resulting retardation of transport, and greater persistence of dissolved PFAS in the environment.

==Transformation==
[[File:AndersonBrusseau1w2Fig3.png | thumb | 600px | Figure 3. Conceptual model of precursor transformation resulting in the formation of PFAAs. Source L. Trozzolo, TRC and C. Higgins, Colorado School of Mines, used with permission.]]
Certain polyfluorinated substances have the potential to transform to other PFAS, with PFAAs as the typical terminal daughter products. These polyfluorinated substances are often referred to as “precursors”. The transformation potential of polyfluorinated precursors is influenced by the presence, location, and number of carbon-hydrogen (C-H) bonds and potentially carbon-oxygen (C-O) bonds throughout the carbon chain. Specifically, PFAS with C-H bonds are subject to a variety of biotic and abiotic reactions that ultimately result in the formation of PFAAs with perfluorinated carbon chains of the same length or shorter than the initial polyfluorinated precursor<ref name="Houtz2013">Houtz, E.F., Higgins, C.P., Field, J.A. and Sedlak, D.L., 2013. Persistence of perfluoroalkyl acid precursors in AFFF-impacted groundwater and soil. Environmental Science and Technology, 47(15), pp. 8187-8195. [https://doi.org/10.1021/es4018877 DOI: 10.1021/es4018877]   Free download from [https://www.researchgate.net/profile/Erika_Houtz/publication/252323955_Persistence_of_Perfluoroalkyl_Acid_Precursors_in_AFFF-Impacted_Groundwater_and_Soil/links/59dbddeeaca2728e2018336d/Persistence-of-Perfluoroalkyl-Acid-Precursors-in-AFFF-Impacted-Groundwater-and-Soil.pdf ReseqarchGate]</ref><ref name="McGuire2014">McGuire, M.E., Schaefer, C., Richards, T., Backe, W.J., Field, J.A., Houtz, E., Sedlak, D.L., Guelfo, J.L., Wunsch, A., and Higgins, C.P., 2014. Evidence of Remediation-Induced Alteration of Subsurface Poly- and Perfluoroalkyl Substance Distribution at a Former Firefighter Training Area. Environmental Science and Technology, 48(12) pp. 6644-6652. [https://doi.org/10.1021/es5006187 DOI: 10.1021/es5006187]   Manuscript available from [https://ir.library.oregonstate.edu/downloads/td96k706f Oregon State University]</ref><ref name="Anderson2016">Anderson, R.H., Long, G.C., Porter, R.C. and Anderson, J.K., 2016. Occurrence of select perfluoroalkyl substances at US Air Force aqueous film-forming foam release sites other than fire-training areas: Field-validation of critical fate and transport properties. Chemosphere, 150, pp. 678-685. [https://doi.org/10.1016/j.chemosphere.2016.01.014 DOI: 10.1016/j.chemosphere.2016.01.014]</ref><ref name="Weber2017" />.

Transformation studies published to date have tested only a small subsample of possible precursors and, therefore, much uncertainty exists regarding 1) the extent to which precursor transformation occurs on a global scale, 2) which environmental compartments represent the majority of transformation, 3) relevant environmental conditions that affect transformation processes, and 4) transformation rates and pathways. Nevertheless, a portion of the precursors are expected to transform to PFAAs over time as shown in Figure 3.

Precursors can be transformed by a variety of abiotic processes including hydrolysis, photolysis, and oxidation. Hydrolysis of some precursors, followed by subsequent biotransformation, can produce perfluoroalkyl sulfonates (PFSAs). An important example is the production of PFOS from perfluorooctane sulfonyl fluoride (POSF)<ref name="Martin2010">Martin, J.W., Asher, B.J., Beesoon, S., Benskin, J.P. and Ross, M.S., 2010. PFOS or PreFOS? Are perfluorooctane sulfonate precursors (PreFOS) important determinants of human and environmental perfluorooctane sulfonate (PFOS) exposure? Journal of Environmental Monitoring, 12(11), pp.1979-2004. [https://doi.org/10.1039/C0EM00295J DOI: 10.1039/C0EM00295J]   Free download from [https://www.researchgate.net/profile/Matthew_Ross3/publication/47415684_PFOS_or_PreFOS_Are_perfluorooctane_sulfonate_precursors_PreFOS_important_determinants_of_human_and_environmental_perfluorooctane_sulfonate_PFOS_exposure/links/00b7d520a6132da945000000.pdf ResearchGate]</ref>. Other hydrolysis reactions produce perfluoroalkyl carboxylates (PFCAs). At neutral pH, the hydrolysis of fluorotelomer-derived polymeric precursors results in the formation of monomeric precursors of PFOA and other PFAAs with half-lives of 50 to 90 years<ref name="Washington2010">Washington, J.W., Ellington, J.J., Jenkins, T.M. and Yoo, H., 2010. Response to Comments on “Degradability of an Acrylate-Linked, Fluorotelomer Polymer in Soil”. Environmental Science and Technology, 44(2), pp. 849-850. [https://doi.org/10.1021/es902672q DOI: 10.1021/es902672q]   [https://pubs.acs.org/doi/pdf/10.1021/es902672q Free Download from ACS].</ref>. Oxidation of precursors by hydroxyl radicals can occur in natural waters, with the fluorotelomer-derived precursors being oxidized relatively rapidly<ref name="Gauthier2005">Gauthier, S.A. and Mabury, S.A., 2005. Aqueous photolysis of 8: 2 fluorotelomer alcohol. Environmental Toxicology and Chemistry, 24(8), pp.1837-1846. [https://doi.org/10.1897/04-591R.1 DOI: 10.1897/04-591R.1]   Free download from [https://www.researchgate.net/profile/Suzanne_Gauthier/publication/7609648_Aqueous_photolysis_of_8_2_fluorotelomer_alcohol/links/5ec16c4792851c11a86d9438/Aqueous-photolysis-of-8-2-fluorotelomer-alcohol.pdf ResearchGate].</ref><ref name="Plumlee2009">Plumlee, M.H., McNeill, K. and Reinhard, M., 2009. Indirect Photolysis of Perfluorochemicals: Hydroxyl Radical-Initiated Oxidation of N-Ethyl Perfluorooctane Sulfonamido Acetate (N-EtFOSAA) and Other Perfluoroalkanesulfonamides. Environmental Science and Technology, 43(10), pp.3662-3668. [https://doi.org/10.1021/es803411w DOI: 10.1021/es803411w]   Free download from [https://www.researchgate.net/profile/Megan_Plumlee/publication/26309488_Indirect_Photolysis_of_Perfluorochemicals_Hydroxyl_Radical-Initiated_Oxidation_of_N-Ethyl_Perfluorooctane_Sulfonamido_Acetate_N-EtFOSAA_and_Other_Perfluoroalkanesulfonamides/links/5aac0437a6fdcc1bc0b8d002/Indirect-Photolysis-of-Perfluorochemicals-Hydroxyl-Radical-Initiated-Oxidation-of-N-Ethyl-Perfluorooctane-Sulfonamido-Acetate-N-EtFOSAA-and-Other-Perfluoroalkanesulfonamides.pdf ResearchGate].</ref>.
Evidence of aerobic biotransformation is provided from studies of PFAS composition throughout the continuum of wastewater treatments<ref name="Arvaniti2015">Arvaniti, O.S. and Stasinakis, A.S., 2015. Review on the occurrence, fate and removal of perfluorinated compounds during wastewater treatment. Science of the Total Environment, 524, pp. 81-92. [https://doi.org/10.1016/j.scitotenv.2015.04.023 DOI: 10.1016/j.scitotenv.2015.04.023]</ref>, from field studies at AFFF-impacted sites<ref name="Houtz2013" /><ref name="McGuire2014" /><ref name="Anderson2016" /><ref name="Weber2017" />, and from microcosm experiments. In general, the literature on aerobic biotransformation collectively demonstrates or indirectly supports the following conclusions as summarized in ITRC 2020<ref name="ITRC2020" />:

[[File:Fate-and-transport_v3.mp4|thumb|500px|left|Figure 4. PFAS Fate and Transport.]]
*Numerous aerobic biotransformation pathways exist with relatively rapid kinetics
*All polyfluorinated precursors studied to date have the potential to aerobically biotransform to PFAAs
*Aerobic biotransformation of various fluorotelomer-derived precursors exclusively results in the formation of PFCAs, including PFOA, without necessarily the conservation of chain-length
*Aerobic biotransformation of various electrochemical fluorination-derived precursors primarily results in the formation of PFAAs, including PFOS, with the conservation of chain-length

Precursor transformation can complicate CSMs (and risk assessments) and should be considered during comprehensive site investigations. For example, atmospheric emissions of volatile precursors can result in long-range transport where subsequent transformation and deposition can result in detectable levels of PFAAs in environmental media independent of obvious point-sources<ref name="Vedagiri2018">Vedagiri, U.K., Anderson, R.H., Loso, H.M. and Schwach, C.M., 2018. Ambient levels of PFOS and PFOA in multiple environmental media. Remediation Journal, 28(2), pp. 9-51. [https://doi.org/10.1002/rem.21548 DOI: 10.1002/rem.21548]</ref>. With respect to site-related precursors, transformation of otherwise unmeasured PFAS into detectable PFAAs is obviously relevant to site investigations to the extent transformation occurs after initial site characterization efforts or if past remedial efforts have accelerated ''in situ'' transformation rates<ref name="McGuire2014" />. Additionally, differential transport rates between precursor PFAS and the corresponding terminal PFAA could also confound CSMs if transformation rates are slower than transport rates as has been suggested<ref name="Weber2017" />.
To account for otherwise unmeasurable precursors, several surrogate analytical methods have been developed.

==References==
<references />

==See Also:==
[https://soundcloud.com/arcadis-north-america/pfas-understanding-fate-and-transport-in-the-environment?utm_source=clipboard&utm_campaign=wtshare&utm_medium=widget&utm_content=https%253A%252F%252Fsoundcloud.com%252Farcadis-north-america%252Fpfas-understanding-fate-and-transport-in-the-environment SERDP & ESTCP PFAS Research and Remediation Podcast: PFAS: Understanding Fate and Transport in the Environment]

[https://soundcloud.com/arcadis-north-america/how-pfas-moves-from-afff-areas-to-groundwater SERDP & ESTCP PFAS Research and Remediation Podcast: How PFAS Moves from AFFF Areas to Groundwater]

PFAS Sources

2026-08-19T11:39:06Z

Admin:

[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]] have been used in coatings for textiles, paper products, and cookware; in some firefighting foams; and have a range of applications in the aerospace, photographic imaging, semiconductor, automotive, construction, electronics, and aviation industries<ref name="ITRC2020">Interstate Technology and Regulatory Council (ITRC), 2020. PFAS Technical and Regulatory Guidance Document and Fact Sheets, PFAS-1. PFAS Team, Washington, DC. [https://pfas-1.itrcweb.org/ Website]   [//www.enviro.wiki/images/7/74/ITRC_PFAS-1_092020.pdf Report.pdf]</ref><ref name="KEMI2015">Swedish Chemicals Agency (KEMI), 2015. Occurrence and use of highly fluorinated substances and alternatives, Report 7/15. ISSN 0284-1185. Article number 361 164. [//www.enviro.wiki/images/d/df/KEMI2015.pdf Report.pdf]</ref><ref name="USEPA2021">US Environmental Protection Agency (USEPA), 2021. Basic Information on PFAS. [https://www.epa.gov/pfas/basic-information-pfas#tab-1 Website]</ref>. Although PFAS and PFAS-containing products have been manufactured since the 1950s, PFAS were not widely documented in environmental samples until the early 2000s. Understanding PFAS manufacturing history, past and current uses, and waste management over the last six to seven decades is necessary for the identification of potential environmental sources of PFAS, possible release mechanisms, and associated pathway-receptor relationships.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]
*[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]
*[[PFAS Ex Situ Water Treatment]]
*[[PFAS Leaching Characterization with the Leaching Environmental Assessment Framework (LEAF)]]
*[[PFAS Soil Remediation Technologies]]
*[[PFAS Transport and Fate]]
*[[PFAS Treatment by Electrical Discharge Plasma]]
*[[Supercritical Water Oxidation (SCWO)]]

'''Contributor(s):''' [[Dr. Dora Chiang | Dr. Dora Chiang]] and [[Dr. Alexandra Salter-Blanc | Dr. Alexandra Salter-Blanc]]

'''Key Resource(s):'''

*[https://pfas-1.itrcweb.org/ Per- and Polyfluoroalkyl Substances (PFAS), PFAS-1. ITRC 2020.]<ref name="ITRC2020" />

==Introduction==
[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]] are a complex family of more than 3,000 manmade fluorinated organic chemicals<ref name="Wang2017">Wang, Z., DeWitt, J.C., Higgins, C.P., and Cousins, I.T., 2017. A Never-Ending Story of Per- and Poly-Fluoroalkyl Substances (PFASs)? Environmental Science and Technology, 51(5), pp. 2508-2518. [https://doi.org/10.1021/acs.est.6b04806 DOI: 10.1021/acs.est.6b04806]   [//www.enviro.wiki/images/e/e8/Wang2017.pdf Open access article.]</ref> although not all of these are currently in use or production. PFAS are produced using several different processes. Fluorosurfactants, which include perfluoroalkyl acids (PFAAs) (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]] article for nomenclature) and side-chain fluorinated polymers, have been manufactured using two major processes: [[Wikipedia: Electrochemical fluorination | electrochemical fluorination (ECF)]] and [[Wikipedia: Telomerization | telomerization]]<ref name="KEMI2015" />. ECF was licensed by 3M in the 1940s<ref name="Banks1994">Banks, R.E., Smart, B.E. and Tatlow, J.C. eds., 1994. Organofluorine Chemistry: Principles and Commercial Applications. Springer Science and Business Media, New York, N. Y. [https://link.springer.com/book/10.1007/978-1-4899-1202-2 DOI: 10.1007/978-1-4899-1202-2]</ref> and used by 3M until 2001. ECF produces a mixture of even and odd numbered carbon chain lengths of approximately 70% linear and 30% branched substances<ref name="Concawe2016">Concawe (Conservation of Clean Air and Water in Europe), 2016. Environmental fate and effects of poly- and perfluoroalkyl substances (PFAS). Report No. 8/16. Brussels, Belgium. [//www.enviro.wiki/images/d/de/Concawe2016.pdf Report.pdf]</ref>. Telomerization was developed in the 1970s<ref name="Benskin2012a">Benskin, J.P., Ahrens, L., Muir, D.C., Scott, B.F., Spencer, C., Rosenberg, B., Tomy, G., Kylin, H., Lohmann, R. and Martin, J.W., 2012. Manufacturing Origin of Perfluorooctanoate (PFOA) in Atlantic and Canadian Arctic Seawater. Environmental Science and Technology, 46(2), pp. 677-685. [https://doi.org/10.1021/es202958p DOI: 10.1021/es202958p]</ref>, and yields mainly even numbered, straight carbon chain isomers<ref name="Kissa2001">Kissa, E., 2001. Fluorinated Surfactants and Repellents, Second Edition. Surfactant Science Series, Vol. 97. Marcel Dekker, Inc., CRC Press, New York. 640 pages. ISBN: 9780824704728</ref><ref name="Parsons2008">Parsons, J.R., Sáez, M., Dolfing, J. and De Voogt, P., 2008. Biodegradation of Perfluorinated Compounds. Reviews of Environmental Contamination and Toxicology, 196, pp. 53-71. Springer, New York, NY. [https://doi.org/10.1007/978-0-387-78444-1_2 DOI: 10.1007/978-0-387-78444-1_2]   Free download from: [https://www.researchgate.net/profile/Jan_Dolfing/publication/23489065_Biodegradation_of_Perfluorinated_Compounds/links/0912f5087a40c9d5df000000.pdf ResearchGate]</ref>. PFAS manufacturers have provided PFAS to secondary manufacturers for production of a vast array of industrial and consumer products.

During manufacturing, PFAS may be released into the atmosphere then redeposited on land where they can also affect surface water and groundwater, or PFAS may be discharged without treatment to wastewater treatment plants or landfills, and eventually be released into the environment by treatment systems that are not designed to mitigate PFAS (see also [[PFAS Transport and Fate]]). Industrial discharges of PFAS were unregulated for many years, but that has begun to change. In January 2016, New York became the first state in the nation to regulate PFOA as a hazardous substance followed by the regulation of PFOS in April 2016. Consumer and industrial uses of PFAS-containing products can also end up releasing PFAS into landfills and into municipal wastewater, where it may accumulate undetected in biosolids which are typically treated by land application.

==Industrial Sources==
[[File: ChiangSalterBlanc1w2Fig0.png | thumb | 700px | Figure 1. Conceptual Site Model for PFAS industrial sites<ref name="ITRC2020" />. Adapted from figure by L. Trozzolo, TRC, used with permission.]]
PFAS are used in many industrial and consumer applications, which may have released PFAS into the environment and impacted drinking water supplies in many areas of the United States<ref name="EWG2017">Environmental Working Group (EWG) and Northeastern University Social Science Environmental Health Research Institute, 2017. Mapping A Contamination Crisis. [https://www.ewg.org/research/mapping-contamination-crisis Website]</ref>. Both in the United States (US) and abroad, primary manufacturing facilities produce PFAS and secondary manufacturing facilities use PFAS to produce goods. Environmental release mechanisms associated with these facilities include air emission and dispersion, spills, and disposal of manufacturing wastes and wastewater. Potential impacts to air, soil, sediment, surface water, stormwater, and groundwater are present not only at primary release points but potentially over the surrounding area<ref name="Shin2011">Shin, H.M., Vieira, V.M., Ryan, P.B., Detwiler, R., Sanders, B., Steenland, K., and Bartell, S.M., 2011. Environmental Fate and Transport Modeling for Perfluorooctanoic Acid Emitted from the Washington Works Facility in West Virginia. Environmental Science and Technology, 45(4), pp. 1435-1442. [https://doi.org/10.1021/es102769t DOI: 10.1021/es102769t]</ref>, as illustrated in Figure 1. Some of the potential primary and secondary sources of PFAS releases to the environment are listed here<ref name="ITRC2020" />:

*'''Textiles and leather:''' Factory or consumer applied coating to repel water, oil, and stains. Applications include protective clothing and outerwear, umbrellas, tents, sails, architectural materials, carpets, and upholstery<ref name="Rao1994">Rao, N.S., and Baker, B.E., 1994. Textile Finishes and Fluorosurfactants. In: Organofluorine Chemistry, Banks, R.E., Smart, B.E., and Tatlow, J.C., Eds. Springer, New York. [https://doi.org/10.1007/978-1-4899-1202-2_15 DOI: 10.1007/978-1-4899-1202-2_15]</ref><ref name="Hekster2003">Hekster, F.M., Laane, R.W. and De Voogt, P., 2003. Environmental and Toxicity Effects of Perfluoroalkylated Substances. Reviews of Environmental Contamination and Toxicology, 179, pp. 99-121. Springer, New York, NY. [https://doi.org/10.1007/0-387-21731-2_4 DOI: 10.1007/0-387-21731-2_4]</ref><ref name="Brooke2004">Brooke, D., Footitt, A., and Nwaogu, T.A., 2004. Environmental Risk Evaluation Report: Perfluorooctanesulphonate (PFOS). Environment Agency (UK), Science Group. Free download from: [http://chm.pops.int/Portals/0/docs/from_old_website/documents/meetings/poprc/submissions/Comments_2006/sia/pfos.uk.risk.eval.report.2004.pdf The Stockholm Convention]   [//www.enviro.wiki/images/d/df/Brooke2004.pdf Report.pdf]</ref><ref name="Poulsen2005">Poulsen, P.B., Jensen, A.A., and Wallström, E., 2005. More environmentally friendly alternatives to PFOS-compounds and PFOA. Danish Environmental Protection Agency, Environmental Project 1013. [//www.enviro.wiki/images/c/c2/Poulsen2005.pdf Report.pdf]</ref><ref name="Prevedouros2006">Prevedouros, K., Cousins, I.T., Buck, R.C. and Korzeniowski, S.H., 2006. Sources, Fate and Transport of Perfluorocarboxylates. Environmental Science and Technology, 40(1), pp. 32-44. [https://doi.org/10.1021/es0512475 DOI: 10.1021/es0512475]   Free download from: [https://www.academia.edu/download/39945519/Sources_Fate_and_Transport_of_Perfluoroc20151112-1647-19vcvbf.pdf Academia.edu]</ref><ref name="Walters2006">Walters, A., and Santillo, D., 2006. Technical Note 06/2006: Uses of Perfluorinated Substances. Greenpeace Research Laboratories. [http://www.greenpeace.to/publications/uses-of-perfluorinated-chemicals.pdf Website]   [//www.enviro.wiki/images/3/3a/Walters2006.pdf Report.pdf]</ref><ref name="Trudel2008">Trudel, D., Horowitz, L., Wormuth, M., Scheringer, M., Cousins, I.T. and Hungerbühler, K., 2008. Estimating Consumer Exposure to PFOS and PFOA. Risk Analysis: An International Journal, 28(2), pp. 251-269. [https://doi.org/10.1111/j.1539-6924.2008.01017.x DOI: 10.1111/j.1539-6924.2008.01017.x]</ref><ref name="Guo2009">Guo, Z., Liu, X., Krebs, K.A. and Roache, N.F., 2009. Perfluorocarboxylic Acid Content in 116 Articles of Commerce, EPA/600/R-09/033. National Risk Management Research Laboratory, US Environmental Protection Agency, Washington, DC. Available from: [https://cfpub.epa.gov/si/si_public_record_report.cfm?Lab=NRMRL&dirEntryId=206124 US EPA.]   [//www.enviro.wiki/images/9/9e/Guo2009.pdf Report.pdf]</ref><ref name="USEPA2009">US Environmental Protection Agency (USEPA), 2009. Long-Chain Perfluorinated Chemicals (PFCs), Action Plan. [https://www.epa.gov/sites/production/files/2016-01/documents/pfcs_action_plan1230_09.pdf Website]   [//www.enviro.wiki/images/b/b8/USEPA2009.pdf Report.pdf]</ref><ref name="Ahrens2011a">Ahrens, L., 2011. Polyfluoroalkyl compounds in the aquatic environment: a review of their occurrence and fate. Journal of Environmental Monitoring, 13(1), pp.20-31.
[http://dx.doi.org/10.1039/C0EM00373E DOI: 10.1039/C0EM00373E]. Free download available from: [https://www.researchgate.net/profile/Lutz_Ahrens/publication/47622154_Polyfluoroalkyl_compounds_in_the_aquatic_environment_A_review_of_their_occurrence_and_fate/links/00b7d53762cfedaf12000000/Polyfluoroalkyl-compounds-in-the-aquatic-environment-A-review-of-their-occurrence-and-fate.pdf ResearchGate]</ref><ref name="Buck2011">Buck, R.C., Franklin, J., Berger, U., Conder, J.M., Cousins, I.T., De Voogt, P., Jensen, A.A., Kannan, K., Mabury, S.A. and van Leeuwen, S.P., 2011. Perfluoroalkyl and Polyfluoroalkyl Substances in the Environment: Terminology, Classification, and Origins. Integrated Environmental Assessment and Management, 7(4), pp. 513-541. [https://doi.org/10.1002/ieam.258 DOI: 10.1002/ieam.258]   [//www.enviro.wiki/images/6/6f/Buck2011.pdf Open access article.]</ref><ref name="UNEP2011">United Nations Environmental Programme (UNEP), 2011. Report of the persistent organic pollutants review committee on the work of its sixth meeting, Addendum, Guidance on alternatives to perfluorooctane sulfonic acid and its derivatives, UNEP/POPS/POPRC.6/13/Add.3/Rev.1 [http://www.pops.int/TheConvention/POPsReviewCommittee/Meetings/POPRC6/POPRC6Documents/tabid/783/ctl/Download/mid/3507/Default.aspx?id=125 Website]   [//www.enviro.wiki/images/e/ee/UNEP2011.pdf Report.pdf]</ref><ref name="Herzke2012">Herzke, D., Olsson, E. and Posner, S., 2012. Perfluoroalkyl and polyfluoroalkyl substances (PFASs) in consumer products in Norway – A pilot study. Chemosphere, 88(8), pp. 980-987. [https://doi.org/10.1016/j.chemosphere.2012.03.035 DOI: 10.1016/j.chemosphere.2012.03.035]</ref><ref name="Patagonia2016">Patagonia, Inc., 2016. An Update on Our DWR Problem. [https://www.patagonia.com/stories/our-dwr-problem-updated/story-17673.html Website]   [//www.enviro.wiki/images/4/41/Patagonia2016.pdf Report.pdf]</ref><ref name="Kotthoff2015">Kotthoff, M., Müller, J., Jürling, H., Schlummer, M., and Fiedler, D., 2015. Perfluoroalkyl and polyfluoroalkyl substances in consumer products. Environmental Science and Pollution Research, 22(19), pp. 14546-14559. [https://doi.org/10.1007/s11356-015-4202-7 DOI: 10.1007/s11356-015-4202-7]   [//www.enviro.wiki/images/c/c8/Kotthoff2015.pdf Open access article.]</ref><ref name="ATSDR2018">Agency for Toxic Substances and Disease Registry (ATSDR), 2018. Toxicological Profile for Perfluoroalkyls, Draft for Public Comment. US Department of Health and Human Services. Free download from: [http://www.atsdr.cdc.gov/toxprofiles/tp200.pdf ATSDR]   [//www.enviro.wiki/images/e/eb/ATSDR2018.pdf Report.pdf]</ref>.

*'''Paper products:''' Surface coatings to repel grease and moisture. Uses include non-food paper packaging (for example, cardboard, carbonless forms, masking papers) and food-contact materials (for example, pizza boxes, fast food wrappers, microwave popcorn bags, baking papers, pet food bags)<ref name="Rao1994" /><ref name="Kissa2001" /><ref name="Hekster2003" /><ref name="Poulsen2005" /><ref name="Trudel2008" /><ref name="Buck2011" /><ref name="UNEP2011" /><ref name="Kotthoff2015" /><ref name="Schaider2017">Schaider, L.A., Balan, S.A., Blum, A., Andrews, D.Q., Strynar, M.J., Dickinson, M.E., Lunderberg, D.M., Lang, J.R., and Peaslee, G.F., 2017. Fluorinated Compounds in US Fast Food Packaging. Environmental Science and Technology Letters, 4(3), pp. 105-111. [https://doi.org/10.1021/acs.estlett.6b00435 DOI: 10.1021/acs.estlett.6b00435]   [//www.enviro.wiki/images/b/b8/Schaider2017.pdf Open access article.]</ref>

*'''Metal Plating & Etching:''' Corrosion prevention, mechanical wear reduction, aesthetic enhancement, surfactant, wetting agent/fume suppressant for chrome, copper, nickel and tin electroplating, and post-plating cleaner<ref name="USEPA1996">US Environmental Protection Agency (USEPA), 1996. Emission Factor Documentation for AP-42, Section 12.20. Office of Air Quality Planning and Standards, Emission Factor and Inventory Group, Research Triangle Park, NC. [//www.enviro.wiki/images/a/a3/USEPA1996.pdf Report.pdf]</ref><ref name="Riordan1998">Riordan, B.J., Karamchandanl, R.T., Zitko, L.J., and Cushnie Jr., G.C., 1998. Capsule Report: Hard Chrome Fume Suppressants and Control Technologies. Center for Environmental Research Information, National Risk Management Research Laboratory, Office of Research and Development. EPA/625/R-98/002 [https://cfpub.epa.gov/si/si_public_record_Report.cfm?Lab=NRMRL&dirEntryID=115419 Website]   [//www.enviro.wiki/images/b/bd/Riordan1998.pdf Report.pdf]</ref><ref name="Kissa2001" /><ref name="Prevedouros2006" /><ref name="USEPA2009a">US Environmental Protection Agency (USEPA), 2009. PFOS Chromium Electroplater Study. US EPA – Region 5, Chicago, IL. [//www.enviro.wiki/images/1/11/USEPA2009a.pdf Report.pdf]</ref><ref name="UNEP2011" /><ref name="OSHA2013">Occupational Safety and Health Agency (OSHA), 2013. Fact Sheet: Controlling Hexavalent Chromium Exposures during Electroplating. United States Department of Labor. [//www.enviro.wiki/images/9/90/OSHA2013.pdf Report.pdf]</ref><ref name="KEMI2015" /><ref name="DEPA2015">Danish Environmental Protection Agency, 2015. Alternatives to perfluoroalkyl and polyfluoroalkyl substances (PFAS) in textiles. [//www.enviro.wiki/images/f/f4/DEPA2015.pdf Report.pdf]</ref>

*'''Wire Manufacturing:''' Coating and insulation<ref name="Kissa2001" /><ref name="vanderPutte2010">van der Putte, I., Murin, M., van Velthoven, M., and Affourtit, F., 2010. Analysis of the risks arising from the industrial use of Perfluorooctanoic acid (PFOA) and Ammonium Perfluorooctanoate (APFO) and from their use in consumer articles. Evaluation of the risk reduction measures for potential restrictions on the manufacture, placing on the market and use of PFOA and APFO. RPS Advies, Delft, The Netherlands for European Commission Enterprise and Industry Directorate-General. [https://ec.europa.eu/docsroom/documents/13037/attachments/1/translations/en/renditions/pdf Website]   [//www.enviro.wiki/images/7/7b/VanderPutte2010.pdf Report.pdf]</ref><ref name="ASTSWMO2015">Association of State and Territorial Solid Waste Management Officials (ASTSWMO), 2015. Perfluorinated Chemicals (PFCs): Perfluorooctanoic Acid (PFOA) and Perfluorooctane Sulfonate (PFOS) Information Paper. Remediation and Reuse Focus Group, Federal Facilities Research Center, Washington, D.C. Free download from: [https://clu-in.org/download/contaminantfocus/pops/POPs-ASTSWMO-PFCs-2015.pdf US EPA]   [//www.enviro.wiki/images/3/3a/Deeb-Article_1-Table_2-L10-Provisional_Groundwater_Remediaton_Objectives_Class_I_Groundwater.pdf Report.pdf]</ref>

*'''Industrial Surfactants, Resins, Molds, Plastics:''' Manufacture of plastics and fluoropolymers, rubber, and compression mold release coatings; plumbing fluxing agents; fluoroplastic coatings, composite resins, and flame retardant for polycarbonate<ref name="Kissa2001" /><ref name="Renner2001">Renner, R., 2001. Growing Concern Over Perfluorinated Chemicals. Environmental Science and Technology, 35(7), pp. 154A-160A. [https://doi.org/10.1021/es012317k DOI: 10.1021/es012317k]   [//www.enviro.wiki/images/f/f5/Renner2001.pdf Open access article.]</ref><ref name="Poulsen2005" /><ref name="Fricke2005">Fricke, M. and Lahl, U., 2005. Risk Evaluation of Perfluorinated Surfactants as Contribution to the current Debate on the EU Commission’s REACH Document. Umweltwissenschaften und Schadstoff-Forschung (UWSF), 17(1), pp. 36-49. [https://doi.org/10.1007/BF03038694 DOI: 10.1007/BF03038694]</ref><ref name="Prevedouros2006" /><ref name="Skutlarek2006">Skutlarek, D., Exner, M. and Färber, H., 2006. Perfluorinated Surfactants in Surface and Drinking Waters. Environmental Science and Pollution Research International, 13(5), pp. 299-307. [https://doi.org/10.1065/espr2006.07.326 DOI: 10.1065/espr2006.07.326]   Free download from: [https://www.researchgate.net/profile/Dirk_Skutlarek/publication/6729263_Perfluorinated_surfactants_in_surface_and_drinking_waters/links/0deec52049b9cba2e4000000.pdf ResearchGate]</ref><ref name="vanderPutte2010" /><ref name="Buck2011" /><ref name="Herzke2012" /><ref name="Kotthoff2015" /><ref name="Chemours2010">Chemours, 2010. The History of Teflon Fluoropolymers. [https://www.teflon.com/en/news-events/history Website]</ref>

*'''Photolithography, Semiconductor Industry:''' Photoresists, top anti-reflective coatings, bottom anti-reflective coatings, and etchants, with other uses including surfactants, wetting agents, and photo-acid generation<ref name="Choi2005">Choi, D.G., Jeong, J.H., Sim, Y.S., Lee, E.S., Kim, W.S. and Bae, B.S., 2005. Fluorinated Organic− Inorganic Hybrid Mold as a New Stamp for Nanoimprint and Soft Lithography. Langmuir, 21(21), pp. 9390-9392. [https://doi.org/10.1021/la0513205 DOI: 10.1021/la0513205]</ref><ref name="Rolland2004">Rolland, J.P., Van Dam, R.M., Schorzman, D.A., Quake, S.R., and DeSimone, J.M., 2004. Solvent-Resistant Photocurable “Liquid Teflon” for Microfluidic Device Fabrication. Journal of the American Chemical Society, 126(8), pp. 2322-2323. [https://doi.org/10.1021/ja031657y DOI: 10.1021/ja031657y]</ref><ref name="Brooke2004" /><ref name="vanderPutte2010" /><ref name="UNEP2011" /><ref name="Herzke2012" />

==Class B Firefighting Foams==
[[File: ChiangSalterBlanc1w2Fig0.5.png | thumb | 700px | Figure 2. Conceptual Site Model for fire training areas<ref name="ITRC2020" />. Adapted from figure by L. Trozzolo, TRC, used with permission.]]
Aqueous film forming foam (AFFF) and other fluorinated Class B firefighting foams are another important source of PFAS to the environment, especially in military and aviation settings. [[Wikipedia: Firefighting foam | Class B firefighting foams]] have been used since the 1960s to extinguish flammable liquid hydrocarbon fires and for vapor suppression. These foams contain complex and variable mixtures of PFAS that act as surfactants. Fluorinated surfactants are both hydrophobic and oleophobic (oil-repelling), as well as thermally stable, chemically stable, and highly surface active<ref name="Moody1999">Moody, C.A. and Field, J.A., 1999. Determination of Perfluorocarboxylates in Groundwater Impacted by Fire-Fighting Activity. Environmental Science and Technology, 33(16), pp. 2800-2806. [https://pubs.acs.org/doi/10.1021/es981355%2B DOI: 10.1021/es981355+]</ref>. These properties make them uniquely suited to fighting hydrocarbon fuel fires. Use of fluorinated Class B foams is prevalent and is a major source of PFAS release to the environment, as shown in Figure 2. Release to the environment typically occurs during firefighting operations, firefighter training, apparatus testing, or leakage during storage. Research into fluorine-free alternatives is underway and Congressional pressure is leading towards banning fluorinated Class B firefighting foams in the United States.

[[File: ChiangSalterBlanc1w2Fig1.png | thumb | 500px | Figure 3. Types of Class B firefighting foams<ref name="ITRC2020" />. Source: S. Thomas, Wood, PLC. Used with permission.]]
When discussing the relationship between firefighting foams and sources of PFAS to the environment, the emphasis is typically on AFFF; however, many different types of Class B firefighting foams exist. These may or may not be fluorinated (contain PFAS). Class B foams are used to extinguish Class B fires, that is, those involving flammable liquids. Fluorinated Class B foams spread across the surface of the flammable liquid forming a thin film and extinguish fires by (1) excluding air from the flammable vapors, (2) suppressing vapor release, (3) physically separating the flames from the fuel source, and (4) cooling the fuel surface and surrounding metal surfaces<ref name="NationalFoam">National Foam, no date. A Firefighter’s Guide to Foam. [http://foamtechnology.us/Firefighters.pdf Website]   [//www.enviro.wiki/images/9/9e/NationalFoam.pdf Report.pdf]</ref>. From a PFAS perspective, Class B firefighting foams can be divided into two broad categories: fluorinated foams (that contain PFAS) and fluorine-free foams (that do not contain PFAS)<ref name="ITRC2020" />. This distinction and examples of each type are shown in Figure 3.

AFFF was developed by the US Navy in the 1960s and in 1969, the US Department of Defense (DoD) issued military specification MIL-F-24385 listing firefighting performance requirements for all AFFF used within the US DoD<ref name="ITRC2020" /><ref name="Navy1969">US Navy, 1969. Military Specification MIL-F-24385(NAVY). Fire Extinguishing Agent, Aqueous Film Forming Foam (AFFF) Liquid Concentrate, Six Percent, for Fresh and Sea Water. Department of Defense, Hyattsville, Maryland. [https://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=17270 Website]   [//www.enviro.wiki/images/c/c5/MilspecAFFF1969.pdf Report.pdf]</ref><ref name="Navy2020">US Navy, 2020. Performance Specification MIL-PRF-24385F(SH) with Amendment 4. Fire Extinguishing Agent, Aqueous Film Forming Foam (AFFF) Liquid Concentrate for Fresh and Sea Water. Department of Defense, Washington, DC. [https://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=17270 Website]   [//www.enviro.wiki/images/5/58/MilspecAFFF2020.pdf Report.pdf]</ref>. These performance standards are often referred to as “Mil-Spec.” Products that meet the Mil-Spec have been added to the US DoD [https://qpldocs.dla.mil/ Qualified Product Listing (QPL)]. In 2006 the US Federal Aviation Administration (FAA) also began requiring that 14-CFR-139-certified commercial airports purchase Mil-Spec compliant AFFF only. Because the US DoD and FAA have been the primary purchasers of AFFF, development of AFFF product mixtures has historically been performance-driven (to comply with the Mil-Spec) rather than formula-driven (the specific PFAS mixtures utilized have varied over time and by manufacturer). Multiple manufacturers in the US and throughout the world produce or have produced AFFF concentrate<ref name="ITRC2020" />. AFFF concentrate is or has been available in 1%, 3%, or 6% formulations, where the percentage designates the recommended percentage of concentrate to be mixed into water during application.

The specific mixtures of PFAS found in AFFF have varied by manufacturer and over time due to differences in production processes and voluntary formula changes. AFFF formulations can generally be grouped into three categories<ref name="ITRC2020" />:

*'''Legacy Perfluorooctane Sulfonate (PFOS) AFFF''' This type of AFFF was manufactured exclusively by 3M under the brand name “Lightwater” from the late 1960s until 2002 using the ECF production process. They contain PFOS and perflouroalkane sulfonates (PFSAs) such as perfluorohexane sulfonate (PFHxS)<ref name="ITRC2020" /><ref name="Backe2013">Backe, W.J., Day, T.C. and Field, J.A., 2013. Zwitterionic, Cationic, and Anionic Fluorinated Chemicals in Aqueous Film Forming Foam Formulations and Groundwater from US Military Bases by Nonaqueous Large-Volume Injection HPLC-MS/MS. Environmental Science and Technology, 47(10), pp. 5226-5234. [https://pubs.acs.org/doi/10.1021/es3034999 DOI: 10.1021/es3034999]</ref>. Legacy PFOS AFFF produced by ECF were voluntarily phased out in 2002, however, use of stockpiled product was permitted after that date<ref name="ITRC2020" />.

*'''Legacy fluorotelomer AFFF''' This group consists of AFFF manufactured and sold in the U.S. from the 1970s until 2016 and includes all brands that were produced using a process known as fluorotelomerization (FT). The FT manufacturing process produces polyfluorinated substances that can degrade in the environment to perfluoroalkyl substances (specifically PFAAs) including Perfluorooctanoic Acid (PFOA). Polyfluoroalkyl substances that degrade to create terminal PFAAs are referred to as “precursors” <ref name="ITRC2020" />.

*'''Modern fluorotelomer AFFF''' This group consists of AFFF developed in response to the USEPA 2010-2015 voluntary PFOA Stewardship Program<ref name="USEPA2018">US Environmental Protection Agency (USEPA), 2018. Fact Sheet: 2010/2015 PFOA Stewardship Program. [https://www.epa.gov/assessing-and-managing-chemicals-under-tsca/fact-sheet-20102015-pfoa-stewardship-program Website]</ref>, which asked companies to commit to first reducing and then eliminating the following: PFOA, precursors that can break down to PFOA, and related chemicals from facility emissions and products. In response, manufacturers began producing only short-chain fluorosurfactants targeting fluorotelomer PFAS with 6 carbons per chain (C6), rather than the traditional long-chain fluorosurfactants (8 or more carbons per chain). These short-chain PFAS do not breakdown in the environment to PFOS or PFOA<ref name="ITRC2020" />. Their toxicity in comparison to long-chain fluorosurfactants is a topic of current research.

In the US, AFFF users including the US DoD (predominantly the Navy and Air Force), civilian airports, oil refineries, other petrochemical industries, and municipal fire departments<ref name="Darwin2011">Darwin, Robert L. 2011. Estimated Inventory of PFOS-based Aqueous Film Forming Foam (AFFF). Fire Fighting Foam Coalition, Inc., Arlington, VA. [//www.enviro.wiki/images/4/49/Darwin2011.pdf Report.pdf]</ref>. AFFF is used, for example, in fire fighting vehicles, in fixed fire suppression systems (including sprinklers and fixed spray systems in or at aircraft hangars, flammable liquid storage areas, engine hush houses, and fuel farms), and onboard military and commercial ships. Fluorinated Class B foams may be introduced to the environment through the following practices<ref name="ITRC2020" />:

*low volume releases of foam concentrate during storage, transfer or operational requirements that mandate periodic equipment calibration
*moderate volume discharge of foam solution for apparatus testing and episodic discharge of AFFF-containing fire suppression systems within large aircraft hangars and buildings
*occasional, high-volume, broadcast discharge of foam solution for firefighting and fire suppression/prevention for emergency response
*periodic, high volume, broadcast discharge for fire training
*accidental leaks from foam distribution piping between storage and pumping locations, and from storage tanks and railcars

The DoD is currently replacing legacy, long-chain AFFF with modern, short-chain fluorotelomer AFFF and disposing of the legacy foams through incineration. While the PFAS included in modern fluorotelomer AFFF formulations are currently understood to be less toxic and less bioaccumulative than those used in legacy formulations, they are also environmentally persistent and can degrade to produce other PFAS that may pose environmental concerns<ref name="ITRC2020" />. While fluorine free alternatives exist, they do not meet the current Mil-Spec<ref name="Navy2020" /> which requires that fluorine-based compounds be used. The US DoD is working to revise the Mil-Spec to allow fluorine-free foams, and several states have passed laws prohibiting the use of fluorinated Class B foams for training and prohibiting future manufacture, sale or distribution of fluorinated foams, with limited exceptions<ref name="Denton2019">Denton, Charles, 2019. Expert Focus: US states outpace EPA on PFAS firefighting foam laws. Chemical Watch. [https://chemicalwatch.com/78075/expert-focus-us-states-outpace-epa-on-pfas-firefighting-foam-laws Website]</ref> (e.g., WA Rev Code § 70.75A.005 (2019); VA § 9.1-207.1 (2019)). Additionally, a bill passed in the US Congress in 2018 directs the FAA to allow fluorine-free foams for use at commercial airports<ref name="FAA2018">FAA Reauthorization Act of 2018. US Public Law No: 115-254 (10/05/2018). [https://www.congress.gov/bill/115th-congress/house-bill/302/text?r=1 Website]   [//www.enviro.wiki/images/0/06/FAA2018.pdf Report.pdf]</ref>. Research into the development of Mil-Spec compliant fluorine-free foams that will be compatible with existing AFFF and supporting equipment is ongoing and includes the following:

*Novel Fluorine-Free Replacement for Aqueous Film Forming Foam (Lead investigator: Dr. Joseph Tsang, Naval Air Warfare Center Weapons Divisions) [https://serdp-estcp.org/Program-Areas/Weapons-Systems-and-Platforms/Waste-Reduction-and-Treatment-in-DoD-Operations/WP-2737 SERDP/ESTCP Project WP-2737]
*Fluorine-Free Aqueous Film Forming Foam (Lead investigator: Dr. John Payne, National Foam) [https://serdp-estcp.org/Program-Areas/Weapons-Systems-and-Platforms/Waste-Reduction-and-Treatment-in-DoD-Operations/WP-2738 SERDP/ESTCP Project WP-2738]
*Fluorine-Free Foams with Oleophobic Surfactants and Additives for Effective Pool fire Suppression (Lead investigator: Dr. Ramagopal Ananth, U.S. Naval Research Laboratory) [https://serdp-estcp.org/Program-Areas/Weapons-Systems-and-Platforms/Waste-Reduction-and-Treatment-in-DoD-Operations/WP-2739 SERDP/ESTCP Project WP-2739]

==Wastewater Treatment Plants==
[[File: ChiangSalterBlanc1w2Fig4.png | thumb | 700px | Figure 4. Conceptual Site Model for landfills and WWTPs<ref name="ITRC2020" />. Adapted from figure by L. Trozzolo, TRC, used with permission.]]
Consumer and/or industrial uses of PFAS-containing materials results in the discharge of PFAS to industrial and municipal wastewater treatment plants (WWTPs). Conventional WWTP treatment processes remove less than 5% of PFAAs<ref name="Ahrens2011a" /><ref name="Schultz2006">Schultz, M.M., Higgins, C.P., Huset, C.A., Luthy, R.G., Barofsky, D.F., and Field, J.A., 2006. Fluorochemical Mass Flows in a Municipal Wastewater Treatment Facility. Environmental Science and Technology, 40(23), pp. 7350-7357. [https://doi.org/10.1021/es061025m DOI: 10.1021/es061025m]   [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2556954/ Author Manuscript]</ref><ref name="MWRA2019">Michigan Waste and Recycling Association (MWRA), 2019. Statewide Study on Landfill Leachate PFOA and PFOS Impact on Water Resource Recovery Facility Influent, Second Revision. [//www.enviro.wiki/images/f/ff/MWRA2019.pdf Report.pdf]</ref>. WWTPs, particularly those that receive industrial wastewater, are possible sources of PFAS release<ref name="Bossi2008">Bossi, R., Strand, J., Sortkjær, O. and Larsen, M.M., 2008. Perfluoroalkyl compounds in Danish wastewater treatment plants and aquatic environments. Environment International, 34(4), pp. 443-450. [https://doi.org/10.1016/j.envint.2007.10.002 DOI: 10.1016/j.envint.2007.10.002] Free download from: [https://www.academia.edu/download/43968517/Perfluoroalkyl_compounds_in_Danish_waste20160321-31116-esz4d1.pdf Academia.edu]</ref><ref name="Lin2014">Lin, A.Y.C., Panchangam, S.C., Tsai, Y.T., and Yu, T.H., 2014. Occurrence of perfluorinated compounds in the aquatic environment as found in science park effluent, river water, rainwater, sediments, and biotissues. Environmental Monitoring and Assessment, 186(5), pp. 3265-3275. [https://doi.org/10.1007/s10661-014-3617-9 DOI: 10.1007/s10661-014-3617-9]</ref><ref name="Ahrens2009">Ahrens, L., Felizeter, S., Sturm, R., Xie, Z. and Ebinghaus, R., 2009. Polyfluorinated compounds in waste water treatment plant effluents and surface waters along the River Elbe, Germany. Marine Pollution Bulletin, 58(9), pp.1326-1333. [https://doi.org/10.1016/j.marpolbul.2009.04.028 DOI: 10.1016/j.marpolbul.2009.04.028]   [//www.enviro.wiki/images/9/9e/Ahrens2009.pdf Author’s manuscript]</ref>, as shown in Figure 4.

Evaluation of full-scale WWTPs has indicated that conventional primary (sedimentation and clarification) and secondary (aerobic biodegradation of organic matter) treatment processes can result in changes in PFAS concentrations and classes. For example, higher concentrations of PFAAs have been observed in effluent than in influent, presumably due to transformation of precursor PFAS<ref name="Schultz2006" />. Some data has indicated that the terminal PFAS compounds PFOS and PFOA were among the most frequently detected PFAS in wastewater<ref name="Hamid2016">Hamid, H. and Li, L., 2016. Role of wastewater treatment plant in environmental cycling of poly- and perfluoroalkyl substances. Ecocycles, 2(2), pp. 43-53. [https://doi.org/10.19040/ecocycles.v2i2.62 DOI: 10.19040/ecocycles.v2i2.62]   [//www.enviro.wiki/images/6/67/Hamid2016.pdf Open access article.]</ref>. A state-wide study in Michigan indicated that PFAS were detected in all of the samples from 42 WWTPs, including influent, effluent, and biosolids/sludge samples, and that the short-chain PFAS were more frequently detected in the liquid process flow (influent and effluent), while long-chain PFAS were more common in biosolids<ref name="EGLE2020">Michigan Department of Environment, Great Lakes and Energy (EGLE), 2020. Summary Report: Initiatives to Evaluate the Presence of PFAS in Municipal Wastewater and Associated Residuals (Sludge/Biosolids) in Michigan. [//www.enviro.wiki/images/7/70/EGLE2020.pdf Report.pdf]  
[https://www.michigan.gov/documents/egle/wrd-pfas-initiatives_691391_7.pdf Website]</ref>.

Multiple studies have found PFAS in municipal sewage sludge<ref name="Higgins2005">Higgins, C.P., Field, J.A., Criddle, C.S., and Luthy, R.G., 2005. Quantitative Determination of Perfluorochemicals in Sediments and Domestic Sludge. Environmental Science and Technology, 39 (11), pp. 3946 – 3956. [https://doi.org/10.1021/es048245p DOI: 10.1021/es048245p]</ref><ref name="EGLE2020" />. The US EPA states that more than half of the sludge produced in the United States is applied to agricultural land as biosolids, therefore there are concerns that biosolids applications may become a potential source of PFAS to the environment<ref name="USEPA2020">US Environmental Protection Agency (USEPA), 2020. Research on Per- and Polyfluoroalkyl Substances (PFAS). [https://www.epa.gov/chemical-research/research-and-polyfluoroalkyl-substances-pfas Website]</ref>. Application of biosolids as a soil amendment can potentially result in transfer of PFAS to soil, surface water and groundwater and can possibly allow PFAS to enter the food chain<ref name="Sepulvado2011">Sepulvado, J.G., Blaine, A.C., Hundal, L.S. and Higgins, C.P., 2011. Occurrence and Fate of Perfluorochemicals in Soil Following the Land Application of Municipal Biosolids. Environmental Science and Technology, 45(19), pp. 8106-8112. [https://doi.org/10.1021/es103903d DOI: 10.1021/es103903d]</ref><ref name="Lindstrom2011">Lindstrom, A.B., Strynar, M.J., Delinsky, A.D., Nakayama, S.F., McMillan, L., Libelo, E.L., Neill, M. and Thomas, L., 2011. Application of WWTP Biosolids and Resulting Perfluorinated Compound Contamination of Surface and Well Water in Decatur, Alabama, USA. Environmental Science and Technology, 45(19), pp. 8015-8021. [https://doi.org/10.1021/es1039425 DOI: 10.1021/es1039425]</ref><ref name="Blaine2013">Blaine, A.C., Rich, C.D., Hundal, L.S., Lau, C., Mills, M.A., Harris, K.M. and Higgins, C.P., 2013. Uptake of Perfluoroalkyl Acids into Edible Crops via Land Applied Biosolids: Field and Greenhouse Studies. Environmental Science and Technology, 47(24), pp.14062-14069. [https://doi.org/10.1021/es403094q DOI: 10.1021/es403094q]   Free download from: [https://www.epa.gov/sites/production/files/2019-11/documents/508_pfascropuptake.pdf US EPA]</ref><ref name="Blaine2014">Blaine, A.C., Rich, C.D., Sedlacko, E.M., Hundal, L.S., Kumar, K., Lau, C., Mills, M.A., Harris, K.M. and Higgins, C.P., 2014. Perfluoroalkyl Acid Distribution in Various Plant Compartments of Edible Crops Grown in Biosolids-Amended Soils. Environmental Science and Technology, 48(14), pp. 7858-7865. [https://doi.org/10.1021/es500016s DOI: 10.1021/es500016s] Free download from: [https://www.researchgate.net/profile/Kuldip_Kumar2/publication/263015815_Perfluoroalkyl_Acid_Distribution_in_Various_Plant_Compartments_of_Edible_Crops_Grown_in_Biosolids-Amended_soils/links/5984cb310f7e9b6c852f4f02/Perfluoroalkyl-Acid-Distribution-in-Various-Plant-Compartments-of-Edible-Crops-Grown-in-Biosolids-Amended-soils.pdf ResearchGate]</ref><ref name="Navarro2017">Navarro, I., de la Torre, A., Sanz, P., Porcel, M.Á., Pro, J., Carbonell, G. and de los Ángeles Martínez, M., 2017. Uptake of perfluoroalkyl substances and halogenated flame retardants by crop plants grown in biosolids-amended soils. Environmental Research, 152, pp. 199-206. [https://doi.org/10.1016/j.envres.2016.10.018 DOI: 10.1016/j.envres.2016.10.018]</ref>. Limited studies have shown that PFAS concentrations can be elevated in surface and groundwater in the vicinity of agricultural fields that received PFAS contaminated biosolids for an extended period<ref name="Washington2010">Washington, J.W., Yoo, H., Ellington, J.J., Jenkins, T.M., and Libelo, E.L., 2010. Concentrations, Distribution, and Persistence of Perfluoroalkylates in Sludge-Applied Soils near Decatur, Alabama, USA. Environmental Science and Technology, 44(22), pp. 8390-8396. [https://doi.org/10.1021/es1003846 DOI: 10.1021/es1003846] Free download from: [https://www.researchgate.net/profile/John_Washington3/publication/47447289_Concentrations_Distribution_and_Persistence_of_Perfluoroalkylates_in_Sludge-Applied_Soils_near_Decatur_Alabama_USA/links/5e3c0184a6fdccd9658add41/Concentrations-Distribution-and-Persistence-of-Perfluoroalkylates-in-Sludge-Applied-Soils-near-Decatur-Alabama-USA.pdf ResearchGate]</ref>. The most abundant PFAS found in biosolids are the long-chain PFAS<ref name="Hamid2016" /><ref name="EGLE2020" />. Based on the persistence and stability of long-chain PFAS and their interaction with biosolids, research is ongoing to determine PFAS leachability from biosolids and their bioavailability for uptake by plants, soil organisms, and the consumers of potentially PFAS-impacted plants and soil organisms.

==Solid Waste Management Facilities==
Industrial, commercial, and consumer products containing PFAS that have been disposed in municipal solid waste (MSW) landfills or other legacy disposal areas since the 1950s are potential sources of PFAS release to the environment. Environmental and drinking water impacts from disposal of legacy PFAS-containing industrial and consumer wastes have been documented<ref name="Oliaei2010">Oliaei, F., Kriens, D. and Weber, R., 2010. Discovery and investigation of PFOS/PFCs contamination from a PFC manufacturing facility in Minnesota—environmental releases and exposure risks. Organohalogen Compd, 72, pp. 1338-1341.</ref><ref name="Shin2011" /><ref name="MDH2020">Minnesota Department of Health (MDH), 2020. Perfluoroalkyl Substances (PFAS) Sites in Minnesota. [https://www.health.state.mn.us/communities/environment/hazardous/topics/sites.html Website]</ref>.

Several studies have identified a wide variety of PFAS in MSW landfill leachates<ref name="Busch2010">Busch, J., Ahrens, L., Sturm, R. and Ebinghaus, R., 2010. Polyfluoroalkyl compounds in landfill leachates. Environmental Pollution, 158(5), pp.1467-1471. [https://doi.org/10.1016/j.envpol.2009.12.031 DOI: 10.1016/j.envpol.2009.12.031]</ref><ref name="Eggen2010">Eggen, T., Moeder, M. and Arukwe, A., 2010. Municipal landfill leachates: A significant source for new and emerging pollutants. Science of the Total Environment, 408(21), pp. 5147-5157. [https://doi.org/10.1016/j.scitotenv.2010.07.049 DOI: 10.1016/j.scitotenv.2010.07.049]</ref>. PFAS composition and concentration in leachates vary depending on waste age, climate, and waste composition<ref name="Allred2015">Allred, B. M., Lang, J. R., Barlaz, M. A., and Field, J. A., 2015. Physical and Biological Release of Poly- and Perfluoroalkyl Substances (PFAS) from Municipal Solid Waste in Anaerobic Model Landfill Reactors. Environmental Science and Technology, 49(13), pp. 7648-7656. [http://pubs.acs.org/doi/abs/10.1021/acs.est.5b01040 DOI: 10.1021/acs.est.5b01040]</ref><ref name="Lang2017">Lang, J.R., Allred, B.M., Field, J.A., Levis, J.W. and Barlaz, M.A., 2017. National Estimate of Per- and Polyfluoroalkyl Substance (PFAS) Release to U.S. Municipal Landfill Leachate. Environmental Science and Technology, 51(4), pp. 2197-2205. [https://doi.org/10.1021/acs.est.6b05005 DOI: 10.1021/acs.est.6b05005]</ref>. The relative concentrations of various PFAS in leachate and groundwater from landfill sites is different from those found at WWTPs and AFFF-contaminated sites. In particular, 5:3 fluorotelomer carboxylic acid (FTCA) is a common and often dominant PFAS found in landfills, and has been released from carpet in model anaerobic landfill reactors. This compound could prove to be an indicator that PFAS in the environment originated from a landfill<ref name="Lang2016">Lang, J.R., Allred, B.M., Peaslee, G.F., Field, J.A., and Barlaz, M.A., 2016. Release of Per-and Polyfluoroalkyl Substances (PFASs) from Carpet and Clothing in Model Anaerobic Landfill Reactors. Environmental Science and Technology, 50(10), pp. 5024-5032. [https://doi.org/10.1021/acs.est.5b06237 DOI: 10.1021/acs.est.5b06237]</ref><ref name="Lang2017" />. PFAS may also be released to the air from landfills, predominantly as fluorotelomer alcohols (FTOHs) and perfluorobutanoate (PFBA). In one study, total airborne PFAS concentrations were 5 to 30 times greater at landfills than at background reference sites<ref name="Ahrens2011b">Ahrens, L., Shoeib, M., Harner, T., Lane, D.A., Guo, R. and Reiner, E.J., 2011. Comparison of Annular Diffusion Denuder and High volume Air Samplers for Measuring Per- and Polyfluoroalkyl Substances in the Atmosphere. Analytical Chemistry, 83(24), pp. 9622-9628. [https://pubs.acs.org/doi/ DOI: 10.1021/ac202414w]   Free download available from: [https://www.informea.org/sites/default/files/imported-documents/UNEP-POPS-POPRC11FU-SUBM-PFOA-Canada-2-20151211.En.pdf InforMEA]</ref>. PFAS release rates within landfills vary over time for a given waste mass, with climate (for example, rainfall) serving as the apparent driving factor for the variations<ref name="Lang2017" /><ref name="Benskin2012">Benskin, J.P., Li, B., Ikonomou, M.G., Grace, J.R. and Li, L.Y., 2012. Per-and Polyfluoroalkyl Substances in Landfill Leachate: Patterns, Time Trends, and Sources. Environmental Science and Technology, 46(21), pp.11532-11540. [https://doi.org/10.1021/es302471n DOI: 10.1021/es302471n]</ref>.

==Commercial and Consumer Products==
PFAS are widely used in consumer products and household applications, with a diverse mixture of PFAS found in varying concentrations depending on the product<ref name="Clara2008">Clara, M., Scharf, S., Weiss, S., Gans, O. and Scheffknecht, C., 2008. Emissions of perfluorinated alkylated substances (PFAS) from point sources - identification of relevant branches. Water Science and Technology, 58(1), pp. 59-66. [https://doi.org/10.2166/wst.2008.641 DOI: 10.2166/wst.2008.641]   [//www.enviro.wiki/images/a/a3/Clara2008.pdf Open access article.]</ref><ref name="Trier2011">Trier, X., Granby, K. and Christensen, J.H., 2011. Polyfluorinated surfactants (PFS) in paper and board coatings for food packaging. Environmental Science and Pollution Research International, 18(7), pp. 1108–1120. [https://doi.org/10.1007/s11356-010-0439-3 DOI: 10.1007/s11356-010-0439-3]</ref><ref name="Fujii2013">Fujii, Y., Harada, K.H. and Koizumi, A., 2013. Occurrence of perfluorinated carboxylic acids (PFCAs) in personal care products and compounding agents. Chemosphere, 93(3), pp. 538-544. [https://doi.org/10.1016/j.chemosphere.2013.06.049 DOI: 10.1016/j.chemosphere.2013.06.049]</ref><ref name="OECD2013">Organisation for Economic Cooperation and Development (OECD), 2013. Synthesis paper on per‐ and polyfluorinated chemicals (PFCs). OECD Environment Directorate/UNEP Global PFC Group. [https://www.oecd.org/env/ehs/risk-management/PFC_FINAL-Web.pdf Website]   [//www.enviro.wiki/images/5/55/OECD2013.pdf Report.pdf]</ref><ref name="ATSDR2018" /><ref name="Kotthoff2015" /><ref name="KEMI2015" /><ref name="USEPA2016">US Environmental Protection Agency (USEPA), 2016. Drinking Water Health Advisory for Perfluorooctane Sulfonate (PFOS), EPA Document Number: 822-R-16-004. Office of Water, Health and Ecological Criteria Division, Washington, DC. [https://www.epa.gov/sites/production/files/2016-05/documents/pfos_health_advisory_final_508.pdf Website]   [//www.enviro.wiki/images/6/63/USEPA2016.pdf Report.pdf]</ref>. Environmental releases associated with the commercial and consumer products are primarily related to their production. To a much lower extent, the environmental releases may be associated with the management of solid waste (for example, disposal of used items in a MSW landfill) and wastewater disposal (for example, discharge to WWTPs, private septic systems, or other subsurface disposal systems).

Studies have shown that physical degradation of some consumer products (such as PFAS-treated paper, textiles, and carpets) may release PFAS in house dust<ref name="Bjorklund2009">Björklund, J.A., Thuresson, K. and De Wit, C.A., 2009. Perfluoroalkyl Compounds (PFCs) in Indoor Dust: Concentrations, Human Exposure Estimates, and Sources. Environmental Science and Technology, 43(7), pp. 2276-2281. [https://doi.org/10.1021/es803201a DOI: 10.1021/es803201a]</ref>. Additionally, studies have also shown that professional ski wax technicians may have significant inhalation exposures to PFAS<ref name="Nilsson2013">Nilsson, H., Kärrman, A., Rotander, A., van Bavel, B., Lindström, G., and Westberg, H., 2013. Professional ski waxers' exposure to PFAS and aerosol concentrations in gas phase and different particle size fractions. Environmental Science: Processes and Impacts, 15(4), pp. 814-822. [https://doi.org/10.1039/C3EM30739E DOI: 10.1039/C3EM30739E]</ref> and snowmelt and surface waters near ski areas could have measurable PFAS impacts<ref name="Kwok2013">Kwok, K.Y., Yamazaki, E., Yamashita, N., Taniyasu, S., Murphy, M.B., Horii, Y., Petrick, G., Kallerborn, R., Kannan, K., Murano, K. and Lam, P.K., 2013. Transport of Perfluoroalkyl substances (PFAS) from an arctic glacier to downstream locations: Implications for sources. Science of the Total Environment, 447, pp. 46-55. [https://doi.org/10.1016/j.scitotenv.2012.10.091 DOI: 10.1016/j.scitotenv.2012.10.091]</ref>.

As increased environmental sampling for PFAS occurs, additional information will become available to further our understanding of the major and minor PFAS contributors to the environment.
 

==References==

<references />

==See Also==

Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)

2026-08-19T11:38:39Z

Admin:

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are a class of man-made chemicals suspected to cause adverse human and ecological health effects. The acronym “PFAS” encompasses thousands of individual compounds. The two most studied and regulated are perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS). PFAS, including PFOA and PFOS, have entered the environment from a variety of sources and release scenarios, including releases from manufacturing facilities and areas where aqueous film-forming foam (AFFF), a type of fire-fighting foam, was applied. Many PFAS have unique physical and chemical properties that render them highly stable and resistant to degradation in the environment. They are typically removed from water supplies using granular activated carbon or ion exchange resins, although research is ongoing to develop ''in situ'' treatment technologies as well as more cost-effective ''ex situ'' treatment methods.

<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
'''Related Article(s):'''

*[[Hydrothermal Alkaline Treatment (HALT)]]
*[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
*[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]
*[[PFAS Ex Situ Water Treatment]]
*[[PFAS Leaching Characterization with the Leaching Environmental Assessment Framework (LEAF)]]
*[[PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)]]
*[[PFAS Soil Remediation Technologies]]
*[[PFAS Sources]]
*[[PFAS Toxicology and Risk Assessment]]
*[[PFAS Transport and Fate]]
*[[PFAS Treatment by Anion Exchange]]
*[[PFAS Treatment by Electrical Discharge Plasma]]
*[[Photoactivated Reductive Defluorination - PFAS Destruction]]
*[[Reverse Osmosis and Nanofiltration Membrane Filtration Systems for PFAS Removal]]
*[[Soil & Groundwater Contaminants]]
*[[Supercritical Water Oxidation (SCWO)]]
*[[Thermal Conduction Heating for Treatment of PFAS-Impacted Soil]]
*[[Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)]]

'''Contributor(s):''' [[Dr. Rula Deeb]], [[Dr. Jennifer Field]], Dr. Lydia Dorrance, [[Elisabeth Hawley]] and [[Dr. Christopher Higgins]]

'''Key Resource(s):'''

*[//www.enviro.wiki/images/2/2a/USEPA-2014-Emerging_Contaminants_-_PFOS_and_PFOA_Fact_Sheet.pdf U.S. EPA Emerging Contaminants - PFOS and PFOA Fact Sheet]<ref name="USEPA2014">U.S. Environmental Protection Agency, 2014. Emerging Contaminants Fact Sheet – Perfluorooctane Sulfonate (PFOS) and Perfluorooctanoic Acid (PFOA). EPA 505-F-14-001. [//www.enviro.wiki/images/2/2a/USEPA-2014-Emerging_Contaminants_-_PFOS_and_PFOA_Fact_Sheet.pdf March Fact Sheet]</ref>
*[https://www.epa.gov/sites/production/files/2017-12/documents/ffrrofactsheet_contaminants_pfos_pfoa_11-20-17_508_0.pdf Technical Fact Sheet: Perfluorooctane Sulfonate (PFOS) and Perfluorooctanoic Acid (PFOA)<ref name="USEPA2017">U.S. Environmental Protection Agency, 2017. Technical Fact Sheet: Perfluorooctane Sulfonate (PFOS) and Perfluorooctanoic Acid (PFOA). EPA 505-F-17-001.</ref>].

==Introduction==
PFAS were first developed in the 1940s and have been used by numerous industrial and commercial sectors for products that benefited from PFAS’ unique properties, including thermal and chemical stability, water resistance, stain resistance, and their [[wikipedia: Surfactant |surfactant]] nature. Awareness of PFAS in the environment first emerged in the late 1990s following developments in analytical instrumentation which enhanced detection of ionized substances such as PFAS<ref>Hansen, K.J., L.A. Clemen, M.E. Ellefson and H.O. Johnson, 2001. Compound-Specific, Quantitative Characterization of Organic Fluorochemicals in Biological Matrices. Environmental Science and Technology 35(4):766-770.</ref>. This environmental awareness was generally concurrent to increased scrutiny into the health effects of PFAS<ref>U.S. Environmental Protection Agency, 2018. Risk Management for Per- and Polyfluoroalkyl Substances (PFASs) under TSCA. https://www.epa.gov/assessing-and-managing-chemicals-under-tsca/risk-management-and-polyfluoroalkyl-substances-pfass</ref>. In 2000, the sole U.S. manufacturer of PFOS voluntarily discontinued production<ref>U.S. Environmental Protection Agency, 2000. EPA and 3M announce phase out of PFOS. News release dated Tuesday May 16. [https://yosemite.epa.gov/opa/admpress.nsf/0/33aa946e6cb11f35852568e1005246b4 U.S. EPA PFOS Phase Out Announcement]</ref>. Shortly thereafter, legal actions were taken against PFAS product manufacturing facilities in the Ohio River Valley in West Virginia<ref>Rich, N., 2016. The lawyer who became DuPont’s worst nightmare. The New York Times Magazine.</ref>. Between 2006 and 2015, in cooperation with the EPA, eight global companies with PFAS-related operations voluntarily phased out the manufacture of PFOA and similarly structured PFAS with longer carbon chains<ref>U.S. Environmental Protection Agency, 2018. Fact Sheet: 2010/2015 PFOA Stewardship Program. https://www.epa.gov/assessing-and-managing-chemicals-under-tsca/fact-sheet-20102015-pfoa-stewardship-program</ref>. In 2011, recognizing the potential impact of PFAS for the Department of Defense due to the military’s ubiquitous use of PFAS-containing AFFF, SERDP/ESTCP research programs began funding PFAS-related research, and the U.S. Air Force began conducting initial site investigations at former fire-fighting training areas<ref>SERDP/ESTCP website on Per- and Polyfluorinated Substances (PFASs). https://www.serdp-estcp.org/Featured-Initiatives/Per-and-Polyfluoroalkyl-Substances-PFASs</ref>. The U.S. Environmental Protection Agency (EPA) issued provisional drinking water health advisories for PFOA and PFOS in 2009 and replaced these with more stringent health advisories in 2016<ref name="USEPA2016">U.S. Environmental Protection Agency, 2016. Drinking water health advisories for PFOA and PFOS. [https://www.epa.gov/ground-water-and-drinking-water/drinking-water-health-advisories-pfoa-and-pfos U.S. EPA Water Health Advisories - PFOA and PFOS]</ref>. Over the past five years, regulating agencies in several states have issued screening levels, notification levels, and health-based guidelines for PFOS, PFOA and other PFAS. Several states have undertaken statewide sampling programs of drinking water systems and groundwater resources in the vicinity of potential source areas including manufacturing facilities, military fire-training facilities, airports, refineries, and landfills<ref>California State Water Resources Control Board, 2019. PFAS Phased Investigation Approach. https://www.waterboards.ca.gov/pfas/docs/7_investigation_plan.pdf</ref><ref>Michigan, 2019. PFAS response. Taking Action, Protecting Michigan. Michigan PFAS Action Response Team (MPART).</ref>.

==Nomenclature==
[[File:PFASupdate2019Fig1.png | thumbnail | left | 700 px |Figure 1. PFAS families of compounds<ref name="OECD2015">Organisation for Economic Cooperation and Development, 2015. Working Towards a Global Emission Inventory of PFASs: Focus on PFCAs – Status Quo and the Way Forward. Paris: Environment, Health and Safety, Environmental Directorate, OECD/UNEP Global PFC Group.</ref>]]
[[File:Deeb-Article 1-Figure 1.JPG|thumbnail|right|400 px|Figure 2. a) Structure of a perfluoroalkyl substance, PFOS, compared with b) the structure of a polyfluoroalkyl substance, 6:2 fluorotelomer sulfonate (6:2 FTSA).]]
[[File:PFAS_naming_red.mp4 | thumb | right | 400px | Figure 3. PFAS naming conventions explained.]]
There are over 3,000 PFAS currently on the global market. A summary of families of compounds that are included in the umbrella terminology “PFAS” is provided in Figure 1<ref name="OECD2015" />. Perfluoroalkyl compounds have a non-polar hydrophobic carbon (alkyl) chain structure that is fully saturated with fluorine atoms (i.e., they are perfluoroalkyl substances) attached to a hydrophilic polar functional group. Polyfluoroalkyl compounds have a similar structure but have at least one carbon that is bound to hydrogen rather than fluorine (Figure 2). Carbon chains may be linear or branched, leading to a variety of isomers. The term PFAS also includes fluoropolymers that may consist of thousands of shorter-chain units bonded together<ref name="ITRC2018">Interstate Technology and Regulatory Council, 2018. Naming Conventions and Physical and Chemical Properties of Per- and Polyfluoroalkyl Substances (PFAS). https://pfas-1.itrcweb.org/wp-content/uploads/2018/03/pfas_fact_sheet_naming_conventions__3_16_18.pdf</ref>.

One of the most studied and regulated families of perfluoroalkyl compounds are the perfluoroalkyl acids (PFAAs), which include perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs). PFOA, a perfluoroalkyl carboxylic acid, and PFOS, a perfluoroalkyl sulfonic acid, are both PFAAs with eight carbons. Other PFCAs with the number of carbons ranging from nine to four include perfluorononanoic acid (PFNA), perfluoroheptanoic acid (PFHpA), perfluorohexanoic acid (PFHxA), and perfluorobutanoic acid (PFBA). PFAAs are sometimes differentiated as “long-chain” or “short-chain.” The term “long-chain” refers to PFCAs with eight or more carbons and PFSAs with six or more carbons. The term “short-chain” refers to PFCAs with seven or fewer carbons and PFSAs with five or fewer carbons<ref name="ITRC2018" />.The PFAS naming conventions are explained in more detail in the video shown in Figure 3.

==Physical and Chemical Properties==
{| class="wikitable" style="float:right; margin-left:10px;"
|+Table 1. Physical and Chemical Properties of PFOS and PFOA.<ref name="USEPA2014" /><ref name="USEPA2017" /><ref name="ITRC2018b">Interstate Technology Regulatory Council, 2018. PFAS Fact Sheets: Environmental Fate and Transport, Table 3-1. https://pfas-1.itrcweb.org/wp-content/uploads/2018/05/ITRCPFASFactSheetFTPartitionTable3-1April18.xlsx</ref><ref name="ATSDR2018">Agency for Toxic Substances and Disease Registry, 2018. ToxGuideTM for Perfluoroalkyls. https://www.atsdr.cdc.gov/toxguides/toxguide-200.pdf </ref>
|-
!Property
!PFOS (Free Acid)
!PFOA (Free Acid)
|-
|Chemical Abstracts Service Number||1763-23-1||335-67-1
|-
|Physical State (at 25° C and 1 atmosphere pressure)||White powder||White powder/waxy white solid
|-
|Molecular weight (g/mol)||500||414
|-
|Water solubility at 25° C (mg/L)||570''a''||9,500''a''
|-
|Melting point (°C)||No data||45 to 54
|-
|Boiling point (°C)||258 to 260||188 to 192
|-
|Vapor pressure at 20° C (mm Hg)||0.002||0.525 to 10
|-
|Organic-carbon partition coefficient (log ''Koc'')||2.4 to 3.7||1.89 to 2.63
|-
|Henry’s Law constant (atm-m3/mol)||Not measurable||3.57x10-6
|-
|Half-life||Atmospheric: 114 days Water: >41 years (at 25° C) Human: 3.1 to 7.4 years||Atmospheric: 90 days''b'' Water: >92 years (at 25° C) Human: 2.1 to 8.5 years
|-
| colspan="3" style="background:white;" |Abbreviations: g/mol = grams per mole; mg/L = milligrams per liter; °C = degrees Celsius; mm Hg = millimeters of mercury; atm-m3/mol = atmosphere-cubic meters per mole
|-
| colspan="3" style="background:white;" |Notes: ''a'' Solubility in purified water. ''b'' The atmospheric half-life value for PFOA was extrapolated from available data measured over short study periods.
|}

The combination of the polar and non-polar structure makes PFAAs “amphiphilic,” associating with both water and oils, while the strength of their [[wikipedia: Carbon-fluorine bond |carbon-fluorine bonds]] lends them extremely high chemical and thermal stabilities. In most groundwater and surface water environments, PFAAs are found as the water-soluble anionic (i.e. deprotonated, negatively charged) form. Other groups of PFAS can be cationic (positively charged) or zwitterionic (possessing both a positive and negative charge) under typical environmental conditions. In general, documented physical properties of PFAS are scarce, and much that is available for PFAAs is related to the acid forms of the compounds, which are not typically found in the environment<ref name="ITRC2018" />. The surfactant properties of PFAAs complicate the prediction of their physiochemical properties, such as vapor pressure and partitioning coefficients. Some relevant properties of PFOS and PFOA are summarized in Table 1. A summary of the general characteristics of PFAS has been compiled in Table 6-2 of the [https://pfas-1.itrcweb.org/wp-content/uploads/2018/03/pfas_fact_sheet_naming_conventions__3_16_18.pdf ITRC fact sheet on PFAS naming conventions and physical and chemical properties]<ref name="ITRC2018" />.

==Environmental Concern==
Environmental concern surrounding PFAS stems from their widespread detection, high degree of environmental stability and mobility, and suspected toxicological effects on humans and the environment. Perfluorinated compounds, including PFAAs, are very stable and do not biodegrade. As a result, these compounds are found throughout the global environment. Trace amounts of perfluorinated compounds have been detected at remote locations like the Arctic, far from potential point sources<ref>Young, C.J., Furdui, V.I., Franklin, J., Koerner, R.M., Muir, D.C. and Mabury, S.A., 2007. Perfluorinated acids in arctic snow: new evidence for atmospheric formation. Environmental Science & Technology, 41(10), 3455-3461. [http://dx.doi.org/10.1021/es0626234 doi: 10.1021/es0626234]</ref>. Other studies have shown that some long-chain perfluorinated substances bioaccumulate and biomagnify in wildlife<ref>Conder, J.M., Hoke, R.A., Wolf, W.D., Russell, M.H. and Buck, R.C., 2008. Are PFCAs bioaccumulative? A critical review and comparison with regulatory criteria and persistent lipophilic compounds. Environmental Science & Technology, 42(4), 995-1003. [http://dx.doi.org/10.1021/es070895g doi: 10.1021/es070895g]</ref>. Because of this, higher trophic wildlife including fish and birds, and humans who consume them, can be particularly susceptible to any deleterious health effects posed by PFAS<ref>Sinclair, E., Mayack, D.T., Roblee, K., Yamashita, N. and Kannan, K., 2006. Occurrence of perfluoroalkyl surfactants in water, fish, and birds from New York State. Archives of Environmental Contamination and Toxicology, 50(3), pp.398-410. [http://dx.doi.org/10.1007/s00244-005-1188-z doi: 10.1007/s00244-005-1188-z]</ref>. The Dutch National Institute for Public Health and the Environment calculated a maximum permissible concentration for PFOS of 0.65 nanograms per liter (ng/L) for fresh water, based on human consumption of fish<ref name="USEPA2014" />. Recent fish and wildlife consumption advisories have been issued at certain locations in the United States associated with PFAS contamination<ref>Minnesota Department of Health, 2018. Media FAQ: Fish Consumption Advisory, PFOS and Lake Elmo Fish 2018. https://www.co.washington.mn.us/DocumentCenter/View/20895/FAQ-2018-Fish-Consumption-Advisory-NR </ref><ref>State of Michigan, PFAS Response, Taking Action, Protecting Michigan, 2019. https://www.michigan.gov/pfasresponse/0,9038,7-365-86512_88981_88982---,00.html </ref>.

Like other aspects of PFAS research, information on the toxicological effects of PFAS on humans is still emerging. PFOA and PFOS have half-lives of 2.1-8.5 years and 3.1-7.4 years, respectively, in humans<ref name="ATSDR2018" />. PFAS typically accumulate in the liver, proteins, and the blood stream<ref name="USEPA2017" />. Toxicological and epidemiological studies of PFOA, PFOS and other PFAAs indicate potential association with a constellation of ailments including decreased fertility, increased cholesterol, suppression of response to vaccines, and certain cancers<ref name="ATSDR2018" /><ref>C8 Science Panel, 2012. C8 Probable Link Reports. http://www.c8sciencepanel.org/prob_link.html </ref>. Both PFOA and PFOS are suspected carcinogens, but their carcinogenicity remains to be classified by the U.S. EPA<ref name="USEPA2017" />. The International Agency for Research on Cancer (IARC) has classified PFOA as a Group 2B carcinogen, i.e., possibly carcinogenic to humans<ref>Benbrahim-Tallaa, L., Lauby-Secretan, B. Loomis, D., Guyton, K.Z., Grosse, Y., Bouvard, F. El Ghissassi, V., Guha, N., Mattock, H., Straif, K., 2014. Carcinogenicity of perfluorooctanoic acid, tetrafluoroethylene, dichloromethane, 1,2-dichloropropane, and 1,3-propane sultone. The Lancet Oncology, 15 (9), 924-925. [http://dx.doi.org/10.1016/s1470-2045(14)70316-x doi: 10.1016/S1470-2045(14)70316-X]</ref><ref>International Agency for Research on Cancer (IARC), 2016. Monographs on the evaluation of carcinogenic risks to humans. Lists of Classifications, Volumes 1 to 116. [//www.enviro.wiki/images/f/fd/IARC-2016-Monographs_on_the_eval_of_carcinogenic_risks_to_humans_List_of_Classifications.pdf List of Classifications.pdf]</ref>. The U.S. EPA published draft oral reference doses of 20 ng/kg-day for both PFOA and PFOS (based on non-cancer hazard)<ref name="USEPA2017" />. Drinking water ingestion, fish consumption, dermal contact with water, and (accidental) ingestion of or contact with contaminated soil are the exposure pathways of concern with respect to human health.

==Uses and Potential Sources to the Environment==
Due to their unique properties, including surfactant qualities, heat and stain resistance, and [[wikipedia: Amphiphile |amphiphilic]] nature, PFAS are used widely by a number of industries, including carpet, textile and leather production, chromium plating, photography, [[wikipedia: Photolithography |photolithography]], paper products, semi-conductor manufacturing, coating additives, and cleaning products<ref name="ITRC2017">Interstate Technology and Regulatory Council, 2017. History and Use of Per- and Polyfluoroalkyl Substances (PFAS). https://pfas-1.itrcweb.org/wp-content/uploads/2017/11/pfas_fact_sheet_history_and_use__11_13_17.pdf </ref><ref>Krafft, M.P. and Riess, J.G., 2015. Selected physicochemical aspects of poly-and perfluoroalkylated substances relevant to performance, environment and sustainability - Part one. Chemosphere, 129, 4-19. [http://dx.doi.org/10.1016/j.chemosphere.2014.08.039 doi: 10.1016/j.chemosphere.2014.08.039]</ref>. Sources to the environment include primary manufacturing facilities, where PFAS is produced, and secondary manufacturing facilities, where PFAS is incorporated into products. PFAS are found in a variety of consumer products including food paper and packaging, furnishings, waterproof clothing, and cosmetics<ref name="BirnbaumGrandjean2015">Birnbaum, L.S. and Grandjean, P., 2015. Alternatives to PFAS: Perspectives on the Science. Environmental Health Perspectives, 123(5), A104-A105. [http://dx.doi.org/10.1289/ehp.1509944 doi: 10.1289/ehp.1509944]</ref>. The presence of PFAS in consumer products has created an urban background concentration in stormwater, wastewater treatment plant influent<ref>Houtz, E.F., 2013. Oxidative measurement of perfluoroalkyl acid precursors: Implications for urban runoff management and remediation of AFFF-contaminated groundwater and soil. Ph.D. Dissertation. Available online at http://escholarship.org/uc/item/4jq0v5qp</ref>, and landfill leachate<ref>Lang, J.R., Allred, B.M., Peaslee, G.F., Field, J.A. and Barlaz, M.A., 2016. Release of Per-and Polyfluoroalkyl Substances (PFAS) from Carpet and Clothing in Model Anaerobic Landfill Reactors. Environmental Science & Technology, 50(10), 5024-5032. [http://dx.doi.org/10.1021/acs.est.5b06237 doi: 10.1021/acs.est.5b06237]</ref>.

An additional widely documented source of PFAS is AFFF. AFFF is as a Class B firefighting foam used to combat flammable liquid fires. AFFF was released in large quantities at firefighting training areas as part of routine handling, fire-suppression training and equipment testing, and during fire emergency responses. While all AFFF contains PFAS<ref>Interstate Technology and Regulatory Council, 2018. Aqueous Film-Forming Foam (AFFF). https://pfas-1.itrcweb.org/wp-content/uploads/2019/03/pfas-fact-sheet-afff-10-3-18.pdf </ref>, the types and concentrations of PFAS in AFFF vary among manufacturers and manufacturing time periods. 3M AFFF products were made using an [[wikipedia: Electrochemical fluorination |electrochemical fluorination]] process that produced a high percentage of PFAS as PFOS, while other formulations were made using a [[wikipedia: Teolmerization |telomerization]] process and contain a different suite of PFAS.

==Regulation==
The U.S. EPA recently developed Drinking Water Health Advisory levels for PFOA and PFOS, replacing previously published provisional values<ref name="USEPA2016" />. However, no Federal enforceable drinking water standards have been set. Recent years have seen increased regulatory activity at the state level, with around 20 states having guidance or advisory levels for one or more PFAS compounds in various environmental media (drinking water, groundwater, etc.). New Jersey is the first state to have promulgated an enforceable maximum concentration level (MCL) for a PFAS by setting an MCL for PFNA in 2018. Several other states, particularly in the eastern United States, are moving towards setting MCLs for various PFAS. Regulatory levels set or proposed by states vary but the majority are equivalent to or lower than the Federal Health Advisory level of 70 parts per trillion (ppt) combined concentration of PFOA and PFOS. The regulatory landscape for PFAS is developing rapidly. A regularly updated repository of [https://pfas-1.itrcweb.org/fact-sheets/ regulatory levels] is maintained by the ITRC (Figure 4). [[File:ITRCfactSheetPFAS.png |thumb|left|400px| link=https://pfas-1.itrcweb.org/fact-sheets/ | [https://pfas-1.itrcweb.org/fact-sheets/ Figure 4. ITRC PFAS Fact Sheets: 1. Naming Conventions and Physical and Chemical Properties, 2. Regulations, Guidance, and Advisories, 3. History and Use, 4. Environmental Fate and Transport, 5. Site Characterization Tools, Sampling Techniques, and Laboratory Analytical Methods, and 6. Remediation Technologies and Methods.]]]

Other regulatory actions have restricted the use and production of PFAS. PFOS was added to list of chemicals under the [[wikipedia: Stockholm Convention on Persistent Organic Pollutants |Stockholm Convention on Persistent Organic Pollutants]] in 2009. Nearly all use of PFOS is therefore banned in Europe, with some exemptions. Substances or mixtures may not contain PFOS above 0.001% by weight (EU 757/2010). In the U.S., because PFOS manufacturing was voluntarily phased out in 2002, AFFF containing PFOS is no longer manufactured. The U.S. military and others still have large quantities of stockpiled AFFF containing PFOS, although its use is discouraged<ref>Darwin, R.L., 2011. Estimated Inventory of PFOS-based Aqueous Film Forming Foam (AFFF). July. https://www.informea.org/sites/default/files/imported-documents/UNEP-POPS-POPRC13FU-SUBM-PFOA-FFFC-3-20180112.En.pdf </ref><ref>Department of Defense, 2018. Alternatives to Aqueous Film Forming Foam Report to Congress, June. https://www.denix.osd.mil/derp/home/documents/alternatives-to-aqueous-film-forming-foam-report-to-congress/ </ref>.

==Litigation==
There have been many lawsuits filed with PFAS-related claims; some resulting in settlements in the hundreds of millions of dollars. In 2017 DuPont and Chemours paid nearly $700 million to settle 3,550 individual lawsuits claiming personal injury as a result of PFOA releases from DuPont’s former Washington Works manufacturing facility in Parkersburg, West Virginia. This settlement was reached after three of the lawsuits went to trial resulting in nearly $20 million in jury awards to the plaintiffs<ref>Reisch, M.S. “DuPont, Chemours settle PFOA suits” Chem. & Eng. News, Feb. 20, 2017. https://cen.acs.org/articles/95/i8/DuPont-Chemours-settle-PFOA-suits.html </ref>. In 2018, 3M agreed to pay $850 million to settle a $5 billion natural resource damages claim related to PFAS impacts brought by Minnesota’s Attorney General<ref>Bellon, T. 3M, Minnesota settle water pollution claims for $850 million, Reuters, Feb. 20, 2018. https://www.reuters.com/article/us-3m-pollution-minnesota/3m-minnesota-settle-water-pollution-claims-for-850-million-idUSKCN1G42UW </ref>. In early 2019, numerous product liability lawsuits against former manufacturers of PFOS and PFOA and manufacturers of AFFF were consolidated into a multi-district litigation (MDL) pending before the U.S District Court of South Carolina<ref>United States District Court of South Carolina. Aqueous Film-Forming Foams (AFFF) Products Liability Litigation MDL No. 2873. https://www.scd.uscourts.gov/mdl-2873/ </ref>. Numerous additional PFAS-related lawsuits have been brought under common law tort, personal injury, product liability and natural resource protection laws across the United States. The proposed designation of PFAS as a hazardous substance under [[wikipedia: Superfund |CERCLA]]<ref>Congressional Research Service, 2019. Regulating Drinking Water Contaminants: EPA PFAS Actions. August. https://fas.org/sgp/crs/misc/IF11219.pdf </ref> may have additional legal implications.

==Sampling and Analytical Methods==
Because of the presence of PFAS in many common consumer items and sampling materials, and the low reporting levels needed to compare to current regulatory guidelines, sampling for PFAS requires extra care to avoid cross contamination from other potential sources of PFAS. Most standard operating procedures and work plans advise avoiding the use of fluoropolymer-based (e.g., Teflon) components and recommend additional precautions related to sample containers, sampler clothing and handling of certain every-day items.

Commercial laboratories analyze PFAS in drinking water samples using the EPA-approved method 537.1, which consists of [[wikipedia: Solid phase extraction |solid phase extraction]] and [[wikipedia: Liquid chromatography-mass spectrometry |liquid chromatography with tandem mass spectrometry]]<ref>U.S. Environmental Protection Agency, 2019. Method 537.1 Determination of Selected Per- and Polyfluorinated Alkyl Substances in Drinking Water by Solid Phase Extraction and Liquid Chromatography/Tandem Mass Spectrometry (LC/MS/MS). August. https://cfpub.epa.gov/si/si_public_record_Report.cfm?dirEntryId=343042&Lab=NERL </ref>. Samples collected from any environmental media other than drinking water require a modified version of 537.1 to quantify approximately 24 individual PFAS compounds. An EPA method for some of these other media, Method 8327, was released for public comment in summer 2019<ref>U.S. Environmental Protection Agency, 2019. SW-486 Update VII Announcements, Phase II – PFAS 8372 and 3512, July. https://www.epa.gov/hw-sw846/sw-846-update-vii-announcements </ref>. Some commercial laboratories can extend the target analyte list to include up to 40 compounds. Some commercial laboratories also offer an analytical method known as the [[Wikipedia: TOP Assay | Total Oxidizable Precursor (TOP) Assay]], which provides a bulk measurement of PFAS mass in a sample, including that of oxidizable precursors<ref>Houtz, E.F., and Sedlak, D.L., 2012. Oxidative conversion as a means of detecting precursors to perfluoroalkyl acids in urban runoff. Environmental Science & Technology, 46(17), 9342-9349. doi/10.1021/es302274g </ref><ref>Houtz, E.F., Higgins, C.P., Field, J.A. and Sedlak, D.L., 2013. Persistence of perfluoroalkyl acid precursors in AFFF-impacted groundwater and soil. Environmental Science & Technology, 47(15), 8187-8195. doi: 10.1021/es4018877 </ref>. Other approaches to quantify the total amount of organic fluorine in water samples include [[wikipedia: Particle-induced gamma emission |particle induced gamma-ray emission]] (PIGE) and absorbable organic fluorine (AOF)<ref name="BirnbaumGrandjean2015" />.

==Fate and Transport==
The processes of [[wikipedia: Sorption |sorption]] and biotransformation as well as the presence of co-contaminants can affect the fate and transport of PFAS. It has been observed that PFAAs exhibit affinity for solid-phase organic carbon to varying degrees depending in part on chain length and structure, with long-chain compounds exhibiting a stronger affinity than short-chain and PFSAs exhibiting a stronger affinity than PFCAs for a given chain length<ref>Higgins, C.P., and Luthy, R.G., 2006. Sorption of perfluorinated surfactants on sediments. Environmental Science & Technology, 40(23), 7251-7256. [http://dx.doi.org/10.1021/es061000n doi: 10.1021/es061000n]</ref>. Interactions with mineral phases, particularly ferric oxide materials, may also be important under certain conditions<ref name="Ferrey2012">Ferrey, M.L., Wilson, J.T., Adair, C., Su, C., Fine, D.D., Liu, X. and Washington, J.W., 2012. Behavior and fate of PFOA and PFOS in sandy aquifer sediment. Groundwater Monitoring & Remediation, 32(4), 63-71. [http://dx.doi.org/10.1111/j.1745-6592.2012.01395.x doi: 10.1111/j.1745-6592.2012.01395.x]</ref><ref>Johnson, R.L., Anschutz, A.J., Smolen, J.M., Simcik, M.F. and Penn, R.L., 2007. The adsorption of perfluorooctane sulfonate onto sand, clay, and iron oxide surfaces. Journal of Chemical & Engineering Data, 52(4), 1165-1170. [http://dx.doi.org/10.1021/je060285g doi: 10.1021/je060285g]</ref>. At present, empirical site-specific sorption estimates are recommended to accurately predict PFAS mobility<ref name="Ferrey2012" />. PFAAs do not readily degrade in the environment. However, polyfluorinated forms may biotically or abiotically degrade to other intermediate forms and/or so-called “terminal,” recalcitrant PFAA forms, including PFOA and PFOS<ref name="Tseng2014">Tseng, N., Wang, N., Szostek, B. and Mahendra, S., 2014. Biotransformation of 6: 2 fluorotelomer alcohol (6: 2 FTOH) by a wood-rotting fungus. Environmental Science & Technology, 48(7), 4012-4020. [http://dx.doi.org/10.1021/es4057483 doi:10.1021/es4057483]</ref><ref>Harding-Marjanovic, K.C., Houtz, E.F., Yi, S., Field, J.A., Sedlak, D.L. and Alvarez-Cohen, L., 2015. Aerobic biotransformation of fluorotelomer thioether amido sulfonate (Lodyne) in AFFF-amended microcosms. Environmental Science & Technology, 49(13), pp.7666-7674. [http://dx.doi.org/10.1021/acs.est.5b01219 doi: 10.1021/acs.est.5b01219]</ref><ref>Ellis, D.A., Martin, J.W., De Silva, A.O., Marbury, S.A., Hurley, M.D., Sulbaek Andersen, M.P., and T.J. Wallington, 2004. Degradation of fluorotelomer alcohols: a likely atmospheric source of perfluoronated carboxylic acids. Environmental Science & Technology 38(12), 3316-3321. doi/10.1021/es049860w </ref>. As a result, these degradable PFAS are sometimes referred as PFAA “precursors.” Remediation of co-contaminants, particularly using techniques involving oxidation, can enhance the degradation of precursors. Interactions between PFAS and non-aqueous phase liquids can retard PFAS migration<ref>Guelfo, J. 2013. Subsurface fate and transport of poly- and perfluoroalkyl substances. Doctor of Philosophy Thesis, Colorado School of Mines. [//www.enviro.wiki/images/d/d8/Guelfo-2013-Subsuface_fate_and_transport_of_Poly-and_perfluoroalkyl_substances.pdf Thesis]</ref>

==Remediation Technologies==
Due to the chemical and thermal stability of PFAS and the complexity of PFAS mixtures, soil and groundwater remediation is challenging and costly. Research is still ongoing to develop effective remedial strategies. Treatment options for soil include 1) treatment and/or direct on-site reuse, 2) temporary on-site storage, and 3) off-site disposal to a soil processing or treatment facility, licensed landfill, or incinerator. For groundwater, management options include the following: 1) ''in situ'' treatment, 2) ''ex situ'' treatment and/or reuse, aquifer reinjection, or discharge to surface water, stormwater, or sewer, 3) temporary on-site storage, and 4) off-site disposal to a hazardous waste treatment and disposal facility. The most common remediation approach is to use pump-and-treat with [[wikipedia: Activated carbon |granular activated carbon]] followed by off-site incineration of the spent activated carbon. This technology has been used for years at full scale<ref name="Appleman2014">Appleman, T.D., Higgins, C.P., Quinones, O., Vanderford, B.J., Kolstad, C., Zeigler-Holady, J.C. and Dickenson, E.R., 2014. Treatment of poly-and perfluoroalkyl substances in US full-scale water treatment systems. Water Research, 51, 246-255. [http://dx.doi.org/10.1016/j.watres.2013.10.067 doi: 10.1016/j.watres.2013.10.067]</ref>. However, granular activated carbon has a relatively low capacity for PFAS particularly when shorter-chain compounds are present. Sorption capacity improvement tests have been conducted on various forms of granular and powdered activated carbon, [[wikipedia: Ion-exchange resin |ion exchange resins]], and other sorbent materials as well as mixtures of clay, powdered activated carbon, and other sorbents<ref>Du, Z., Deng, S., Bei, Y., Huang, Q., Wang, B., Huang, J. and Yu, G., 2014. Adsorption behavior and mechanism of perfluorinated compounds on various adsorbents-A review. Journal of Hazardous Materials, 274, 443-454. [http://dx.doi.org/10.1016/j.jhazmat.2014.04.038 doi:10.1016/j.jhazmat.2014.04.038]</ref>. Other methods for ''ex situ'' PFAS removal include high-pressure membrane treatment using [[wikipedia: Nanofiltration |nanofiltration]] or [[wikipedia: Reverse osmosis |reverse osmosis]]<ref name="Appleman2014" /><ref>Department of the Navy (DON). 2015. Interim perfluorinated compounds (PFCs) guidance/frequently asked questions. [//www.enviro.wiki/images/b/b1/Dept_of_Navy-_2015-Interim_Perfluorinated_Compounds_Frequently_asked_questions.pdf FAQs]</ref><ref>Steinle-Darling, E. and Reinhard, M., 2008. Nanofiltration for trace organic contaminant removal: structure, solution, and membrane fouling effects on the rejection of perfluorochemicals. Environmental Science & Technology, 42 (14), 5292–5297. [http://dx.doi.org/10.1021/es703207s doi: 10.1021/es703207s]</ref>. Research into other PFAS treatment technologies, including ''in situ'' barriers (sequestration), biological treatment<ref>Huang, Shan and Jaffe, Peter R., 2019. Defluorination of Perfluorooctanoic Acid (PFOA) and Perfluorooctane Sulfonate (PFOS) by ''Acidimicrobium'' sp. Strain A6. Environmental Science and Technology, 53, pp 11410-11419. DOI:10.1021/acs.est.9b04047 https://pubs.acs.org/doi/full/10.1021/acs.est.9b04047 </ref>, [[wikipedia: Advanced oxidation process |advanced oxidation processes]] and [[Chemical Reduction (In Situ - ISCR) |''in situ'' chemical reduction]] is ongoing.

==Summary==
PFAS are ubiquitous in a variety of industrial and commercial products, have been detected in many environmental media, pose potential risks to human and environmental health, and present challenges with respect to remediation. They are highly stable and mobile in the environment, may bioaccumulate and biomagnify in wildlife, and are the subject of litigation and regulatory actions at the local, state and Federal level. Health-based drinking water advisory levels are low, i.e., ng/L concentrations. As awareness of PFAS grows and regulatory criteria progress, site managers are conducting site investigations, improving analytical techniques, and designing and operating remediation systems. Current research, including that funded by SERDP/ESTCP, aims to demonstrate effective treatment technologies for PFAS and improve technology cost-effectiveness.

==References==
<references />

==See Also==

*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-2423. In situ treatment train for remediation of perfluoroalkyl contaminated groundwater: In situ chemical oxidation of sorbed contaminants (ISCO-SC). SERDP/ESTCP Project ER-2423]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-2426/ER-2426/(language)/eng-US. Quantification of In Situ Chemical Reductive Defluorination (ISCRD) of perfluoroalkyl acids in groundwater impacted by AFFFs. SERDP/ESTCP Project ER-2426]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-2422/ER-2422/(language)/eng-US. Bioaugmentation with vaults: Novel In Situ Remediation Strategy for Transformation of Perfluoroalkyl Compounds. SERDP/ESTCP Project ER-2422]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-2424/ER-2424/(language)/eng-US. Investigating Electrocatalytic and Catalytic Approaches for In Situ Treatment of Perfluoroalkyl Contaminants in Groundwater. SERDP/ESTCP project ER-2424]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-2425/ER-2425/(language)/eng-US. Development of a Novel Approach for In Situ Remediation of Pfc Contaminated Groundwater Systems. SERDP/ESTCP project ER-2425]
*[https://serdp-estcp.mil/projects/details/d8fdde05-10b6-43d4-a4d3-2a1a60329392/pfas-podcast-series-serdp-and-estcp-research-and-demonstrations PFAS Podcast Series: SERDP and ESTCP Research and Demonstrations. SERDP/ESTCP project ER23-7692]

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[[File:WH Picture1.JPG|thumb|center|x350px|link=Matrix Diffusion|Molecular diffusion slowly transports solutes into clay-rich, lower permeability zones]]
[[File:WH Picture2.JPG|thumb|center|x350px|link=Subgrade Biogeochemical Reactor (SBGR)|Typical subgrade biogeochemical reactor (SBGR) layout. The SBGR is an in situ remediation technology for treatment of contaminated source areas and groundwater plume hot spots ]]
[[File:WH Picture3.JPG|thumb|center|x350px|link=Direct Push Logging|An Hydraulic Profiling Tool (HPT) log with electrical conductivity (EC) on left, injection pressure in middle, and flow rate on the right]]
[[File:WH Picture4.JPG|thumb|center|x350px|link=PH Buffering in Aquifers|Diagram of mineral surface exchanging hydrogen ions with varying pH. The surface of most aquifer minerals carries an electrical charge that varies with pH]]
[[File:WH Picture5.JPG|thumb|center|x350px|link=Biodegradation - Hydrocarbons|Comparison of the longitudinal redox zonation concept (A) and the plume fringe concept (B). Both concepts describe the spatial distribution of electron acceptors and respiration processes in a hydrocarbon contaminant plume]]
[[File:WH Picture6.JPG|thumb|center|x350px|link=Direct Push Logging|Schematic of an Hydraulic Profiling Tool (HPT) probe. HPT were developed to better understand formation permeability and the distribution of permeable and low permeability zones in unconsolidated formations]]
[[File:WH Picture7.JPG|thumb|center|x350px|link=Chemical Oxidation Design Considerations(In Situ - ISCO)|In situ chemical oxidation using (a) direct-push injection probes or (b) well-to-well flushing to delivery oxidants (shown in blue) into a target treatment zone of groundwater contaminated by dense nonaqueous phase liquid compounds (shown in red)]]
[[File:WH Picture8.JPG|thumb|center|x350px|link=Geophysical Methods - Case_Studies|High-resolution 3D cross-borehole electrical imaging of contaminated fractured rock at the former Naval Air Warfare Center in New Jersey. Cross-borehole resistivity tomography imaging is a geophysical technique that can be used for site characterization and monitoring by observing variations in the electrical properties of subsurface materials]]
[[File:WH Picture9.JPG|thumb|center|x350px|link=Stable_Isotope_Probing_(SIP)|Stable isotope probing (SIP) in use: Loading, deployment and recovery of Bio-Trap® passive sampler with 13C-labeled benzene. Stable isotope probing (SIP) is used to conclusively determine whether in situ biodegradation of a contaminant is occurring]]
[[File:WH Picture10.JPG|thumb|center|x350px|link=1,2,3-Trichloropropane|Summary of anticipated, primary reaction pathways for degradation of 1,2,3-Trichloropropane (TCP). TCP is a man-made chemical that was used in the past primarily as a solvent and extractive agent, a paint and varnish remover, and as a cleaning and degreasing agent]]
[[File:WH Picture11.JPG|thumb|center|x350px|link=Monitored Natural Attenuation (MNA) of Fuels|Distribution of BTEX plume lengths from 604 hydrocarbon sites. Monitored Natural Attenuation (MNA) is one of the most commonly used remediation approaches for groundwater contaminated with petroleum hydrocarbons (PHCs) and certain fuel additives such as fuel oxygenates or lead scavengers]]
[[File:WH Picture12.JPG|thumb|center|x350px|link=Groundwater Sampling - No-Purge/Passive|No-purge and passive sampling methods eliminate the pre-purging step for groundwater sample collection and represent alternatives to conventional sampling methods that rely on low-flow purging of a well prior to collection. The Snap SamplerTM is an example of a passive grab sampler]]
[[File:WH Picture13.JPG|thumb|center|x350px|link=Natural Source Zone Depletion (NSZD)|Conceptualization of Vapor Transport-related Natural Source Zone Depletion (NSZD) processes at a Petroleum Release Site]]
[[File:WH Picture14.JPG|thumb|center|x350px|link=Soil Vapor Extraction (SVE)|Conceptual diagram of basic Soil Vapor Extraction (SVE) system for vadose zone remediation. (SVE) is a common and typically effective physical treatment process for remediation of volatile contaminants in vadose zone (unsaturated) soils]]
[[File:WH Picture15.JPG|thumb|center|x350px|link=Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation|Emulsified Vegetable Oil (EVO) mixed in field during early pilot test. EVO is commonly added as a slowly fermentable substrate to stimulate the in situ anaerobic bioremediation of chlorinated solvents, explosives, perchlorate, chromate, and other contaminants]]
[[File:WH Picture16.JPG|thumb|center|x350px|link=Vapor_Intrusion_(VI)|Key elements of vapor intrusion pathways]]
[[File:WH Picture17.JPG|thumb|center|x350px|link=Sorption_of_Organic_Contaminants|Batch reactor experiments to generate points on a sorption isotherm]]
[[File:WH Picture18.JPG|thumb|center|x350px|link=Metagenomics|Results for metagenomic analysis of a groundwater sample obtained from a site impacted with petroleum hydrocarbons]]
[[File:WH Picture19.JPG|thumb|center|x350px|link=Perchlorate|Perchlorate releases and drinking water detections]]
[[File:WH Picture20.JPG|thumb|center|x350px|link=Mass_Flux_and_Mass_Discharge|Data input screen for ESTCP Mass Flux Toolkit]]
[[File:WH Picture21.JPG|thumb|center|x350px|link=Bioremediation_-_Anaerobic_Design_Considerations|Amendment addition for biobarrier]]
[[File:WH Picture22.JPG|thumb|center|x350px|link=Thermal Conduction Heating (TCH)|Thermal Remediation - Desorption schematic]]
[[File:WH_Picture23.jpg|thumb|center|x350px|link=Contaminated_Sediments_-_Introduction |Key exposure pathways for human health risk from contaminated sediments]]
[[File:WH_Picture24.jpg|thumb|center|x350px|link=Perfluoroalkyl_and_Polyfluoroalkyl_Substances_(PFAS)| The PFAS family of compounds]]

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'''[[Transport & Attenuation Processes | Attenuation & Transport Processes]]'''

*[[Biodegradation - 1,4-Dioxane]]
*[[Biodegradation - Cometabolic]]
*[[Biodegradation - Hydrocarbons]]
*[[Biodegradation - Reductive Processes]]
*[[Groundwater Flow and Solute Transport]]
*[[Matrix Diffusion]]
*[[Metals and Metalloids - Mobility in Groundwater | Mobility of Metals and Metalloids]]
*[[pH Buffering in Aquifers]]
*[[Sorption of Organic Contaminants]]
*[[Vapor Intrusion (VI)]]
**[[Vapor Intrusion - Separation Distances from Petroleum Sources]]
**[[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways|Vapor Intrusion - Sewers and Utility Tunnels as Preferential Pathways]]
**[[Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)|Vapor Intrusion - Assessing VI Impacts in Neighborhoods with Groundwater Contaminated CVOCs]]

'''[[Characterization, Assessment & Monitoring]]'''

*[[Characterization Methods – Hydraulic Conductivity]]
*[[Compound Specific Isotope Analysis (CSIA)|Compound Specific Isotope Analysis (CSIA)]]
*[[Direct Push (DP) Technology]]
**[[Direct Push Logging |Direct Push Logging]]
**[[Direct Push Sampling |Direct Push Sampling]]
*[[Geophysical Methods | Geophysical Methods]]
**[[Geophysical Methods - Case Studies |Case Studies]]
**[[Hydrogeophysical Methods for Characterization and Monitoring of Groundwater-Surface Water Exchanges]]
*[[Groundwater Sampling - No-Purge/Passive]]
*[[Long-Term Monitoring (LTM)|Long-Term Monitoring (LTM)]]
**[[Long-Term Monitoring (LTM) - Data Analysis |LTM Data Analysis]]
**[[Long-Term Monitoring (LTM) - Data Variability |LTM Data Variability]]
*[[Molecular Biological Tools - MBTs |Molecular Biological Tools (MBTs)]]
**[[Metagenomics]]
**[[Proteomics and Proteogenomics]]
**[[Quantitative Polymerase Chain Reaction (qPCR)]]
**[[Stable Isotope Probing (SIP)]]
*[[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill |Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
*[[OPTically-based In-situ Characterization System (OPTICS)]]

'''[[Coastal and Estuarine Ecology]]'''

*[[Phytoplankton (Algae) Blooms]]

'''[[Contaminated Sediments - Introduction | Contaminated Sediments]]'''

*[[Contaminated Sediment Risk Assessment]]
*[[In Situ Toxicity Identification Evaluation (iTIE) | In Situ Toxicity Identification Evaluation]]
*[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
*[[Mercury in Sediments]]
*[[Passive Sampling of Sediments]]
**[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]
*[[Sediment Capping]]

| style="width:33%; vertical-align:top; " |

'''[[Light Non-Aqueous Phase Liquids (LNAPLs)]]'''

*[[LNAPL Conceptual Site Models]]
*[[LNAPL Remediation Technologies]]
*[[NAPL Mobility]]

'''[[Munitions Constituents]]'''

*[[Munitions Constituents - Abiotic Reduction|Abiotic Reduction]]
*[[Munitions Constituents - Alkaline Degradation|Alkaline Degradation]]
**[[Pyrogenic Carbonaceous Matter Enhanced Alkaline Hydrolysis]]
*[[Munitions Constituents - Composting|Composting]]
*[[Munitions Constituents - Deposition |Deposition]]
*[[Munitions Constituents - Dissolution |Dissolution]]
*[[Munitions Constituents - Electrochemical Treatment|Electrochemical Treatment]]
*[[Metal(loid)s - Small Arms Ranges]]
*[[Passive Sampling of Munitions Constituents|Passive Sampling]]
*[[Munitions Constituents – Photolysis |Photolysis]]
*[[Remediation of Stormwater Runoff Contaminated by Munition Constituents |Remediation of Stormwater Runoff ]]
*[[Munitions Constituents – Sample Extraction and Analytical Techniques|Sample Extraction and Analytical Techniques]]
*[[Munitions Constituents - Soil Sampling |Soil Sampling]]
*[[Munitions Constituents - Sorption |Sorption]]
*[[Munitions Constituents - IM Toxicology |Toxicology]]
*[[Munitions Constituents- TREECS™ Fate and Risk Modeling|TREECS™]]

'''[[Monitored Natural Attenuation (MNA)]]'''

*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| MNA of Chlorinated Solvents]]
**[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA) of Fuels| MNA of Fuels]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| MNA of Metals and Metalloids]]
*[[Natural Source Zone Depletion (NSZD)]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies| Transitioning from Active Remedies]]

'''[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]'''

*[[Hydrothermal Alkaline Treatment (HALT)]]
*[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
*[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]
*[[PFAS Ex Situ Water Treatment]]
**[[PFAS Treatment by Anion Exchange]]
*[[PFAS Leaching Characterization with the Leaching Environmental Assessment Framework (LEAF)]]
*[[PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)]]
*[[PFAS Soil Remediation Technologies]]
*[[PFAS Sources]]
*[[PFAS Toxicology and Risk Assessment]]
*[[PFAS Transport and Fate]]
*[[PFAS Treatment by Electrical Discharge Plasma]]
*[[Photoactivated Reductive Defluorination - PFAS Destruction | Photoactivated Reductive Defluorination]]
*[[Reverse Osmosis and Nanofiltration Membrane Filtration Systems for PFAS Removal]]
*[[Thermal Conduction Heating for Treatment of PFAS-Impacted Soil]]
*[[Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)| Transition of Aqueous Film Forming Foam Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances]]

| style="width:33%; vertical-align:top; " |
'''[[Regulatory Issues and Site Management]]'''

*[[Alternative Endpoints]]
*[[Mass Flux and Mass Discharge]]
*[[Plume Response Modeling]]
*[[REMChlor - MD | REMChlor-MD]]
*[[Source Zone Modeling]]
*[[Sustainable Remediation]]

'''[[Remediation Technologies]]'''
*[[Amendment Distribution in Low Conductivity Materials]]
*[[Bioremediation - Anaerobic|Anaerobic Bioremediation]]
**[[Bioremediation - Anaerobic Design Considerations | Design Considerations]]
**[[Design Tool - Base Addition for ERD]]
**[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
**[[Low pH Inhibition of Reductive Dechlorination]]
**[[Bioremediation - Anaerobic Secondary Water Quality Impacts | Secondary Water Quality Impacts]]
*[[Chemical Oxidation (In Situ - ISCO) | In Situ Chemical Oxidation (ISCO)]]
**[[Chemical Oxidation Design Considerations(In Situ - ISCO) | Design Considerations]]
**[[Chemical Oxidation Oxidant Selection (In Situ - ISCO) | Oxidant Selection]]
*[[Chemical Reduction (In Situ - ISCR) | In Situ Chemical Reduction (ISCR)]]
**[[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR) | Zero-Valent Iron (ZVI)]]
**[[Zerovalent Iron Permeable Reactive Barriers]]
*[[In Situ Groundwater Treatment with Activated Carbon]]
*[[Injection Techniques for Liquid Amendments]]
*[[Injection Techniques - Viscosity Modification]]
*[[Landfarming]]
*[[Metal and Metalloids - Remediation | Remediation of Metals and Metalloids]]
*[[Remediation Performance Assessment at Chlorinated Solvent Sites]]
*[[Soil Vapor Extraction (SVE)]]
*[[Stream Restoration]]
*[[Subgrade Biogeochemical Reactor (SBGR)]]
*[[Supercritical Water Oxidation (SCWO)]]
*[[Thermal Remediation]]
**[[Thermal Remediation - Combined Remedies | Combined Remedies]]
**[[Thermal Remediation - Electrical Resistance Heating | Electrical Resistance Heating (ERH)]]
**[[Thermal Remediation - Smoldering | Smoldering]]
**[[Thermal Remediation - Steam | Steam Enhanced Extraction (SEE)]]
**[[Thermal Conduction Heating (TCH)]]

'''[[Soil & Groundwater Contaminants]]'''

*[[1,2,3-Trichloropropane]]
*[[1,4-Dioxane]]
*[[Chlorinated Solvents]]
*[[Metal and Metalloid Contaminants|Metals and Metalloids]]
*[[N-nitrosodimethylamine (NDMA)]]
*[[Perchlorate|Perchlorate]]
*[[Petroleum Hydrocarbons (PHCs)]]
*[[Polycyclic Aromatic Hydrocarbons (PAHs)]]
|}
|}
|}

PFAS Leaching Characterization with the Leaching Environmental Assessment Framework (LEAF)

2026-08-17T14:53:33Z

Admin:

[[Wikipedia: Firefighting_foam#Synthetic_foams | Aqueous film-forming foams (AFFFs)]] are a major source of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | per- and poly-fluoroalkyl substances (PFAS)]] impacts in soil and groundwater. Standardized tools are needed to rapidly assess the potential for retention, leaching, and transport of PFAS from the source zone to downgradient regions, so that this information can be applied towards critical facets of site management such as prioritizing PFAS-impacted sites for further investigation and remediation. Existing standard leaching methods were developed prior to concerns regarding PFAS. Therefore, studies are needed to ensure that leaching methods are compatible for use with PFAS and that resulting data are representative of the risk of PFAS leaching at impacted sites.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]
*[[PFAS Sources]]
*[[PFAS Transport and Fate]]

'''Contributors:''' Dr. Jennifer L. Guelfo, Dr. David Kosson, Dr. Andy Garrabrants, Dr. Fangfei Liu, Darlington Yawson, and Dr. Md. Isreq Real

'''Key Resources:'''
*Development of Leaching Tests for Materials Containing SVOCs and PFAS, EPA 600/R-23/382<ref name="GarrabrantsEtAl2024"/>

*[https://www.epa.gov/hw-sw846/leaching-environmental-assessment-framework-leaf-methods-and-guidance Leaching Environmental Assessment Framework (LEAF) Methods and Guidance] (EPA website)

==Introduction to LEAF==
The [https://www.epa.gov/ U.S. Environmental Protection Agency (EPA)] [https://www.epa.gov/hw-sw846/leaching-environmental-assessment-framework-leaf-methods-and-guidance Leaching Environmental Assessment Framework (LEAF)] is a suite of standardized test methods for evaluating contaminant release from solids under environmentally relevant conditions (Table 1). The four leaching methods within LEAF were originally validated for inorganic constituents<ref>Garrabrants, A.C., Kosson, D.S., Stefanski, L., DeLapp, R., Seignette, P.F.A.B., van der Sloot, H.A., Kariher, P., Baldwin, M., 2012. Interlaboratory Validation of the Leaching Environmental Assessment Framework (LEAF) Method 1313 and Method 1316, EPA/600/R-12/623, U.S. Environmental Protection Agency, Air Pollution and Control Division. [[Media: EPA 600_R-12_623.pdf | Free Download EPA 600/R-12/623]]</ref><ref>Garrabrants, A.C., Kosson, D.S., DeLapp, R., Kariher, P., Seignette, P.F.A.B., van der Sloot, H.A., Stefanski, L., Baldwin, M., 2012. Interlaboratory Validation of the Leaching Environmental Assessment Framework (LEAF) Method 1314 and Method 1315, EPA/600/R-12/624, U.S. Environmental Protection Agency, Air Pollution and Control Division. [[Media: EPA 600_R-12_624.pdf | Free Download EPA 600/R-12/624]]</ref> and included as standard leaching methods EPA 1313 – 1316 within Update V of SW-846<ref>USEPA, 2026. Hazardous Waste Test Methods / SW-846. [https://www.epa.gov/hw-sw846 USEPA SW-846 website]</ref>. To address the need for standardized tests to evaluate PFAS leaching and mobility, LEAF methods have been optimized and demonstrated for use with PFAS (Methods 1313A-1316A)<ref name="GarrabrantsEtAl2024">Garrabrants, A.C., Liu, F., Warne, R., DeLapp, R., Brown, L., Rubin, Z., Yawson, D., Kosson, D.S., Guelfo, J.L., Real, M.I., van der Sloot, H.A., Touati, A., Thorneloe, S., 2024. Development of Leaching Tests for Materials Containing SVOCs and PFAS, EPA 600/R-23/382, USEPA, Washington, D.C. [[Media: EPA 600_R-23_382.PDF | Free Download EPA 600/R-23/382]]</ref>. This article will focus on Methods 1313A, 1314A, and 1316A. Demonstration of Method 1315A for compacted granular materials (including concrete and asphalt) is ongoing.

{| class="wikitable" style="float:left; margin-left:10px;"
|+Table 1. EPA SW-846 methods that comprise the LEAF framework
|-
!Method
!Description
|-
| 1313 || Liquid-solid partitioning as a function of '''''extract pH''''' using a parallel batch extraction (i.e., equilibrium) procedure (Figure 1)
|-
| 1314 || Liquid-solid partitioning as a function of '''''liquid-solid ratio (L/S)''''' for constituents in solid materials using an up-flow '''''percolation''''' column procedure (Figure 3)
|-
| 1315 || '''''Mass transfer rates''''' of constituents in monolithic or compacted granular materials using a semi-dynamic tank leaching procedure
|-
| 1316 || Liquid-solid partitioning as a function of '''''L/S''''' using a parallel batch extraction (i.e., '''''equilibrium''''') procedure (Figure 2)
|-
| colspan="2" style="background:white;" | Note: Text shown in '''''bold''''' indicates primary condition evaluated in each method.
|}
 

==Method Development for PFAS==
Complete details of the development of LEAF Methods 1313A, 1314A, and 1316A for use with PFAS are available in Garrabrants ''et al.'', 2024<ref name="GarrabrantsEtAl2024"/>. Representative method modifications include:
* Materials of construction for experimental apparatus: containers used for leaching vessels (Methods 1313A, 1316A) and column construction materials (Method 1314A) evaluated for background PFAS and PFAS uptake.
* Reagents and eluant composition: eluant composition was optimized to use 1 mM CaCl2 to reduce formation of colloidal matter; Method 1313A pH adjustment now conducted with nonoxidizing HCl.
* Experimental conditions: Longer equilibration times (e.g., Method 1313, 1316) may be required due to slow desorption kinetics of certain PFAS from soil and organic matrices, implementation of settling to facilitate separation of solids from eluates.
* Eluate processing (all methods): Use of centrifugation in lieu of eluate filtering, sonication of bottle prior to eluate subsampling.

==Batch Test Demonstration Studies==
PFAS-specific adaptations were tested in batch test demonstration studies, which included triplicate implementation of Methods 1313A and 1316A in four AFFF-impacted site soils.

[[File: GuelfoFig1.png | thumb | 500 px | Figure 1: Figure 1. a) Overview of LEAF Method 1313A and b) PFHxS leaching as a function of pH<ref name="GarrabrantsEtAl2024"/>. Definitions: lower limit of quantification (LLOQ) and method detection limit (MDL)]]
'''Draft Method 1313A''' was used to evaluate pH-dependent leaching in PFAS-contaminated soils in parallel batch extractions where each set of batch reactors is prepared and equilibrated at different pH (Figure 1). Short-chain PFAS (≤6 fluorinated carbons) generally showed little to no variation in leaching across the tested pH range of 2-13 (e.g., [[Wikipedia: Perfluorohexanesulfonic acid | PFHxS]], Figure 1), whereas long-chain PFAS exhibited increased leaching at higher pH<ref name="GarrabrantsEtAl2024"/>. This trend is consistent with previous findings showing that soil-water partitioning coefficients (''Kd'') decrease as pH increases (e.g., Higgins and Luthy 2006)<ref name="HigginsLuthy2006">Higgins, C.P., Luthy, R.G., 2006. Sorption of Perfluorinated Surfactants on Sediments. Environmental Science and Technology, 40(23), pp. 7251–7256. [https://doi.org/10.1021/es061000n doi: 10.1021/es061000n]</ref>. The most pronounced pH effects were observed for perfluoroalkyl sulfonamides (FASAs) such as [[Wikipedia: Perfluorooctanesulfonamide | perfluorooctane sulfonamide (FOSA)]], which transition from neutral to anionic forms within the circumneutral pH range (~pH 6). The anionic form has a lower ''Kd'' and results in higher leaching concentrations<ref name="GarrabrantsEtAl2024"/><ref name="NguyenEtAl2020">Nguyen, T.M.H., Bräunig, J., Thompson, K., Thompson, J., Kabiri, S., Navarro, D.A., Kookana, R.S., Grimison, C., Barnes, C.M., Higgins, C.P., McLaughlin, M.J., Mueller, J.F., 2020. Influences of Chemical Properties, Soil Properties, and Solution pH on Soil–Water Partitioning Coefficients of Per- and Polyfluoroalkyl Substances (PFASs). Environmental Science and Technology, 54(24), pp. 15883–15892. [https://doi.org/10.1021/acs.est.0c05705 doi: 10.1021/acs.est.0c05705]  [[Media: NguyenEtAl2020.pdf | Open Access Article]]</ref>. For many site management scenarios where pH is circumneutral, variations in anionic PFAS leaching are expected to be small over the relevant pH range. In such cases, when testing time and costs are primary considerations, Method 1313A may be a lower priority relative to evaluating leaching as a function of L/S (Method 1316A, Method 1314A). Different considerations may be needed where FASAs or PFAS with multiple, ionizable functional groups (i.e., [[Wikipedia: Zwitterion | zwitterions]]) are of concern.

[[File: GuelfoFig2.png | thumb | 500 px | Figure 2: a) Overview of LEAF Method 1316A and b) PFHxS leaching as a function of L/S ratio evaluated in parallel batch leaching vessels<ref name="GarrabrantsEtAl2024"/>]]
'''Draft Method 1316A''' was used to evaluate L/S-dependent leaching of PFAS in impacted soils using parallel batch extractions where each set of batch reactors is prepared and equilibrated at a different L/S. (Figure 2). Methods 1314A and 1316A are similar in intent as they both evaluate leaching as a function of L/S; however, the experimental approach differs. Method 1314A uses a flow-through column configuration (Figure 3; discussed further below). Method 1314A may better simulate field conditions, but Method 1316A is simpler and less costly to implement. Trends in Method 1314A and 1316A are expected to be qualitatively similar but leaching concentrations are expected to exhibit differences. Despite this, leaching studies comparing Methods 1314A and 1316A for inorganics showed that cumulative release results were within one order of magnitude<ref>Lopez Meza, S., Garrabrants, A.C., van der Sloot, H., Kosson, D.S., 2008. Comparison of the Release of Constituents from Granular Materials under Batch and Column Testing. Waste Management, 28(10), pp. 1853–1867. [https://doi.org/10.1016/j.wasman.2007.11.009 doi: 10.1016/j.wasman.2007.11.009]</ref>.

Method 1316A and Method 1314A may also provide different insights into transport mechanisms. Because Method 1316A is performed using equilibrated batch reactors at varying L/S, results can be used to develop equilibrium desorption isotherms and calculate desorption coefficients (e.g., ''Kd,desorption''). Studies have shown that ''Kd,desorption'' values for PFAS may be greater than ''Kd'', an effect often attributed to desorption hysteresis<ref>Schaefer, C.E., Nguyen, D., Christie, E., Shea, S., Higgins, C.P., Field, J., 2022. Desorption Isotherms for Poly- and Perfluoroalkyl Substances in Soil Collected from an Aqueous Film-Forming Foam Source Area. Journal of Environmental Engineering, 148(1), Article 04021074. [https://doi.org/10.1061/(ASCE)EE.1943-7870.0001952 doi: 10.1061/(ASCE)EE.1943-7870.0001952]</ref>. Consequently Method 1316A provides a straightforward method to estimate site-specific desorption parameters. Although sorption parameters can also be inferred from column (Method 1314A) data, interpretation is often complicated by nonequilibrium processes. Conversely, the column data can be valuable for quantifying those additional mechanisms providing transport parameters that can describe rate-limited transport (e.g., fraction of non-equilibrium sorption sites and sorption rates) and other dynamic behavior.

As anticipated, trends in PFAS leaching obtained during the Method 1316A and Method 1314A demonstrations were qualitatively similar. These are further discussed below.

==Column Test Demonstration Studies==
[[File: GuelfoFig3.png | thumb | 500 px | Figure 3. a) Overview of LEAF Method 1314A and b) PFHxS leaching as a function of ∑(L/S) evaluated in saturated up-flow column tests<ref name="GarrabrantsEtAl2024"/>. Note that acrylic here simply refers to the column material of construction used in this round of Method 1314 testing.]]
PFAS-specific adaptations were tested in column test demonstration studies, which included triplicate implementation of Method 1314A in three AFFF-impacted site soils.

'''Draft Method 1314A''' was implemented in saturated, up-flow columns to evaluate leaching of PFAS as a function of cumulative L/S (∑(L/S)); Figure 3; total volume of water that has passed through the column divided by the soil mass in the column). As noted, the intent of Method 1314a and 1316a is similar, and in both tests, similar qualitative results were observed. For example, short-chain PFAS exhibited high initial concentrations that decreased rapidly. However, in Method 1314a, these rapid drops in short-chain PFAS tended to occur by ∑(L/S) ≈ 2 (e.g., Site 1 and 3 soils, Figure 3) whereas in some cases, such as for PFHxS, Method 1316A produced slightly flatter elution curves than Method 1314A (Figures 2b and 3b). Long-chain PFAS generally displayed flatter elution profiles than short-chain PFAS across both methods. These trends are consistent with chain length dependent sorption documented in the literature<ref name="HigginsLuthy2006"/><ref name="NguyenEtAl2020"/><ref>Guelfo, J.L., Higgins, C.P., 2013. Subsurface Transport Potential of Perfluoroalkyl Acids at Aqueous Film-Forming Foam (AFFF)-Impacted Sites. Environmental Science and Technology, 47(9), pp. 4164–4171. [https://doi.org/10.1021/es3048043 doi: 10.1021/es3048043]</ref>. Although column modeling is beyond the scope of this article, prior studies have shown that saturated transport can be influenced by rate-limited desorption, particularly for long-chain PFAS<ref>Doria-Manzur, A., Gray, E.P., Streets, S.S., Guelfo, J.L., 2025. Per- and Polyfluoroalkyl Substances (PFAS) Transport from Biosolids-Amended Soils: An Experimental and Numerical Approach. Water Research, 288(Part B), Article 124674. [https://doi.org/10.1016/j.watres.2025.124674 doi: 10.1016/j.watres.2025.124674]  [[Media: Doria-ManzurEtAl2026.pdf | Open Access Article]]</ref><ref>Guelfo, J.L., Wunsch, A., McCray, J., Stults, J.F., Higgins, C.P., 2020. Subsurface Transport Potential of Perfluoroalkyl Acids (PFAAs): Column Experiments and Modeling. Journal of Contaminant Hydrology, 233, Article 103661. [https://doi.org/10.1016/j.jconhyd.2020.103661 doi: 10.1016/j.jconhyd.2020.103661]  [[Media: GuelfoEtAl2020.pdf | Open Access Manuscript]]</ref>. As noted, data from Methods 1316A and 1314A can support estimation of transport parameters representing equilibrium and nonequilibrium behavior, respectively.

==LEAF Screening Evaluations==
[[File: GuelfoFig4.png | thumb | 500 px | Figure 4. Example screening assessment for perfluorooctane sulfonate (PFOS) using total content and data from Methods 1313A and 1314A. Figure format adapted from Garrabrants ''et al''. 2021<ref>Garrabrants, A.C., Kosson, D.S., Brown, K.G., Fagnant, D.P., Helms, G., Thorneloe, S.A., 2021. Methodology for Scenario-Based Assessments and Demonstration of Treatment Effectiveness Using the Leaching Environmental Assessment Framework (LEAF). Journal of Hazardous Materials, 406, Article 124635. [https://doi.org/10.1016/j.jhazmat.2020.124635 doi: 10.1016/j.jhazmat.2020.124635]  [[Media: GarrabrantsEtAl2021.pdf | Open Access Manuscript]]</ref>.]]
LEAF provides a standardized, robust approach for evaluating PFAS release from impacted granular materials under a range of environmental conditions. The tests are complementary, capture a range of conditions, and vary in ease of implementation. This provides the flexibility for users to select the test or test combinations that best suit their project objectives, timeline, and budget. A common use of LEAF data is in screening level assessments. These are stepwise assessments that establish increasingly refined maximum leaching concentrations, ''Cleach,max'' (Figure 4), which can then be compared to regulatory limits such as maximum contaminant levels, when available. For example, a stepwise screening assessment might include:
#Assume the maximum leaching concentration is represented by the total content (total initial mass of contaminant present) leaching into the first L/S.
#Assume only the available content leaches into the first L/S where available content is the maximum mass released over pH 2-13 measured using Method 1313a. For many PFAS, total content is equal to available content meaning that all of the PFAS mass is available for leaching.
#Assume the leaching concentration at natural pH (measured in Method 1313A at natural pH or Method 1316A at L/S of 10) is maximum leaching concentration adjusted to the first L/S.
#Consider the maximum leaching concentration over the L/S range (Method 1314A or Method 1316A) and the upper estimate of leaching, or ''Cleach,max'', is the concentration from either Step 3 or Step 4, whichever is greater.

Screening assessments may be sufficient to meet project goals, but when additional refinements of leaching estimates are needed, site-specific data (e.g., infiltration) can be combined with test data and computational approaches (e.g., fate and transport models) for more site-specific estimates of leaching. Example scenarios where LEAF may be used to evaluate PFAS-impacted solids include 1) estimating PFAS release from AFFF-impacted soils, 2) estimating PFAS release from biosolids-amended soils at land application sites, 3) providing transport parameters to model PFAS transport from the source zone to the saturated zone, and 4) evaluating PFAS release from treatment residuals such as soils or sediments treated by soil washing or thermal approaches.

==Summary and Ongoing Research==
The LEAF framework offers a reliable, replicable approach to evaluating PFAS release from solids. With recent adaptations for PFAS-specific considerations, LEAF methods provide valuable tools for regulators and practitioners in managing PFAS-contaminated materials and assessing long-term environmental risks. However, there are key areas of ongoing research, including:
*An interlab validation of Methods 1313A, 1314A, and 1316A in coordination with the EPA
*Optimization and demonstration of Method 1315A for use with PFAS-impacted solids
*Evaluation of an unsaturated Method 1314A protocol to assess the need for and ability of LEAF testing to capture air-water interfacial partitioning of PFAS
*Application of the total oxidizable precursor (TOP) assay for evaluating the maximum additional PFAS leaching that may occur as a result of polyfluoroalkyl precursor transformation
*Comparison of LEAF testing data to previously collected field-scale enhanced flushing data collected from the same site

Additionally, there are key areas for consideration in future research:
*Collection of paired field-laboratory data under ambient conditions to further validate the applicability of LEAF assessments for estimation of field-relevant PFAS leaching and mobility
*Consideration of biotransformation in modeling and interpretation of LEAF data, as the state of the science regarding biotransformation of polyfluoroalkyl substances to terminal perfluoroalkyl acids advances

==Other LEAF Resources==
There are numerous resources describing the development of the LEAF leaching methods for inorganics. They are not specific to PFAS, but are still valuable resources focused on LEAF implementation, applications, and management of LEAF data. They include:
*[https://www.vanderbilt.edu/leaching/leach-xs-lite/ Leach XS Lite] - a tool for LEAF data management and visualization; free to download after registering for a free license key
*[https://www.epa.gov/hw-sw846/how-guide-leaching-environmental-assessment-framework LEAF “How-To” Guide] - guidance on LEAF background, implementation, test result interpretation; includes case studies
*[https://www.epa.gov/hw-sw846/leaching-environmental-assessment-framework-leaf-methods-and-guidance USEPA LEAF Methods and Guidance] homepage
 

==References==
<references />

==See Also==

PFAS Destruction by Ultraviolet/Sulfite Treatment

2026-08-15T14:43:23Z

Admin:

<onlyinclude>The ultraviolet (UV)/sulfite based reductive defluorination process has emerged as an effective and practical option for generating hydrated electrons (''eaq<big>-</big>'' ) which can destroy [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | per- and polyfluoroalkyl substances (PFAS)]] in water. </onlyinclude>It offers significant advantages for PFAS destruction, including high percentages of defluorination, high treatment efficiency for long-, short-, and ultra-short chain PFAS without mass transfer limitations, selective reactivity by hydrated electrons, low energy consumption, low capital and operation costs, and no production of harmful byproducts. <onlyinclude>A UV/sulfite treatment system </onlyinclude>designed and developed by Haley and Aldrich (EradiFluorTM<ref name="EradiFluor">Haley and Aldrich, Inc. (commercial business), 2024. EradiFluor. [https://www.haleyaldrich.com/about-us/applied-research-program/eradifluor/ Comercial Website]</ref>) <onlyinclude>has been demonstrated in two field demonstrations in which it achieved near-complete defluorination and greater than 99% destruction of 40 PFAS analytes measured by EPA method 1633. </onlyinclude>
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]
*[[PFAS Ex Situ Water Treatment]]
*[[PFAS Sources]]
*[[PFAS Treatment by Electrical Discharge Plasma]]
*[[Photoactivated Reductive Defluorination - PFAS Destruction]]
*[[Supercritical Water Oxidation (SCWO)]]

'''Contributor(s):''' [[Dr. John Xiong]], [[Dr. Yida Fang]], [[Dr. Raul Tenorio]], Isobel Li, and [[Dr. Jinyong Liu]]

'''Key Resource(s):'''
*[https://www.haleyaldrich.com/about-us/applied-research-program/eradifluor/ EradiFluorTM]<ref name="EradiFluor"/>
*Defluorination of Per- and Polyfluoroalkyl Substances (PFAS) with Hydrated Electrons: Structural Dependence and Implications to PFAS Remediation and Management<ref name="BentelEtAl2019">Bentel, M.J., Yu, Y., Xu, L., Li, Z., Wong, B.M., Men, Y., and Liu, J., 2019. Defluorination of Per- and Polyfluoroalkyl Substances (PFASs) with Hydrated Electrons: Structural Dependence and Implications to PFAS Remediation and Management. Environmental Science and Technology, 53(7), pp. 3718-28. [https://doi.org/10.1021/acs.est.8b06648 doi: 10.1021/acs.est.8b06648]  [[Media: BentelEtAl2019.pdf | Article pdf]]</ref>
*Accelerated Degradation of Perfluorosulfonates and Perfluorocarboxylates by UV/Sulfite + Iodide: Reaction Mechanisms and System Efficiencies<ref>Liu, Z., Chen, Z., Gao, J., Yu, Y., Men, Y., Gu, C., and Liu, J., 2022. Accelerated Degradation of Perfluorosulfonates and Perfluorocarboxylates by UV/Sulfite + Iodide: Reaction Mechanisms and System Efficiencies. Environmental Science and Technology, 56(6), pp. 3699-3709. [https://doi.org/10.1021/acs.est.1c07608 doi: 10.1021/acs.est.1c07608]  [[Media: LiuZEtAl2022.pdf | Article pdf]]</ref>
*Destruction of Per- and Polyfluoroalkyl Substances (PFAS) in Aqueous Film-Forming Foam (AFFF) with UV-Sulfite Photoreductive Treatment<ref>Tenorio, R., Liu, J., Xiao, X., Maizel, A., Higgins, C.P., Schaefer, C.E., and Strathmann, T.J., 2020. Destruction of Per- and Polyfluoroalkyl Substances (PFASs) in Aqueous Film-Forming Foam (AFFF) with UV-Sulfite Photoreductive Treatment. Environmental Science and Technology, 54(11), pp. 6957-67. [https://doi.org/10.1021/acs.est.0c00961 doi: 10.1021/acs.est.0c00961]</ref>
*[https://serdp-estcp.mil/projects/details/4c073623-e73e-4f07-a36d-e35c7acc75b6/er21-5152-project-overview Demonstration of a UV/Sulfite System (EradiFluor™) for PFAS Destruction in Concentrated Waste Streams - ESTCP Project ER31-5152]
*[https://serdp-estcp.mil/projects/details/b66d1399-3904-4d68-9d03-b77d16f3f90a/er18-1289-project-overview Treatment of Legacy and Emerging Fluoroalkyl Chemicals in Groundwater with Integrated Approaches: Rapid and Regenerable Adsorption and UV-induced Defluorination - SERDP Project ER18-1289]
*[https://serdp-estcp.mil/projects/details/50228f09-a6db-4c72-a9c5-15f82e34bac3/er21-1117-project-overview Thermal-Enhanced Photochemical and Alkaline Destruction of PFAS in Sorbent Regenerants and Membrane Concentrates - SERDP Project ER21-1117]

==Introduction==
The hydrated electron (''eaq<big>'''-'''</big>'' ) can be described as an electron in solution surrounded by a small number of water molecules<ref name="BuxtonEtAl1988">Buxton, G.V., Greenstock, C.L., Phillips Helman, W., and Ross, A.B., 1988. Critical Review of Rate Constants for Reactions of Hydrated Electrons, Hydrogen Atoms and Hydroxyl Radicals (⋅OH/⋅O-) in Aqueous Solution. Journal of Physical and Chemical Reference Data, 17(2), pp. 513-886. [https://doi.org/10.1063/1.555805 doi: 10.1063/1.555805]</ref>. <onlyinclude>Hydrated electrons can be produced by photoirradiation of solutes, including sulfite, iodide, dithionite, and ferrocyanide, and have been reported in literature to effectively decompose per- and polyfluoroalkyl substances (PFAS) in water. The hydrated electron is one of the most reactive reducing species, with a standard reduction potential of about −2.9 volts. Though short-lived, hydrated electrons react rapidly with many species having more positive reduction potentials</onlyinclude><ref name="BuxtonEtAl1988"/><onlyinclude>. </onlyinclude>

Among the electron source chemicals, sulfite (SO32−) has emerged as one of the most effective and practical options for generating hydrated electrons to destroy PFAS in water. The mechanism of hydrated electron production in a sulfite solution under ultraviolet is shown in Equation 1 (UV is denoted as ''hv, SO3<big>'''•-'''</big>'' is the sulfur trioxide radical anion):

::<big>'''Equation 1:'''</big>   [[File: XiongEq1.png | 200 px]]

The hydrated electron has demonstrated excellent performance in destroying PFAS such as [[Wikipedia:Perfluorooctanesulfonic acid | perfluorooctanesulfonic acid (PFOS)]], [[Wikipedia:Perfluorooctanoic acid|perfluorooctanoic acid (PFOA)]]<ref>Gao, J., Rao, D., Liu, Z., Yin, E., Zhang, Z., Fu, Q., Nogales, M., and Liu, J., 2025. Temperature Effect on Per- and Polyfluoroalkyl Substance Degradation by Ultraviolet/Sulfite: Insights on Lamp Heat, Molecular Transformation, and Photochemical Principles. Environmental Science & Technology, 59(49), pp. 26865-26874. [https://doi.org/10.1021/acs.est.5c11519 doi: 10.1021/acs.est.5c11519]</ref> and [[Wikipedia: GenX|GenX]]<ref>Bentel. M., Yu, Y., Xu, L., Kwon, H., Li, Z., Wong, B.M., Men, Y., and Liu, J., 2020. Degradation of Perfluoroalkyl Ether Carboxylic Acids with Hydrated Electrons: Structure–Reactivity Relationships and Environmental Implications. Environmental Science and Technology, 54(4), pp. 2489-2499. [https://doi.org/10.1021/acs.est.9b05869 doi: 110.1021/acs.est.9b05869]  [[Media: BentelEtAl2020.pdf | Open Access Article]]</ref>. Mechanisms include cleaving carbon-to-fluorine (C-F) bonds (i.e., hydrogen/fluorine atom exchange) and chain shortening (i.e., [[Wikipedia: Decarboxylation | decarboxylation]], [[Wikipedia: Hydroxylation | hydroxylation]], [[Wikipedia: Elimination reaction | elimination]], and [[Wikipedia: Hydrolysis | hydrolysis]])<ref name="BentelEtAl2019"/>.

==Process Description==
A commercial UV/sulfite treatment system designed and developed by Haley and Aldrich (EradiFluorTM<ref name="EradiFluor"/>) includes an optional pre-oxidation step to transform PFAS precursors (when present) and a main treatment step to destroy PFAS by UV/sulfite. The effluent from the treatment process can be sent back to the influent of a pre-treatment separation system (such as a [[Wikipedia: Foam fractionation | foam fractionation]], [[PFAS Treatment by Anion Exchange | regenerable ion exchange]], or a [[Reverse Osmosis and Nanofiltration Membrane Filtration Systems for PFAS Removal | membrane filtration system]]) for further concentration or sent for off-site disposal in accordance with relevant disposal regulations. A conceptual treatment process diagram is shown in Figure 1. [[File: XiongFig1.png | thumb | left | 600 px | Figure 1: Conceptual Treatment Process for a Concentrated PFAS Stream]] 

==Advantages==
A UV/sulfite treatment system offers significant advantages for PFAS destruction compared to other technologies, including high defluorination percentage, high treatment efficiency for short-chain PFAS without mass transfer limitation, selective reactivity by ''eaq<big>'''-'''</big>'', low energy consumption, and the production of no harmful byproducts. A summary of these advantages is provided below:
*'''High efficiency for short- and ultrashort-chain PFAS:''' While the degradation efficiency for short-chain PFAS is challenging for other technologies that degrade PFAS at the heterogeneous solid-water or gas-water interface, the UV/sulfite process demonstrates excellent defluorination efficiency for both short- and ultrashort-chain PFAS, including [[Wikipedia: Trifluoroacetic acid | trifluoroacetic acid (TFA)]] and [[Wikipedia: Perfluoropropionic acid | perfluoropropionic acid (PFPrA)]].
*'''High defluorination ratio:''' As shown in Figures 2 and 3, the UV/sulfite treatment system has demonstrated near 100% defluorination for various PFAS under both laboratory and field conditions.
*'''No harmful byproducts:''' While some oxidative technologies, such as electrochemical oxidation, generate toxic byproducts, including perchlorate, bromate, and chlorate, the UV/sulfite system employs a reductive mechanism and does not generate these byproducts.
*'''Ambient pressure and low temperature:''' The system operates under ambient pressure and low temperature (<60°C), as it utilizes UV light and common chemicals to degrade PFAS.
*'''Low energy consumption:''' The electrical energy per order values for the degradation of [[Wikipedia: Perfluoroalkyl carboxylic acids | perfluorocarboxylic acids (PFCAs)]] by UV/sulfite have been reduced to less than 1.5 kilowatt-hours (kWh) per cubic meter under laboratory conditions. The energy consumption is orders of magnitude lower than that for many other destructive PFAS treatment technologies (e.g., [[Supercritical Water Oxidation (SCWO) | supercritical water oxidation]])<ref>Nzeribe, B.N., Crimi, M., Mededovic Thagard, S., and Holsen, T.M., 2019. Physico-Chemical Processes for the Treatment of Per- And Polyfluoroalkyl Substances (PFAS): A Review. Critical Reviews in Environmental Science and Technology, 49(10), pp. 866-915. [https://doi.org/10.1080/10643389.2018.1542916 doi: 10.1080/10643389.2018.1542916]</ref>.
*'''Co-contaminant destruction:''' The UV/sulfite system has also been reported effective in destroying certain co-contaminants in wastewater. For example, UV/sulfite is reported to be effective in reductive dechlorination of chlorinated volatile organic compounds, such as trichloroethene, 1,2-dichloroethane, and vinyl chloride<ref>Jung, B., Farzaneh, H., Khodary, A., and Abdel-Wahab, A., 2015. Photochemical degradation of trichloroethylene by sulfite-mediated UV irradiation. Journal of Environmental Chemical Engineering, 3(3), pp. 2194-2202. [https://doi.org/10.1016/j.jece.2015.07.026 doi: 10.1016/j.jece.2015.07.026]</ref><ref>Liu, X., Yoon, S., Batchelor, B., and Abdel-Wahab, A., 2013. Photochemical degradation of vinyl chloride with an Advanced Reduction Process (ARP) – Effects of reagents and pH. Chemical Engineering Journal, 215-216, pp. 868-875. [https://doi.org/10.1016/j.cej.2012.11.086 doi: 10.1016/j.cej.2012.11.086]</ref><ref>Li, X., Ma, J., Liu, G., Fang, J., Yue, S., Guan, Y., Chen, L., and Liu, X., 2012. Efficient Reductive Dechlorination of Monochloroacetic Acid by Sulfite/UV Process. Environmental Science and Technology, 46(13), pp. 7342-49. [https://doi.org/10.1021/es3008535 doi: 10.1021/es3008535]</ref><ref>Li, X., Fang, J., Liu, G., Zhang, S., Pan, B., and Ma, J., 2014. Kinetics and efficiency of the hydrated electron-induced dehalogenation by the sulfite/UV process. Water Research, 62, pp. 220-228. [https://doi.org/10.1016/j.watres.2014.05.051 doi: 10.1016/j.watres.2014.05.051]</ref>.

==Limitations==
Several environmental factors and potential issues have been identified that may impact the performance of the UV/sulfite treatment system, as listed below. Solutions to address these issues are also proposed.
*Environmental factors, such as the presence of elevated concentrations of natural organic matter (NOM), dissolved oxygen, or nitrate, can inhibit the efficacy of UV/sulfite treatment systems by scavenging available hydrated electrons. Those interferences are commonly managed through chemical additions, reaction optimization, and/or dilution, and are therefore not considered likely to hinder treatment success.
*Coloration in waste streams may also impact the effectiveness of the UV/sulfite treatment system by blocking the transmission of UV light, thus reducing the UV lamp's effective path length. To address this, pre-treatment may be necessary to enable UV/sulfite destruction of PFAS in the waste stream. Pre-treatment may include the use of strong oxidants or coagulants to consume or remove UV-absorbing constituents.
*The degradation efficiency is strongly influenced by PFAS molecular structure, with fluorotelomer sulfonates (FTS) and [[Wikipedia: Perfluorobutanesulfonic acid | perfluorobutanesulfonate (PFBS)]] exhibiting greater resistance to degradation by UV/sulfite treatment compared to other PFAS compounds.

==State of the Practice==
[[File: XiongFig2.png | thumb | 500 px | Figure 2. Field demonstration of EradiFluorTM<ref name="EradiFluor"/> for PFAS destruction in a concentrated waste stream in a Mid-Atlantic Naval Air Station: a) Target PFAS at each step of the treatment shows that about 99% of PFAS were destroyed; meanwhile, the final degradation product, i.e., fluoride, increased to 15 mg/L in concentration, demonstrating effective PFAS destruction; b) AOF concentrations at each step of the treatment provided additional evidence to show near-complete mineralization of PFAS. Average results from multiple batches of treatment are shown here.]]
[[File: XiongFig3.png | thumb | 500 px | Figure 3. Field demonstration of a treatment train (SAFF + EradiFluorTM<ref name="EradiFluor"/>) for groundwater PFAS separation and destruction at an Air Force base in California: a) Two main components of the treatment train, i.e. SAFF and EradiFluorTM<ref name="EradiFluor"/>; b) Results showed the effective destruction of various PFAS in the foam fractionate. The target PFAS at each step of the treatment shows that about 99.9% of PFAS were destroyed. Meanwhile, the final degradation product, i.e., fluoride, increased to 30 mg/L in concentration, demonstrating effective destruction of PFAS in a foam fractionate concentrate. After a polishing treatment step (GAC) via the onsite groundwater extraction and treatment system, all PFAS were removed to concentrations below their MCLs.]]
The effectiveness of UV/sulfite technology for treating PFAS has been evaluated in two field demonstrations using the EradiFluorTM<ref name="EradiFluor"/> system. Aqueous samples collected from the system were analyzed using EPA Method 1633, the [[Wikipedia: TOP Assay | total oxidizable precursor (TOP) assay]], adsorbable organic fluorine (AOF) method, and non-target analysis. A summary of each demonstration and their corresponding PFAS treatment efficiency is provided below.
*<onlyinclude>Under the [https://serdp-estcp.mil/ Environmental Security Technology Certification Program (ESTCP)] [https://serdp-estcp.mil/projects/details/4c073623-e73e-4f07-a36d-e35c7acc75b6/er21-5152-project-overview Project ER21-5152], a field demonstration </onlyinclude>of EradiFluorTM<ref name="EradiFluor"/> <onlyinclude>was conducted at a Navy site on the east coast, and results showed that the technology was highly effective in destroying various PFAS in a liquid concentrate produced from an ''in situ'' foam fractionation groundwater treatment system. </onlyinclude>As shown in Figure 2a, total PFAS concentrations were reduced from 17,366 micrograms per liter (µg/L) to 195 µg/L at the end of the UV/sulfite reaction, representing 99% destruction. After the ion exchange resin polishing step, all residual PFAS had been removed to the non-detect level, except one compound (PFOS) reported as 1.5 nanograms per liter (ng/L), which is below the current Maximum Contaminant Level (MCL) of 4 ng/L. Meanwhile, the fluoride concentration increased up to 15 milligrams per liter (mg/L), confirming near complete defluorination. Figure 2b shows the adsorbable organic fluorine results from the same treatment test, which similarly demonstrates destruction of 99% of PFAS.
*<onlyinclude>Another field demonstration was completed at an Air Force base in California, where a treatment train </onlyinclude>combining [https://serdp-estcp.mil/projects/details/263f9b50-8665-4ecc-81bd-d96b74445ca2 Surface Active Foam Fractionation (SAFF)] and EradiFluorTM<ref name="EradiFluor"/> <onlyinclude>was used to treat PFAS in groundwater. </onlyinclude>As shown in Figure 3, <onlyinclude>PFAS analytical data and fluoride results demonstrated near-complete destruction of various PFAS. In addition, this demonstration showed: a) high PFAS destruction ratio was achieved in the foam fractionate, even in very high concentration (up to 1,700 mg/L of booster), and b) the effluent </onlyinclude>from EradiFluorTM<ref name="EradiFluor"/> <onlyinclude>was sent back to the influent of the </onlyinclude>SAFF <onlyinclude>system for further concentration and treatment, resulting in a closed-loop treatment system and no waste discharge</onlyinclude> from EradiFluorTM<ref name="EradiFluor"/><onlyinclude>. </onlyinclude>This field demonstration was conducted with the approval of three regulatory agencies (United States Environmental Protection Agency, California Regional Water Quality Control Board, and California Department of Toxic Substances Control).

==References==
<references />

==See Also==

PFAS Leaching Characterization with the Leaching Environmental Assessment Framework (LEAF)

2026-08-15T14:43:02Z

Admin:

[[Wikipedia: Firefighting_foam#Synthetic_foams | Aqueous film-forming foams (AFFFs)]] are a major source of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | per- and poly-fluoroalkyl substances (PFAS)]] impacts in soil and groundwater. Standardized tools are needed to rapidly assess the potential for retention, leaching, and transport of PFAS from the source zone to downgradient regions, so that this information can be applied towards critical facets of site management such as prioritizing PFAS-impacted sites for further investigation and remediation. Existing standard leaching methods were developed prior to concerns regarding PFAS. Therefore, studies are needed to ensure that leaching methods are compatible for use with PFAS and that resulting data are representative of the risk of PFAS leaching at impacted sites.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]
*[[PFAS Sources]]
*[[PFAS Transport and Fate]]

'''Contributors:''' Dr. Jennifer L. Guelfo, Dr. David Kosson, Dr. Andy Garrabrants, Ms. Fangfei Liu, Mr. Darlington Yawson, and Dr. Md. Isreq Real

'''Key Resources:'''
*Development of Leaching Tests for Materials Containing SVOCs and PFAS, EPA 600/R-23/382<ref name="GarrabrantsEtAl2024"/>

*[https://www.epa.gov/hw-sw846/leaching-environmental-assessment-framework-leaf-methods-and-guidance Leaching Environmental Assessment Framework (LEAF) Methods and Guidance] (EPA website)

==Introduction to LEAF==
The [https://www.epa.gov/ U.S. Environmental Protection Agency (EPA)] [https://www.epa.gov/hw-sw846/leaching-environmental-assessment-framework-leaf-methods-and-guidance Leaching Environmental Assessment Framework (LEAF)] is a suite of standardized test methods for evaluating contaminant release from solids under environmentally relevant conditions (Table 1). The four leaching methods within LEAF were originally validated for inorganic constituents<ref>Garrabrants, A.C., Kosson, D.S., Stefanski, L., DeLapp, R., Seignette, P.F.A.B., van der Sloot, H.A., Kariher, P., Baldwin, M., 2012. Interlaboratory Validation of the Leaching Environmental Assessment Framework (LEAF) Method 1313 and Method 1316, EPA/600/R-12/623, U.S. Environmental Protection Agency, Air Pollution and Control Division. [[Media: EPA 600_R-12_623.pdf | Free Download EPA 600/R-12/623]]</ref><ref>Garrabrants, A.C., Kosson, D.S., DeLapp, R., Kariher, P., Seignette, P.F.A.B., van der Sloot, H.A., Stefanski, L., Baldwin, M., 2012. Interlaboratory Validation of the Leaching Environmental Assessment Framework (LEAF) Method 1314 and Method 1315, EPA/600/R-12/624, U.S. Environmental Protection Agency, Air Pollution and Control Division. [[Media: EPA 600_R-12_624.pdf | Free Download EPA 600/R-12/624]]</ref> and included as standard leaching methods EPA 1313 – 1316 within Update V of SW-846<ref>USEPA, 2026. Hazardous Waste Test Methods / SW-846. [https://www.epa.gov/hw-sw846 USEPA SW-846 website]</ref>. To address the need for standardized tests to evaluate PFAS leaching and mobility, LEAF methods have been optimized and demonstrated for use with PFAS (Methods 1313A-1316A)<ref name="GarrabrantsEtAl2024">Garrabrants, A.C., Liu, F., Warne, R., DeLapp, R., Brown, L., Rubin, Z., Yawson, D., Kosson, D.S., Guelfo, J.L., Real, M.I., van der Sloot, H.A., Touati, A., Thorneloe, S., 2024. Development of Leaching Tests for Materials Containing SVOCs and PFAS, EPA 600/R-23/382, USEPA, Washington, D.C. [[Media: EPA 600_R-23_382.PDF | Free Download EPA 600/R-23/382]]</ref>. This article will focus on Methods 1313A, 1314A, and 1316A. Demonstration of Method 1315A for compacted granular materials (including concrete and asphalt) is ongoing.

{| class="wikitable" style="float:left; margin-left:10px;"
|+Table 1. EPA SW-846 methods that comprise the LEAF framework
|-
!Method
!Description
|-
| 1313 || Liquid-solid partitioning as a function of '''''extract pH''''' using a parallel batch extraction (i.e., equilibrium) procedure (Figure 1)
|-
| 1314 || Liquid-solid partitioning as a function of '''''liquid-solid ratio (L/S)''''' for constituents in solid materials using an up-flow '''''percolation''''' column procedure (Figure 3)
|-
| 1315 || '''''Mass transfer rates''''' of constituents in monolithic or compacted granular materials using a semi-dynamic tank leaching procedure
|-
| 1316 || Liquid-solid partitioning as a function of '''''L/S''''' using a parallel batch extraction (i.e., '''''equilibrium''''') procedure (Figure 2)
|-
| colspan="2" style="background:white;" | Note: Text shown in '''''bold''''' indicates primary condition evaluated in each method.
|}
 

==Method Development for PFAS==
Complete details of the development of LEAF Methods 1313A, 1314A, and 1316A for use with PFAS are available in Garrabrants ''et al.'', 2024<ref name="GarrabrantsEtAl2024"/>. Representative method modifications include:
* Materials of construction for experimental apparatus: containers used for leaching vessels (Methods 1313A, 1316A) and column construction materials (Method 1314A) evaluated for background PFAS and PFAS uptake.
* Reagents and eluant composition: eluant composition was optimized to use 1 mM CaCl2 to reduce formation of colloidal matter; Method 1313A pH adjustment now conducted with nonoxidizing HCl.
* Experimental conditions: Longer equilibration times (e.g., Method 1313, 1316) may be required due to slow desorption kinetics of certain PFAS from soil and organic matrices, implementation of settling to facilitate separation of solids from eluates.
* Eluate processing (all methods): Use of centrifugation in lieu of eluate filtering, sonication of bottle prior to eluate subsampling.

==Batch Test Demonstration Studies==
PFAS-specific adaptations were tested in batch test demonstration studies, which included triplicate implementation of Methods 1313A and 1316A in four AFFF-impacted site soils.

[[File: GuelfoFig1.png | thumb | 500 px | Figure 1: Figure 1. a) Overview of LEAF Method 1313A and b) PFHxS leaching as a function of pH<ref name="GarrabrantsEtAl2024"/>. Definitions: lower limit of quantification (LLOQ) and method detection limit (MDL)]]
'''Draft Method 1313A''' was used to evaluate pH-dependent leaching in PFAS-contaminated soils in parallel batch extractions where each set of batch reactors is prepared and equilibrated at different pH (Figure 1). Short-chain PFAS (≤6 fluorinated carbons) generally showed little to no variation in leaching across the tested pH range of 2-13 (e.g., [[Wikipedia: Perfluorohexanesulfonic acid | PFHxS]], Figure 1), whereas long-chain PFAS exhibited increased leaching at higher pH<ref name="GarrabrantsEtAl2024"/>. This trend is consistent with previous findings showing that soil-water partitioning coefficients (''Kd'') decrease as pH increases (e.g., Higgins and Luthy 2006)<ref name="HigginsLuthy2006">Higgins, C.P., Luthy, R.G., 2006. Sorption of Perfluorinated Surfactants on Sediments. Environmental Science and Technology, 40(23), pp. 7251–7256. [https://doi.org/10.1021/es061000n doi: 10.1021/es061000n]</ref>. The most pronounced pH effects were observed for perfluoroalkyl sulfonamides (FASAs) such as [[Wikipedia: Perfluorooctanesulfonamide | perfluorooctane sulfonamide (FOSA)]], which transition from neutral to anionic forms within the circumneutral pH range (~pH 6). The anionic form has a lower ''Kd'' and results in higher leaching concentrations<ref name="GarrabrantsEtAl2024"/><ref name="NguyenEtAl2020">Nguyen, T.M.H., Bräunig, J., Thompson, K., Thompson, J., Kabiri, S., Navarro, D.A., Kookana, R.S., Grimison, C., Barnes, C.M., Higgins, C.P., McLaughlin, M.J., Mueller, J.F., 2020. Influences of Chemical Properties, Soil Properties, and Solution pH on Soil–Water Partitioning Coefficients of Per- and Polyfluoroalkyl Substances (PFASs). Environmental Science and Technology, 54(24), pp. 15883–15892. [https://doi.org/10.1021/acs.est.0c05705 doi: 10.1021/acs.est.0c05705]  [[Media: NguyenEtAl2020.pdf | Open Access Article]]</ref>. For many site management scenarios where pH is circumneutral, variations in anionic PFAS leaching are expected to be small over the relevant pH range. In such cases, when testing time and costs are primary considerations, Method 1313A may be a lower priority relative to evaluating leaching as a function of L/S (Method 1316A, Method 1314A). Different considerations may be needed where FASAs or PFAS with multiple, ionizable functional groups (i.e., [[Wikipedia: Zwitterion | zwitterions]]) are of concern.

[[File: GuelfoFig2.png | thumb | 500 px | Figure 2: a) Overview of LEAF Method 1316A and b) PFHxS leaching as a function of L/S ratio evaluated in parallel batch leaching vessels<ref name="GarrabrantsEtAl2024"/>]]
'''Draft Method 1316A''' was used to evaluate L/S-dependent leaching of PFAS in impacted soils using parallel batch extractions where each set of batch reactors is prepared and equilibrated at a different L/S. (Figure 2). Methods 1314A and 1316A are similar in intent as they both evaluate leaching as a function of L/S; however, the experimental approach differs. Method 1314A uses a flow-through column configuration (Figure 3; discussed further below). Method 1314A may better simulate field conditions, but Method 1316A is simpler and less costly to implement. Trends in Method 1314A and 1316A are expected to be qualitatively similar but leaching concentrations are expected to exhibit differences. Despite this, leaching studies comparing Methods 1314A and 1316A for inorganics showed that cumulative release results were within one order of magnitude<ref>Lopez Meza, S., Garrabrants, A.C., van der Sloot, H., Kosson, D.S., 2008. Comparison of the Release of Constituents from Granular Materials under Batch and Column Testing. Waste Management, 28(10), pp. 1853–1867. [https://doi.org/10.1016/j.wasman.2007.11.009 doi: 10.1016/j.wasman.2007.11.009]</ref>.

Method 1316A and Method 1314A may also provide different insights into transport mechanisms. Because Method 1316A is performed using equilibrated batch reactors at varying L/S, results can be used to develop equilibrium desorption isotherms and calculate desorption coefficients (e.g., ''Kd,desorption''). Studies have shown that ''Kd,desorption'' values for PFAS may be greater than ''Kd'', an effect often attributed to desorption hysteresis<ref>Schaefer, C.E., Nguyen, D., Christie, E., Shea, S., Higgins, C.P., Field, J., 2022. Desorption Isotherms for Poly- and Perfluoroalkyl Substances in Soil Collected from an Aqueous Film-Forming Foam Source Area. Journal of Environmental Engineering, 148(1), Article 04021074. [https://doi.org/10.1061/(ASCE)EE.1943-7870.0001952 doi: 10.1061/(ASCE)EE.1943-7870.0001952]</ref>. Consequently Method 1316A provides a straightforward method to estimate site-specific desorption parameters. Although sorption parameters can also be inferred from column (Method 1314A) data, interpretation is often complicated by nonequilibrium processes. Conversely, the column data can be valuable for quantifying those additional mechanisms providing transport parameters that can describe rate-limited transport (e.g., fraction of non-equilibrium sorption sites and sorption rates) and other dynamic behavior.

As anticipated, trends in PFAS leaching obtained during the Method 1316A and Method 1314A demonstrations were qualitatively similar. These are further discussed below.

==Column Test Demonstration Studies==
[[File: GuelfoFig3.png | thumb | 500 px | Figure 3. a) Overview of LEAF Method 1314A and b) PFHxS leaching as a function of ∑(L/S) evaluated in saturated up-flow column tests<ref name="GarrabrantsEtAl2024"/>. Note that acrylic here simply refers to the column material of construction used in this round of Method 1314 testing.]]
PFAS-specific adaptations were tested in column test demonstration studies, which included triplicate implementation of Method 1314A in three AFFF-impacted site soils.

'''Draft Method 1314A''' was implemented in saturated, up-flow columns to evaluate leaching of PFAS as a function of cumulative L/S (∑(L/S)); Figure 3; total volume of water that has passed through the column divided by the soil mass in the column). As noted, the intent of Method 1314a and 1316a is similar, and in both tests, similar qualitative results were observed. For example, short-chain PFAS exhibited high initial concentrations that decreased rapidly. However, in Method 1314a, these rapid drops in short-chain PFAS tended to occur by ∑(L/S) ≈ 2 (e.g., Site 1 and 3 soils, Figure 3) whereas in some cases, such as for PFHxS, Method 1316A produced slightly flatter elution curves than Method 1314A (Figures 2b and 3b). Long-chain PFAS generally displayed flatter elution profiles than short-chain PFAS across both methods. These trends are consistent with chain length dependent sorption documented in the literature<ref name="HigginsLuthy2006"/><ref name="NguyenEtAl2020"/><ref>Guelfo, J.L., Higgins, C.P., 2013. Subsurface Transport Potential of Perfluoroalkyl Acids at Aqueous Film-Forming Foam (AFFF)-Impacted Sites. Environmental Science and Technology, 47(9), pp. 4164–4171. [https://doi.org/10.1021/es3048043 doi: 10.1021/es3048043]</ref>. Although column modeling is beyond the scope of this article, prior studies have shown that saturated transport can be influenced by rate-limited desorption, particularly for long-chain PFAS<ref>Doria-Manzur, A., Gray, E.P., Streets, S.S., Guelfo, J.L., 2025. Per- and Polyfluoroalkyl Substances (PFAS) Transport from Biosolids-Amended Soils: An Experimental and Numerical Approach. Water Research, 288(Part B), Article 124674. [https://doi.org/10.1016/j.watres.2025.124674 doi: 10.1016/j.watres.2025.124674]  [[Media: Doria-ManzurEtAl2026.pdf | Open Access Article]]</ref><ref>Guelfo, J.L., Wunsch, A., McCray, J., Stults, J.F., Higgins, C.P., 2020. Subsurface Transport Potential of Perfluoroalkyl Acids (PFAAs): Column Experiments and Modeling. Journal of Contaminant Hydrology, 233, Article 103661. [https://doi.org/10.1016/j.jconhyd.2020.103661 doi: 10.1016/j.jconhyd.2020.103661]  [[Media: GuelfoEtAl2020.pdf | Open Access Manuscript]]</ref>. As noted, data from Methods 1316A and 1314A can support estimation of transport parameters representing equilibrium and nonequilibrium behavior, respectively.

==LEAF Screening Evaluations==
[[File: GuelfoFig4.png | thumb | 500 px | Figure 4. Example screening assessment for perfluorooctane sulfonate (PFOS) using total content and data from Methods 1313A and 1314A. Figure format adapted from Garrabrants ''et al''. 2021<ref>Garrabrants, A.C., Kosson, D.S., Brown, K.G., Fagnant, D.P., Helms, G., Thorneloe, S.A., 2021. Methodology for Scenario-Based Assessments and Demonstration of Treatment Effectiveness Using the Leaching Environmental Assessment Framework (LEAF). Journal of Hazardous Materials, 406, Article 124635. [https://doi.org/10.1016/j.jhazmat.2020.124635 doi: 10.1016/j.jhazmat.2020.124635]  [[Media: GarrabrantsEtAl2021.pdf | Open Access Manuscript]]</ref>.]]
LEAF provides a standardized, robust approach for evaluating PFAS release from impacted granular materials under a range of environmental conditions. The tests are complementary, capture a range of conditions, and vary in ease of implementation. This provides the flexibility for users to select the test or test combinations that best suit their project objectives, timeline, and budget. A common use of LEAF data is in screening level assessments. These are stepwise assessments that establish increasingly refined maximum leaching concentrations, ''Cleach,max'' (Figure 4), which can then be compared to regulatory limits such as maximum contaminant levels, when available. For example, a stepwise screening assessment might include:
#Assume the maximum leaching concentration is represented by the total content (total initial mass of contaminant present) leaching into the first L/S.
#Assume only the available content leaches into the first L/S where available content is the maximum mass released over pH 2-13 measured using Method 1313a. For many PFAS, total content is equal to available content meaning that all of the PFAS mass is available for leaching.
#Assume the leaching concentration at natural pH (measured in Method 1313A at natural pH or Method 1316A at L/S of 10) is maximum leaching concentration adjusted to the first L/S.
#Consider the maximum leaching concentration over the L/S range (Method 1314A or Method 1316A) and the upper estimate of leaching, or ''Cleach,max'', is the concentration from either Step 3 or Step 4, whichever is greater.

Screening assessments may be sufficient to meet project goals, but when additional refinements of leaching estimates are needed, site-specific data (e.g., infiltration) can be combined with test data and computational approaches (e.g., fate and transport models) for more site-specific estimates of leaching. Example scenarios where LEAF may be used to evaluate PFAS-impacted solids include 1) estimating PFAS release from AFFF-impacted soils, 2) estimating PFAS release from biosolids-amended soils at land application sites, 3) providing transport parameters to model PFAS transport from the source zone to the saturated zone, and 4) evaluating PFAS release from treatment residuals such as soils or sediments treated by soil washing or thermal approaches.

==Summary and Ongoing Research==
The LEAF framework offers a reliable, replicable approach to evaluating PFAS release from solids. With recent adaptations for PFAS-specific considerations, LEAF methods provide valuable tools for regulators and practitioners in managing PFAS-contaminated materials and assessing long-term environmental risks. However, there are key areas of ongoing research, including:
*An interlab validation of Methods 1313A, 1314A, and 1316A in coordination with the EPA
*Optimization and demonstration of Method 1315A for use with PFAS-impacted solids
*Evaluation of an unsaturated Method 1314A protocol to assess the need for and ability of LEAF testing to capture air-water interfacial partitioning of PFAS
*Application of the total oxidizable precursor (TOP) assay for evaluating the maximum additional PFAS leaching that may occur as a result of polyfluoroalkyl precursor transformation
*Comparison of LEAF testing data to previously collected field-scale enhanced flushing data collected from the same site

Additionally, there are key areas for consideration in future research:
*Collection of paired field-laboratory data under ambient conditions to further validate the applicability of LEAF assessments for estimation of field-relevant PFAS leaching and mobility
*Consideration of biotransformation in modeling and interpretation of LEAF data, as the state of the science regarding biotransformation of polyfluoroalkyl substances to terminal perfluoroalkyl acids advances

==Other LEAF Resources==
There are numerous resources describing the development of the LEAF leaching methods for inorganics. They are not specific to PFAS, but are still valuable resources focused on LEAF implementation, applications, and management of LEAF data. They include:
*[https://www.vanderbilt.edu/leaching/leach-xs-lite/ Leach XS Lite] - a tool for LEAF data management and visualization; free to download after registering for a free license key
*[https://www.epa.gov/hw-sw846/how-guide-leaching-environmental-assessment-framework LEAF “How-To” Guide] - guidance on LEAF background, implementation, test result interpretation; includes case studies
*[https://www.epa.gov/hw-sw846/leaching-environmental-assessment-framework-leaf-methods-and-guidance USEPA LEAF Methods and Guidance] homepage
 

==References==
<references />

==See Also==

PFAS Leaching Characterization with the Leaching Environmental Assessment Framework (LEAF)

2026-08-15T14:42:33Z

Admin:

[[Wikipedia: Firefighting_foam#Synthetic_foams | Aqueous film-forming foams (AFFFs)]] are a major source of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | per- and poly-fluoroalkyl substances (PFAS)]] impacts in soil and groundwater. Standardized tools are needed to rapidly assess the potential for retention, leaching, and transport of PFAS from the source zone to downgradient regions, so that this information can be applied towards critical facets of site management such as prioritizing PFAS-impacted sites for further investigation and remediation. Existing standard leaching methods were developed prior to concerns regarding PFAS. Therefore, studies are needed to ensure that leaching methods are compatible for use with PFAS and that resulting data are representative of the risk of PFAS leaching at impacted sites.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]
*[[PFAS Sources]]
*[[PFAS Transport and Fate]]

'''Contributors:''' Dr. Jennifer L. Guelfo, Dr. David Kosson, Dr. Andy Garrabrants, Ms. Fangfei Liu, Mr. Darlington Yawson, and Dr. Md. Isreq Real

'''Key Resources:'''
*Development of Leaching Tests for Materials Containing SVOCs and PFAS, EPA 600/R-23/382<ref name="GarrabrantsEtAl2024"/>

*[https://www.epa.gov/hw-sw846/leaching-environmental-assessment-framework-leaf-methods-and-guidance Leaching Environmental Assessment Framework (LEAF) Methods and Guidance] (EPA website)

==Introduction to LEAF==
The [https://www.epa.gov/ U.S. Environmental Protection Agency (EPA)] [https://www.epa.gov/hw-sw846/leaching-environmental-assessment-framework-leaf-methods-and-guidance Leaching Environmental Assessment Framework (LEAF)] is a suite of standardized test methods for evaluating contaminant release from solids under environmentally relevant conditions (Table 1). The four leaching methods within LEAF were originally validated for inorganic constituents<ref>Garrabrants, A.C., Kosson, D.S., Stefanski, L., DeLapp, R., Seignette, P.F.A.B., van der Sloot, H.A., Kariher, P., Baldwin, M., 2012. Interlaboratory Validation of the Leaching Environmental Assessment Framework (LEAF) Method 1313 and Method 1316, EPA/600/R-12/623, U.S. Environmental Protection Agency, Air Pollution and Control Division. [[Media: EPA 600_R-12_623.pdf | Free Download EPA 600/R-12/623]]</ref><ref>Garrabrants, A.C., Kosson, D.S., DeLapp, R., Kariher, P., Seignette, P.F.A.B., van der Sloot, H.A., Stefanski, L., Baldwin, M., 2012. Interlaboratory Validation of the Leaching Environmental Assessment Framework (LEAF) Method 1314 and Method 1315, EPA/600/R-12/624, U.S. Environmental Protection Agency, Air Pollution and Control Division. [[Media: EPA 600_R-12_624.pdf | Free Download EPA 600/R-12/624]]</ref> and included as standard leaching methods EPA 1313 – 1316 within Update V of SW-846<ref>USEPA, 2026. Hazardous Waste Test Methods / SW-846. [https://www.epa.gov/hw-sw846 USEPA SW-846 website]</ref>. To address the need for standardized tests to evaluate PFAS leaching and mobility, LEAF methods have been optimized and demonstrated for use with PFAS (Methods 1313A-1316A)<ref name="GarrabrantsEtAl2024">Garrabrants, A.C., Liu, F., Warne, R., DeLapp, R., Brown, L., Rubin, Z., Yawson, D., Kosson, D.S., Guelfo, J.L., Real, M.I., van der Sloot, H.A., Touati, A., Thorneloe, S., 2024. Development of Leaching Tests for Materials Containing SVOCs and PFAS, EPA 600/R-23/382, USEPA, Washington, D.C. [[Media: EPA 600_R-23_382.PDF | Free Download EPA 600/R-23/382]]</ref>. This article will focus on Methods 1313A, 1314A, and 1316A. Demonstration of Method 1315A for compacted granular materials (including concrete and asphalt) is ongoing.

{| class="wikitable" style="float:left; margin-left:10px;"
|+Table 1. EPA SW-846 methods that comprise the LEAF framework
|-
!Method
!Description
|-
| 1313 || Liquid-solid partitioning as a function of '''''extract pH''''' using a parallel batch extraction (i.e., equilibrium) procedure (Figure 1)
|-
| 1314 || Liquid-solid partitioning as a function of '''''liquid-solid ratio (L/S)''''' for constituents in solid materials using an up-flow '''''percolation''''' column procedure (Figure 3)
|-
| 1315 || '''''Mass transfer rates''''' of constituents in monolithic or compacted granular materials using a semi-dynamic tank leaching procedure
|-
| 1316 || Liquid-solid partitioning as a function of '''''L/S''''' using a parallel batch extraction (i.e., '''''equilibrium''''') procedure (Figure 2)
|-
| colspan="2" style="background:white;" | Note: Text shown in '''''bold''''' indicates primary condition evaluated in each method.
|}
 

==Method Development for PFAS==
Complete details of the development of LEAF Methods 1313A, 1314A, and 1316A for use with PFAS are available in Garrabrants ''et al.'', 2024<ref name="GarrabrantsEtAl2024"/>. Representative method modifications include:
* Materials of construction for experimental apparatus: containers used for leaching vessels (Methods 1313A, 1316A) and column construction materials (Method 1314A) evaluated for background PFAS and PFAS uptake.
* Reagents and eluant composition: eluant composition was optimized to use 1 mM CaCl2 to reduce formation of colloidal matter; Method 1313A pH adjustment now conducted with nonoxidizing HCl.
* Experimental conditions: Longer equilibration times (e.g., Method 1313, 1316) may be required due to slow desorption kinetics of certain PFAS from soil and organic matrices, implementation of settling to facilitate separation of solids from eluates.
* Eluate processing (all methods): Use of centrifugation in lieu of eluate filtering, sonication of bottle prior to eluate subsampling.

==Batch Test Demonstration Studies==
PFAS-specific adaptations were tested in batch test demonstration studies, which included triplicate implementation of Methods 1313A and 1316A in four AFFF-impacted site soils.

[[File: GuelfoFig1.png | thumb | 500 px | Figure 1: Figure 1. a) Overview of LEAF Method 1313A and b) PFHxS leaching as a function of pH<ref name="GarrabrantsEtAl2024"/>. Definitions: lower limit of quantification (LLOQ) and method detection limit (MDL)]]
'''Draft Method 1313A''' was used to evaluate pH-dependent leaching in PFAS-contaminated soils in parallel batch extractions where each set of batch reactors is prepared and equilibrated at different pH (Figure 1). Short-chain PFAS (≤6 fluorinated carbons) generally showed little to no variation in leaching across the tested pH range of 2-13 (e.g., [[Wikipedia: Perfluorohexanesulfonic acid | PFHxS]], Figure 1), whereas long-chain PFAS exhibited increased leaching at higher pH<ref name="GarrabrantsEtAl2024"/>. This trend is consistent with previous findings showing that soil-water partitioning coefficients (''Kd'') decrease as pH increases (e.g., Higgins and Luthy 2006)<ref name="HigginsLuthy2006">Higgins, C.P., Luthy, R.G., 2006. Sorption of Perfluorinated Surfactants on Sediments. Environmental Science and Technology, 40(23), pp. 7251–7256. [https://doi.org/10.1021/es061000n doi: 10.1021/es061000n]</ref>. The most pronounced pH effects were observed for perfluoroalkyl sulfonamides (FASAs) such as [[Wikipedia: Perfluorooctanesulfonamide | perfluorooctane sulfonamide (FOSA)]], which transition from neutral to anionic forms within the circumneutral pH range (~pH 6). The anionic form has a lower ''Kd'' and results in higher leaching concentrations<ref name="GarrabrantsEtAl2024"/><ref name="NguyenEtAl2020">Nguyen, T.M.H., Bräunig, J., Thompson, K., Thompson, J., Kabiri, S., Navarro, D.A., Kookana, R.S., Grimison, C., Barnes, C.M., Higgins, C.P., McLaughlin, M.J., Mueller, J.F., 2020. Influences of Chemical Properties, Soil Properties, and Solution pH on Soil–Water Partitioning Coefficients of Per- and Polyfluoroalkyl Substances (PFASs). Environmental Science and Technology, 54(24), pp. 15883–15892. [https://doi.org/10.1021/acs.est.0c05705 doi: 10.1021/acs.est.0c05705]  [[Media: NguyenEtAl2020.pdf | Open Access Article]]</ref>. For many site management scenarios where pH is circumneutral, variations in anionic PFAS leaching are expected to be small over the relevant pH range. In such cases, when testing time and costs are primary considerations, Method 1313A may be a lower priority relative to evaluating leaching as a function of L/S (Method 1316A, Method 1314A). Different considerations may be needed where FASAs or PFAS with multiple, ionizable functional groups (i.e., [[Wikipedia: Zwitterion | zwitterions]]) are of concern.

[[File: GuelfoFig2.png | thumb | 500 px | Figure 2: a) Overview of LEAF Method 1316A and b) PFHxS leaching as a function of L/S ratio evaluated in parallel batch leaching vessels<ref name="GarrabrantsEtAl2024"/>]]
'''Draft Method 1316A''' was used to evaluate L/S-dependent leaching of PFAS in impacted soils using parallel batch extractions where each set of batch reactors is prepared and equilibrated at a different L/S. (Figure 2). Methods 1314A and 1316A are similar in intent as they both evaluate leaching as a function of L/S; however, the experimental approach differs. Method 1314A uses a flow-through column configuration (Figure 3; discussed further below). Method 1314A may better simulate field conditions, but Method 1316A is simpler and less costly to implement. Trends in Method 1314A and 1316A are expected to be qualitatively similar but leaching concentrations are expected to exhibit differences. Despite this, leaching studies comparing Methods 1314A and 1316A for inorganics showed that cumulative release results were within one order of magnitude<ref>Lopez Meza, S., Garrabrants, A.C., van der Sloot, H., Kosson, D.S., 2008. Comparison of the Release of Constituents from Granular Materials under Batch and Column Testing. Waste Management, 28(10), pp. 1853–1867. [https://doi.org/10.1016/j.wasman.2007.11.009 doi: 10.1016/j.wasman.2007.11.009]</ref>.

Method 1316A and Method 1314A may also provide different insights into transport mechanisms. Because Method 1316A is performed using equilibrated batch reactors at varying L/S, results can be used to develop equilibrium desorption isotherms and calculate desorption coefficients (e.g., ''Kd,desorption''). Studies have shown that ''Kd,desorption'' values for PFAS may be greater than ''Kd'', an effect often attributed to desorption hysteresis<ref>Schaefer, C.E., Nguyen, D., Christie, E., Shea, S., Higgins, C.P., Field, J., 2022. Desorption Isotherms for Poly- and Perfluoroalkyl Substances in Soil Collected from an Aqueous Film-Forming Foam Source Area. Journal of Environmental Engineering, 148(1), Article 04021074. [https://doi.org/10.1061/(ASCE)EE.1943-7870.0001952 doi: 10.1061/(ASCE)EE.1943-7870.0001952]</ref>. Consequently Method 1316A provides a straightforward method to estimate site-specific desorption parameters. Although sorption parameters can also be inferred from column (Method 1314A) data, interpretation is often complicated by nonequilibrium processes. Conversely, the column data can be valuable for quantifying those additional mechanisms providing transport parameters that can describe rate-limited transport (e.g., fraction of non-equilibrium sorption sites and sorption rates) and other dynamic behavior.

As anticipated, trends in PFAS leaching obtained during the Method 1316A and Method 1314A demonstrations were qualitatively similar. These are further discussed below.

==Column Test Demonstration Studies==
[[File: GuelfoFig3.png | thumb | 500 px | Figure 3. a) Overview of LEAF Method 1314A and b) PFHxS leaching as a function of ∑(L/S) evaluated in saturated up-flow column tests<ref name="GarrabrantsEtAl2024"/>. Note that acrylic here simply refers to the column material of construction used in this round of Method 1314 testing.]]
PFAS-specific adaptations were tested in column test demonstration studies, which included triplicate implementation of Method 1314A in three AFFF-impacted site soils.

'''Draft Method 1314A''' was implemented in saturated, up-flow columns to evaluate leaching of PFAS as a function of cumulative L/S (∑(L/S)); Figure 3; total volume of water that has passed through the column divided by the soil mass in the column). As noted, the intent of Method 1314a and 1316a is similar, and in both tests, similar qualitative results were observed. For example, short-chain PFAS exhibited high initial concentrations that decreased rapidly. However, in Method 1314a, these rapid drops in short-chain PFAS tended to occur by ∑(L/S) ≈ 2 (e.g., Site 1 and 3 soils, Figure 3) whereas in some cases, such as for PFHxS, Method 1316A produced slightly flatter elution curves than Method 1314A (Figures 2b and 3b). Long-chain PFAS generally displayed flatter elution profiles than short-chain PFAS across both methods. These trends are consistent with chain length dependent sorption documented in the literature<ref name="HigginsLuthy2006"/><ref name="NguyenEtAl2020"/><ref>Guelfo, J.L., Higgins, C.P., 2013. Subsurface Transport Potential of Perfluoroalkyl Acids at Aqueous Film-Forming Foam (AFFF)-Impacted Sites. Environmental Science and Technology, 47(9), pp. 4164–4171. [https://doi.org/10.1021/es3048043 doi: 10.1021/es3048043]</ref>. Although column modeling is beyond the scope of this article, prior studies have shown that saturated transport can be influenced by rate-limited desorption, particularly for long-chain PFAS<ref>Doria-Manzur, A., Gray, E.P., Streets, S.S., Guelfo, J.L., 2025. Per- and Polyfluoroalkyl Substances (PFAS) Transport from Biosolids-Amended Soils: An Experimental and Numerical Approach. Water Research, 288(Part B), Article 124674. [https://doi.org/10.1016/j.watres.2025.124674 doi: 10.1016/j.watres.2025.124674]  [[Media: Doria-ManzurEtAl2026.pdf | Open Access Article]]</ref><ref>Guelfo, J.L., Wunsch, A., McCray, J., Stults, J.F., Higgins, C.P., 2020. Subsurface Transport Potential of Perfluoroalkyl Acids (PFAAs): Column Experiments and Modeling. Journal of Contaminant Hydrology, 233, Article 103661. [https://doi.org/10.1016/j.jconhyd.2020.103661 doi: 10.1016/j.jconhyd.2020.103661]  [[Media: GuelfoEtAl2020.pdf | Open Access Manuscript]]</ref>. As noted, data from Methods 1316A and 1314A can support estimation of transport parameters representing equilibrium and nonequilibrium behavior, respectively.

==LEAF Screening Evaluations==
[[File: GuelfoFig4.png | thumb | 500 px | Figure 4. Example screening assessment for perfluorooctane sulfonate (PFOS) using total content and data from Methods 1313A and 1314A. Figure format adapted from Garrabrants ''et al''. 2021<ref>Garrabrants, A.C., Kosson, D.S., Brown, K.G., Fagnant, D.P., Helms, G., Thorneloe, S.A., 2021. Methodology for Scenario-Based Assessments and Demonstration of Treatment Effectiveness Using the Leaching Environmental Assessment Framework (LEAF). Journal of Hazardous Materials, 406, Article 124635. [https://doi.org/10.1016/j.jhazmat.2020.124635 doi: 10.1016/j.jhazmat.2020.124635]  [[Media: GarrabrantsEtAl2021.pdf | Open Access Manuscript]]</ref>.]]
LEAF provides a standardized, robust approach for evaluating PFAS release from impacted granular materials under a range of environmental conditions. The tests are complementary, capture a range of conditions, and vary in ease of implementation. This provides the flexibility for users to select the test or test combinations that best suit their project objectives, timeline, and budget. A common use of LEAF data is in screening level assessments. These are stepwise assessments that establish increasingly refined maximum leaching concentrations, ''Cleach,max'' (Figure 4), which can then be compared to regulatory limits such as maximum contaminant levels, when available. For example, a stepwise screening assessment might include:
#Assume the maximum leaching concentration is represented by the total content (total initial mass of contaminant present) leaching into the first L/S.
#Assume only the available content leaches into the first L/S where available content is the maximum mass released over pH 2-13 measured using Method 1313a. For many PFAS, total content is equal to available content meaning that all of the PFAS mass is available for leaching.
#Assume the leaching concentration at natural pH (measured in Method 1313A at natural pH or Method 1316A at L/S of 10) is maximum leaching concentration adjusted to the first L/S.
#Consider the maximum leaching concentration over the L/S range (Method 1314A or Method 1316A) and the upper estimate of leaching, or ''Cleach,max'', is the concentration from either Step 3 or Step 4, whichever is greater.

Screening assessments may be sufficient to meet project goals, but when additional refinements of leaching estimates are needed, site-specific data (e.g., infiltration) can be combined with test data and computational approaches (e.g., fate and transport models) for more site-specific estimates of leaching. Example scenarios where LEAF may be used to evaluate PFAS-impacted solids include 1) estimating PFAS release from AFFF-impacted soils, 2) estimating PFAS release from biosolids-amended soils at land application sites, 3) providing transport parameters to model PFAS transport from the source zone to the saturated zone, and 4) evaluating PFAS release from treatment residuals such as soils or sediments treated by soil washing or thermal approaches.

==Summary and Ongoing Research==
The LEAF framework offers a reliable, replicable approach to evaluating PFAS release from solids. With recent adaptations for PFAS-specific considerations, LEAF methods provide valuable tools for regulators and practitioners in managing PFAS-contaminated materials and assessing long-term environmental risks. However, there are key areas of ongoing research, including:
*An interlab validation of Methods 1313A, 1314A, and 1316A in coordination with the EPA
*Optimization and demonstration of Method 1315A for use with PFAS-impacted solids
*Evaluation of an unsaturated Method 1314A protocol to assess the need for and ability of LEAF testing to capture air-water interfacial partitioning of PFAS
*Application of the total oxidizable precursor (TOP) assay for evaluating the maximum additional PFAS leaching that may occur as a result of polyfluoroalkyl precursor transformation
*Comparison of LEAF testing data to previously collected field-scale enhanced flushing data collected from the same site

Additionally, there are key areas for consideration in future research:
*Collection of paired field-laboratory data under ambient conditions to further validate the applicability of LEAF assessments for estimation of field-relevant PFAS leaching and mobility
*Consideration of biotransformation in modeling and interpretation of LEAF data, as the state of the science regarding biotransformation of polyfluoroalkyl substances to terminal perfluoroalkyl acids advances

==Other LEAF Resources==
There are numerous resources describing the development of the LEAF leaching methods for inorganics. They are not specific to PFAS, but are still valuable resources focused on LEAF implementation, applications, and management of LEAF data. They include:
*[https://www.vanderbilt.edu/leaching/leach-xs-lite/ Leach XS Lite] - a tool for LEAF data management and visualization; free to download after registering for a free license key
*[https://www.epa.gov/hw-sw846/how-guide-leaching-environmental-assessment-framework LEAF “How-To” Guide] - guidance on LEAF background, implementation, test result interpretation; includes case studies
*[https://www.epa.gov/hw-sw846/leaching-environmental-assessment-framework-leaf-methods-and-guidance USEPA LEAF Methods and Guidance] homepage
 

==References==
<references />

==See Also==

PFAS Leaching Characterization with the Leaching Environmental Assessment Framework (LEAF)

2026-08-15T14:42:17Z

Admin:

[[Wikipedia: Firefighting_foam#Synthetic_foams | Aqueous film-forming foams (AFFFs)]] are a major source of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | per- and poly-fluoroalkyl substances (PFAS)]] impacts in soil and groundwater. Standardized tools are needed to rapidly assess the potential for retention, leaching, and transport of PFAS from the source zone to downgradient regions, so that this information can be applied towards critical facets of site management such as prioritizing PFAS-impacted sites for further investigation and remediation. Existing standard leaching methods were developed prior to concerns regarding PFAS. Therefore, studies are needed to ensure that leaching methods are compatible for use with PFAS and that resulting data are representative of the risk of PFAS leaching at impacted sites.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]
*[[PFAS Sources]]
*[[PFAS Transport and Fate]]

'''Contributors:''' Dr. Jennifer L. Guelfo, Dr. David Kosson, Dr. Andy Garrabrants, Ms. Fangfei Liu, Mr. Darlington Yawson, and Dr. Md. Isreq Real

'''Key Resources:'''
*Development of Leaching Tests for Materials Containing SVOCs and PFAS, EPA 600/R-23/382<ref name="GarrabrantsEtAl2024"/>

*[https://www.epa.gov/hw-sw846/leaching-environmental-assessment-framework-leaf-methods-and-guidance Leaching Environmental Assessment Framework (LEAF) Methods and Guidance] (EPA website)

==Introduction to LEAF==
The [https://www.epa.gov/ U.S. Environmental Protection Agency (EPA)] [https://www.epa.gov/hw-sw846/leaching-environmental-assessment-framework-leaf-methods-and-guidance Leaching Environmental Assessment Framework (LEAF)] is a suite of standardized test methods for evaluating contaminant release from solids under environmentally relevant conditions (Table 1). The four leaching methods within LEAF were originally validated for inorganic constituents<ref>Garrabrants, A.C., Kosson, D.S., Stefanski, L., DeLapp, R., Seignette, P.F.A.B., van der Sloot, H.A., Kariher, P., Baldwin, M., 2012. Interlaboratory Validation of the Leaching Environmental Assessment Framework (LEAF) Method 1313 and Method 1316, EPA/600/R-12/623, U.S. Environmental Protection Agency, Air Pollution and Control Division. [[Media: EPA 600_R-12_623.pdf | Free Download EPA 600/R-12/623]]</ref><ref>Garrabrants, A.C., Kosson, D.S., DeLapp, R., Kariher, P., Seignette, P.F.A.B., van der Sloot, H.A., Stefanski, L., Baldwin, M., 2012. Interlaboratory Validation of the Leaching Environmental Assessment Framework (LEAF) Method 1314 and Method 1315, EPA/600/R-12/624, U.S. Environmental Protection Agency, Air Pollution and Control Division. [[Media: EPA 600_R-12_624.pdf | Free Download EPA 600/R-12/624]]</ref> and included as standard leaching methods EPA 1313 – 1316 within Update V of SW-846<ref>USEPA, 2026. Hazardous Waste Test Methods / SW-846. [https://www.epa.gov/hw-sw846 USEPA SW-846 website]</ref>. To address the need for standardized tests to evaluate PFAS leaching and mobility, LEAF methods have been optimized and demonstrated for use with PFAS (Methods 1313A-1316A)<ref name="GarrabrantsEtAl2024">Garrabrants, A.C., Liu, F., Warne, R., DeLapp, R., Brown, L., Rubin, Z., Yawson, D., Kosson, D.S., Guelfo, J.L., Real, M.I., van der Sloot, H.A., Touati, A., Thorneloe, S., 2024. Development of Leaching Tests for Materials Containing SVOCs and PFAS, EPA 600/R-23/382, USEPA, Washington, D.C. [[Media: EPA 600_R-23_382.PDF | Free Download EPA 600/R-23/382]]</ref>. This article will focus on Methods 1313A, 1314A, and 1316A. Demonstration of Method 1315A for compacted granular materials (including concrete and asphalt) is ongoing.

{| class="wikitable" style="float:left; margin-left:10px;"
|+Table 1. EPA SW-846 methods that comprise the LEAF framework
|-
!Method
!Description
|-
| 1313 || Liquid-solid partitioning as a function of '''''extract pH''''' using a parallel batch extraction (i.e., equilibrium) procedure (Figure 1)
|-
| 1314 || Liquid-solid partitioning as a function of '''''liquid-solid ratio (L/S)''''' for constituents in solid materials using an up-flow '''''percolation''''' column procedure (Figure 3)
|-
| 1315 || '''''Mass transfer rates''''' of constituents in monolithic or compacted granular materials using a semi-dynamic tank leaching procedure
|-
| 1316 || Liquid-solid partitioning as a function of '''''L/S''''' using a parallel batch extraction (i.e., '''''equilibrium''''') procedure (Figure 2)
|-
| colspan="2" style="background:white;" | Note: Text shown in '''''bold''''' indicates primary condition evaluated in each method.
|}
 

==Method Development for PFAS==
Complete details of the development of LEAF Methods 1313A, 1314A, and 1316A for use with PFAS are available in Garrabrants ''et al.'', 2024<ref name="GarrabrantsEtAl2024"/>. Representative method modifications include:
* Materials of construction for experimental apparatus: containers used for leaching vessels (Methods 1313A, 1316A) and column construction materials (Method 1314A) evaluated for background PFAS and PFAS uptake.
* Reagents and eluant composition: eluant composition was optimized to use 1 mM CaCl2 to reduce formation of colloidal matter; Method 1313A pH adjustment now conducted with nonoxidizing HCl.
* Experimental conditions: Longer equilibration times (e.g., Method 1313, 1316) may be required due to slow desorption kinetics of certain PFAS from soil and organic matrices, implementation of settling to facilitate separation of solids from eluates.
* Eluate processing (all methods): Use of centrifugation in lieu of eluate filtering, sonication of bottle prior to eluate subsampling.

==Batch Test Demonstration Studies==
PFAS-specific adaptations were tested in batch test demonstration studies, which included triplicate implementation of Methods 1313A and 1316A in four AFFF-impacted site soils.

[[File: GuelfoFig1.png | thumb | 500 px | Figure 1: Figure 1. a) Overview of LEAF Method 1313A and b) PFHxS leaching as a function of pH<ref name="GarrabrantsEtAl2024"/>. Definitions: lower limit of quantification (LLOQ) and method detection limit (MDL)]]
'''Draft Method 1313A''' was used to evaluate pH-dependent leaching in PFAS-contaminated soils in parallel batch extractions where each set of batch reactors is prepared and equilibrated at different pH (Figure 1). Short-chain PFAS (≤6 fluorinated carbons) generally showed little to no variation in leaching across the tested pH range of 2-13 (e.g., [[Wikipedia: Perfluorohexanesulfonic acid | PFHxS]], Figure 1), whereas long-chain PFAS exhibited increased leaching at higher pH<ref name="GarrabrantsEtAl2024"/>. This trend is consistent with previous findings showing that soil-water partitioning coefficients (''Kd'') decrease as pH increases (e.g., Higgins and Luthy 2006)<ref name="HigginsLuthy2006">Higgins, C.P., Luthy, R.G., 2006. Sorption of Perfluorinated Surfactants on Sediments. Environmental Science and Technology, 40(23), pp. 7251–7256. [https://doi.org/10.1021/es061000n doi: 10.1021/es061000n]</ref>. The most pronounced pH effects were observed for perfluoroalkyl sulfonamides (FASAs) such as [[Wikipedia: Perfluorooctanesulfonamide | perfluorooctane sulfonamide (FOSA)]], which transition from neutral to anionic forms within the circumneutral pH range (~pH 6). The anionic form has a lower ''Kd'' and results in higher leaching concentrations<ref name="GarrabrantsEtAl2024"/><ref name="NguyenEtAl2020">Nguyen, T.M.H., Bräunig, J., Thompson, K., Thompson, J., Kabiri, S., Navarro, D.A., Kookana, R.S., Grimison, C., Barnes, C.M., Higgins, C.P., McLaughlin, M.J., Mueller, J.F., 2020. Influences of Chemical Properties, Soil Properties, and Solution pH on Soil–Water Partitioning Coefficients of Per- and Polyfluoroalkyl Substances (PFASs). Environmental Science and Technology, 54(24), pp. 15883–15892. [https://doi.org/10.1021/acs.est.0c05705 doi: 10.1021/acs.est.0c05705]  [[Media: NguyenEtAl2020.pdf | Open Access Article]]</ref>. For many site management scenarios where pH is circumneutral, variations in anionic PFAS leaching are expected to be small over the relevant pH range. In such cases, when testing time and costs are primary considerations, Method 1313A may be a lower priority relative to evaluating leaching as a function of L/S (Method 1316A, Method 1314A). Different considerations may be needed where FASAs or PFAS with multiple, ionizable functional groups (i.e., [[Wikipedia: Zwitterion | zwitterions]]) are of concern.

[[File: GuelfoFig2.png | thumb | 500 px | Figure 2: a) Overview of LEAF Method 1316A and b) PFHxS leaching as a function of L/S ratio evaluated in parallel batch leaching vessels<ref name="GarrabrantsEtAl2024"/>]]
'''Draft Method 1316A''' was used to evaluate L/S-dependent leaching of PFAS in impacted soils using parallel batch extractions where each set of batch reactors is prepared and equilibrated at a different L/S. (Figure 2). Methods 1314A and 1316A are similar in intent as they both evaluate leaching as a function of L/S; however, the experimental approach differs. Method 1314A uses a flow-through column configuration (Figure 3; discussed further below). Method 1314A may better simulate field conditions, but Method 1316A is simpler and less costly to implement. Trends in Method 1314A and 1316A are expected to be qualitatively similar but leaching concentrations are expected to exhibit differences. Despite this, leaching studies comparing Methods 1314A and 1316A for inorganics showed that cumulative release results were within one order of magnitude<ref>Lopez Meza, S., Garrabrants, A.C., van der Sloot, H., Kosson, D.S., 2008. Comparison of the Release of Constituents from Granular Materials under Batch and Column Testing. Waste Management, 28(10), pp. 1853–1867. [https://doi.org/10.1016/j.wasman.2007.11.009 doi: 10.1016/j.wasman.2007.11.009]</ref>.

Method 1316A and Method 1314A may also provide different insights into transport mechanisms. Because Method 1316A is performed using equilibrated batch reactors at varying L/S, results can be used to develop equilibrium desorption isotherms and calculate desorption coefficients (e.g., ''Kd,desorption''). Studies have shown that ''Kd,desorption'' values for PFAS may be greater than ''Kd'', an effect often attributed to desorption hysteresis<ref>Schaefer, C.E., Nguyen, D., Christie, E., Shea, S., Higgins, C.P., Field, J., 2022. Desorption Isotherms for Poly- and Perfluoroalkyl Substances in Soil Collected from an Aqueous Film-Forming Foam Source Area. Journal of Environmental Engineering, 148(1), Article 04021074. [https://doi.org/10.1061/(ASCE)EE.1943-7870.0001952 doi: 10.1061/(ASCE)EE.1943-7870.0001952]</ref>. Consequently Method 1316A provides a straightforward method to estimate site-specific desorption parameters. Although sorption parameters can also be inferred from column (Method 1314A) data, interpretation is often complicated by nonequilibrium processes. Conversely, the column data can be valuable for quantifying those additional mechanisms providing transport parameters that can describe rate-limited transport (e.g., fraction of non-equilibrium sorption sites and sorption rates) and other dynamic behavior.

As anticipated, trends in PFAS leaching obtained during the Method 1316A and Method 1314A demonstrations were qualitatively similar. These are further discussed below.

==Column Test Demonstration Studies==
[[File: GuelfoFig3.png | thumb | 500 px | Figure 3. a) Overview of LEAF Method 1314A and b) PFHxS leaching as a function of ∑(L/S) evaluated in saturated up-flow column tests<ref name="GarrabrantsEtAl2024"/>. Note that acrylic here simply refers to the column material of construction used in this round of Method 1314 testing.]]
PFAS-specific adaptations were tested in column test demonstration studies, which included triplicate implementation of Method 1314A in three AFFF-impacted site soils.

'''Draft Method 1314A''' was implemented in saturated, up-flow columns to evaluate leaching of PFAS as a function of cumulative L/S (∑(L/S)); Figure 3; total volume of water that has passed through the column divided by the soil mass in the column). As noted, the intent of Method 1314a and 1316a is similar, and in both tests, similar qualitative results were observed. For example, short-chain PFAS exhibited high initial concentrations that decreased rapidly. However, in Method 1314a, these rapid drops in short-chain PFAS tended to occur by ∑(L/S) ≈ 2 (e.g., Site 1 and 3 soils, Figure 3) whereas in some cases, such as for PFHxS, Method 1316A produced slightly flatter elution curves than Method 1314A (Figures 2b and 3b). Long-chain PFAS generally displayed flatter elution profiles than short-chain PFAS across both methods. These trends are consistent with chain length dependent sorption documented in the literature<ref name="HigginsLuthy2006"/><ref name="NguyenEtAl2020"/><ref>Guelfo, J.L., Higgins, C.P., 2013. Subsurface Transport Potential of Perfluoroalkyl Acids at Aqueous Film-Forming Foam (AFFF)-Impacted Sites. Environmental Science and Technology, 47(9), pp. 4164–4171. [https://doi.org/10.1021/es3048043 doi: 10.1021/es3048043]</ref>. Although column modeling is beyond the scope of this article, prior studies have shown that saturated transport can be influenced by rate-limited desorption, particularly for long-chain PFAS<ref>Doria-Manzur, A., Gray, E.P., Streets, S.S., Guelfo, J.L., 2025. Per- and Polyfluoroalkyl Substances (PFAS) Transport from Biosolids-Amended Soils: An Experimental and Numerical Approach. Water Research, 288(Part B), Article 124674. [https://doi.org/10.1016/j.watres.2025.124674 doi: 10.1016/j.watres.2025.124674]  [[Media: Doria-ManzurEtAl2026.pdf | Open Access Article]]</ref><ref>Guelfo, J.L., Wunsch, A., McCray, J., Stults, J.F., Higgins, C.P., 2020. Subsurface Transport Potential of Perfluoroalkyl Acids (PFAAs): Column Experiments and Modeling. Journal of Contaminant Hydrology, 233, Article 103661. [https://doi.org/10.1016/j.jconhyd.2020.103661 doi: 10.1016/j.jconhyd.2020.103661]  [[Media: GuelfoEtAl2020.pdf | Open Access Manuscript]]</ref>. As noted, data from Methods 1316A and 1314A can support estimation of transport parameters representing equilibrium and nonequilibrium behavior, respectively.

==LEAF Screening Evaluations==
[[File: GuelfoFig4.png | thumb | 500 px | Figure 4. Example screening assessment for perfluorooctane sulfonate (PFOS) using total content and data from Methods 1313A and 1314A. Figure format adapted from Garrabrants ''et al''. 2021<ref>Garrabrants, A.C., Kosson, D.S., Brown, K.G., Fagnant, D.P., Helms, G., Thorneloe, S.A., 2021. Methodology for Scenario-Based Assessments and Demonstration of Treatment Effectiveness Using the Leaching Environmental Assessment Framework (LEAF). Journal of Hazardous Materials, 406, Article 124635. [https://doi.org/10.1016/j.jhazmat.2020.124635 doi: 10.1016/j.jhazmat.2020.124635]  [[Media: GarrabrantsEtAl2021.pdf | Open Access Manuscript]]</ref>.]]
LEAF provides a standardized, robust approach for evaluating PFAS release from impacted granular materials under a range of environmental conditions. The tests are complementary, capture a range of conditions, and vary in ease of implementation. This provides the flexibility for users to select the test or test combinations that best suit their project objectives, timeline, and budget. A common use of LEAF data is in screening level assessments. These are stepwise assessments that establish increasingly refined maximum leaching concentrations, ''Cleach,max'' (Figure 4), which can then be compared to regulatory limits such as maximum contaminant levels, when available. For example, a stepwise screening assessment might include:
#Assume the maximum leaching concentration is represented by the total content (total initial mass of contaminant present) leaching into the first L/S.
#Assume only the available content leaches into the first L/S where available content is the maximum mass released over pH 2-13 measured using Method 1313a. For many PFAS, total content is equal to available content meaning that all of the PFAS mass is available for leaching.
#Assume the leaching concentration at natural pH (measured in Method 1313A at natural pH or Method 1316A at L/S of 10) is maximum leaching concentration adjusted to the first L/S.
#Consider the maximum leaching concentration over the L/S range (Method 1314A or Method 1316A) and the upper estimate of leaching, or ''Cleach,max'', is the concentration from either Step 3 or Step 4, whichever is greater.

Screening assessments may be sufficient to meet project goals, but when additional refinements of leaching estimates are needed, site-specific data (e.g., infiltration) can be combined with test data and computational approaches (e.g., fate and transport models) for more site-specific estimates of leaching. Example scenarios where LEAF may be used to evaluate PFAS-impacted solids include 1) estimating PFAS release from AFFF-impacted soils, 2) estimating PFAS release from biosolids-amended soils at land application sites, 3) providing transport parameters to model PFAS transport from the source zone to the saturated zone, and 4) evaluating PFAS release from treatment residuals such as soils or sediments treated by soil washing or thermal approaches.

==Summary and Ongoing Research==
The LEAF framework offers a reliable, replicable approach to evaluating PFAS release from solids. With recent adaptations for PFAS-specific considerations, LEAF methods provide valuable tools for regulators and practitioners in managing PFAS-contaminated materials and assessing long-term environmental risks. However, there are key areas of ongoing research, including:
*An interlab validation of Methods 1313A, 1314A, and 1316A in coordination with the EPA
*Optimization and demonstration of Method 1315A for use with PFAS-impacted solids
*Evaluation of an unsaturated Method 1314A protocol to assess the need for and ability of LEAF testing to capture air-water interfacial partitioning of PFAS
*Application of the total oxidizable precursor (TOP) assay for evaluating the maximum additional PFAS leaching that may occur as a result of polyfluoroalkyl precursor transformation
*Comparison of LEAF testing data to previously collected field-scale enhanced flushing data collected from the same site

Additionally, there are key areas for consideration in future research:
*Collection of paired field-laboratory data under ambient conditions to further validate the applicability of LEAF assessments for estimation of field-relevant PFAS leaching and mobility
*Consideration of biotransformation in modeling and interpretation of LEAF data, as the state of the science regarding biotransformation of polyfluoroalkyl substances to terminal perfluoroalkyl acids advances

==Other LEAF Resources==
There are numerous resources describing the development of the LEAF leaching methods for inorganics. They are not specific to PFAS, but are still valuable resources focused on LEAF implementation, applications, and management of LEAF data. They include:
*[https://www.vanderbilt.edu/leaching/leach-xs-lite/ Leach XS Lite] - a tool for LEAF data management and visualization; free to download after registering for a free license key
*[https://www.epa.gov/hw-sw846/how-guide-leaching-environmental-assessment-framework LEAF “How-To” Guide] - guidance on LEAF background, implementation, test result interpretation; includes case studies
*[https://www.epa.gov/hw-sw846/leaching-environmental-assessment-framework-leaf-methods-and-guidance USEPA LEAF Methods and Guidance] homepage
 

==References==
<references />

==See Also==

PFAS Transport and Fate

2026-07-18T20:10:49Z

Admin:

The transport and fate of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]] in the environment is controlled by the nature of the PFAS source, characteristics of the individual PFAS, and environmental conditions where the PFAS are present. Transport, partitioning, and transformation are the primary processes controlling PFAS fate in the environment. PFAS compounds can also be taken up by both plants and animals, and in some cases, bioaccumulate through the food chain.
Understanding PFAS transport and fate is necessary for evaluating the potential risk from a PFAS release and for predictions about PFAS occurrence, migration, and persistence, and about the potential vectors for exposure. This knowledge is important for site characterization, identification of potential sources of PFAS to the site, development of an appropriate conceptual site model (CSM), and selection and predicted performance of remediation strategies.

<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s): '''

*[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]
*[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
*[[PFAS Ex Situ Water Treatment]]
*[[PFAS Soil Remediation Technologies]]
*[[PFAS Sources]]
*[[PFAS Treatment by Electrical Discharge Plasma]]
*[[Supercritical Water Oxidation (SCWO)]]

'''Contributor(s): '''
[[Dr. Richard Anderson]] and [[Dr. Mark Brusseau]]

'''Key Resource(s): '''

*[https://pfas-1.itrcweb.org/ Per- and Polyfluoroalkyl Substances (PFAS), PFAS-1. ITRC 2020]<ref name="ITRC2020">Interstate Technology and Regulatory Council (ITRC), 2020. Technical/Regulatory Guidance: Per- and Polyfluoroalkyl Substances (PFAS), PFAS-1. ITRC, PFAS Team, Washington DC. [//www.enviro.wiki/images/2/2e/ITRC_PFAS-1.pdf Report.pdf]</ref>.

*[//www.enviro.wiki/images/d/de/Brusseau2018manuscript.pdf Assessing the Potential Contributions of Additional Retention Processes to PFAS Retardation in the Subsurface]<ref name="Brusseau2018">Brusseau, M.L., 2018. Assessing the Potential Contributions of Additional Retention Processes to PFAS Retardation in the Subsurface. Science of the Total Environment, 613-614, pp. 176-185. [https://doi.org/10.1016/j.scitotenv.2017.09.065 DOI: 10.1016/j.scitotenv.2017.09.065]  [//www.enviro.wiki/images/d/de/Brusseau2018manuscript.pdf Article pdf]</ref>.

==Introduction==
The transport and fate of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]] is a rapidly evolving field of science, with many questions that are not yet resolved. Much of the currently available information is based on a few well-studied PFAS compounds. However, there is a large number and variety of PFAS with a wide range of physical and chemical characteristics that affect their behavior in the environment. The transport and fate of some PFAS could differ significantly from the compounds studied to date. Nevertheless, information about the behavior of some PFAS in the environment can be ascertained from the results of currently available research.

PFAS transport and fate in the environment is controlled by the nature of the PFAS source, characteristics of the individual PFAS, and environmental conditions where the PFAS are present. Perfluoroalkyl acids (PFAAs) (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]] for nomenclature) are strong acids and are anionic in the environmentally-relevant pH range. They are extremely persistent in the environment and do not degrade or transform under typical environmental conditions. Polyfluoroalkyl substances (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]] for nomenclature) include compounds that have the potential to degrade to PFAAs. These compounds are commonly referred to as PFAA precursors or just ‘precursors’. Because some polyfluoroalkyl substances can degrade into PFAA via biotic or abiotic degradation pathways, PFAAs are sometimes referred to as “terminal PFAS” or “terminal degradation products”.
The most important molecular properties controlling PFAA transport are the carbon chain length and functional moieties of the headgroups (e.g., sulfonate, carboxylate). The molecular properties of PFAA precursors are more varied, with different carbon chain lengths, headgroups and ionic states<ref name="Buck2011">Buck, R.C., Franklin, J., Berger, U., Conder, J.M., Cousins, I.T., de Voogt, P., Jensen, A.A., Kannan, K., Mabury, S.A., and van Leeuwen, S.P.J., 2011. Perfluoroalkyl and Polyfluoroalkyl Substances in the Environment: Terminology, Classification, and Origins. Integrated Environmental Assessment and Management, 7(4): pp. 513-541. [https://doi.org/10.1002/ieam.258 DOI: 10.1002/ieam.258]   [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/ieam.258 Open Access Article]</ref><ref name="Wang2017">Wang, Z., DeWitt, J.C., Higgins, C.P., and Cousins, I.T., 2017. A Never-Ending Story of Per- and Polyfluoroalkyl Substances (PFASs)? Environmental Science and Technology, 51(5), pp. 2508-2518. American Chemical Society. [https://doi.org/10.1021/acs.est.6b04806 DOI: 10.1021/acs.est.6b04806]   [https://pubs.acs.org/doi/pdf/10.1021/acs.est.6b04806 Free Download from ACS]</ref> (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]]). All of these properties can influence transport and fate of PFAA precursors in the environment.

Important environmental characteristics include the nature of the source (mode of input into the environment), the length of time that the source was active, and the magnitude of the input, as well as precipitation and infiltration rates, depth to groundwater, surface water and groundwater flow rates and interactions, prevailing atmospheric conditions, the properties of the porous-media (e.g., soil and sediment) and aqueous solution, microbiological factors, and the presence of additional fluid phases such as air and non-aqueous phase liquids [[Wikipedia: Non-aqueous phase liquid | (NAPLs)]] in the vadose zone and water-saturated source. In the subsurface, soil characteristics (texture, organic carbon content, clay mineralogy, metal-oxide content, solid surface area, surface charge, and exchange capacity) and solution characteristics (pH, redox potential, major ion chemistry, and co-contaminants) can influence PFAS transport and fate.

==PFAS Transport and Fate Processes==
[[File:AndersonBrusseau1w2Fig1.png | thumb | 600px | Figure 1. Illustration of PFAS partitioning and transformation processes. Source: D. Adamson, GSI, used with permission.]]
Transport, partitioning, and transformation are the primary processes controlling PFAS fate in the environment (Figure 1). PFAS compounds can also be taken up by both plants and animals, and in some cases, bioaccumulate through the food chain. However, PFAS uptake and bioaccumulation is not discussed in this article (see “Environmental Concern” section of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]).

*'''Transport:''' PFAS can be transported substantial distances in the atmosphere<ref name="Ahrens2012">Ahrens, L., Harner, T., Shoeib, M., Lane, D.A. and Murphy, J.G., 2012. Improved Characterization of Gas–Particle Partitioning for Per- and Polyfluoroalkyl Substances in the Atmosphere Using Annular Diffusion Denuder Samplers. Environmental Science and Technology, 46(13), pp. 7199-7206. [https://doi.org/10.1021/es300898s DOI: 10.1021/es300898s]   Free download available from [https://www.researchgate.net/profile/Tom_Harner/publication/225046057_Improved_Characterization_of_Gas-Particle_Partitioning_for_Per-_and_Polyfluoroalkyl_Substances_in_the_Atmosphere_Using_Annular_Diffusion_Denuder_Samplers/links/5cc730c4299bf12097893fdc/Improved-Characterization-of-Gas-Particle-Partitioning-for-Per-and-Polyfluoroalkyl-Substances-in-the-Atmosphere-Using-Annular-Diffusion-Denuder-Samplers.pdf ResearchGate].</ref>, surface water<ref name="Taniyasu2013">Taniyasu, S., Yamashita, N., Moon, H.B., Kwok, K.Y., Lam, P.K., Horii, Y., Petrick, G. and Kannan, K., 2013. Does wet precipitation represent local and regional atmospheric transportation by perfluorinated alkyl substances? Environment International, 55, pp. 25-32. [https://doi.org/10.1016/j.envint.2013.02.005 DOI: 10.1016/j.envint.2013.02.005]</ref>, soil<ref name="Braunig2017">Bräunig, J., Baduel, C., Heffernan, A., Rotander, A., Donaldson, E. and Mueller, J.F., 2017. Fate and redistribution of perfluoroalkyl acids through AFFF-impacted groundwater. Science of the Total Environment, 596, pp. 360-368. [https://doi.org/10.1016/j.scitotenv.2017.04.095 DOI: 10.1016/j.scitotenv.2017.04.095]</ref>, and groundwater<ref name="Weber2017">Weber, A.K., Barber, L.B., LeBlanc, D.R., Sunderland, E.M. and Vecitis, C.D., 2017. Geochemical and Hydrologic Factors Controlling Subsurface Transport of Poly- and Perfluoroalkyl Substances, Cape Cod, Massachusetts. Environmental Science and Technology, 51(8), pp. 4269-4279. [https://doi.org/10.1021/acs.est.6b05573 DOI: 10.1021/acs.est.6b05573]   [https://bgc.seas.harvard.edu/assets/weber2017_final.pdf Free Download]</ref>. The primary mechanisms controlling PFAS transport are [[Wikipedia:Advection | advection]] and [[Wikipedia:Dispersive_mass_transfer | dispersion]], similar to other dissolved compounds. For additional information on transport in groundwater, see [[Advection and Groundwater Flow]] and [[Dispersion and Diffusion]].

*'''Partitioning:''' Partitioning of PFAS between the mobile and immobile phases is one of the most important processes controlling the rate of migration in the environment. The primary mobile phases are typically air and water. Relatively immobile phases include stream sediments, soils, aquifer material, NAPLs, and interfaces between different phases (air-water, NAPL-water). Partitioning of a significant portion of the PFAS mass into an immobile phase increases the amount of material stored in the system and slows the apparent rate of migration in the mobile phase – a phenomenon that has been observed in field metadata<ref name="Anderson2019">Anderson, R.H., Adamson, D.T. and Stroo, H.F., 2019. Partitioning of poly-and perfluoroalkyl substances from soil to groundwater within aqueous film-forming foam source zones. Journal of Contaminant Hydrology, 220, pp. 59-65. [https://doi.org/10.1016/j.jconhyd.2018.11.011 DOI: 10.1016/j.jconhyd.2018.11.011]   Manuscript available from [https://www.researchgate.net/profile/Hans_Stroo3/publication/329227107_Partitioning_of_poly-_and_perfluoroalkyl_substances_from_soil_to_groundwater_WITHIN_aqueous_film-forming_foam_source_zones/links/5e56996b299bf1bdb83e2f69/Partitioning-of-poly-and-perfluoroalkyl-substances-from-soil-to-groundwater-WITHIN-aqueous-film-forming-foam-source-zones.pdf ResearchGate]</ref>.

*'''Transformation:''' Transformation of PFAS is controlled by the molecular structure of the individual compounds. Perfluorinated compounds, including PFAAs, are resistant to abiotic and biotic transformation reactions under typical conditions and highly persistent in the environment. In contrast, precursors can be transformed by both abiotic and biotic processes, often resulting in the production of so-called “terminal” PFAA daughter products.

==Transport and Partitioning in the Atmosphere==
Air serves as a transport media for PFAS, particularly for uncharged polyfluorinated PFAS. Airborne PFAS transport contributes to global distribution and can lead to localized deposition to soils and surface water in the vicinity of emission sources<ref name="Simcik2005">Simcik, M.F. and Dorweiler, K.J., 2005. Ratio of Perfluorochemical Concentrations as a Tracer of Atmospheric Deposition to Surface Waters. Environmental Science and Technology, 39(22), pp. 8678-8683. [https://doi.org/10.1021/es0511218 DOI: 10.1021/es0511218]   Free download available from [https://www.researchgate.net/profile/Matt_Simcik/publication/7444956_Ratio_of_Perfluorochemical_Concentrations_as_a_Tracer_of_Atmospheric_Deposition_to_Surface_Waters/links/5f035861299bf1881603c3be/Ratio-of-Perfluorochemical-Concentrations-as-a-Tracer-of-Atmospheric-Deposition-to-Surface-Waters.pdf ResearchGate]</ref><ref name="Prevedouros2006">Prevedouros, K., Cousins, I.T., Buck, R.C. and Korzeniowski, S.H., 2006. Sources, Fate and Transport of Perfluorocarboxylates. Environmental Science and Technology, 40(1), pp. 32-44. [https://doi.org/10.1021/es0512475 DOI: 10.1021/es0512475]   Free download available from [https://d1wqtxts1xzle7.cloudfront.net/39945519/Sources_Fate_and_Transport_of_Perfluoroc20151112-1647-19vcvbf.pdf?1447365456=&response-content-disposition=inline%3B+filename%3DSources_Fate_and_Transport_of_Perfluoroc.pdf&Expires=1605023809&Signature=Z6KqgaDN6lKdAazoe6qoASoCtVystG5i~5EnrTcb~qMg3xZPz4O49Kghh62WmMzqEKE788~6EwrnlBVo9o6cM0hjf2vymFYxg4mx-eSIOEonfFjk6RonSaWp5gRbA6m~SNjwsjaKXID3OQyWIlLVpUd2LzAdI5rLGFA~gIXXtNPyCArLuGn-kbPYUIcBUg5TIkTZ6TDLXF~ujmzK9tNv~55UYabsJL4pmwIGC2sNGkEyJrYMfU577fbactdrmQXTJH7XbgpfDSfd4-xWkDZTdvVf~TypDDqUCZdtCkY8wINdpqtfe1KEzLrAj7rxxALAHUYxlVbPB45XTkLAGe5qww__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA Academia]</ref><ref name="Ahrens2011">Ahrens, L., Shoeib, M., Harner, T., Lane, D.A., Guo, R. and Reiner, E.J., 2011. Comparison of Annular Diffusion Denuder and High Volume Air Samplers for Measuring Per- and Polyfluoroalkyl Substances in the Atmosphere." Analytical Chemistry, 83(24), pp. 9622-9628. [https://doi.org/10.1021/ac202414w DOI: 10.1021/ac202414w]   Free download available from [https://www.informea.org/sites/default/files/imported-documents/UNEP-POPS-POPRC11FU-SUBM-PFOA-Canada-2-20151211.En.pdf Informea].</ref><ref name="Rauert2018">Rauert, C., Shoieb, M., Schuster, J.K., Eng, A. and Harner, T., 2018. Atmospheric concentrations and trends of poly-and perfluoroalkyl substances (PFAS) and volatile methyl siloxanes (VMS) over 7 years of sampling in the Global Atmospheric Passive Sampling (GAPS) network. Environmental Pollution, 238, pp. 94-102. [https://doi.org/10.1016/j.envpol.2018.03.017 DOI: 10.1016/j.envpol.2018.03.017]   Open access article available from [https://reader.elsevier.com/reader/sd/pii/S0269749117352521?token=4C770E6E8AEDB0B3BA6A1D5B2C20ED5385F81823612551FA3380AAA1DA7A978F9CB36834AF6B7F91F35FF57E32013252 ScienceDirect]   [//www.enviro.wiki/images/e/e6/Rauert2018.pdf Report.pdf]</ref>.

PFAAs, which are ionic and possess a negative charge under ambient environmental conditions, are far less volatile than many other groundwater contaminants. An online database of vapor pressures and Henry’s Law constants for different PFAS, including PFAAs, is maintained by the Interstate Technology Regulatory Council<ref name="ITRC2020" />. In general, vapor pressures of PFAS are low and water solubilities are high, limiting partitioning from water to air<ref name="ITRC2020" />. However, under certain conditions, particularly within industrial stack emissions, PFAS can be transported through the atmosphere in both the gas phase and associated with fugitive particulates. In particular, volatile compounds including fluorotelomer alcohols (FTOHs) may be present in the gas phase, whereas, PFAAs can aerosolize and be transported as particulates<ref name="Ahrens2012" />. In addition, precursors can be transformed to PFAAs in the atmosphere, which can result in PFAA deposition.
Short-range atmospheric transport and deposition can result in PFAS contamination in terrestrial and aquatic systems near points of significant emissions, impacting soil, groundwater, and other media of concern<ref name="Fang2018">Fang, X., Wang, Q., Zhao, Z., Tang, J., Tian, C., Yao, Y., Yu, J. and Sun, H., 2018. Distribution and dry deposition of alternative and legacy perfluoroalkyl and polyfluoroalkyl substances in the air above the Bohai and Yellow Seas, China. Atmospheric Environment, 192, pp. 128-135. [https://doi.org/10.1016/j.atmosenv.2018.08.052 DOI: 10.1016/j.atmosenv.2018.08.052]</ref><ref name="Brandsma2019">Brandsma, S.H., Koekkoek, J.C., van Velzen, M.J.M. and de Boer, J., 2019. The PFOA substitute GenX detected in the environment near a fluoropolymer manufacturing plant in the Netherlands. Chemosphere, 220, pp. 493-500. [https://doi.org/10.1016/j.chemosphere.2018.12.135 DOI: 10.1016/j.chemosphere.2018.12.135]   Open access article available from [https://reader.elsevier.com/reader/sd/pii/S0045653518324706?token=E541D5C4B200C8626A86F41049FE9DCA92652BC9A8BA7D9E47832C08070AB5AF256F4872474C50B5C4908F5CA4C24947 ScienceDirect].   [//www.enviro.wiki/images/4/4a/Brandsma2019.pdf Report.pdf]</ref>. Releases of ionic PFAS from factories are likely tied to particulate matter, which settle to the ground in dry weather and are also wet-scavenged by precipitation<ref name="Barton2006">Barton, C.A., Butler, L.E., Zarzecki, C.J., Flaherty, J. and Kaiser, M., 2006. Characterizing Perfluorooctanoate in Ambient Air near the Fence Line of a Manufacturing Facility: Comparing Modeled and Monitored Values. Journal of the Air and Waste Management Association, 56(1), pp. 48-55. [https://doi.org/10.1080/10473289.2006.10464429 DOI: 10.1080/10473289.2006.10464429]   Free access article available from [https://www.tandfonline.com/doi/pdf/10.1080/10473289.2006.10464429?needAccess=true Taylor and Francis Online]   [//www.enviro.wiki/images/b/b2/Barton2006.pdf Report.pdf]</ref>. The impact of other potential sources, such as combustion emissions or wind-blown fire-fighting foam from fire training and fire response sites, on the fate and transport of PFAS in air may need to be assessed.

Long-range transport processes are responsible for the wide distribution of neutral and ionic PFAS across the Earth as evidenced by their occurrence in biota, surface snow, ice cores, seawater, and other environmental media in regions as remote as the Arctic and Antarctic<ref name="Bossi2016">Bossi, R., Vorkamp, K. and Skov, H., 2016. Concentrations of organochlorine pesticides, polybrominated diphenyl ethers and perfluorinated compounds in the atmosphere of North Greenland. Environmental Pollution, 217, pp. 4-10. [https://doi.org/10.1016/j.envpol.2015.12.026 DOI: 10.1016/j.envpol.2015.12.026]</ref><ref name="Ahrens2010">Ahrens, L., Gerwinski, W., Theobald, N. and Ebinghaus, R., 2010. Sources of polyfluoroalkyl compounds in the North Sea, Baltic Sea and Norwegian Sea: Evidence from their spatial distribution in surface water. Marine Pollution Bulletin, 60(2), pp. 255-260. [https://doi.org/10.1016/j.marpolbul.2009.09.013 DOI: 10.1016/j.marpolbul.2009.09.013]</ref>.
Distribution of PFAS to remote regions far removed from direct industrial input is believed to occur from both: a) long-range atmospheric transport and subsequent degradation of volatile precursors; and b) transport via ocean currents and release into the air as marine aerosols (sea spray)<ref name="DeSilva2009">De Silva, A.O., Muir, D.C. and Mabury, S.A., 2009. Distribution of perfluorocarboxylate isomers in select samples from the North American environment. Environmental Toxicology and Chemistry: An International Journal 28(9), pp. 1801-1814. [https://doi.org/10.1897/08-500.1 DOI: 10.1897/08-500.1]</ref><ref name="Armitage2009">Armitage, J.M., 2009. Modeling the global fate and transport of perfluoroalkylated substances (PFAS). Doctoral Dissertation, Institutionen för tillämpad miljövetenskap (ITM), Stockholm University. [//www.enviro.wiki/images/e/ef/Armitage2009.pdf Report.pdf]</ref>.

==Transport and Partitioning in Aqueous Systems==
PFAS adsorb from water to a variety of solid materials including organic materials, clay minerals, metal oxides, and granular activated carbon<ref name="Du2014">Du, Z., Deng, S., Bei, Y., Huang, Q., Wang, B., Huang, J. and Yu, G., 2014. Adsorption behavior and mechanism of perfluorinated compounds on various adsorbents – A review. Journal of Hazardous Materials, 274, pp. 443-454. [https://doi.org/10.1016/j.jhazmat.2014.04.038 DOI: 10.1016/j.jhazmat.2014.04.038]</ref>. This process is thought to occur through two primary mechanisms: 1) sorption to organic-carbon components of the solids; and 2) electrostatic (and other) interactions with inorganic constituents of the solids, including clay minerals and metal-oxides<ref name="Guelfo2013">Guelfo, J.L. and Higgins, C.P., 2013. Subsurface Transport Potential of Perfluoroalkyl Acids at Aqueous Film-Forming Foam (AFFF)-Impacted Sites. Environmental Science and Technology, 47(9), pp. 4164-4171. [https://doi.org/10.1021/es3048043 DOI: 10.1021/es3048043]   [https://mountainscholar.org/bitstream/handle/11124/80055/Guelfo_mines_0052E_10298.pdf?sequence=1#page=64 Doctoral Dissertation]</ref><ref name="Zhao2014">Zhao, L., Bian, J., Zhang, Y., Zhu, L. and Liu, Z., 2014. Comparison of the sorption behaviors and mechanisms of perfluorosulfonates and perfluorocarboxylic acids on three kinds of clay minerals. Chemosphere, 114, pp. 51-58. [https://doi.org/10.1016/j.chemosphere.2014.03.098 DOI: 10.1016/j.chemosphere.2014.03.098]   Free download available from [https://www.researchgate.net/profile/Lixia_Zhao8/publication/262148355_Comparison_of_the_sorption_behaviors_and_mechanisms_of_perfluorosulfonates_and_perfluorocarboxylic_acids_on_three_kinds_of_clay_minerals/links/5b1be5dca6fdcca67b681a4f/Comparison-of-the-sorption-behaviors-and-mechanisms-of-perfluorosulfonates-and-perfluorocarboxylic-acids-on-three-kinds-of-clay-minerals.pdf ResearchGate].</ref>. The relative contribution of each mechanism varies depending on surface chemistry and other geochemical factors, as well as the molecular properties of the PFAS. In general, the impact of electrostatic interactions with charged soil constituents is more important for PFAS than non-polar, hydrophobic organic contaminants (e.g. hydrocarbons, chlorinated solvents). Adsorption of PFAS by solids is often nonlinear, with greater sorption at lower solute concentrations. The impacts of adsorption kinetics and their potential reversibility on PFAS transport have not yet been examined for most PFAS compounds.

Sorption of hydrocarbons, chlorinated solvents and other hydrophobic organics is often controlled the by organic-carbon components of the solid phase (see [[Sorption of Organic Contaminants]]). However, studies of PFAS sorption to solid phase organic carbon have reported conflicting results. In a study of field sites with aqueous film-forming foam (AFFF, a type of fire-fighting foam) releases, solid phase organic carbon content was found to significantly influence PFAS soil-to-groundwater concentration ratios. Statistical modeling was then used to derive apparent organic carbon partition coefficients for 18 different PFAS<ref name="Anderson2019" />. A recent compilation of published organic carbon partition coefficients found a good correspondence to PFAS molecular structure<ref name="Brusseau2019a">Brusseau, M.L., 2019. Estimating the relative magnitudes of adsorption to solid-water and air/oil-water interfaces for per-and poly-fluoroalkyl substances. Environmental Pollution, 254B, p. 113102. [https://doi.org/10.1016/j.envpol.2019.113102 DOI: 10.1016/j.envpol.2019.113102]</ref>. However, other studies have shown a general lack of correlation between solid phase partition coefficients and organic carbon<ref name="Li2018">Li, Y., Oliver, D.P. and Kookana, R.S., 2018. A critical analysis of published data to discern the role of soil and sediment properties in determining sorption of per and polyfluoroalkyl substances (PFASs). Science of the Total Environment, 628, pp. 110-120. [https://doi.org/10.1016/j.scitotenv.2018.01.167 DOI: 10.1016/j.scitotenv.2018.01.167]</ref>. It is possible that greater variability may be observed for broader data sets that incorporate different ranges of PFAS concentrations, different solution conditions, different measurement methods, and field-based data which often have less well-defined conditions and may also be influenced by other retention processes<ref name="Anderson2019" /><ref name="Brusseau2019a" />.

[[File:AndersonBrusseau1w2Fig2.png | thumb | 500px | Figure 2. Example of expected orientation and accumulation of PFAS at air-water interface. Source: D. Adamson, GSI, used with permission.]]
Most solids present in the environment contain both fixed-charged (negative) and variably charged surfaces. At neutral to high pH, variably charged clay minerals have a net-negative charge. As a result, negatively charged PFAAs do not strongly interact electrostatically in most soils, although as the soil pH decreases electrostatic sorption would be expected to increase in soils with variably charged clay minerals. Cationic and zwitterionic precursors are expected to be more strongly sorbed than anionic PFAAs in most environments due to well-established cation exchange reactions. Other factors, including ionic strength, composition, and the presence of co-solutes, can affect adsorption of PFAS<ref name="Higgins2006">Higgins, C.P. and Luthy, R.G., 2006. Sorption of Perfluorinated Surfactants on Sediments. Environmental Science and Technology, 40(23), pp. 7251-7256. [https://doi.org/10.1021/es061000n DOI: 10.1021/es061000n]</ref><ref name="Chen2009">Chen, H., Chen, S., Quan, X., Zhao, Y. and Zhao, H., 2009. Sorption of perfluorooctane sulfonate (PFOS) on oil and oil-derived black carbon: Influence of solution pH and [Ca2+]. Chemosphere, 77(10), pp. 1406-1411. [https://doi.org/10.1016/j.chemosphere.2009.09.008 DOI: 10.1016/j.chemosphere.2009.09.008]</ref><ref name="Pan2009">Pan, G., Jia, C., Zhao, D., You, C., Chen, H. and Jiang, G., 2009. Effect of cationic and anionic surfactants on the sorption and desorption of perfluorooctane sulfonate (PFOS) on natural sediments. Environmental Pollution, 157(1), pp.325-330. [https://doi.org/10.1016/j.envpol.2008.06.035 DOI: 10.1016/j.envpol.2008.06.035]   Free download available from [https://www.researchgate.net/profile/Gang_Pan2/publication/23189567_Effect_of_cationic_and_anionic_surfactants_on_the_sorption_and_desorption_of_perfluorooctane_sulfonate_PFOS_on_natural_sediments/links/5be19d23a6fdcc3a8dc2550d/Effect-of-cationic-and-anionic-surfactants-on-the-sorption-and-desorption-of-perfluorooctane-sulfonate-PFOS-on-natural-sediments.pdf ResearchGate]</ref><ref name="Guelfo2013" /><ref name="Zhao2014" />.

Most PFAS compounds act as surface-active agents (or [[Wikipedia:Surfactant | surfactants]]) due to the presence of a hydrophilic headgroup and a hydrophobic tail. The hydrophilic headgroup will preferentially partition to the aqueous phase and the hydrophobic tail will preferentially partition to the non-aqueous phase (air or organic material). As a result, PFAS tend to accumulate at interfaces (air-water, water-NAPL, water-solid) (Figure 2). This tendency to accumulate at interfaces can influence transport in the atmosphere (on water droplets and hydrated aerosols), in the vadose or unsaturated zone at air-water interfaces, in the presence of NAPLs, and in wastewater treatment systems<ref name="Brusseau2018" /><ref name="Brusseau2019b">Brusseau, M.L., 2019. The Influence of Molecular Structure on the Adsorption of PFAS to Fluid-Fluid Interfaces: Using QSPR to Predict Interfacial Adsorption Coefficients. Water Research, 152, pp. 148-158. [https://doi.org/10.1016/j.watres.2018.12.057 DOI: 10.1016/j.watres.2018.12.057]   [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6374777/ Author’s Manuscript]</ref>.

In theoretical and experimental studies of transport in unsaturated porous media, adsorption at the air-water interface increased PFOS and PFOA retention<ref name="Brusseau2018" /><ref name="Lyu2018">Lyu, Y., Brusseau, M.L., Chen, W., Yan, N., Fu, X., and Lin, X., 2018. Adsorption of PFOA at the Air-Water Interface during Transport in Unsaturated Porous Media. Environmental Science and Technology, 52(14), pp. 7745-7753. [https://doi.org/10.1021/acs.est.8b02348 DOI: 10.1021/acs.est.8b02348]   [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6312111/ Author’s Manuscript]</ref><ref name="BrusseauEtAl2019">Brusseau, M.L., Yan, N., Van Glubt, S., Wang, Y., Chen, W., Lyu, Y., Dungan, B., Carroll, K.C., and Holguin, F.O., 2019. Comprehensive Retention Model for PFAS Transport in Subsurface Systems. Water Research, 148, pp. 41-50. [https://doi.org/10.1016/j.watres.2018.10.035 DOI: 10.1016/j.watres.2018.10.035]   [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6294326/ Author’s Manuscript]</ref>, contributing approximately 20% to 80% of total retention in sands and soil. The impact of oil-water interfacial adsorption on PFAS transport was also quantitatively characterized in recent studies and shown to contribute to total retention on a similar scale as air-water interfacial adsorption<ref name="Brusseau2018" /><ref name="BrusseauEtAl2019" />. These processes may result in increased PFAS mass retained in NAPL source zones, increased PFAS sorption with the resulting retardation of transport, and greater persistence of dissolved PFAS in the environment.

==Transformation==
[[File:AndersonBrusseau1w2Fig3.png | thumb | 600px | Figure 3. Conceptual model of precursor transformation resulting in the formation of PFAAs. Source L. Trozzolo, TRC and C. Higgins, Colorado School of Mines, used with permission.]]
Certain polyfluorinated substances have the potential to transform to other PFAS, with PFAAs as the typical terminal daughter products. These polyfluorinated substances are often referred to as “precursors”. The transformation potential of polyfluorinated precursors is influenced by the presence, location, and number of carbon-hydrogen (C-H) bonds and potentially carbon-oxygen (C-O) bonds throughout the carbon chain. Specifically, PFAS with C-H bonds are subject to a variety of biotic and abiotic reactions that ultimately result in the formation of PFAAs with perfluorinated carbon chains of the same length or shorter than the initial polyfluorinated precursor<ref name="Houtz2013">Houtz, E.F., Higgins, C.P., Field, J.A. and Sedlak, D.L., 2013. Persistence of perfluoroalkyl acid precursors in AFFF-impacted groundwater and soil. Environmental Science and Technology, 47(15), pp. 8187-8195. [https://doi.org/10.1021/es4018877 DOI: 10.1021/es4018877]   Free download from [https://www.researchgate.net/profile/Erika_Houtz/publication/252323955_Persistence_of_Perfluoroalkyl_Acid_Precursors_in_AFFF-Impacted_Groundwater_and_Soil/links/59dbddeeaca2728e2018336d/Persistence-of-Perfluoroalkyl-Acid-Precursors-in-AFFF-Impacted-Groundwater-and-Soil.pdf ReseqarchGate]</ref><ref name="McGuire2014">McGuire, M.E., Schaefer, C., Richards, T., Backe, W.J., Field, J.A., Houtz, E., Sedlak, D.L., Guelfo, J.L., Wunsch, A., and Higgins, C.P., 2014. Evidence of Remediation-Induced Alteration of Subsurface Poly- and Perfluoroalkyl Substance Distribution at a Former Firefighter Training Area. Environmental Science and Technology, 48(12) pp. 6644-6652. [https://doi.org/10.1021/es5006187 DOI: 10.1021/es5006187]   Manuscript available from [https://ir.library.oregonstate.edu/downloads/td96k706f Oregon State University]</ref><ref name="Anderson2016">Anderson, R.H., Long, G.C., Porter, R.C. and Anderson, J.K., 2016. Occurrence of select perfluoroalkyl substances at US Air Force aqueous film-forming foam release sites other than fire-training areas: Field-validation of critical fate and transport properties. Chemosphere, 150, pp. 678-685. [https://doi.org/10.1016/j.chemosphere.2016.01.014 DOI: 10.1016/j.chemosphere.2016.01.014]</ref><ref name="Weber2017" />.

Transformation studies published to date have tested only a small subsample of possible precursors and, therefore, much uncertainty exists regarding 1) the extent to which precursor transformation occurs on a global scale, 2) which environmental compartments represent the majority of transformation, 3) relevant environmental conditions that affect transformation processes, and 4) transformation rates and pathways. Nevertheless, a portion of the precursors are expected to transform to PFAAs over time as shown in Figure 3.

Precursors can be transformed by a variety of abiotic processes including hydrolysis, photolysis, and oxidation. Hydrolysis of some precursors, followed by subsequent biotransformation, can produce perfluoroalkyl sulfonates (PFSAs). An important example is the production of PFOS from perfluorooctane sulfonyl fluoride (POSF)<ref name="Martin2010">Martin, J.W., Asher, B.J., Beesoon, S., Benskin, J.P. and Ross, M.S., 2010. PFOS or PreFOS? Are perfluorooctane sulfonate precursors (PreFOS) important determinants of human and environmental perfluorooctane sulfonate (PFOS) exposure? Journal of Environmental Monitoring, 12(11), pp.1979-2004. [https://doi.org/10.1039/C0EM00295J DOI: 10.1039/C0EM00295J]   Free download from [https://www.researchgate.net/profile/Matthew_Ross3/publication/47415684_PFOS_or_PreFOS_Are_perfluorooctane_sulfonate_precursors_PreFOS_important_determinants_of_human_and_environmental_perfluorooctane_sulfonate_PFOS_exposure/links/00b7d520a6132da945000000.pdf ResearchGate]</ref>. Other hydrolysis reactions produce perfluoroalkyl carboxylates (PFCAs). At neutral pH, the hydrolysis of fluorotelomer-derived polymeric precursors results in the formation of monomeric precursors of PFOA and other PFAAs with half-lives of 50 to 90 years<ref name="Washington2010">Washington, J.W., Ellington, J.J., Jenkins, T.M. and Yoo, H., 2010. Response to Comments on “Degradability of an Acrylate-Linked, Fluorotelomer Polymer in Soil”. Environmental Science and Technology, 44(2), pp. 849-850. [https://doi.org/10.1021/es902672q DOI: 10.1021/es902672q]   [https://pubs.acs.org/doi/pdf/10.1021/es902672q Free Download from ACS].</ref>. Oxidation of precursors by hydroxyl radicals can occur in natural waters, with the fluorotelomer-derived precursors being oxidized relatively rapidly<ref name="Gauthier2005">Gauthier, S.A. and Mabury, S.A., 2005. Aqueous photolysis of 8: 2 fluorotelomer alcohol. Environmental Toxicology and Chemistry, 24(8), pp.1837-1846. [https://doi.org/10.1897/04-591R.1 DOI: 10.1897/04-591R.1]   Free download from [https://www.researchgate.net/profile/Suzanne_Gauthier/publication/7609648_Aqueous_photolysis_of_8_2_fluorotelomer_alcohol/links/5ec16c4792851c11a86d9438/Aqueous-photolysis-of-8-2-fluorotelomer-alcohol.pdf ResearchGate].</ref><ref name="Plumlee2009">Plumlee, M.H., McNeill, K. and Reinhard, M., 2009. Indirect Photolysis of Perfluorochemicals: Hydroxyl Radical-Initiated Oxidation of N-Ethyl Perfluorooctane Sulfonamido Acetate (N-EtFOSAA) and Other Perfluoroalkanesulfonamides. Environmental Science and Technology, 43(10), pp.3662-3668. [https://doi.org/10.1021/es803411w DOI: 10.1021/es803411w]   Free download from [https://www.researchgate.net/profile/Megan_Plumlee/publication/26309488_Indirect_Photolysis_of_Perfluorochemicals_Hydroxyl_Radical-Initiated_Oxidation_of_N-Ethyl_Perfluorooctane_Sulfonamido_Acetate_N-EtFOSAA_and_Other_Perfluoroalkanesulfonamides/links/5aac0437a6fdcc1bc0b8d002/Indirect-Photolysis-of-Perfluorochemicals-Hydroxyl-Radical-Initiated-Oxidation-of-N-Ethyl-Perfluorooctane-Sulfonamido-Acetate-N-EtFOSAA-and-Other-Perfluoroalkanesulfonamides.pdf ResearchGate].</ref>.
Evidence of aerobic biotransformation is provided from studies of PFAS composition throughout the continuum of wastewater treatments<ref name="Arvaniti2015">Arvaniti, O.S. and Stasinakis, A.S., 2015. Review on the occurrence, fate and removal of perfluorinated compounds during wastewater treatment. Science of the Total Environment, 524, pp. 81-92. [https://doi.org/10.1016/j.scitotenv.2015.04.023 DOI: 10.1016/j.scitotenv.2015.04.023]</ref>, from field studies at AFFF-impacted sites<ref name="Houtz2013" /><ref name="McGuire2014" /><ref name="Anderson2016" /><ref name="Weber2017" />, and from microcosm experiments. In general, the literature on aerobic biotransformation collectively demonstrates or indirectly supports the following conclusions as summarized in ITRC 2020<ref name="ITRC2020" />:

[[File:Fate-and-transport_v3.mp4|thumb|500px|left|Figure 4. PFAS Fate and Transport.]]
*Numerous aerobic biotransformation pathways exist with relatively rapid kinetics
*All polyfluorinated precursors studied to date have the potential to aerobically biotransform to PFAAs
*Aerobic biotransformation of various fluorotelomer-derived precursors exclusively results in the formation of PFCAs, including PFOA, without necessarily the conservation of chain-length
*Aerobic biotransformation of various electrochemical fluorination-derived precursors primarily results in the formation of PFAAs, including PFOS, with the conservation of chain-length

Precursor transformation can complicate CSMs (and risk assessments) and should be considered during comprehensive site investigations. For example, atmospheric emissions of volatile precursors can result in long-range transport where subsequent transformation and deposition can result in detectable levels of PFAAs in environmental media independent of obvious point-sources<ref name="Vedagiri2018">Vedagiri, U.K., Anderson, R.H., Loso, H.M. and Schwach, C.M., 2018. Ambient levels of PFOS and PFOA in multiple environmental media. Remediation Journal, 28(2), pp. 9-51. [https://doi.org/10.1002/rem.21548 DOI: 10.1002/rem.21548]</ref>. With respect to site-related precursors, transformation of otherwise unmeasured PFAS into detectable PFAAs is obviously relevant to site investigations to the extent transformation occurs after initial site characterization efforts or if past remedial efforts have accelerated ''in situ'' transformation rates<ref name="McGuire2014" />. Additionally, differential transport rates between precursor PFAS and the corresponding terminal PFAA could also confound CSMs if transformation rates are slower than transport rates as has been suggested<ref name="Weber2017" />.
To account for otherwise unmeasurable precursors, several surrogate analytical methods have been developed.

==References==
<references />

==See Also:==
[https://soundcloud.com/arcadis-north-america/pfas-understanding-fate-and-transport-in-the-environment?utm_source=clipboard&utm_campaign=wtshare&utm_medium=widget&utm_content=https%253A%252F%252Fsoundcloud.com%252Farcadis-north-america%252Fpfas-understanding-fate-and-transport-in-the-environment SERDP & ESTCP PFAS Research and Remediation Podcast: PFAS: Understanding Fate and Transport in the Environment]

[https://soundcloud.com/arcadis-north-america/how-pfas-moves-from-afff-areas-to-groundwater SERDP & ESTCP PFAS Research and Remediation Podcast: How PFAS Moves from AFFF Areas to Groundwater]

PFAS Transport and Fate

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The transport and fate of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]] in the environment is controlled by the nature of the PFAS source, characteristics of the individual PFAS, and environmental conditions where the PFAS are present. Transport, partitioning, and transformation are the primary processes controlling PFAS fate in the environment. PFAS compounds can also be taken up by both plants and animals, and in some cases, bioaccumulate through the food chain.
Understanding PFAS transport and fate is necessary for evaluating the potential risk from a PFAS release and for predictions about PFAS occurrence, migration, and persistence, and about the potential vectors for exposure. This knowledge is important for site characterization, identification of potential sources of PFAS to the site, development of an appropriate conceptual site model (CSM), and selection and predicted performance of remediation strategies.

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'''Related Article(s): '''

*[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]
*[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
*[[PFAS Ex Situ Water Treatment]]
*[[PFAS Soil Remediation Technologies]]
*[[PFAS Sources]]
*[[PFAS Treatment by Electrical Discharge Plasma]]
*[[Supercritical Water Oxidation (SCWO)]]

'''Contributor(s): '''
[[Dr. Richard Anderson]] and [[Dr. Mark Brusseau]]

'''Key Resource(s): '''

*[https://pfas-1.itrcweb.org/ Per- and Polyfluoroalkyl Substances (PFAS), PFAS-1. ITRC 2020]<ref name="ITRC2020">Interstate Technology and Regulatory Council (ITRC), 2020. Technical/Regulatory Guidance: Per- and Polyfluoroalkyl Substances (PFAS), PFAS-1. ITRC, PFAS Team, Washington DC. [//www.enviro.wiki/images/2/2e/ITRC_PFAS-1.pdf Report.pdf]</ref>.

*[//www.enviro.wiki/images/d/de/Brusseau2018manuscript.pdf Assessing the Potential Contributions of Additional Retention Processes to PFAS Retardation in the Subsurface]<ref name="Brusseau2018">Brusseau, M.L., 2018. Assessing the Potential Contributions of Additional Retention Processes to PFAS Retardation in the Subsurface. Science of the Total Environment, 613-614, pp. 176-185. [https://doi.org/10.1016/j.scitotenv.2017.09.065 DOI: 10.1016/j.scitotenv.2017.09.065]  [//www.enviro.wiki/images/d/de/Brusseau2018manuscript.pdf Article pdf]</ref>.

==Introduction==
The transport and fate of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]] is a rapidly evolving field of science, with many questions that are not yet resolved. Much of the currently available information is based on a few well-studied PFAS compounds. However, there is a large number and variety of PFAS with a wide range of physical and chemical characteristics that affect their behavior in the environment. The transport and fate of some PFAS could differ significantly from the compounds studied to date. Nevertheless, information about the behavior of some PFAS in the environment can be ascertained from the results of currently available research.

PFAS transport and fate in the environment is controlled by the nature of the PFAS source, characteristics of the individual PFAS, and environmental conditions where the PFAS are present. Perfluoroalkyl acids (PFAAs) (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]] for nomenclature) are strong acids and are anionic in the environmentally-relevant pH range. They are extremely persistent in the environment and do not degrade or transform under typical environmental conditions. Polyfluoroalkyl substances (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]] for nomenclature) include compounds that have the potential to degrade to PFAAs. These compounds are commonly referred to as PFAA precursors or just ‘precursors’. Because some polyfluoroalkyl substances can degrade into PFAA via biotic or abiotic degradation pathways, PFAAs are sometimes referred to as “terminal PFAS” or “terminal degradation products”.
The most important molecular properties controlling PFAA transport are the carbon chain length and functional moieties of the headgroups (e.g., sulfonate, carboxylate). The molecular properties of PFAA precursors are more varied, with different carbon chain lengths, headgroups and ionic states<ref name="Buck2011">Buck, R.C., Franklin, J., Berger, U., Conder, J.M., Cousins, I.T., de Voogt, P., Jensen, A.A., Kannan, K., Mabury, S.A., and van Leeuwen, S.P.J., 2011. Perfluoroalkyl and Polyfluoroalkyl Substances in the Environment: Terminology, Classification, and Origins. Integrated Environmental Assessment and Management, 7(4): pp. 513-541. [https://doi.org/10.1002/ieam.258 DOI: 10.1002/ieam.258]   [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/ieam.258 Open Access Article]</ref><ref name="Wang2017">Wang, Z., DeWitt, J.C., Higgins, C.P., and Cousins, I.T., 2017. A Never-Ending Story of Per- and Polyfluoroalkyl Substances (PFASs)? Environmental Science and Technology, 51(5), pp. 2508-2518. American Chemical Society. [https://doi.org/10.1021/acs.est.6b04806 DOI: 10.1021/acs.est.6b04806]   [https://pubs.acs.org/doi/pdf/10.1021/acs.est.6b04806 Free Download from ACS]</ref> (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]]). All of these properties can influence transport and fate of PFAA precursors in the environment.

Important environmental characteristics include the nature of the source (mode of input into the environment), the length of time that the source was active, and the magnitude of the input, as well as precipitation and infiltration rates, depth to groundwater, surface water and groundwater flow rates and interactions, prevailing atmospheric conditions, the properties of the porous-media (e.g., soil and sediment) and aqueous solution, microbiological factors, and the presence of additional fluid phases such as air and non-aqueous phase liquids [[Wikipedia: Non-aqueous phase liquid | (NAPLs)]] in the vadose zone and water-saturated source. In the subsurface, soil characteristics (texture, organic carbon content, clay mineralogy, metal-oxide content, solid surface area, surface charge, and exchange capacity) and solution characteristics (pH, redox potential, major ion chemistry, and co-contaminants) can influence PFAS transport and fate.

==PFAS Transport and Fate Processes==
[[File:AndersonBrusseau1w2Fig1.png | thumb | 600px | Figure 1. Illustration of PFAS partitioning and transformation processes. Source: D. Adamson, GSI, used with permission.]]
Transport, partitioning, and transformation are the primary processes controlling PFAS fate in the environment (Figure 1). PFAS compounds can also be taken up by both plants and animals, and in some cases, bioaccumulate through the food chain. However, PFAS uptake and bioaccumulation is not discussed in this article (see “Environmental Concern” section of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]).

*'''Transport:''' PFAS can be transported substantial distances in the atmosphere<ref name="Ahrens2012">Ahrens, L., Harner, T., Shoeib, M., Lane, D.A. and Murphy, J.G., 2012. Improved Characterization of Gas–Particle Partitioning for Per- and Polyfluoroalkyl Substances in the Atmosphere Using Annular Diffusion Denuder Samplers. Environmental Science and Technology, 46(13), pp. 7199-7206. [https://doi.org/10.1021/es300898s DOI: 10.1021/es300898s]   Free download available from [https://www.researchgate.net/profile/Tom_Harner/publication/225046057_Improved_Characterization_of_Gas-Particle_Partitioning_for_Per-_and_Polyfluoroalkyl_Substances_in_the_Atmosphere_Using_Annular_Diffusion_Denuder_Samplers/links/5cc730c4299bf12097893fdc/Improved-Characterization-of-Gas-Particle-Partitioning-for-Per-and-Polyfluoroalkyl-Substances-in-the-Atmosphere-Using-Annular-Diffusion-Denuder-Samplers.pdf ResearchGate].</ref>, surface water<ref name="Taniyasu2013">Taniyasu, S., Yamashita, N., Moon, H.B., Kwok, K.Y., Lam, P.K., Horii, Y., Petrick, G. and Kannan, K., 2013. Does wet precipitation represent local and regional atmospheric transportation by perfluorinated alkyl substances? Environment International, 55, pp. 25-32. [https://doi.org/10.1016/j.envint.2013.02.005 DOI: 10.1016/j.envint.2013.02.005]</ref>, soil<ref name="Braunig2017">Bräunig, J., Baduel, C., Heffernan, A., Rotander, A., Donaldson, E. and Mueller, J.F., 2017. Fate and redistribution of perfluoroalkyl acids through AFFF-impacted groundwater. Science of the Total Environment, 596, pp. 360-368. [https://doi.org/10.1016/j.scitotenv.2017.04.095 DOI: 10.1016/j.scitotenv.2017.04.095]</ref>, and groundwater<ref name="Weber2017">Weber, A.K., Barber, L.B., LeBlanc, D.R., Sunderland, E.M. and Vecitis, C.D., 2017. Geochemical and Hydrologic Factors Controlling Subsurface Transport of Poly- and Perfluoroalkyl Substances, Cape Cod, Massachusetts. Environmental Science and Technology, 51(8), pp. 4269-4279. [https://doi.org/10.1021/acs.est.6b05573 DOI: 10.1021/acs.est.6b05573]   [https://bgc.seas.harvard.edu/assets/weber2017_final.pdf Free Download]</ref>. The primary mechanisms controlling PFAS transport are [[Wikipedia:Advection | advection]] and [[Wikipedia:Dispersive_mass_transfer | dispersion]], similar to other dissolved compounds. For additional information on transport in groundwater, see [[Advection and Groundwater Flow]] and [[Dispersion and Diffusion]].

*'''Partitioning:''' Partitioning of PFAS between the mobile and immobile phases is one of the most important processes controlling the rate of migration in the environment. The primary mobile phases are typically air and water. Relatively immobile phases include stream sediments, soils, aquifer material, NAPLs, and interfaces between different phases (air-water, NAPL-water). Partitioning of a significant portion of the PFAS mass into an immobile phase increases the amount of material stored in the system and slows the apparent rate of migration in the mobile phase – a phenomenon that has been observed in field metadata<ref name="Anderson2019">Anderson, R.H., Adamson, D.T. and Stroo, H.F., 2019. Partitioning of poly-and perfluoroalkyl substances from soil to groundwater within aqueous film-forming foam source zones. Journal of Contaminant Hydrology, 220, pp. 59-65. [https://doi.org/10.1016/j.jconhyd.2018.11.011 DOI: 10.1016/j.jconhyd.2018.11.011]   Manuscript available from [https://www.researchgate.net/profile/Hans_Stroo3/publication/329227107_Partitioning_of_poly-_and_perfluoroalkyl_substances_from_soil_to_groundwater_WITHIN_aqueous_film-forming_foam_source_zones/links/5e56996b299bf1bdb83e2f69/Partitioning-of-poly-and-perfluoroalkyl-substances-from-soil-to-groundwater-WITHIN-aqueous-film-forming-foam-source-zones.pdf ResearchGate]</ref>.

*'''Transformation:''' Transformation of PFAS is controlled by the molecular structure of the individual compounds. Perfluorinated compounds, including PFAAs, are resistant to abiotic and biotic transformation reactions under typical conditions and highly persistent in the environment. In contrast, precursors can be transformed by both abiotic and biotic processes, often resulting in the production of so-called “terminal” PFAA daughter products.

==Transport and Partitioning in the Atmosphere==
Air serves as a transport media for PFAS, particularly for uncharged polyfluorinated PFAS. Airborne PFAS transport contributes to global distribution and can lead to localized deposition to soils and surface water in the vicinity of emission sources<ref name="Simcik2005">Simcik, M.F. and Dorweiler, K.J., 2005. Ratio of Perfluorochemical Concentrations as a Tracer of Atmospheric Deposition to Surface Waters. Environmental Science and Technology, 39(22), pp. 8678-8683. [https://doi.org/10.1021/es0511218 DOI: 10.1021/es0511218]   Free download available from [https://www.researchgate.net/profile/Matt_Simcik/publication/7444956_Ratio_of_Perfluorochemical_Concentrations_as_a_Tracer_of_Atmospheric_Deposition_to_Surface_Waters/links/5f035861299bf1881603c3be/Ratio-of-Perfluorochemical-Concentrations-as-a-Tracer-of-Atmospheric-Deposition-to-Surface-Waters.pdf ResearchGate]</ref><ref name="Prevedouros2006">Prevedouros, K., Cousins, I.T., Buck, R.C. and Korzeniowski, S.H., 2006. Sources, Fate and Transport of Perfluorocarboxylates. Environmental Science and Technology, 40(1), pp. 32-44. [https://doi.org/10.1021/es0512475 DOI: 10.1021/es0512475]   Free download available from [https://d1wqtxts1xzle7.cloudfront.net/39945519/Sources_Fate_and_Transport_of_Perfluoroc20151112-1647-19vcvbf.pdf?1447365456=&response-content-disposition=inline%3B+filename%3DSources_Fate_and_Transport_of_Perfluoroc.pdf&Expires=1605023809&Signature=Z6KqgaDN6lKdAazoe6qoASoCtVystG5i~5EnrTcb~qMg3xZPz4O49Kghh62WmMzqEKE788~6EwrnlBVo9o6cM0hjf2vymFYxg4mx-eSIOEonfFjk6RonSaWp5gRbA6m~SNjwsjaKXID3OQyWIlLVpUd2LzAdI5rLGFA~gIXXtNPyCArLuGn-kbPYUIcBUg5TIkTZ6TDLXF~ujmzK9tNv~55UYabsJL4pmwIGC2sNGkEyJrYMfU577fbactdrmQXTJH7XbgpfDSfd4-xWkDZTdvVf~TypDDqUCZdtCkY8wINdpqtfe1KEzLrAj7rxxALAHUYxlVbPB45XTkLAGe5qww__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA Academia]</ref><ref name="Ahrens2011">Ahrens, L., Shoeib, M., Harner, T., Lane, D.A., Guo, R. and Reiner, E.J., 2011. Comparison of Annular Diffusion Denuder and High Volume Air Samplers for Measuring Per- and Polyfluoroalkyl Substances in the Atmosphere." Analytical Chemistry, 83(24), pp. 9622-9628. [https://doi.org/10.1021/ac202414w DOI: 10.1021/ac202414w]   Free download available from [https://www.informea.org/sites/default/files/imported-documents/UNEP-POPS-POPRC11FU-SUBM-PFOA-Canada-2-20151211.En.pdf Informea].</ref><ref name="Rauert2018">Rauert, C., Shoieb, M., Schuster, J.K., Eng, A. and Harner, T., 2018. Atmospheric concentrations and trends of poly-and perfluoroalkyl substances (PFAS) and volatile methyl siloxanes (VMS) over 7 years of sampling in the Global Atmospheric Passive Sampling (GAPS) network. Environmental Pollution, 238, pp. 94-102. [https://doi.org/10.1016/j.envpol.2018.03.017 DOI: 10.1016/j.envpol.2018.03.017]   Open access article available from [https://reader.elsevier.com/reader/sd/pii/S0269749117352521?token=4C770E6E8AEDB0B3BA6A1D5B2C20ED5385F81823612551FA3380AAA1DA7A978F9CB36834AF6B7F91F35FF57E32013252 ScienceDirect]   [//www.enviro.wiki/images/e/e6/Rauert2018.pdf Report.pdf]</ref>.

PFAAs, which are ionic and possess a negative charge under ambient environmental conditions, are far less volatile than many other groundwater contaminants. An online database of vapor pressures and Henry’s Law constants for different PFAS, including PFAAs, is maintained by the Interstate Technology Regulatory Council<ref name="ITRC2020" />. In general, vapor pressures of PFAS are low and water solubilities are high, limiting partitioning from water to air<ref name="ITRC2020" />. However, under certain conditions, particularly within industrial stack emissions, PFAS can be transported through the atmosphere in both the gas phase and associated with fugitive particulates. In particular, volatile compounds including fluorotelomer alcohols (FTOHs) may be present in the gas phase, whereas, PFAAs can aerosolize and be transported as particulates<ref name="Ahrens2012" />. In addition, precursors can be transformed to PFAAs in the atmosphere, which can result in PFAA deposition.
Short-range atmospheric transport and deposition can result in PFAS contamination in terrestrial and aquatic systems near points of significant emissions, impacting soil, groundwater, and other media of concern<ref name="Fang2018">Fang, X., Wang, Q., Zhao, Z., Tang, J., Tian, C., Yao, Y., Yu, J. and Sun, H., 2018. Distribution and dry deposition of alternative and legacy perfluoroalkyl and polyfluoroalkyl substances in the air above the Bohai and Yellow Seas, China. Atmospheric Environment, 192, pp. 128-135. [https://doi.org/10.1016/j.atmosenv.2018.08.052 DOI: 10.1016/j.atmosenv.2018.08.052]</ref><ref name="Brandsma2019">Brandsma, S.H., Koekkoek, J.C., van Velzen, M.J.M. and de Boer, J., 2019. The PFOA substitute GenX detected in the environment near a fluoropolymer manufacturing plant in the Netherlands. Chemosphere, 220, pp. 493-500. [https://doi.org/10.1016/j.chemosphere.2018.12.135 DOI: 10.1016/j.chemosphere.2018.12.135]   Open access article available from [https://reader.elsevier.com/reader/sd/pii/S0045653518324706?token=E541D5C4B200C8626A86F41049FE9DCA92652BC9A8BA7D9E47832C08070AB5AF256F4872474C50B5C4908F5CA4C24947 ScienceDirect].   [//www.enviro.wiki/images/4/4a/Brandsma2019.pdf Report.pdf]</ref>. Releases of ionic PFAS from factories are likely tied to particulate matter, which settle to the ground in dry weather and are also wet-scavenged by precipitation<ref name="Barton2006">Barton, C.A., Butler, L.E., Zarzecki, C.J., Flaherty, J. and Kaiser, M., 2006. Characterizing Perfluorooctanoate in Ambient Air near the Fence Line of a Manufacturing Facility: Comparing Modeled and Monitored Values. Journal of the Air and Waste Management Association, 56(1), pp. 48-55. [https://doi.org/10.1080/10473289.2006.10464429 DOI: 10.1080/10473289.2006.10464429]   Free access article available from [https://www.tandfonline.com/doi/pdf/10.1080/10473289.2006.10464429?needAccess=true Taylor and Francis Online]   [//www.enviro.wiki/images/b/b2/Barton2006.pdf Report.pdf]</ref>. The impact of other potential sources, such as combustion emissions or wind-blown fire-fighting foam from fire training and fire response sites, on the fate and transport of PFAS in air may need to be assessed.

Long-range transport processes are responsible for the wide distribution of neutral and ionic PFAS across the Earth as evidenced by their occurrence in biota, surface snow, ice cores, seawater, and other environmental media in regions as remote as the Arctic and Antarctic<ref name="Bossi2016">Bossi, R., Vorkamp, K. and Skov, H., 2016. Concentrations of organochlorine pesticides, polybrominated diphenyl ethers and perfluorinated compounds in the atmosphere of North Greenland. Environmental Pollution, 217, pp. 4-10. [https://doi.org/10.1016/j.envpol.2015.12.026 DOI: 10.1016/j.envpol.2015.12.026]</ref><ref name="Ahrens2010">Ahrens, L., Gerwinski, W., Theobald, N. and Ebinghaus, R., 2010. Sources of polyfluoroalkyl compounds in the North Sea, Baltic Sea and Norwegian Sea: Evidence from their spatial distribution in surface water. Marine Pollution Bulletin, 60(2), pp. 255-260. [https://doi.org/10.1016/j.marpolbul.2009.09.013 DOI: 10.1016/j.marpolbul.2009.09.013]</ref>.
Distribution of PFAS to remote regions far removed from direct industrial input is believed to occur from both: a) long-range atmospheric transport and subsequent degradation of volatile precursors; and b) transport via ocean currents and release into the air as marine aerosols (sea spray)<ref name="DeSilva2009">De Silva, A.O., Muir, D.C. and Mabury, S.A., 2009. Distribution of perfluorocarboxylate isomers in select samples from the North American environment. Environmental Toxicology and Chemistry: An International Journal 28(9), pp. 1801-1814. [https://doi.org/10.1897/08-500.1 DOI: 10.1897/08-500.1]</ref><ref name="Armitage2009">Armitage, J.M., 2009. Modeling the global fate and transport of perfluoroalkylated substances (PFAS). Doctoral Dissertation, Institutionen för tillämpad miljövetenskap (ITM), Stockholm University. [//www.enviro.wiki/images/e/ef/Armitage2009.pdf Report.pdf]</ref>.

==Transport and Partitioning in Aqueous Systems==
PFAS adsorb from water to a variety of solid materials including organic materials, clay minerals, metal oxides, and granular activated carbon<ref name="Du2014">Du, Z., Deng, S., Bei, Y., Huang, Q., Wang, B., Huang, J. and Yu, G., 2014. Adsorption behavior and mechanism of perfluorinated compounds on various adsorbents – A review. Journal of Hazardous Materials, 274, pp. 443-454. [https://doi.org/10.1016/j.jhazmat.2014.04.038 DOI: 10.1016/j.jhazmat.2014.04.038]</ref>. This process is thought to occur through two primary mechanisms: 1) sorption to organic-carbon components of the solids; and 2) electrostatic (and other) interactions with inorganic constituents of the solids, including clay minerals and metal-oxides<ref name="Guelfo2013">Guelfo, J.L. and Higgins, C.P., 2013. Subsurface Transport Potential of Perfluoroalkyl Acids at Aqueous Film-Forming Foam (AFFF)-Impacted Sites. Environmental Science and Technology, 47(9), pp. 4164-4171. [https://doi.org/10.1021/es3048043 DOI: 10.1021/es3048043]   [https://mountainscholar.org/bitstream/handle/11124/80055/Guelfo_mines_0052E_10298.pdf?sequence=1#page=64 Doctoral Dissertation]</ref><ref name="Zhao2014">Zhao, L., Bian, J., Zhang, Y., Zhu, L. and Liu, Z., 2014. Comparison of the sorption behaviors and mechanisms of perfluorosulfonates and perfluorocarboxylic acids on three kinds of clay minerals. Chemosphere, 114, pp. 51-58. [https://doi.org/10.1016/j.chemosphere.2014.03.098 DOI: 10.1016/j.chemosphere.2014.03.098]   Free download available from [https://www.researchgate.net/profile/Lixia_Zhao8/publication/262148355_Comparison_of_the_sorption_behaviors_and_mechanisms_of_perfluorosulfonates_and_perfluorocarboxylic_acids_on_three_kinds_of_clay_minerals/links/5b1be5dca6fdcca67b681a4f/Comparison-of-the-sorption-behaviors-and-mechanisms-of-perfluorosulfonates-and-perfluorocarboxylic-acids-on-three-kinds-of-clay-minerals.pdf ResearchGate].</ref>. The relative contribution of each mechanism varies depending on surface chemistry and other geochemical factors, as well as the molecular properties of the PFAS. In general, the impact of electrostatic interactions with charged soil constituents is more important for PFAS than non-polar, hydrophobic organic contaminants (e.g. hydrocarbons, chlorinated solvents). Adsorption of PFAS by solids is often nonlinear, with greater sorption at lower solute concentrations. The impacts of adsorption kinetics and their potential reversibility on PFAS transport have not yet been examined for most PFAS compounds.

Sorption of hydrocarbons, chlorinated solvents and other hydrophobic organics is often controlled the by organic-carbon components of the solid phase (see [[Sorption of Organic Contaminants]]). However, studies of PFAS sorption to solid phase organic carbon have reported conflicting results. In a study of field sites with aqueous film-forming foam (AFFF, a type of fire-fighting foam) releases, solid phase organic carbon content was found to significantly influence PFAS soil-to-groundwater concentration ratios. Statistical modeling was then used to derive apparent organic carbon partition coefficients for 18 different PFAS<ref name="Anderson2019" />. A recent compilation of published organic carbon partition coefficients found a good correspondence to PFAS molecular structure<ref name="Brusseau2019a">Brusseau, M.L., 2019. Estimating the relative magnitudes of adsorption to solid-water and air/oil-water interfaces for per-and poly-fluoroalkyl substances. Environmental Pollution, 254B, p. 113102. [https://doi.org/10.1016/j.envpol.2019.113102 DOI: 10.1016/j.envpol.2019.113102]</ref>. However, other studies have shown a general lack of correlation between solid phase partition coefficients and organic carbon<ref name="Li2018">Li, Y., Oliver, D.P. and Kookana, R.S., 2018. A critical analysis of published data to discern the role of soil and sediment properties in determining sorption of per and polyfluoroalkyl substances (PFASs). Science of the Total Environment, 628, pp. 110-120. [https://doi.org/10.1016/j.scitotenv.2018.01.167 DOI: 10.1016/j.scitotenv.2018.01.167]</ref>. It is possible that greater variability may be observed for broader data sets that incorporate different ranges of PFAS concentrations, different solution conditions, different measurement methods, and field-based data which often have less well-defined conditions and may also be influenced by other retention processes<ref name="Anderson2019" /><ref name="Brusseau2019a" />.

[[File:AndersonBrusseau1w2Fig2.png | thumb | 500px | Figure 2. Example of expected orientation and accumulation of PFAS at air-water interface. Source: D. Adamson, GSI, used with permission.]]
Most solids present in the environment contain both fixed-charged (negative) and variably charged surfaces. At neutral to high pH, variably charged clay minerals have a net-negative charge. As a result, negatively charged PFAAs do not strongly interact electrostatically in most soils, although as the soil pH decreases electrostatic sorption would be expected to increase in soils with variably charged clay minerals. Cationic and zwitterionic precursors are expected to be more strongly sorbed than anionic PFAAs in most environments due to well-established cation exchange reactions. Other factors, including ionic strength, composition, and the presence of co-solutes, can affect adsorption of PFAS<ref name="Higgins2006">Higgins, C.P. and Luthy, R.G., 2006. Sorption of Perfluorinated Surfactants on Sediments. Environmental Science and Technology, 40(23), pp. 7251-7256. [https://doi.org/10.1021/es061000n DOI: 10.1021/es061000n]</ref><ref name="Chen2009">Chen, H., Chen, S., Quan, X., Zhao, Y. and Zhao, H., 2009. Sorption of perfluorooctane sulfonate (PFOS) on oil and oil-derived black carbon: Influence of solution pH and [Ca2+]. Chemosphere, 77(10), pp. 1406-1411. [https://doi.org/10.1016/j.chemosphere.2009.09.008 DOI: 10.1016/j.chemosphere.2009.09.008]</ref><ref name="Pan2009">Pan, G., Jia, C., Zhao, D., You, C., Chen, H. and Jiang, G., 2009. Effect of cationic and anionic surfactants on the sorption and desorption of perfluorooctane sulfonate (PFOS) on natural sediments. Environmental Pollution, 157(1), pp.325-330. [https://doi.org/10.1016/j.envpol.2008.06.035 DOI: 10.1016/j.envpol.2008.06.035]   Free download available from [https://www.researchgate.net/profile/Gang_Pan2/publication/23189567_Effect_of_cationic_and_anionic_surfactants_on_the_sorption_and_desorption_of_perfluorooctane_sulfonate_PFOS_on_natural_sediments/links/5be19d23a6fdcc3a8dc2550d/Effect-of-cationic-and-anionic-surfactants-on-the-sorption-and-desorption-of-perfluorooctane-sulfonate-PFOS-on-natural-sediments.pdf ResearchGate]</ref><ref name="Guelfo2013" /><ref name="Zhao2014" />.

Most PFAS compounds act as surface-active agents (or [[Wikipedia:Surfactant | surfactants]]) due to the presence of a hydrophilic headgroup and a hydrophobic tail. The hydrophilic headgroup will preferentially partition to the aqueous phase and the hydrophobic tail will preferentially partition to the non-aqueous phase (air or organic material). As a result, PFAS tend to accumulate at interfaces (air-water, water-NAPL, water-solid) (Figure 2). This tendency to accumulate at interfaces can influence transport in the atmosphere (on water droplets and hydrated aerosols), in the vadose or unsaturated zone at air-water interfaces, in the presence of NAPLs, and in wastewater treatment systems<ref name="Brusseau2018" /><ref name="Brusseau2019b">Brusseau, M.L., 2019. The Influence of Molecular Structure on the Adsorption of PFAS to Fluid-Fluid Interfaces: Using QSPR to Predict Interfacial Adsorption Coefficients. Water Research, 152, pp. 148-158. [https://doi.org/10.1016/j.watres.2018.12.057 DOI: 10.1016/j.watres.2018.12.057]   [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6374777/ Author’s Manuscript]</ref>.

In theoretical and experimental studies of transport in unsaturated porous media, adsorption at the air-water interface increased PFOS and PFOA retention<ref name="Brusseau2018" /><ref name="Lyu2018">Lyu, Y., Brusseau, M.L., Chen, W., Yan, N., Fu, X., and Lin, X., 2018. Adsorption of PFOA at the Air-Water Interface during Transport in Unsaturated Porous Media. Environmental Science and Technology, 52(14), pp. 7745-7753. [https://doi.org/10.1021/acs.est.8b02348 DOI: 10.1021/acs.est.8b02348]   [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6312111/ Author’s Manuscript]</ref><ref name="BrusseauEtAl2019">Brusseau, M.L., Yan, N., Van Glubt, S., Wang, Y., Chen, W., Lyu, Y., Dungan, B., Carroll, K.C., and Holguin, F.O., 2019. Comprehensive Retention Model for PFAS Transport in Subsurface Systems. Water Research, 148, pp. 41-50. [https://doi.org/10.1016/j.watres.2018.10.035 DOI: 10.1016/j.watres.2018.10.035]   [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6294326/ Author’s Manuscript]</ref>, contributing approximately 20% to 80% of total retention in sands and soil. The impact of oil-water interfacial adsorption on PFAS transport was also quantitatively characterized in recent studies and shown to contribute to total retention on a similar scale as air-water interfacial adsorption<ref name="Brusseau2018" /><ref name="BrusseauEtAl2019" />. These processes may result in increased PFAS mass retained in NAPL source zones, increased PFAS sorption with the resulting retardation of transport, and greater persistence of dissolved PFAS in the environment.

==Transformation==
[[File:AndersonBrusseau1w2Fig3.png | thumb | 600px | Figure 3. Conceptual model of precursor transformation resulting in the formation of PFAAs. Source L. Trozzolo, TRC and C. Higgins, Colorado School of Mines, used with permission.]]
Certain polyfluorinated substances have the potential to transform to other PFAS, with PFAAs as the typical terminal daughter products. These polyfluorinated substances are often referred to as “precursors”. The transformation potential of polyfluorinated precursors is influenced by the presence, location, and number of carbon-hydrogen (C-H) bonds and potentially carbon-oxygen (C-O) bonds throughout the carbon chain. Specifically, PFAS with C-H bonds are subject to a variety of biotic and abiotic reactions that ultimately result in the formation of PFAAs with perfluorinated carbon chains of the same length or shorter than the initial polyfluorinated precursor<ref name="Houtz2013">Houtz, E.F., Higgins, C.P., Field, J.A. and Sedlak, D.L., 2013. Persistence of perfluoroalkyl acid precursors in AFFF-impacted groundwater and soil. Environmental Science and Technology, 47(15), pp. 8187-8195. [https://doi.org/10.1021/es4018877 DOI: 10.1021/es4018877]   Free download from [https://www.researchgate.net/profile/Erika_Houtz/publication/252323955_Persistence_of_Perfluoroalkyl_Acid_Precursors_in_AFFF-Impacted_Groundwater_and_Soil/links/59dbddeeaca2728e2018336d/Persistence-of-Perfluoroalkyl-Acid-Precursors-in-AFFF-Impacted-Groundwater-and-Soil.pdf ReseqarchGate]</ref><ref name="McGuire2014">McGuire, M.E., Schaefer, C., Richards, T., Backe, W.J., Field, J.A., Houtz, E., Sedlak, D.L., Guelfo, J.L., Wunsch, A., and Higgins, C.P., 2014. Evidence of Remediation-Induced Alteration of Subsurface Poly- and Perfluoroalkyl Substance Distribution at a Former Firefighter Training Area. Environmental Science and Technology, 48(12) pp. 6644-6652. [https://doi.org/10.1021/es5006187 DOI: 10.1021/es5006187]   Manuscript available from [https://ir.library.oregonstate.edu/downloads/td96k706f Oregon State University]</ref><ref name="Anderson2016">Anderson, R.H., Long, G.C., Porter, R.C. and Anderson, J.K., 2016. Occurrence of select perfluoroalkyl substances at US Air Force aqueous film-forming foam release sites other than fire-training areas: Field-validation of critical fate and transport properties. Chemosphere, 150, pp. 678-685. [https://doi.org/10.1016/j.chemosphere.2016.01.014 DOI: 10.1016/j.chemosphere.2016.01.014]</ref><ref name="Weber2017" />.

Transformation studies published to date have tested only a small subsample of possible precursors and, therefore, much uncertainty exists regarding 1) the extent to which precursor transformation occurs on a global scale, 2) which environmental compartments represent the majority of transformation, 3) relevant environmental conditions that affect transformation processes, and 4) transformation rates and pathways. Nevertheless, a portion of the precursors are expected to transform to PFAAs over time as shown in Figure 3.

Precursors can be transformed by a variety of abiotic processes including hydrolysis, photolysis, and oxidation. Hydrolysis of some precursors, followed by subsequent biotransformation, can produce perfluoroalkyl sulfonates (PFSAs). An important example is the production of PFOS from perfluorooctane sulfonyl fluoride (POSF)<ref name="Martin2010">Martin, J.W., Asher, B.J., Beesoon, S., Benskin, J.P. and Ross, M.S., 2010. PFOS or PreFOS? Are perfluorooctane sulfonate precursors (PreFOS) important determinants of human and environmental perfluorooctane sulfonate (PFOS) exposure? Journal of Environmental Monitoring, 12(11), pp.1979-2004. [https://doi.org/10.1039/C0EM00295J DOI: 10.1039/C0EM00295J]   Free download from [https://www.researchgate.net/profile/Matthew_Ross3/publication/47415684_PFOS_or_PreFOS_Are_perfluorooctane_sulfonate_precursors_PreFOS_important_determinants_of_human_and_environmental_perfluorooctane_sulfonate_PFOS_exposure/links/00b7d520a6132da945000000.pdf ResearchGate]</ref>. Other hydrolysis reactions produce perfluoroalkyl carboxylates (PFCAs). At neutral pH, the hydrolysis of fluorotelomer-derived polymeric precursors results in the formation of monomeric precursors of PFOA and other PFAAs with half-lives of 50 to 90 years<ref name="Washington2010">Washington, J.W., Ellington, J.J., Jenkins, T.M. and Yoo, H., 2010. Response to Comments on “Degradability of an Acrylate-Linked, Fluorotelomer Polymer in Soil”. Environmental Science and Technology, 44(2), pp. 849-850. [https://doi.org/10.1021/es902672q DOI: 10.1021/es902672q]   [https://pubs.acs.org/doi/pdf/10.1021/es902672q Free Download from ACS].</ref>. Oxidation of precursors by hydroxyl radicals can occur in natural waters, with the fluorotelomer-derived precursors being oxidized relatively rapidly<ref name="Gauthier2005">Gauthier, S.A. and Mabury, S.A., 2005. Aqueous photolysis of 8: 2 fluorotelomer alcohol. Environmental Toxicology and Chemistry, 24(8), pp.1837-1846. [https://doi.org/10.1897/04-591R.1 DOI: 10.1897/04-591R.1]   Free download from [https://www.researchgate.net/profile/Suzanne_Gauthier/publication/7609648_Aqueous_photolysis_of_8_2_fluorotelomer_alcohol/links/5ec16c4792851c11a86d9438/Aqueous-photolysis-of-8-2-fluorotelomer-alcohol.pdf ResearchGate].</ref><ref name="Plumlee2009">Plumlee, M.H., McNeill, K. and Reinhard, M., 2009. Indirect Photolysis of Perfluorochemicals: Hydroxyl Radical-Initiated Oxidation of N-Ethyl Perfluorooctane Sulfonamido Acetate (N-EtFOSAA) and Other Perfluoroalkanesulfonamides. Environmental Science and Technology, 43(10), pp.3662-3668. [https://doi.org/10.1021/es803411w DOI: 10.1021/es803411w]   Free download from [https://www.researchgate.net/profile/Megan_Plumlee/publication/26309488_Indirect_Photolysis_of_Perfluorochemicals_Hydroxyl_Radical-Initiated_Oxidation_of_N-Ethyl_Perfluorooctane_Sulfonamido_Acetate_N-EtFOSAA_and_Other_Perfluoroalkanesulfonamides/links/5aac0437a6fdcc1bc0b8d002/Indirect-Photolysis-of-Perfluorochemicals-Hydroxyl-Radical-Initiated-Oxidation-of-N-Ethyl-Perfluorooctane-Sulfonamido-Acetate-N-EtFOSAA-and-Other-Perfluoroalkanesulfonamides.pdf ResearchGate].</ref>.
Evidence of aerobic biotransformation is provided from studies of PFAS composition throughout the continuum of wastewater treatments<ref name="Arvaniti2015">Arvaniti, O.S. and Stasinakis, A.S., 2015. Review on the occurrence, fate and removal of perfluorinated compounds during wastewater treatment. Science of the Total Environment, 524, pp. 81-92. [https://doi.org/10.1016/j.scitotenv.2015.04.023 DOI: 10.1016/j.scitotenv.2015.04.023]</ref>, from field studies at AFFF-impacted sites<ref name="Houtz2013" /><ref name="McGuire2014" /><ref name="Anderson2016" /><ref name="Weber2017" />, and from microcosm experiments. In general, the literature on aerobic biotransformation collectively demonstrates or indirectly supports the following conclusions as summarized in ITRC 2020<ref name="ITRC2020" />:

*Numerous aerobic biotransformation pathways exist with relatively rapid kinetics
*All polyfluorinated precursors studied to date have the potential to aerobically biotransform to PFAAs
*Aerobic biotransformation of various fluorotelomer-derived precursors exclusively results in the formation of PFCAs, including PFOA, without necessarily the conservation of chain-length
*Aerobic biotransformation of various electrochemical fluorination-derived precursors primarily results in the formation of PFAAs, including PFOS, with the conservation of chain-length

[[File:Fate-and-transport_v3.mp4|thumb|500px|left|Figure 4. PFAS Fate and Transport.]]
Precursor transformation can complicate CSMs (and risk assessments) and should be considered during comprehensive site investigations. For example, atmospheric emissions of volatile precursors can result in long-range transport where subsequent transformation and deposition can result in detectable levels of PFAAs in environmental media independent of obvious point-sources<ref name="Vedagiri2018">Vedagiri, U.K., Anderson, R.H., Loso, H.M. and Schwach, C.M., 2018. Ambient levels of PFOS and PFOA in multiple environmental media. Remediation Journal, 28(2), pp. 9-51. [https://doi.org/10.1002/rem.21548 DOI: 10.1002/rem.21548]</ref>. With respect to site-related precursors, transformation of otherwise unmeasured PFAS into detectable PFAAs is obviously relevant to site investigations to the extent transformation occurs after initial site characterization efforts or if past remedial efforts have accelerated ''in situ'' transformation rates<ref name="McGuire2014" />. Additionally, differential transport rates between precursor PFAS and the corresponding terminal PFAA could also confound CSMs if transformation rates are slower than transport rates as has been suggested<ref name="Weber2017" />.
To account for otherwise unmeasurable precursors, several surrogate analytical methods have been developed.

==References==
<references />

==See Also:==
[https://soundcloud.com/arcadis-north-america/pfas-understanding-fate-and-transport-in-the-environment?utm_source=clipboard&utm_campaign=wtshare&utm_medium=widget&utm_content=https%253A%252F%252Fsoundcloud.com%252Farcadis-north-america%252Fpfas-understanding-fate-and-transport-in-the-environment SERDP & ESTCP PFAS Research and Remediation Podcast: PFAS: Understanding Fate and Transport in the Environment]

[https://soundcloud.com/arcadis-north-america/how-pfas-moves-from-afff-areas-to-groundwater SERDP & ESTCP PFAS Research and Remediation Podcast: How PFAS Moves from AFFF Areas to Groundwater]

File:Fate-and-transport v3.mp4

2026-07-18T20:07:03Z

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2026-05-07T17:14:24Z

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Monitored Natural Attenuation - Transitioning from Active Remedies

2026-05-07T17:08:14Z

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Many contaminated sites use active remedies such as pump-and-treat or ''in situ'' remediation to clean up impacted groundwater. Natural attenuation processes such as natural degradation or [[Dispersion and Diffusion | hydrodynamic dispersion]] also contribute to the cleanup. As remediation progresses, a point is often reached when the time required to reach the remedial objectives using the active remedy is roughly the same as the time required if the active remedy is shut down, and the continuing remediation of the site is provided by natural attenuation processes alone. From that point forward, the extra effort and expense of the active remedy provides no benefit over natural attenuation, and it may be appropriate to transition the site to [[Monitored Natural Attenuation (MNA)]]. This article deals with currently available tools and approaches that can be used to support a decision to transition from active remediation to MNA.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s)''':

*[[Alternative Endpoints]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Plume Response Modeling]]
*[[REMChlor - MD]]
*[[Source Zone Modeling]]

'''Contributor(s):''' [[Dr. John Wilson]] and [[Dr. David Adamson, P.E.]]

'''Key Resource(s)''':

*[//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies]<ref name="Newell2002">Newell, C.J., Rifai, H.S., Wilson, J.T., Connor, J.A., Aziz, J.A., Suarez, M.P., 2002. Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies. 28p. EPA/540/S-02/500. [//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Report.pdf]</ref>

*[https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS) Version 2.3.3]<ref name="Widdowson2008">Widdowson, M.A., Mendez, E., Chapelle, F.H., Casey, C.C., 2008. Natural Attenuation Software (NAS) Version 2.3.3. Virginia Polytechnic Institute and State University, the United States Geological Survey, and the United States Naval Facilities Engineering Command. NAS webpage: https://www.nas.cee.vt.edu/index.php See also: https://toxics.usgs.gov/highlights/nas_2.2.0/index.html</ref>

*[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf BIOCHLOR Natural Attenuation Support System, Version 2.2]<ref name="Aziz2002">Aziz, C.E., Newell, C.J. and Gonzales, J.R., 2002. BIOCHLOR Natural Attenuation Decision Support System Version 2.2 User’s Manual Addendum. Groundwater Services, Inc., Houston, Texas for the Air Force Center for Environmental Excellence.[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf Report.pdf] Available at: https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system</ref>

*[https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC User's Guide and Tool Website]<ref name="BioPIC2021">Danko, A., Adamson, D., Newell, C., Wilson, J., Wilson, B., Freedman, D., Lebrón, C., 2021. Quick BioPIC User’s Guide, ESTCP Project ER-201730. [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 Project Website]   [//www.enviro.wiki/images/c/c9/ER-201730_BioPIC_User%27s_Guide.pdf User’s Guide]</ref>

*[https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool Website]<ref name="TATA2024">Adamson, D.T., Newell, C.J., Hort, H.M, Wilson, J.T., 2024. TA2: The SERDP Transition Assessment Teaching Assistant. Strategic Environmental Research and Development Program (SERDP) Project ER20-1429. [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview Project Website]  [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Online Tool]</ref>

==Introduction==
Many active remedies are effective at treating higher concentrations of contaminants, but as the contaminant concentrations decrease, the rate of cleanup may slow before the site reaches the cleanup goal. At some sites, the rate of cleanup may slow until it is not significantly different from the rate of cleanup provided by the natural attenuation processes that occur at the site. At other sites, the concentration of contaminants in water produced by a pumping system is below the cleanup goal, but the concentration in monitoring wells in the source area are still above the goal. At some sites, active treatment has stopped further expansion of the plume toward a receptor, and concentrations are declining over time throughout the plume, but back diffusion is sustaining concentrations in the plume that are above the cleanup goal.

In 2013, a significant National Research Council (NRC) report noted that despite years of effort and considerable investment, many sites “will require long-term management that could extend for decades or longer”<ref name="NRC2013">National Research Council (NRC), 2013. Alternatives for Managing the Nation's Complex Contaminated Groundwater Sites. Committee on Future Options for Management in the Nation's Subsurface Remediation Effort, Water Science, Technology Board, Division on Earth and Life Studies, NRC. National Academies Press, 422 pages, ISBN 978-0-309-27874-4 [https://doi.org/10.17226/14668 doi: 10.17226/14668]. [//www.enviro.wiki/images/4/48/NRC2013.pdf Report.pdf]</ref>. The authors of the report discussed the need for developments that can aid in “transition from active remediation to more passive strategies and provide more cost-effective and protective long-term management of complex sites”<ref name="NRC2013" />.

The United States Environmental Protection Agency<ref> U.S. Environmental Protection Agency (USEPA), 1999. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. OSWER Directive 9200.4-17P. 39pp.[//www.enviro.wiki/images/a/aa/1999_USEPA-_Use_of_monitored_natural_attenuation_at_superfund.pdf Report.pdf]</ref> allows the use of [[Monitored Natural Attenuation (MNA) | monitored natural attenuation (MNA)]] to attain the cleanup goals when the site-specific remediation objectives can be attained within a time frame that is reasonable compared to that offered by other more active methods. Many CERCLA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Summary of the Comprehensive Environmental Response, Compensation, and Liability Act (Superfund) https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act</ref> and RCRA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Resource Conservation and Recovery Act (RCRA) Laws and Regulations https://www.epa.gov/rcra</ref> sites take advantage of this policy. An active remedy is typically used initially to treat high concentrations of contaminants followed by MNA to treat the lower concentrations that remain.

Unfortunately, there is no well-established approach to determine when it is appropriate to discontinue the active remedy. The NRC report<ref name="NRC2013" /> emphasized the use of more rigorous evaluations of existing data to support these efforts. This can include a quantitative assessment of the performance of active remedies (e.g., evidence of asymptotic performance) as well as documenting site conditions that may be contributing to these performance limitations. Importantly, it also identifies alternative approaches for managing the site, which could include MNA if the natural attenuation processes can meaningfully contribute to the achievement of site cleanup objectives.

This article reviews available tools and approaches to evaluate a transition to MNA. The tools and approaches depend on calculations of rate constants for natural attenuation with distance in flowing groundwater or rate constants for attenuation over time in individual monitoring wells.

==Background on Rate Constants==
[[File:Wilson1w2Fig1.png|thumb|400px| Figure 1. Attenuation of Trichloroethene (TCE) over time in a monitoring well at a site in Michigan. The concentration vs. time rate constant is 0.326 per year and largely represents the rate of the attenuation of the source of contaminants in the aquifer.]]
At sites where a transition to MNA is being considered, a key step is estimating attenuation rate constants and understanding how they are extracted from monitoring data. A general formula to describe the rate of a chemical reaction is:

:{|
|-
|'''Equation 1:'''|| ||<big>''r = k [C]''</big>'' m''
|-
|where:
|-
| ||''r''    ||is the rate of the reaction,
|-
| ||''k''||is the rate constant,
|-
| ||''C''||is the concentration of the chemical undergoing the reaction, and
|-
|the exponent    ||''m''||is the order of the reaction.
|}

When the rate of the reaction is proportional to the concentration of the contaminant, the value of ''m'' is 1. Therefore, the reaction is described as a first-order reaction, and the rate constant is described as a first-order rate constant. In Equation 1, concentration could go up or down, but ''k'' is a constant of proportionality for the rate of increase in concentration. The rate constant for attenuation is the negative of ''k''. If the rate of degradation is a fixed value regardless of concentration, the value of ''m'' is 0, and degradation is a zero-order process.

Natural attenuation of concentrations over time in monitoring wells is frequently described by a first-order rate constant, and natural biological or abiotic degradation of contaminants in flowing groundwater is typically also described by a first-order rate constant. Figure 1 provides an example of monitoring data that is described by a first-order rate constant.

The rate constant for attenuation over time in a single well and the rate constant for attenuation with distance along a flow path in an aquifer describe different situations that are controlled by different processes. ''Attenuation over time'' in a well is largely controlled by the rate of attenuation of the source of contamination in the aquifer. ''Attenuation with distance'' along a flow path includes attenuation of concentrations in the source along with contributions from biological degradation processes, abiotic degradation processes and hydrodynamic dispersion of the contaminated groundwater into clean groundwater<ref name="Newell2002" />.

The first-order rate constant for attenuation over time in a single well is commonly referred to as ''kpoint''<ref name="Newell2002" />. A time series chart in Microsoft EXCEL of the concentrations of a contaminant (''y'' axis) on the date of sampling (''x'' axis) can be used to extract a value for ''kpoint''. Select the data, then insert an exponential trend line and display the equation on the chart. The value of ''kpoint'' can also be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Note that the rate constants extracted in EXCEL are constants for the rate of change, not the rate of attenuation. Take the negative of the rate of change to get ''kpoint''. In the example in Figure 1, the unit of time on the X axis is years, and the value of ''kpoint'' is 0.326 per year.

Attenuation versus distance rate coefficients describe a bulk attenuation rate including both degradation and non-destructive processes such as dispersion. To extract values for rate constants for degradation alone, it is necessary to calibrate a groundwater flow and transport model to the data at the site. The model is calibrated with values for the hydrogeological properties of the aquifer (effective porosity, hydraulic gradient, hydraulic conductivity, hydrodynamic dispersion and the organic carbon content of the aquifer matrix). After the hydrogeological properties of the aquifer are fixed in the model, the most appropriate values for the degradation rate constants are the values that produce the best fit between the contaminant concentrations that are predicted by the model and the contaminant monitoring data at the site.

There are a number of reasons why natural attenuation processes are better described as first-order relationship instead of zero-order or some other order. The attenuation over time in a monitoring well tracks the attenuation over time of the source of contamination that sustains the plume<ref name="Newell2002" />. Sites go through a lifecycle, and attenuation of sources at mature sites is often a first-order process<ref>Sale, T., Newell, C., Stroo, H., Hinchee, R. and Johnson, P., 2008. Frequently Asked Questions Regarding Management of Chlorinated Solvents in Soils and Groundwater. Environmental Security Technology Certification Program (ESTCP, Project ER-200530), Department of Defense (DoD), Arlington, VA. [//www.enviro.wiki/images/c/cb/2008-Sale-Frequently_Asked_Questions_Regarding_Management_of_Chlorinated_Solvent_in_Soils_and_Groundwater.pdf Report.pdf]   Project Overview Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200530</ref>. If a chlorinated solvent site is mature, the contamination in the source area that was originally present as nonaqueous phase liquids (NAPL) has been redistributed and is now sequestered in a sorbed phase to aquifer solids or has diffused into non-transmissive portions of the aquifer matrix. Transfer of contaminants back into the more transmissive portions of the aquifer occurs by diffusion along a fixed path length, and the rate of transfer is controlled by the concentration of the contaminant remaining in the source material. Because the rate of transfer is proportional to the concentration of contaminant in the source material, attenuation of the source is a first-order process. These processes are discussed in more detail in [[Source Zone Modeling]].

Degradation processes are also usually first order. Abiotic reactions are almost always first order with respect to the concentration of the target chemical. Biodegradation reactions are zero order at high concentrations because the available enzymes are saturated with substrate, but are first order at lower concentrations that are typical of natural attenuation conditions in groundwater.

==Goals for MNA at Sites==

The information necessary to evaluate whether a site can be transitioned to MNA depends on the goal for MNA at the site. For many cleanup actions, the goal is to confine contamination within a waste management area where the contamination is left in place, in which case the cleanup goal applies to point-of-compliance wells that are outside the waste management area. For other cleanup actions, the entire site must be cleaned up, in which case the cleanup goal applies to any monitoring well on the site. The time by which the goal is to be attained is specified at CERCLA sites in the Record of Decision (the ROD). At RCRA sites, the time allowed for the cleanup to be attained may be specified in the permit.

==When the Goal Applies to Point-of-Compliance Wells==
Consider the following framework for evaluating a transition to MNA:

#Use a computer model to extract rate constants for the natural degradation of the contaminant that occurred in groundwater at the site before the active remedy was installed.
#Assume that the same rate constants will apply after the active remedy is no longer in operation. Note that this assumption may not be valid if the active remedy changes the geochemistry of the aquifer in the flow path to the point-of-compliance well.
#Calibrate a computer groundwater flow and transport model with the hydrogeological properties of the aquifer that pertain after the active remedy is no longer in operation, the concentration of contaminant after the active remedy, and the rate constants for natural degradation that are assumed to apply after the active remedy.
#Use the computer model to project the concentrations of the contaminant at the point-of-compliance well over time.
#If the concentrations at the point-of-compliance wells are predicted to be less than the goal before the specified date, that is a quantitative line of evidence in support of a transition to MNA.

There are several computer applications that are particularly useful to extract rate constants at a site from monitoring data that were collected before the active remedy was installed. For example, [https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS)]<ref name="Widdowson2008" />, [https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system BIOCHLOR]<ref name="Aziz2002" /> and [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC]<ref name="BioPIC2021" /> can be downloaded from the internet at no cost. Another recent example, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ TA2 Tool]<ref name="TATA2024" />, is discussed in detail later in this article.

[[File:Wilson1w2Fig2.png|thumb|left|400px| Figure 2. Example calibration of NAS to natural attenuation of total BTEX at a site (Figure 17 of NAS User’s Manual).]]
[[File:Wilson1w2Fig3.png|thumb|400px| Figure 3. The data input screen for BIOCHLOR before remediation with cis-1,2-Dichloroethene (DCE) and vinyl chloride (VC) source concentrations of 500 and 87 mg/L respectively at the source when the release first occurred.]]
[[File:Wilson1w2Fig4.png|thumb|left|400px| Figure 4. Output of the RUN CENTERLINE simulation in BIOCHLOR comparing the fit between the simulation and the field data for vinyl chloride before an active remedy was implemented]]
[[File:Wilson1w2Fig5.png|thumb|400px| Figure 5. Output of the RUN CENTERLINE simulation of conditions after an active remedy was implemented with a source concentration of 1.1 mg/L, projecting the concentration of vinyl chloride at a distance corresponding to a point-of-compliance well.]]
 
In NAS, the user inputs the hydrogeological data, the distance of wells along the flow path, and the concentrations of contaminants in the wells. The NAS application extracts rate constants and makes projections at the point-of-compliance. With NAS, it is possible to extract different rate constants for specific geochemical environments along the flow path.

Figure 2 provides an example calibration of NAS. The concentrations in the monitoring wells used to calibrate the model are compared to the simulation provided by the model. The values of the rate constants that are extracted from the field data are available in the “Output” tab under “Data and Results Table.”

Figure 3 depicts the input screen for BIOCHLOR. The user inputs the hydrogeological parameters, the first-order rate constants (1st Order Decay Coefficient), the distribution of the wells along the flow path, and the concentrations of contaminants in the wells. The model is set up for conditions that apply before the installation of the active remedy.

BIOCHLOR does not automatically fit the rate constants to the field data. Instead, the user examines the output of the model, and adjusts the rate constants until they provide the best fit between the model prediction and the monitoring data for wells at the site. This comparison is illustrated in Figure 4.

If the distance from the source well to the point-of-compliance well is set as the “Modeled Area Length” in Section 5 of the input screen, the “Run Centerline” output will provide the projected concentrations at that length. Assume the distance from the source well to the point-of-compliance well is 250 feet. The projected concentration in Figure 3 of vinyl chloride at a point-of-compliance well is 0.042 mg/L. If the regulatory goal were the federal drinking water maximum contaminant level (MCL)<ref>U. S. Environmental Protection Agency (USEPA), 2009. National Primary Drinking Water Regulations. EPA 816-F-09-004. [//www.enviro.wiki/images/a/ae/2009-USEPA-national_Primary_Drinking_Water_Regulations.pdf Report.pdf]</ref> of 0.002 mg/L, the projected concentration would exceed the goal, and MNA would not be adequate as a remedy.

For the sake of illustration, assume that an active remedy has been implemented, and the concentrations in the source well are 5.4 mg/L for DCE and 1.1 mg/L for vinyl chloride. To evaluate whether it is now appropriate to transition to MNA, BIOCHLOR could be calibrated with these concentrations to predict concentrations in the point-of-compliance well. (See Figure 5). In this example, the projected concentration at the point-of-compliance well does meet the goal.

Some active remedies are subject to rebound. If this is the case, the evaluation should begin at the point in time when it is clear that the trend in concentrations is downward.

A new EXCEL-based tool that does many of the same basic calculations as BIOCHLOR was recently developed as part of an update to the BioPIC<ref name="BioPIC2021" /> decision support software. This tool, the MNA Rate Constant Estimator, extracts rate constants from concentration versus distance data for a variety of different chemicals, including chlorinated ethenes (e.g., PCE and TCE), chlorinated ethanes (e.g., 1,1,1-TCA), and 1,4-dioxane. This tool was developed to run using current versions of EXCEL, whereas BIOCHLOR must be run using older versions of EXCEL that may be unavailable to many users. The MNA Rate Constant Estimator can be used to estimate degradation rate constants and/or predict plume footprints over time. Consequently, it is a useful addition to the BioPIC decision framework for understanding if MNA is appropriate remedy for a site, and it can also be helpful for estimating rate constants as part of a transition assessment.

==When the Goal Applies to All the Wells==
At sites where a concentration-based cleanup goal must be achieved at all wells, each well at the site is evaluated independently, and the rate constant that is applicable is the rate constant for attenuation over time in the well (''kpoint''). To evaluate whether the region in an aquifer that is sampled by a particular monitoring well is ready to transition to MNA, it is necessary to have monitoring data from a period of time before the remedy was implemented. This data is used to extract a value for ''kpoint'' in the aquifer under natural attenuation conditions. The evaluation of a transition to MNA will assume that the same value for ''kpoint'' will apply after the active remedy is complete. This assumption may not be appropriate if the active remedy caused a permanent change in the geochemistry of the aquifer. The assumption is usually appropriate for pump-and-treat remedies.

If ''kpoint'' before implementation of the active remedy describes the time course of natural attenuation after the active remedy is completed, the time required to attain the cleanup goal is predicted from the following:

:{|
| || || rowspan="2" |<big>''ln (''</big>
| style="border-style:solid; border-width: 0px 0px 1px 0px" |''Cgoal''
| rowspan="2" |<big>'')''</big>||                                                                       
|-
|'''Equation 2:'''        ||''<big>t =''    
| style="vertical-align:top;" |''Ccurrent''||
|-
| || || colspan="3" style="text-align:center; border-style:solid; border-width: 1px 0 0 0" |''<big>-k</big>point''||
|-
|where:
|-
| ||''Ccurrent''    || colspan="5" |is the current concentration after active remediation,
|-
| ||''Cgoal''|| colspan="5" |is the cleanup goal, and
|-
| ||''t''|| colspan="5" |is the time required for concentrations to attenuate from ''Ccurrent'' to ''Cgoal.''
|}

If the value of ''t'' estimated using Equation 2 is less than the difference between the current date and the date specified by the site stakeholders to attain the goal, that is evidence in support of a transition to MNA.

Some active remedies are subject to contaminant concentration rebound. If this is the case, the evaluation should use a value of ''C''current that is attained after the rebound has stabilized.

This approach depends on a robust value for ''kpoint''. It is worthwhile to do a sensitivity analysis on ''kpoint'' where the lower 95% or 90% confidence interval on ''kpoint'' is used in Equation 2 to see if that changes the outcome of the evaluation. The confidence intervals can be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Wilson<ref name="Wilson2011">Wilson, J.T. 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [//www.enviro.wiki/images/e/e3/Wilson-2011-An_Approach_for_Evaluating_Progress.pdf Report.pdf]</ref> provides detailed discussion of the use of linear regression to extract ''kpoint'' and confidence intervals on ''kpoint''. Wilson<ref name="Wilson2011" /> also discusses the use of goodness-of-fit tests to determine if there is evidence that a first-order rate equation is not the best fit to the monitoring data, and as a result the use of Equation 2 would not be appropriate. The TA2 Tool<ref name="TATA2024" /> also has the capability to calculate ''kpoint'' with a user-specified confidence interval, as described below.

At many sites, there is no specified date when the cleanup goal must be attained. In this situation, the monitoring data can be evaluated to determine if the current rate of attenuation under the active remedy is faster than the rate of natural attenuation before the active remedy was installed. The monitoring data can be examined to identify a time interval when the benefit of the active remedy has approached an asymptote. A second value of for ''kpoint'' can be extracted for that time interval. The two values for ''kpoint'' can be evaluated statistically to see if the current rate is faster at some appropriate level of confidence. If there is no statistical evidence that the rate of attenuation is faster, that determination can support a decision to transition to MNA.
[[File:Wilson1w2Fig6.png|thumb|400px| Figure 6. Example calibration of NAS to predict the reduced concentration at the source that is necessary to meet the remediation goal at a point-of-compliance well (Figure 19 of NAS User’s Manual).]]

==Extent of Treatment Necessary to Transition to MNA==
There are several computer applications that can predict the extent of treatment that must be achieved by the active remedy before it is worthwhile to evaluate the site for transition to MNA. For example, based on the distribution of contamination along the flow path, the NAS application will automatically predict a reduced concentration at the source well that will bring concentrations to the goal in the point-of-compliance well (Figure 6). A table that opens under the “DOS/TOS” tab provides the “Time of Equilibration” required to meet the goal at the reduced concentration. Modules in NAS allow the user to evaluate the effect of various pump-and-treat and source removal scenarios on the time required to attain the goal at the point-of-compliance well.

The [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018">Falta, R.W., Farhat, S.K., Newell, C.J. and Lynch, K., 2018. A Practical Approach for Modeling Matrix Diffusion Effects in REMChlor. SERDP/ESTCP Project ER-201426 [//www.enviro.wiki/images/0/0b/2018-Falta-REMChlor_Modeling_Matrix_Diffusion_Effects.pdf Report.pdf]   Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426</ref> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012">Falta, R.W., Ahsanuzzaman, A.N., Stacy, M.B., Earle, R.C. and Wilson, J.T., 2012. Remediation Evaluation Model for Fuel Hydrocarbons (REMFuel). Users Manual Version 1.0. U.S. Environmental Protection Agency. EPA/600/R-12/028. [//www.enviro.wiki/images/6/67/2012-Falta-REMFuel_Remediation_Evaluation-Model_for_Fuel_hydrocarbons_users_manual.PDF Report.pdf]   Website: https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel</ref> models are flexible screening tools that allow a simultaneous evaluation of the extent of treatment provided by (1) source removal, (2) ''in situ'' remediation of the contaminated groundwater, or (3) natural attenuation processes in three discrete intervals along the flow path and three discrete time periods. Both [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018" /> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012" /> can be downloaded from the internet at no cost. Liang ''et al.''<ref>Liang, H., Falta, R.W., Newell, C.J., Farhat, S.K., Rao, P.S. and Basu, N., 2010. Decision & Management Tools for DNAPL Sites: Optimization of Chlorinated Solvent Source and Plume Remediation Considering Uncertainty. SERDP/ESTCP Project ER-200704. [//www.enviro.wiki/images/c/ce/2010-Liang-Decision_and_Management_Tools_for_DNAPL_sites-ER-200704-FR.pdf Report.pdf]   Project Overview Website: https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200704/(language)/eng-US</ref> provide a modeling program that uses Monte Carlo simulations to evaluate the effects of the uncertainties in the modeling parameters on the predictions of REMChlor-MD.

==The Transition Assessment Teaching Assistant (TA2) Tool==
[[File:Wilson1w2Fig7.png|thumb|500px| Figure 7. Home Page for TA2 Tool. Users can click on buttons to access various modules that are designed to answer specific questions or research relevant topics.]]
[[File:Wilson1w2Fig8.png|thumb|500px| Figure 8. Example of an asymptote analysis using concentration versus time data in Tool 1 of the TA2 Tool. The source attenuation rate and corresponding remediation timeframe can be estimated for different monitoring periods.]]
A learning and decision-making tool was recently released as part of [https://serdp-estcp.mil/ Strategic Environmental Development and Research Program (SERDP)] Project [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview ER-201429] to help stakeholders gather information for the purposes of a site-specific transition assessment. This free software, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool]<ref name="TATA2024" />, was developed using the elements identified in the 2013 NRC report<ref name="NRC2013" /> as the critical learning objectives for end users.

The Tool is a web-based app that includes a collection of individual modules designed to answer specific questions or research relevant topics (Figure 7). The Tool has been developed as an R Shiny app (version 1.8.0)<ref> Chang, W., Cheng, J., Allaire, J., Sievert, C., Schloerke, B., Xie, Y., Allen, J., McPherson, J., Dipert, A., Borges, B., 2023. shiny: Web Application Framework for R. R package version 1.8.0, https://github.com/rstudio/shiny, https://shiny.posit.co/</ref>, which is an interactive platform using R programming to perform all quantitative functions. The user can then view the results in a simple interface that easily accommodates plots, charts, and various mapping features in a Web browser. The Tool is free and does not require the user to install R software.

The modules within the TA2 Tool include:

'''Five Quantitative Tools''' that focus on assessing asymptotic groundwater concentrations from monitoring data, evaluating plume stability, estimating remediation timeframes after a hypothetical source removal project, forecasting remediation performance if a technology is applied in the field, or projecting concentrations at downgradient points of compliance.

For example, Tool 1 in the TA2 Tool uses concentration versus time data from monitoring wells to estimate attenuation rate constants (''kpoint'') and evaluate if asymptotic conditions are present at particular locations or across the site. This helps to assess whether performance has plateaued at wells where a pump-and-treat system or other active treatment is in place. The user has the option to choose a “change point” within the monitoring record to determine if the attenuation rate has changed over time (e.g., once most of the accessible mass has been removed) (Figure 8). The user can either use visual interpretation to manually select the date when this apparent change occurred or have the date selected automatically using a binary segmentation protocol that is incorporated into the tool. The tool will calculate a rate for both the early period and a rate for the later period (after the change point), and then go through five different lines of evidence for asymptotic behavior (e.g., are the two rates of attenuation significantly different?). The user can then use the collective results as a technical justification demonstrating that the performance of the active remedy has plateaued as the first step in the transition assessment. The tool will also estimate the time to reach a user-specified cleanup goal if the overall attenuation rate (or the attenuation rate in the later period) were to continue.

Another module (Tool 5) focuses on evaluating sites where the concentration goal applies at a downgradient point of compliance, which is a key criterion for sites where MNA is being used as part of a risk-based strategy. The tool includes several different options to estimate a site-specific attenuation rate constant, including data from the pre-remediation period when natural attenuation processes were the sole means for reducing concentrations. Attenuation rate constants are then used to project the concentration versus distance from the contaminant source. Based on the predicted concentration at the downgradient point of compliance, the user can then see if the natural assimilative capacity along the aquifer flow path is sufficient to achieve the concentration goal in the absence of active treatment. For example, in the tab labeled “Use Pre-Remediation Rate Constant”, the logarithms of the concentrations from the period before active treatment began are plotted against the distance from the source well. The slope of the regression line is the rate constant for natural attenuation (including the contributions of degradation and dispersion). This rate constant can then be used to project the concentration moving downgradient from the well of concern after the end of active treatment. Similar approaches are provided within Tool 5 for using rate constants estimated from lab-based testing or derived from post-remediation data (after steady state has been reestablished).

'''Four Qualitative Tools''' provide information on matrix diffusion, enhanced attenuation options, geologic heterogeneity, and related research on transition assessments. Many of these modules are based on the current understanding of the role of matrix diffusion in influencing long-term concentration trends and remedial performance at contaminated groundwater sites. This includes summaries of different modeling options for better quantifying the effects of matrix diffusion. Sites impacted by matrix diffusion are generally challenging to treat using active remedies and thus are better candidates for less intensive management strategies that focus on reducing mass discharge rates, stabilizing the plume, and protecting potential downgradient receptors. As a result, matrix diffusion is critical to understanding and quantifying how natural attenuation processes are contributing to concentration trends.

'''One Summary Tool''' (Tool 10) compiles metrics from the other tools into a “Remediation Transition Assessment Index” (RTAI) and provides additional guidance on conducting site-specific transition assessments. The RTAI is a simple metric with a value from 1 to 5, where higher values reflect greater persistence of contamination due to matrix diffusion and other site-specific factors. An RTAI value is assigned to each of the results from the different tools that have been completed by the user. An RTAI of 5 suggests that the site is a strong candidate for transitioning to MNA or enhanced attenuation approaches, while a site with an RTAI value of 1 is a poor candidate. The user can assign an overall RTAI for the site based on the preponderance of evidence after reviewing the RTAI values generated by each tool, or calculate a site RTAI based on simple averaging, weighting, or other methods.

Tool 10 also contains a flowchart and a checklist for performing site-specific transition assessments that start with evaluating relevant bright line criteria, such as (1) can the concentration goals be met at the point of compliance by MNA; and (2) is the remediation timeframe for MNA reasonable and/or similar to the timeframe if source remediation were used. This checklist ensures that the user has gathered all relevant information that would be needed to support a technically rigorous site-specific Transition Assessment.

The TA2Tool provides a framework for remedial decision makers to evaluate different types of sites, including those where active treatment (e.g., pump and treat) is in use, as well as sites where future active source zone remediation is being considered. It also includes a description of enhanced MNA alternatives for sites where MNA alone may not be sufficient to control risk. As shown in Figure 8, the tool can be used to answer specific questions that have a primarily quantitative basis or to provide focused qualitative information for researching specific topics. Users can engage with just the modules that might be pertinent to assessment of an individual site, or they can go through all the modules to perform a more thorough, step-by-step analysis of the relevant issues for their site.

==Summary==
Tools and approaches are available that can be adapted to determine when a site is ready to transition from active remedy to MNA. However, these tools and approaches have not been applied for this purpose at a significant number of sites, and at the present time, they are not generally accepted by regulatory authorities. There is an opportunity to establish and implement a logical and consistent framework that can be widely implemented to evaluate sites for transition from active remedy to MNA.

==References==
<references />

==See Also==

*[http://dx.doi.org/10.1007/978-1-4614-6922-3 Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. In: Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward. SERDP ESTCP Environmental Remediation Technology, vol 7. Springer, New York, NY. pgs. 459-508. doi: 10.1007/978-1-4614-6922-3]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436/(language)/eng-US Kram, Mark, and Widdowson, Mark, 2008. Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation. ESTCP ER-200436]

Monitored Natural Attenuation - Transitioning from Active Remedies

2026-05-07T17:07:57Z

Admin:

Many contaminated sites use active remedies such as pump-and-treat or ''in situ'' remediation to clean up impacted groundwater. Natural attenuation processes such as natural degradation or [[Dispersion and Diffusion | hydrodynamic dispersion]] also contribute to the cleanup. As remediation progresses, a point is often reached when the time required to reach the remedial objectives using the active remedy is roughly the same as the time required if the active remedy is shut down, and the continuing remediation of the site is provided by natural attenuation processes alone. From that point forward, the extra effort and expense of the active remedy provides no benefit over natural attenuation, and it may be appropriate to transition the site to [[Monitored Natural Attenuation (MNA)]]. This article deals with currently available tools and approaches that can be used to support a decision to transition from active remediation to MNA.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s)''':

*[[Alternative Endpoints]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Plume Response Modeling]]
*[[REMChlor - MD]]
*[[Source Zone Modeling]]

'''Contributor(s):''' *[[Dr. John Wilson]] and *[[Dr. David Adamson, P.E.]]

'''Key Resource(s)''':

*[//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies]<ref name="Newell2002">Newell, C.J., Rifai, H.S., Wilson, J.T., Connor, J.A., Aziz, J.A., Suarez, M.P., 2002. Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies. 28p. EPA/540/S-02/500. [//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Report.pdf]</ref>

*[https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS) Version 2.3.3]<ref name="Widdowson2008">Widdowson, M.A., Mendez, E., Chapelle, F.H., Casey, C.C., 2008. Natural Attenuation Software (NAS) Version 2.3.3. Virginia Polytechnic Institute and State University, the United States Geological Survey, and the United States Naval Facilities Engineering Command. NAS webpage: https://www.nas.cee.vt.edu/index.php See also: https://toxics.usgs.gov/highlights/nas_2.2.0/index.html</ref>

*[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf BIOCHLOR Natural Attenuation Support System, Version 2.2]<ref name="Aziz2002">Aziz, C.E., Newell, C.J. and Gonzales, J.R., 2002. BIOCHLOR Natural Attenuation Decision Support System Version 2.2 User’s Manual Addendum. Groundwater Services, Inc., Houston, Texas for the Air Force Center for Environmental Excellence.[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf Report.pdf] Available at: https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system</ref>

*[https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC User's Guide and Tool Website]<ref name="BioPIC2021">Danko, A., Adamson, D., Newell, C., Wilson, J., Wilson, B., Freedman, D., Lebrón, C., 2021. Quick BioPIC User’s Guide, ESTCP Project ER-201730. [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 Project Website]   [//www.enviro.wiki/images/c/c9/ER-201730_BioPIC_User%27s_Guide.pdf User’s Guide]</ref>

*[https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool Website]<ref name="TATA2024">Adamson, D.T., Newell, C.J., Hort, H.M, Wilson, J.T., 2024. TA2: The SERDP Transition Assessment Teaching Assistant. Strategic Environmental Research and Development Program (SERDP) Project ER20-1429. [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview Project Website]  [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Online Tool]</ref>

==Introduction==
Many active remedies are effective at treating higher concentrations of contaminants, but as the contaminant concentrations decrease, the rate of cleanup may slow before the site reaches the cleanup goal. At some sites, the rate of cleanup may slow until it is not significantly different from the rate of cleanup provided by the natural attenuation processes that occur at the site. At other sites, the concentration of contaminants in water produced by a pumping system is below the cleanup goal, but the concentration in monitoring wells in the source area are still above the goal. At some sites, active treatment has stopped further expansion of the plume toward a receptor, and concentrations are declining over time throughout the plume, but back diffusion is sustaining concentrations in the plume that are above the cleanup goal.

In 2013, a significant National Research Council (NRC) report noted that despite years of effort and considerable investment, many sites “will require long-term management that could extend for decades or longer”<ref name="NRC2013">National Research Council (NRC), 2013. Alternatives for Managing the Nation's Complex Contaminated Groundwater Sites. Committee on Future Options for Management in the Nation's Subsurface Remediation Effort, Water Science, Technology Board, Division on Earth and Life Studies, NRC. National Academies Press, 422 pages, ISBN 978-0-309-27874-4 [https://doi.org/10.17226/14668 doi: 10.17226/14668]. [//www.enviro.wiki/images/4/48/NRC2013.pdf Report.pdf]</ref>. The authors of the report discussed the need for developments that can aid in “transition from active remediation to more passive strategies and provide more cost-effective and protective long-term management of complex sites”<ref name="NRC2013" />.

The United States Environmental Protection Agency<ref> U.S. Environmental Protection Agency (USEPA), 1999. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. OSWER Directive 9200.4-17P. 39pp.[//www.enviro.wiki/images/a/aa/1999_USEPA-_Use_of_monitored_natural_attenuation_at_superfund.pdf Report.pdf]</ref> allows the use of [[Monitored Natural Attenuation (MNA) | monitored natural attenuation (MNA)]] to attain the cleanup goals when the site-specific remediation objectives can be attained within a time frame that is reasonable compared to that offered by other more active methods. Many CERCLA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Summary of the Comprehensive Environmental Response, Compensation, and Liability Act (Superfund) https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act</ref> and RCRA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Resource Conservation and Recovery Act (RCRA) Laws and Regulations https://www.epa.gov/rcra</ref> sites take advantage of this policy. An active remedy is typically used initially to treat high concentrations of contaminants followed by MNA to treat the lower concentrations that remain.

Unfortunately, there is no well-established approach to determine when it is appropriate to discontinue the active remedy. The NRC report<ref name="NRC2013" /> emphasized the use of more rigorous evaluations of existing data to support these efforts. This can include a quantitative assessment of the performance of active remedies (e.g., evidence of asymptotic performance) as well as documenting site conditions that may be contributing to these performance limitations. Importantly, it also identifies alternative approaches for managing the site, which could include MNA if the natural attenuation processes can meaningfully contribute to the achievement of site cleanup objectives.

This article reviews available tools and approaches to evaluate a transition to MNA. The tools and approaches depend on calculations of rate constants for natural attenuation with distance in flowing groundwater or rate constants for attenuation over time in individual monitoring wells.

==Background on Rate Constants==
[[File:Wilson1w2Fig1.png|thumb|400px| Figure 1. Attenuation of Trichloroethene (TCE) over time in a monitoring well at a site in Michigan. The concentration vs. time rate constant is 0.326 per year and largely represents the rate of the attenuation of the source of contaminants in the aquifer.]]
At sites where a transition to MNA is being considered, a key step is estimating attenuation rate constants and understanding how they are extracted from monitoring data. A general formula to describe the rate of a chemical reaction is:

:{|
|-
|'''Equation 1:'''|| ||<big>''r = k [C]''</big>'' m''
|-
|where:
|-
| ||''r''    ||is the rate of the reaction,
|-
| ||''k''||is the rate constant,
|-
| ||''C''||is the concentration of the chemical undergoing the reaction, and
|-
|the exponent    ||''m''||is the order of the reaction.
|}

When the rate of the reaction is proportional to the concentration of the contaminant, the value of ''m'' is 1. Therefore, the reaction is described as a first-order reaction, and the rate constant is described as a first-order rate constant. In Equation 1, concentration could go up or down, but ''k'' is a constant of proportionality for the rate of increase in concentration. The rate constant for attenuation is the negative of ''k''. If the rate of degradation is a fixed value regardless of concentration, the value of ''m'' is 0, and degradation is a zero-order process.

Natural attenuation of concentrations over time in monitoring wells is frequently described by a first-order rate constant, and natural biological or abiotic degradation of contaminants in flowing groundwater is typically also described by a first-order rate constant. Figure 1 provides an example of monitoring data that is described by a first-order rate constant.

The rate constant for attenuation over time in a single well and the rate constant for attenuation with distance along a flow path in an aquifer describe different situations that are controlled by different processes. ''Attenuation over time'' in a well is largely controlled by the rate of attenuation of the source of contamination in the aquifer. ''Attenuation with distance'' along a flow path includes attenuation of concentrations in the source along with contributions from biological degradation processes, abiotic degradation processes and hydrodynamic dispersion of the contaminated groundwater into clean groundwater<ref name="Newell2002" />.

The first-order rate constant for attenuation over time in a single well is commonly referred to as ''kpoint''<ref name="Newell2002" />. A time series chart in Microsoft EXCEL of the concentrations of a contaminant (''y'' axis) on the date of sampling (''x'' axis) can be used to extract a value for ''kpoint''. Select the data, then insert an exponential trend line and display the equation on the chart. The value of ''kpoint'' can also be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Note that the rate constants extracted in EXCEL are constants for the rate of change, not the rate of attenuation. Take the negative of the rate of change to get ''kpoint''. In the example in Figure 1, the unit of time on the X axis is years, and the value of ''kpoint'' is 0.326 per year.

Attenuation versus distance rate coefficients describe a bulk attenuation rate including both degradation and non-destructive processes such as dispersion. To extract values for rate constants for degradation alone, it is necessary to calibrate a groundwater flow and transport model to the data at the site. The model is calibrated with values for the hydrogeological properties of the aquifer (effective porosity, hydraulic gradient, hydraulic conductivity, hydrodynamic dispersion and the organic carbon content of the aquifer matrix). After the hydrogeological properties of the aquifer are fixed in the model, the most appropriate values for the degradation rate constants are the values that produce the best fit between the contaminant concentrations that are predicted by the model and the contaminant monitoring data at the site.

There are a number of reasons why natural attenuation processes are better described as first-order relationship instead of zero-order or some other order. The attenuation over time in a monitoring well tracks the attenuation over time of the source of contamination that sustains the plume<ref name="Newell2002" />. Sites go through a lifecycle, and attenuation of sources at mature sites is often a first-order process<ref>Sale, T., Newell, C., Stroo, H., Hinchee, R. and Johnson, P., 2008. Frequently Asked Questions Regarding Management of Chlorinated Solvents in Soils and Groundwater. Environmental Security Technology Certification Program (ESTCP, Project ER-200530), Department of Defense (DoD), Arlington, VA. [//www.enviro.wiki/images/c/cb/2008-Sale-Frequently_Asked_Questions_Regarding_Management_of_Chlorinated_Solvent_in_Soils_and_Groundwater.pdf Report.pdf]   Project Overview Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200530</ref>. If a chlorinated solvent site is mature, the contamination in the source area that was originally present as nonaqueous phase liquids (NAPL) has been redistributed and is now sequestered in a sorbed phase to aquifer solids or has diffused into non-transmissive portions of the aquifer matrix. Transfer of contaminants back into the more transmissive portions of the aquifer occurs by diffusion along a fixed path length, and the rate of transfer is controlled by the concentration of the contaminant remaining in the source material. Because the rate of transfer is proportional to the concentration of contaminant in the source material, attenuation of the source is a first-order process. These processes are discussed in more detail in [[Source Zone Modeling]].

Degradation processes are also usually first order. Abiotic reactions are almost always first order with respect to the concentration of the target chemical. Biodegradation reactions are zero order at high concentrations because the available enzymes are saturated with substrate, but are first order at lower concentrations that are typical of natural attenuation conditions in groundwater.

==Goals for MNA at Sites==

The information necessary to evaluate whether a site can be transitioned to MNA depends on the goal for MNA at the site. For many cleanup actions, the goal is to confine contamination within a waste management area where the contamination is left in place, in which case the cleanup goal applies to point-of-compliance wells that are outside the waste management area. For other cleanup actions, the entire site must be cleaned up, in which case the cleanup goal applies to any monitoring well on the site. The time by which the goal is to be attained is specified at CERCLA sites in the Record of Decision (the ROD). At RCRA sites, the time allowed for the cleanup to be attained may be specified in the permit.

==When the Goal Applies to Point-of-Compliance Wells==
Consider the following framework for evaluating a transition to MNA:

#Use a computer model to extract rate constants for the natural degradation of the contaminant that occurred in groundwater at the site before the active remedy was installed.
#Assume that the same rate constants will apply after the active remedy is no longer in operation. Note that this assumption may not be valid if the active remedy changes the geochemistry of the aquifer in the flow path to the point-of-compliance well.
#Calibrate a computer groundwater flow and transport model with the hydrogeological properties of the aquifer that pertain after the active remedy is no longer in operation, the concentration of contaminant after the active remedy, and the rate constants for natural degradation that are assumed to apply after the active remedy.
#Use the computer model to project the concentrations of the contaminant at the point-of-compliance well over time.
#If the concentrations at the point-of-compliance wells are predicted to be less than the goal before the specified date, that is a quantitative line of evidence in support of a transition to MNA.

There are several computer applications that are particularly useful to extract rate constants at a site from monitoring data that were collected before the active remedy was installed. For example, [https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS)]<ref name="Widdowson2008" />, [https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system BIOCHLOR]<ref name="Aziz2002" /> and [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC]<ref name="BioPIC2021" /> can be downloaded from the internet at no cost. Another recent example, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ TA2 Tool]<ref name="TATA2024" />, is discussed in detail later in this article.

[[File:Wilson1w2Fig2.png|thumb|left|400px| Figure 2. Example calibration of NAS to natural attenuation of total BTEX at a site (Figure 17 of NAS User’s Manual).]]
[[File:Wilson1w2Fig3.png|thumb|400px| Figure 3. The data input screen for BIOCHLOR before remediation with cis-1,2-Dichloroethene (DCE) and vinyl chloride (VC) source concentrations of 500 and 87 mg/L respectively at the source when the release first occurred.]]
[[File:Wilson1w2Fig4.png|thumb|left|400px| Figure 4. Output of the RUN CENTERLINE simulation in BIOCHLOR comparing the fit between the simulation and the field data for vinyl chloride before an active remedy was implemented]]
[[File:Wilson1w2Fig5.png|thumb|400px| Figure 5. Output of the RUN CENTERLINE simulation of conditions after an active remedy was implemented with a source concentration of 1.1 mg/L, projecting the concentration of vinyl chloride at a distance corresponding to a point-of-compliance well.]]
 
In NAS, the user inputs the hydrogeological data, the distance of wells along the flow path, and the concentrations of contaminants in the wells. The NAS application extracts rate constants and makes projections at the point-of-compliance. With NAS, it is possible to extract different rate constants for specific geochemical environments along the flow path.

Figure 2 provides an example calibration of NAS. The concentrations in the monitoring wells used to calibrate the model are compared to the simulation provided by the model. The values of the rate constants that are extracted from the field data are available in the “Output” tab under “Data and Results Table.”

Figure 3 depicts the input screen for BIOCHLOR. The user inputs the hydrogeological parameters, the first-order rate constants (1st Order Decay Coefficient), the distribution of the wells along the flow path, and the concentrations of contaminants in the wells. The model is set up for conditions that apply before the installation of the active remedy.

BIOCHLOR does not automatically fit the rate constants to the field data. Instead, the user examines the output of the model, and adjusts the rate constants until they provide the best fit between the model prediction and the monitoring data for wells at the site. This comparison is illustrated in Figure 4.

If the distance from the source well to the point-of-compliance well is set as the “Modeled Area Length” in Section 5 of the input screen, the “Run Centerline” output will provide the projected concentrations at that length. Assume the distance from the source well to the point-of-compliance well is 250 feet. The projected concentration in Figure 3 of vinyl chloride at a point-of-compliance well is 0.042 mg/L. If the regulatory goal were the federal drinking water maximum contaminant level (MCL)<ref>U. S. Environmental Protection Agency (USEPA), 2009. National Primary Drinking Water Regulations. EPA 816-F-09-004. [//www.enviro.wiki/images/a/ae/2009-USEPA-national_Primary_Drinking_Water_Regulations.pdf Report.pdf]</ref> of 0.002 mg/L, the projected concentration would exceed the goal, and MNA would not be adequate as a remedy.

For the sake of illustration, assume that an active remedy has been implemented, and the concentrations in the source well are 5.4 mg/L for DCE and 1.1 mg/L for vinyl chloride. To evaluate whether it is now appropriate to transition to MNA, BIOCHLOR could be calibrated with these concentrations to predict concentrations in the point-of-compliance well. (See Figure 5). In this example, the projected concentration at the point-of-compliance well does meet the goal.

Some active remedies are subject to rebound. If this is the case, the evaluation should begin at the point in time when it is clear that the trend in concentrations is downward.

A new EXCEL-based tool that does many of the same basic calculations as BIOCHLOR was recently developed as part of an update to the BioPIC<ref name="BioPIC2021" /> decision support software. This tool, the MNA Rate Constant Estimator, extracts rate constants from concentration versus distance data for a variety of different chemicals, including chlorinated ethenes (e.g., PCE and TCE), chlorinated ethanes (e.g., 1,1,1-TCA), and 1,4-dioxane. This tool was developed to run using current versions of EXCEL, whereas BIOCHLOR must be run using older versions of EXCEL that may be unavailable to many users. The MNA Rate Constant Estimator can be used to estimate degradation rate constants and/or predict plume footprints over time. Consequently, it is a useful addition to the BioPIC decision framework for understanding if MNA is appropriate remedy for a site, and it can also be helpful for estimating rate constants as part of a transition assessment.

==When the Goal Applies to All the Wells==
At sites where a concentration-based cleanup goal must be achieved at all wells, each well at the site is evaluated independently, and the rate constant that is applicable is the rate constant for attenuation over time in the well (''kpoint''). To evaluate whether the region in an aquifer that is sampled by a particular monitoring well is ready to transition to MNA, it is necessary to have monitoring data from a period of time before the remedy was implemented. This data is used to extract a value for ''kpoint'' in the aquifer under natural attenuation conditions. The evaluation of a transition to MNA will assume that the same value for ''kpoint'' will apply after the active remedy is complete. This assumption may not be appropriate if the active remedy caused a permanent change in the geochemistry of the aquifer. The assumption is usually appropriate for pump-and-treat remedies.

If ''kpoint'' before implementation of the active remedy describes the time course of natural attenuation after the active remedy is completed, the time required to attain the cleanup goal is predicted from the following:

:{|
| || || rowspan="2" |<big>''ln (''</big>
| style="border-style:solid; border-width: 0px 0px 1px 0px" |''Cgoal''
| rowspan="2" |<big>'')''</big>||                                                                       
|-
|'''Equation 2:'''        ||''<big>t =''    
| style="vertical-align:top;" |''Ccurrent''||
|-
| || || colspan="3" style="text-align:center; border-style:solid; border-width: 1px 0 0 0" |''<big>-k</big>point''||
|-
|where:
|-
| ||''Ccurrent''    || colspan="5" |is the current concentration after active remediation,
|-
| ||''Cgoal''|| colspan="5" |is the cleanup goal, and
|-
| ||''t''|| colspan="5" |is the time required for concentrations to attenuate from ''Ccurrent'' to ''Cgoal.''
|}

If the value of ''t'' estimated using Equation 2 is less than the difference between the current date and the date specified by the site stakeholders to attain the goal, that is evidence in support of a transition to MNA.

Some active remedies are subject to contaminant concentration rebound. If this is the case, the evaluation should use a value of ''C''current that is attained after the rebound has stabilized.

This approach depends on a robust value for ''kpoint''. It is worthwhile to do a sensitivity analysis on ''kpoint'' where the lower 95% or 90% confidence interval on ''kpoint'' is used in Equation 2 to see if that changes the outcome of the evaluation. The confidence intervals can be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Wilson<ref name="Wilson2011">Wilson, J.T. 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [//www.enviro.wiki/images/e/e3/Wilson-2011-An_Approach_for_Evaluating_Progress.pdf Report.pdf]</ref> provides detailed discussion of the use of linear regression to extract ''kpoint'' and confidence intervals on ''kpoint''. Wilson<ref name="Wilson2011" /> also discusses the use of goodness-of-fit tests to determine if there is evidence that a first-order rate equation is not the best fit to the monitoring data, and as a result the use of Equation 2 would not be appropriate. The TA2 Tool<ref name="TATA2024" /> also has the capability to calculate ''kpoint'' with a user-specified confidence interval, as described below.

At many sites, there is no specified date when the cleanup goal must be attained. In this situation, the monitoring data can be evaluated to determine if the current rate of attenuation under the active remedy is faster than the rate of natural attenuation before the active remedy was installed. The monitoring data can be examined to identify a time interval when the benefit of the active remedy has approached an asymptote. A second value of for ''kpoint'' can be extracted for that time interval. The two values for ''kpoint'' can be evaluated statistically to see if the current rate is faster at some appropriate level of confidence. If there is no statistical evidence that the rate of attenuation is faster, that determination can support a decision to transition to MNA.
[[File:Wilson1w2Fig6.png|thumb|400px| Figure 6. Example calibration of NAS to predict the reduced concentration at the source that is necessary to meet the remediation goal at a point-of-compliance well (Figure 19 of NAS User’s Manual).]]

==Extent of Treatment Necessary to Transition to MNA==
There are several computer applications that can predict the extent of treatment that must be achieved by the active remedy before it is worthwhile to evaluate the site for transition to MNA. For example, based on the distribution of contamination along the flow path, the NAS application will automatically predict a reduced concentration at the source well that will bring concentrations to the goal in the point-of-compliance well (Figure 6). A table that opens under the “DOS/TOS” tab provides the “Time of Equilibration” required to meet the goal at the reduced concentration. Modules in NAS allow the user to evaluate the effect of various pump-and-treat and source removal scenarios on the time required to attain the goal at the point-of-compliance well.

The [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018">Falta, R.W., Farhat, S.K., Newell, C.J. and Lynch, K., 2018. A Practical Approach for Modeling Matrix Diffusion Effects in REMChlor. SERDP/ESTCP Project ER-201426 [//www.enviro.wiki/images/0/0b/2018-Falta-REMChlor_Modeling_Matrix_Diffusion_Effects.pdf Report.pdf]   Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426</ref> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012">Falta, R.W., Ahsanuzzaman, A.N., Stacy, M.B., Earle, R.C. and Wilson, J.T., 2012. Remediation Evaluation Model for Fuel Hydrocarbons (REMFuel). Users Manual Version 1.0. U.S. Environmental Protection Agency. EPA/600/R-12/028. [//www.enviro.wiki/images/6/67/2012-Falta-REMFuel_Remediation_Evaluation-Model_for_Fuel_hydrocarbons_users_manual.PDF Report.pdf]   Website: https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel</ref> models are flexible screening tools that allow a simultaneous evaluation of the extent of treatment provided by (1) source removal, (2) ''in situ'' remediation of the contaminated groundwater, or (3) natural attenuation processes in three discrete intervals along the flow path and three discrete time periods. Both [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018" /> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012" /> can be downloaded from the internet at no cost. Liang ''et al.''<ref>Liang, H., Falta, R.W., Newell, C.J., Farhat, S.K., Rao, P.S. and Basu, N., 2010. Decision & Management Tools for DNAPL Sites: Optimization of Chlorinated Solvent Source and Plume Remediation Considering Uncertainty. SERDP/ESTCP Project ER-200704. [//www.enviro.wiki/images/c/ce/2010-Liang-Decision_and_Management_Tools_for_DNAPL_sites-ER-200704-FR.pdf Report.pdf]   Project Overview Website: https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200704/(language)/eng-US</ref> provide a modeling program that uses Monte Carlo simulations to evaluate the effects of the uncertainties in the modeling parameters on the predictions of REMChlor-MD.

==The Transition Assessment Teaching Assistant (TA2) Tool==
[[File:Wilson1w2Fig7.png|thumb|500px| Figure 7. Home Page for TA2 Tool. Users can click on buttons to access various modules that are designed to answer specific questions or research relevant topics.]]
[[File:Wilson1w2Fig8.png|thumb|500px| Figure 8. Example of an asymptote analysis using concentration versus time data in Tool 1 of the TA2 Tool. The source attenuation rate and corresponding remediation timeframe can be estimated for different monitoring periods.]]
A learning and decision-making tool was recently released as part of [https://serdp-estcp.mil/ Strategic Environmental Development and Research Program (SERDP)] Project [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview ER-201429] to help stakeholders gather information for the purposes of a site-specific transition assessment. This free software, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool]<ref name="TATA2024" />, was developed using the elements identified in the 2013 NRC report<ref name="NRC2013" /> as the critical learning objectives for end users.

The Tool is a web-based app that includes a collection of individual modules designed to answer specific questions or research relevant topics (Figure 7). The Tool has been developed as an R Shiny app (version 1.8.0)<ref> Chang, W., Cheng, J., Allaire, J., Sievert, C., Schloerke, B., Xie, Y., Allen, J., McPherson, J., Dipert, A., Borges, B., 2023. shiny: Web Application Framework for R. R package version 1.8.0, https://github.com/rstudio/shiny, https://shiny.posit.co/</ref>, which is an interactive platform using R programming to perform all quantitative functions. The user can then view the results in a simple interface that easily accommodates plots, charts, and various mapping features in a Web browser. The Tool is free and does not require the user to install R software.

The modules within the TA2 Tool include:

'''Five Quantitative Tools''' that focus on assessing asymptotic groundwater concentrations from monitoring data, evaluating plume stability, estimating remediation timeframes after a hypothetical source removal project, forecasting remediation performance if a technology is applied in the field, or projecting concentrations at downgradient points of compliance.

For example, Tool 1 in the TA2 Tool uses concentration versus time data from monitoring wells to estimate attenuation rate constants (''kpoint'') and evaluate if asymptotic conditions are present at particular locations or across the site. This helps to assess whether performance has plateaued at wells where a pump-and-treat system or other active treatment is in place. The user has the option to choose a “change point” within the monitoring record to determine if the attenuation rate has changed over time (e.g., once most of the accessible mass has been removed) (Figure 8). The user can either use visual interpretation to manually select the date when this apparent change occurred or have the date selected automatically using a binary segmentation protocol that is incorporated into the tool. The tool will calculate a rate for both the early period and a rate for the later period (after the change point), and then go through five different lines of evidence for asymptotic behavior (e.g., are the two rates of attenuation significantly different?). The user can then use the collective results as a technical justification demonstrating that the performance of the active remedy has plateaued as the first step in the transition assessment. The tool will also estimate the time to reach a user-specified cleanup goal if the overall attenuation rate (or the attenuation rate in the later period) were to continue.

Another module (Tool 5) focuses on evaluating sites where the concentration goal applies at a downgradient point of compliance, which is a key criterion for sites where MNA is being used as part of a risk-based strategy. The tool includes several different options to estimate a site-specific attenuation rate constant, including data from the pre-remediation period when natural attenuation processes were the sole means for reducing concentrations. Attenuation rate constants are then used to project the concentration versus distance from the contaminant source. Based on the predicted concentration at the downgradient point of compliance, the user can then see if the natural assimilative capacity along the aquifer flow path is sufficient to achieve the concentration goal in the absence of active treatment. For example, in the tab labeled “Use Pre-Remediation Rate Constant”, the logarithms of the concentrations from the period before active treatment began are plotted against the distance from the source well. The slope of the regression line is the rate constant for natural attenuation (including the contributions of degradation and dispersion). This rate constant can then be used to project the concentration moving downgradient from the well of concern after the end of active treatment. Similar approaches are provided within Tool 5 for using rate constants estimated from lab-based testing or derived from post-remediation data (after steady state has been reestablished).

'''Four Qualitative Tools''' provide information on matrix diffusion, enhanced attenuation options, geologic heterogeneity, and related research on transition assessments. Many of these modules are based on the current understanding of the role of matrix diffusion in influencing long-term concentration trends and remedial performance at contaminated groundwater sites. This includes summaries of different modeling options for better quantifying the effects of matrix diffusion. Sites impacted by matrix diffusion are generally challenging to treat using active remedies and thus are better candidates for less intensive management strategies that focus on reducing mass discharge rates, stabilizing the plume, and protecting potential downgradient receptors. As a result, matrix diffusion is critical to understanding and quantifying how natural attenuation processes are contributing to concentration trends.

'''One Summary Tool''' (Tool 10) compiles metrics from the other tools into a “Remediation Transition Assessment Index” (RTAI) and provides additional guidance on conducting site-specific transition assessments. The RTAI is a simple metric with a value from 1 to 5, where higher values reflect greater persistence of contamination due to matrix diffusion and other site-specific factors. An RTAI value is assigned to each of the results from the different tools that have been completed by the user. An RTAI of 5 suggests that the site is a strong candidate for transitioning to MNA or enhanced attenuation approaches, while a site with an RTAI value of 1 is a poor candidate. The user can assign an overall RTAI for the site based on the preponderance of evidence after reviewing the RTAI values generated by each tool, or calculate a site RTAI based on simple averaging, weighting, or other methods.

Tool 10 also contains a flowchart and a checklist for performing site-specific transition assessments that start with evaluating relevant bright line criteria, such as (1) can the concentration goals be met at the point of compliance by MNA; and (2) is the remediation timeframe for MNA reasonable and/or similar to the timeframe if source remediation were used. This checklist ensures that the user has gathered all relevant information that would be needed to support a technically rigorous site-specific Transition Assessment.

The TA2Tool provides a framework for remedial decision makers to evaluate different types of sites, including those where active treatment (e.g., pump and treat) is in use, as well as sites where future active source zone remediation is being considered. It also includes a description of enhanced MNA alternatives for sites where MNA alone may not be sufficient to control risk. As shown in Figure 8, the tool can be used to answer specific questions that have a primarily quantitative basis or to provide focused qualitative information for researching specific topics. Users can engage with just the modules that might be pertinent to assessment of an individual site, or they can go through all the modules to perform a more thorough, step-by-step analysis of the relevant issues for their site.

==Summary==
Tools and approaches are available that can be adapted to determine when a site is ready to transition from active remedy to MNA. However, these tools and approaches have not been applied for this purpose at a significant number of sites, and at the present time, they are not generally accepted by regulatory authorities. There is an opportunity to establish and implement a logical and consistent framework that can be widely implemented to evaluate sites for transition from active remedy to MNA.

==References==
<references />

==See Also==

*[http://dx.doi.org/10.1007/978-1-4614-6922-3 Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. In: Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward. SERDP ESTCP Environmental Remediation Technology, vol 7. Springer, New York, NY. pgs. 459-508. doi: 10.1007/978-1-4614-6922-3]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436/(language)/eng-US Kram, Mark, and Widdowson, Mark, 2008. Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation. ESTCP ER-200436]

Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions

2026-05-07T17:07:21Z

Admin:

The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective ''in situ'' approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Chlorinated Solvents]]
*[[Matrix Diffusion]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| Monitored Natural Attenuation of Chlorinated Solvents]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
*[[REMChlor - MD]]

'''Contributor(s):''' [[Dani Tran]], [[Dr. Charles Schaefer]], and [[Dr. Charles Werth]]

'''Key Resource(s):'''
*Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils<ref name="SchaeferEtAl2025"/>

==Introduction==
Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.

For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as [[REMChlor - MD]]<ref>Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]  [[Media: FaltaWang2017.pdf | Open Access Manuscript]]</ref><ref>Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. [https://doi.org/10.1016/j.jconhyd.2022.103986 doi: 10.1016/j.jconhyd.2022.103986]  [[Media: KulkarniEtAl2022.pdf | Open Access Manuscript]]</ref> to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.

==Recommended Approach==
[[File: TranFig1.png | thumb | 500 px | Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions. Circles are data from Schaefer ''et al.'', 2021<ref>Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2021. Abiotic dechlorination in the presence of ferrous minerals. Journal of Contaminant Hydrology, 241, 103839. [https://doi.org/10.1016/j.jconhyd.2021.103839 doi: 10.1016/j.jconhyd.2021.103839]  [[Media: SchaeferEtAl2021.pdf | Open Access Manuscript]]</ref>, filled squares from Schaefer ''et al.'', 2018<ref name="SchaeferEtAl2018"/>, and Schaefer ''et al.'', 2017<ref>Schaefer, C.E., Ho., Gurr, C., Berns, E., Werth, C., 2017. Abiotic dechlorination of chlorinated ethenes in natural clayey soils: impacts of mineralogy and temperature. Journal of Contaminant Hydrology, 206, pp. 10-17. [https://doi.org/10.1016/j.jconhyd.2017.09.007 doi: 10.1016/j.jconhyd.2017.09.007]  [[Media: SchaeferEtAl2017.pdf | Open Access Manuscript]]</ref>, and open squares from Schaefer ''et al.'', 2025<ref name="SchaeferEtAl2025"/>. ]]
[[File: TranFig2.png | thumb | 600 px | Figure 2: Flowchart diagram of field screening procedures]]
The recommended approach builds upon the methodology and findings of a recent study<ref name="SchaeferEtAl2025">Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. [https://doi.org/10.1111/gwmr.12709 doi: 10.1111/gwmr.12709]</ref>, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:
#Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
#Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
#Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
#Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)r) is estimated as shown in Equation 1:

::'''Equation 1:'''       <big>''Fe(II)r = DA + XRDpyr - XRDbiotite''</big>

where ''DA'' is the ferrous content from the dilute acid (1% HCl) extraction, ''XRDpyr'' is the pyrite content from XRD analysis, and ''XRDbiotite'' is the biotite content from XRD analysis<ref name="SchaeferEtAl2025"/>.

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1<ref name="SchaeferEtAl2018">Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. [https://doi.org/10.1021/acs.est.8b04108 doi: 10.1021/acs.est.8b04108]</ref><ref>Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. [https://doi.org/10.1016/j.jconhyd.2021.103889 doi: 10.1016/j.jconhyd.2021]  [[Media: BordenCha2021.pdf | Open Access Manuscript]]</ref>. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s ''r'' = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

If Fe(II)r is ≥ 100 mg/kg, a first-order dechlorination rate constant can be estimated and subsequently used within a contaminant fate and transport model. However, if acetylene is detected in the clay, even with Fe(II)r less than 100 mg/kg, then bench-scale testing using methods similar to those described in a recent study<ref name="SchaeferEtAl2025"/> is recommended, as such results would likely be inconsistent with those shown in Figure 1, suggesting some other mechanism might be involved, or that the system mineralogy might be more complex than anticipated. Even if Fe(II)r ≥ 100 mg/kg, confirmatory bench-scale testing may be conducted for additional verification and to refine estimation of the abiotic dechlorination rate constant.

==Summary and Recommendations==
The approach outlined above is intended to serve as a generalized guide for practitioners and site managers to cost-effectively determine the extent to which beneficial abiotic reductive dechlorination reactions are likely occurring in low permeability (e.g., clayey) zones. This approach may be contraindicated if co-contaminants are present. It is currently unclear whether other classes of potentially reactive chemicals, such as trinitrotoluene (TNT) or chlorinated ethanes, could interact competitively with PCE and TCE.

In addition, it remains unclear how other classes of compounds such as per- and polyfluoroalkyl substances (PFAS) may interact or sorb with ferrous minerals and potentially inhibit abiotic dechlorination reactions. Coupling these recommended activities with conventional site investigation tasks would provide an opportunity to perform many of the up-front screening activities with minimal additional project costs. It is important to note that the guidance proposed herein pertains to particularly low permeability media. Sites with complex or varying lithology, where the mineralogy and/or redox conditions may vary, might require evaluation of multiple samples to provide appropriate site-wide information.
 

==References==
<references />

==See Also==
*[https://serdp-estcp.mil/projects/details/a7e3f7b5-ed82-4591-adaa-6196ff33dd60 ESTCP Project ER20-5031 – In Situ Verification and Quantification of Naturally Occurring Dechlorination Rates in Clays: Demonstrating Processes that Mitigate Back-Diffusion and Plume Persistence]

Matrix Diffusion

2026-05-07T17:07:00Z

Admin:

Matrix diffusion occurs when dissolved groundwater contaminants present in zones with greater hydraulic conductivity (''K'') are transported by [[wikipedia:Molecular diffusion | molecular diffusion]] into lower ''K'' zones, slowing the rate of contaminant migration in the high ''K'' zone. However, once the contaminant source is eliminated, contaminants diffuse back out of low ''K'' zones, slowing the cleanup rate in the high ''K'' zone. In some cases, matrix diffusion can maintain contaminant concentrations in more permeable zones at greater than target cleanup goals for decades or potentially even centuries after the primary sources have been addressed<ref name="Chapman2005">Chapman, S.W. and Parker, B.L., 2005. Plume persistence due to aquitard back diffusion following dense nonaqueous phase liquid source removal or isolation. Water Resources Research, 41(12), Report W12411. [https://doi.org/10.1029/2005WR004224 DOI: 10.1029/2005WR004224] [//www.enviro.wiki/images/a/a0/Chapman2005.pdf Report.pdf] Free access article from [https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2005WR004224 American Geophysical Union]</ref>. Field and laboratory results have illustrated the importance of this process. Analytical and numerical modeling tools are available for evaluating matrix diffusion.
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'''Related Article(s):'''
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Groundwater Flow and Solute Transport]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Plume Response Modeling]]
*[[REMChlor - MD]]

'''Contributor(s):''' [[Dr. Charles Newell, P.E.|Dr. Charles Newell]] and [[Dr. Robert Borden, P.E.|Dr. Robert Borden]]

'''Key Resource(s):'''

*[https://www.serdp-estcp.org/content/download/23838/240653/file/ER-1740 Management of Contaminants Stored in Low Permeability Zones – A State of the Science Review]<ref name="Sale2013">Sale, T., Parker, B.L., Newell, C.J. and Devlin, J.F., 2013. Management of Contaminants Stored in Low Permeability Zones – A State of the Science Review. Strategic Environmental Research and Development Program (SERDP) Project ER-1740. [//www.enviro.wiki/images/2/23/Sale2013ER-1740.pdf Report.pdf] Website: [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-1740 ER-1740]</ref>

==Introduction==
[[File:NewellMatrixDiffFig1.PNG | thumb |500px| Figure 1. Diffusion of a dissolved solute (chlorinated solvent) into lower ''K'' zones during loading period, followed by diffusion back out into higher ''K'' zones once the source is removed <ref name="Sale2007">Sale, T.C., Illangasekare, T.H., Zimbron, J., Rodriguez, D., Wilking, B., and Marinelli, F., 2007. AFCEE Source Zone Initiative. Air Force Center for Environmental Excellence, Brooks City-Base, San Antonio, TX. [https://www.enviro.wiki/images/0/08/AFCEE-2007-Sale.pdf Report.pdf]</ref>]]
Matrix Diffusion can have major impacts on solute migration in groundwater and on cleanup time following source removal. As a groundwater plume advances downgradient, dissolved contaminants are transported by [[Wikipedia: Molecular diffusion | molecular diffusion]] from zones with larger hydraulic conductivity (''K'') into lower ''K'' zones, slowing the rate of contaminant migration in the high ''K'' zone. However, once the contaminant source is eliminated, contaminants diffuse out of low ''K'' zones, slowing the cleanup rate in the high ''K'' zone (Figure 1). This process, termed ‘back diffusion’, can greatly extend cleanup times.

The impacts of back diffusion on aquifer cleanup have been examined in controlled laboratory experiments by several investigators<ref name="Doner2008">Doner, L.A., 2008. Tools to resolve water quality benefits of upgradient contaminant flux reduction. Master’s Thesis, Department of Civil and Environmental Engineering, Colorado State University.</ref><ref name="Yang2015">Yang, M., Annable, M.D. and Jawitz, J.W., 2015. Back Diffusion from Thin Low Permeability Zones. Environmental Science and Technology, 49(1), pp. 415-422. [https://doi.org/10.1021/es5045634 DOI: 10.1021/es5045634] Free download available from: [https://www.researchgate.net/publication/269189924_Back_Diffusion_from_Thin_Low_Permeability_Zones ResearchGate]</ref><ref name="Yang2016">Yang, M., Annable, M.D. and Jawitz, J.W., 2016. Solute source depletion control of forward and back diffusion through low-permeability zones. Journal of Contaminant Hydrology, 193, pp. 54-62. [https://doi.org/10.1016/j.jconhyd.2016.09.004 DOI: 10.1016/j.jconhyd.2016.09.004] Free download available from: [https://www.researchgate.net/profile/Minjune_Yang/publication/308004091_Solute_source_depletion_control_of_forward_and_back_diffusion_through_low-permeability_zones/links/5a2ed2c44585155b6179f489/Solute-source-depletion-control-of-forward-and-back-diffusion-through-low-permeability-zones.pdf ResearchGate]</ref><ref name="Tatti2018">Tatti, F., Papini, M.P., Sappa, G., Raboni, M., Arjmand, F., and Viotti, P., 2018. Contaminant back-diffusion from low-permeability layers as affected by groundwater velocity: A laboratory investigation by box model and image analysis. Science of The Total Environment, 622, pp. 164-171. [https://doi.org/10.1016/j.scitotenv.2017.11.347 DOI: 10.1016/j.scitotenv.2017.11.347]</ref>. The video in Figure 2 shows the results of a 122-day tracer test in a laboratory flow cell (sand tank)<ref name="Doner2008" />. The flow cell contained several clay zones (''K'' = 10-8 cm/s) surrounded by sand (''K'' = 0.02 cm/s). During the loading period, water containing a green fluorescent tracer migrated from left to right with the water flowing through the flow cell, while also diffusing into the clay. After 22 days, the fluorescent tracer is eliminated from the feed, and most of the green tracer is quickly flushed from the tank’s sandy zones. However, small amounts of tracer continue to diffuse out of the clay layers for over 100 days. This illustrates how back diffusion of contaminants out of low ''K'' zones can maintain low contaminant concentrations long after the contaminant source as been eliminated.

[[File: GreenTank.mp4 | thumb |500px| Figure 2. Video of dye tank simulation of matrix diffusion]]
In some cases, matrix diffusion can maintain contaminant concentrations in more permeable zones above target cleanup goals for decades or potentially even centuries after the primary sources have been addressed. At a site impacted by [[Wikipedia: Dense non-aqueous phase liquid | Dense Non-Aqueous Phase Liquids (DNAPL)]], [[Chlorinated Solvents | trichloroethene (TCE)]] concentrations in downgradient wells declined by roughly an order-of-magnitude (OoM) when the upgradient source area was isolated with sheet piling. However, after this initial decline, TCE concentrations appeared to plateau or decline more slowly, consistent with back diffusion from an underlying aquitard. Numerical simulations indicated that back diffusion would cause TCE concentrations in downgradient wells at the site to remain above target cleanup levels for centuries<ref name="Chapman2005" />.

One other implication of matrix diffusion is that plume migration is attenuated by the loss of contaminants into low permeability zones, leading to slower plume migration compared to a case where no matrix diffusion occurs. This phenomena was observed as far back as 1985 when Sudicky et al. observed that “A second consequence of the solute-storage effect offered by transverse diffusion into low-permeability layers is a rate of migration of the frontal portion of a contaminant in the permeable layers that is less than the groundwater velocity.”<ref name="Sudicky1985"> Sudicky, E.A., Gillham, R.W., and Frind, E.O., 1985. Experimental Investigation of Solute Transport in Stratified Porous Media: 1. The Nonreactive Case. Water Resources Research, 21(7), pp. 1035-1041. [https://doi.org/10.1029/WR021i007p01035 DOI: 10.1029/WR021i007p01035]</ref> In cases where there is an attenuating source, matrix diffusion can also reduce the peak concentrations observed in downgradient monitoring wells. The attenuation caused by matrix diffusion may be particularly important for implementing [[Monitored Natural Attenuation (MNA)]] for contaminants that do not completely degrade, such as [[Metal and Metalloid Contaminants | heavy metals]] and [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]].

==SERPD/ESTCP Research==

The SERDP/ESTCP programs have funded several projects focusing on how matrix diffusion can impede progress towards reaching site closure, including:
{|
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|
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-1740 SERDP Management of Contaminants Stored in Low Permeability Zones, A State-of-the-Science Review] <ref name="Sale2013" />
|-
|
*[https://www.serdp-estcp.org/Tools-and-Training/Environmental-Restoration/Groundwater-Plume-Treatment/Matrix-Diffusion-Tool-Kit ESTCP Matrix Diffusion Toolkit]<ref name="Farhat2012">Farhat, S.K., Newell, C.J., Seyedabbasi, M.A., McDade, J.M., Mahler, N.T., Sale, T.C., Dandy, D.S. and Wahlberg, J.J., 2012. Matrix Diffusion Toolkit. Environmental Security Technology Certification Program (ESTCP) Project ER-201126. [//www.enviro.wiki/images/3/3b/Farhat2012ER-201126UsersManual.pdf User’s Manual.pdf] Website: [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201126 ER-201126]</ref>
|-
|
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200530 ESTCP Decision Guide]<ref>Sale, T. and Newell, C., 2011. A Guide for Selecting Remedies for Subsurface Releases of Chlorinated Solvents. Environmental Security Technology Certification Program (ESTCP) Project ER-200530. [//www.enviro.wiki/images/6/6d/Sale2011ER-200530.pdf Report.pdf] Website: [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200530 ER-200530]</ref>
|-
|
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426 ESTCP REMChlor-MD: the USEPA’s REMChlor model with a new matrix diffusion term for the plume]<ref name="Farhat2018">Farhat, S. K., Newell, C. J., Falta, R. W., and Lynch, K., 2018. A Practical Approach for Modeling Matrix Diffusion Effects in REMChlor. Environmental Security Technology Certification Program (ESTCP) Project ER-201426. [https://enviro.wiki/images/0/0b/2018-Falta-REMChlor_Modeling_Matrix_Diffusion_Effects.pdf User’s Manual.pdf] Website: [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426 ER-201426]</ref>
|}

==Transport Modeling==
Several different modeling approaches have been developed to simulate the diffusive transport of dissolved solutes into and out of lower ''K'' zones<ref>Falta, R.W., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 DOI: 10.1016/j.jconhyd.2016.12.007]</ref><ref>Muskus, N. and Falta, R.W., 2018. Semi-analytical method for matrix diffusion in heterogeneous and fractured systems with parent-daughter reactions. Journal of Contaminant Hydrology, 218, pp. 94-109. [https://doi.org/10.1016/j.jconhyd.2018.10.002 DOI: 10.1016/j.jconhyd.2018.10.002]</ref>. The [https://www.serdp-estcp.org/Tools-and-Training/Environmental-Restoration/Groundwater-Plume-Treatment/Matrix-Diffusion-Tool-Kit Matrix Diffusion Toolkit]<ref name="Farhat2012" /> is a Microsoft Excel based tool for simulating forward and back diffusion using two different analytical models<ref name="Parker1994">Parker, B.L., Gillham, R.W., and Cherry, J.A., 1994. Diffusive Disappearance of Immiscible Phase Organic Liquids in Fractured Geologic Media. Groundwater, 32(5), pp. 805-820. [https://doi.org/10.1111/j.1745-6584.1994.tb00922.x DOI: 10.1111/j.1745-6584.1994.tb00922.x]</ref><ref>Sale, T.C., Zimbron, J.A., and Dandy, D.S., 2008. Effects of reduced contaminant loading on downgradient water quality in an idealized two-layer granular porous media. Journal of Contaminant Hydrology, 102(1), pp. 72-85. [https://doi.org/10.1016/j.jconhyd.2008.08.002 DOI: 10.1016/j.jconhyd.2008.08.002]</ref>. Numerical models including [https://en.wikipedia.org/wiki/MODFLOW MODFLOW]/[https://xmswiki.com/wiki/GMS:MT3DMS MT3DMS]<ref name="Zheng1999">Zheng, C. and Wang, P.P., 1999. MT3DMS: A Modular Three-Dimensional Multispecies Transport Model for Simulation of Advection, Dispersion, and Chemical Reactions of Contaminants in Groundwater Systems; Documentation and User’s Guide. Contract Report SERDP-99-1 U.S. Army Engineer Research and Development Center, Vicksburg, MS. [https://www.enviro.wiki/images/3/32/Mt3dmanual.pdf User’s Guide.pdf] [https://xmswiki.com/wiki/GMS:MT3DMS MT3DMS website]</ref> have been shown to be effective in simulating back diffusion processes and can accurately predict concentration changes over 3 orders-of-magnitude in heterogeneous sand tank experiments<ref>Chapman, S.W., Parker, B.L., Sale, T.C., Doner, L.A., 2012. Testing high resolution numerical models for analysis of contaminant storage and release from low permeability zones. Journal of Contaminant Hydrology, 136, pp. 106-116. [https://doi.org/10.1016/j.jconhyd.2012.04.006 DOI: 10.1016/j.jconhyd.2012.04.006]</ref>. However, numerical models require a fine vertical discretization with short time steps to accurately simulate back diffusion, greatly increasing computation times<ref>Farhat, S.K., Adamson, D.T., Gavaskar, A.R., Lee, S.A., Falta, R.W. and Newell, C.J., 2020. Vertical Discretization Impact in Numerical Modeling of Matrix Diffusion in Contaminated Groundwater. Groundwater Monitoring and Remediation, 40(2), pp. 52-64. [https://doi.org/10.1111/gwmr.12373 DOI: 10.1111/gwmr.12373]</ref>. These issues can be addressed by incorporating a local 1-D model domain within a general 3D numerical model<ref>Carey, G.R., Chapman, S.W., Parker, B.L. and McGregor, R., 2015. Application of an Adapted Version of MT3DMS for Modeling Back‐Diffusion Remediation Timeframes. Remediation, 25(4), pp. 55-79. [https://doi.org/10.1002/rem.21440 DOI: 10.1002/rem.21440]</ref>.

The [[REMChlor - MD]] toolkit is capable of simulating matrix diffusion in groundwater contaminant plumes by using a semi-analytical method for estimating mass transfer between high and low permeability zones that provides computationally accurate predictions, with much shorter run times than traditional fine grid numerical models<ref name="Farhat2018" />.

==Impacts on Breakthrough Curves==
[[File:ADRFig3.png | thumb| left |400px| Figure 3. Comparison of tracer breakthrough (upper graph) and cleanup curves (lower graph) from advection-dispersion based (gray lines) and advection-diffusion based (black lines) solute transport<ref name="ITRC2011">Interstate Technology and Regulatory Council (ITRC), 2011. Integrated DNAPL Site Strategy (IDSS-1), Integrated DNAPL Site Strategy Team, ITRC, Washington, DC. [https://www.enviro.wiki/images/d/d9/ITRC-2011-Integrated_DNAPL.pdf Report.pdf] Free download from: [https://itrcweb.org/GuidanceDocuments/IntegratedDNAPLStrategy_IDSSDoc/IDSS-1.pdf ITRC]</ref>.]]
The impacts of matrix diffusion on the initial breakthrough of the solute plume and on later cleanup are illustrated in Figure 3<ref name="ITRC2011" />. Using a traditional advection-dispersion model, the breakthrough curve for a pulse tracer injection appears as a bell-shaped ([[wikipedia:Gaussian function |Gaussian]]) curve (gray line on the right side of the upper graph) where the peak arrival time corresponds to the average groundwater velocity. Using an advection-diffusion approach, the breakthrough curve for a pulse injection is asymmetric (solid black line) with the peak tracer concentration arriving earlier than would be expected based on the average groundwater velocity, but with a long extended tail to the flushout curve.

The lower graph shows the predicted cleanup concentration profiles following complete elimination of a source area. The advection-dispersion model (gray line) predicts a clean-water front arriving at a time corresponding to the average groundwater velocity. The advection-diffusion model (black line) predicts that concentrations will start to decline more rapidly than expected (based on the average groundwater velocity) as clean water rapidly migrates through the highest-permeability strata. However, low but significant contaminant concentrations linger much longer (tailing) due to diffusive contaminant mass exchange between zones of high and low permeability. A similar response to source remediation is seen in models such as the sand tank experiment shown in Figure 2, and also in field observations of plume contaminant concentrations in heterogeneous aquifers.

 

==References==

<references />

==See Also==

*[https://www.youtube.com/watch?v=iLwsIjkVybU Matrix Diffusion Movie]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-1737 Impact of Clay-DNAPL Interactions on Transport and Storage of Chlorinated Solvents in Low Permeability Zones]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200320 Prediction of Groundwater Quality Improvement Down-Gradient of ''In Situ'' Permeable Treatment Barriers and Fully Remediated Source Zones]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201032 Determining Source Attenuation History to Support Closure by Natural Attenuation]
*[https://www.coursera.org/learn/natural-attenuation-of-groundwater-contaminants/lecture/2R7yh/matrix-diffusion-principles Coursera Matrix Diffusion Online Lecture]

Monitored Natural Attenuation - Transitioning from Active Remedies

2026-05-07T17:06:38Z

Admin:

Many contaminated sites use active remedies such as pump-and-treat or ''in situ'' remediation to clean up impacted groundwater. Natural attenuation processes such as natural degradation or [[Dispersion and Diffusion | hydrodynamic dispersion]] also contribute to the cleanup. As remediation progresses, a point is often reached when the time required to reach the remedial objectives using the active remedy is roughly the same as the time required if the active remedy is shut down, and the continuing remediation of the site is provided by natural attenuation processes alone. From that point forward, the extra effort and expense of the active remedy provides no benefit over natural attenuation, and it may be appropriate to transition the site to [[Monitored Natural Attenuation (MNA)]]. This article deals with currently available tools and approaches that can be used to support a decision to transition from active remediation to MNA.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s)''':

*[[Alternative Endpoints]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Plume Response Modeling]]
*[[REMChlor - MD]]
*[[Source Zone Modeling]]

'''Contributor(s):'''

*[[Dr. John Wilson]]
*[[Dr. David Adamson, P.E.]]

'''Key Resource(s)''':

*[//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies]<ref name="Newell2002">Newell, C.J., Rifai, H.S., Wilson, J.T., Connor, J.A., Aziz, J.A., Suarez, M.P., 2002. Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies. 28p. EPA/540/S-02/500. [//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Report.pdf]</ref>

*[https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS) Version 2.3.3]<ref name="Widdowson2008">Widdowson, M.A., Mendez, E., Chapelle, F.H., Casey, C.C., 2008. Natural Attenuation Software (NAS) Version 2.3.3. Virginia Polytechnic Institute and State University, the United States Geological Survey, and the United States Naval Facilities Engineering Command. NAS webpage: https://www.nas.cee.vt.edu/index.php See also: https://toxics.usgs.gov/highlights/nas_2.2.0/index.html</ref>

*[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf BIOCHLOR Natural Attenuation Support System, Version 2.2]<ref name="Aziz2002">Aziz, C.E., Newell, C.J. and Gonzales, J.R., 2002. BIOCHLOR Natural Attenuation Decision Support System Version 2.2 User’s Manual Addendum. Groundwater Services, Inc., Houston, Texas for the Air Force Center for Environmental Excellence.[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf Report.pdf] Available at: https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system</ref>

*[https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC User's Guide and Tool Website]<ref name="BioPIC2021">Danko, A., Adamson, D., Newell, C., Wilson, J., Wilson, B., Freedman, D., Lebrón, C., 2021. Quick BioPIC User’s Guide, ESTCP Project ER-201730. [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 Project Website]   [//www.enviro.wiki/images/c/c9/ER-201730_BioPIC_User%27s_Guide.pdf User’s Guide]</ref>

*[https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool Website]<ref name="TATA2024">Adamson, D.T., Newell, C.J., Hort, H.M, Wilson, J.T., 2024. TA2: The SERDP Transition Assessment Teaching Assistant. Strategic Environmental Research and Development Program (SERDP) Project ER20-1429. [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview Project Website]  [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Online Tool]</ref>

==Introduction==
Many active remedies are effective at treating higher concentrations of contaminants, but as the contaminant concentrations decrease, the rate of cleanup may slow before the site reaches the cleanup goal. At some sites, the rate of cleanup may slow until it is not significantly different from the rate of cleanup provided by the natural attenuation processes that occur at the site. At other sites, the concentration of contaminants in water produced by a pumping system is below the cleanup goal, but the concentration in monitoring wells in the source area are still above the goal. At some sites, active treatment has stopped further expansion of the plume toward a receptor, and concentrations are declining over time throughout the plume, but back diffusion is sustaining concentrations in the plume that are above the cleanup goal.

In 2013, a significant National Research Council (NRC) report noted that despite years of effort and considerable investment, many sites “will require long-term management that could extend for decades or longer”<ref name="NRC2013">National Research Council (NRC), 2013. Alternatives for Managing the Nation's Complex Contaminated Groundwater Sites. Committee on Future Options for Management in the Nation's Subsurface Remediation Effort, Water Science, Technology Board, Division on Earth and Life Studies, NRC. National Academies Press, 422 pages, ISBN 978-0-309-27874-4 [https://doi.org/10.17226/14668 doi: 10.17226/14668]. [//www.enviro.wiki/images/4/48/NRC2013.pdf Report.pdf]</ref>. The authors of the report discussed the need for developments that can aid in “transition from active remediation to more passive strategies and provide more cost-effective and protective long-term management of complex sites”<ref name="NRC2013" />.

The United States Environmental Protection Agency<ref> U.S. Environmental Protection Agency (USEPA), 1999. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. OSWER Directive 9200.4-17P. 39pp.[//www.enviro.wiki/images/a/aa/1999_USEPA-_Use_of_monitored_natural_attenuation_at_superfund.pdf Report.pdf]</ref> allows the use of [[Monitored Natural Attenuation (MNA) | monitored natural attenuation (MNA)]] to attain the cleanup goals when the site-specific remediation objectives can be attained within a time frame that is reasonable compared to that offered by other more active methods. Many CERCLA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Summary of the Comprehensive Environmental Response, Compensation, and Liability Act (Superfund) https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act</ref> and RCRA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Resource Conservation and Recovery Act (RCRA) Laws and Regulations https://www.epa.gov/rcra</ref> sites take advantage of this policy. An active remedy is typically used initially to treat high concentrations of contaminants followed by MNA to treat the lower concentrations that remain.

Unfortunately, there is no well-established approach to determine when it is appropriate to discontinue the active remedy. The NRC report<ref name="NRC2013" /> emphasized the use of more rigorous evaluations of existing data to support these efforts. This can include a quantitative assessment of the performance of active remedies (e.g., evidence of asymptotic performance) as well as documenting site conditions that may be contributing to these performance limitations. Importantly, it also identifies alternative approaches for managing the site, which could include MNA if the natural attenuation processes can meaningfully contribute to the achievement of site cleanup objectives.

This article reviews available tools and approaches to evaluate a transition to MNA. The tools and approaches depend on calculations of rate constants for natural attenuation with distance in flowing groundwater or rate constants for attenuation over time in individual monitoring wells.

==Background on Rate Constants==
[[File:Wilson1w2Fig1.png|thumb|400px| Figure 1. Attenuation of Trichloroethene (TCE) over time in a monitoring well at a site in Michigan. The concentration vs. time rate constant is 0.326 per year and largely represents the rate of the attenuation of the source of contaminants in the aquifer.]]
At sites where a transition to MNA is being considered, a key step is estimating attenuation rate constants and understanding how they are extracted from monitoring data. A general formula to describe the rate of a chemical reaction is:

:{|
|-
|'''Equation 1:'''|| ||<big>''r = k [C]''</big>'' m''
|-
|where:
|-
| ||''r''    ||is the rate of the reaction,
|-
| ||''k''||is the rate constant,
|-
| ||''C''||is the concentration of the chemical undergoing the reaction, and
|-
|the exponent    ||''m''||is the order of the reaction.
|}

When the rate of the reaction is proportional to the concentration of the contaminant, the value of ''m'' is 1. Therefore, the reaction is described as a first-order reaction, and the rate constant is described as a first-order rate constant. In Equation 1, concentration could go up or down, but ''k'' is a constant of proportionality for the rate of increase in concentration. The rate constant for attenuation is the negative of ''k''. If the rate of degradation is a fixed value regardless of concentration, the value of ''m'' is 0, and degradation is a zero-order process.

Natural attenuation of concentrations over time in monitoring wells is frequently described by a first-order rate constant, and natural biological or abiotic degradation of contaminants in flowing groundwater is typically also described by a first-order rate constant. Figure 1 provides an example of monitoring data that is described by a first-order rate constant.

The rate constant for attenuation over time in a single well and the rate constant for attenuation with distance along a flow path in an aquifer describe different situations that are controlled by different processes. ''Attenuation over time'' in a well is largely controlled by the rate of attenuation of the source of contamination in the aquifer. ''Attenuation with distance'' along a flow path includes attenuation of concentrations in the source along with contributions from biological degradation processes, abiotic degradation processes and hydrodynamic dispersion of the contaminated groundwater into clean groundwater<ref name="Newell2002" />.

The first-order rate constant for attenuation over time in a single well is commonly referred to as ''kpoint''<ref name="Newell2002" />. A time series chart in Microsoft EXCEL of the concentrations of a contaminant (''y'' axis) on the date of sampling (''x'' axis) can be used to extract a value for ''kpoint''. Select the data, then insert an exponential trend line and display the equation on the chart. The value of ''kpoint'' can also be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Note that the rate constants extracted in EXCEL are constants for the rate of change, not the rate of attenuation. Take the negative of the rate of change to get ''kpoint''. In the example in Figure 1, the unit of time on the X axis is years, and the value of ''kpoint'' is 0.326 per year.

Attenuation versus distance rate coefficients describe a bulk attenuation rate including both degradation and non-destructive processes such as dispersion. To extract values for rate constants for degradation alone, it is necessary to calibrate a groundwater flow and transport model to the data at the site. The model is calibrated with values for the hydrogeological properties of the aquifer (effective porosity, hydraulic gradient, hydraulic conductivity, hydrodynamic dispersion and the organic carbon content of the aquifer matrix). After the hydrogeological properties of the aquifer are fixed in the model, the most appropriate values for the degradation rate constants are the values that produce the best fit between the contaminant concentrations that are predicted by the model and the contaminant monitoring data at the site.

There are a number of reasons why natural attenuation processes are better described as first-order relationship instead of zero-order or some other order. The attenuation over time in a monitoring well tracks the attenuation over time of the source of contamination that sustains the plume<ref name="Newell2002" />. Sites go through a lifecycle, and attenuation of sources at mature sites is often a first-order process<ref>Sale, T., Newell, C., Stroo, H., Hinchee, R. and Johnson, P., 2008. Frequently Asked Questions Regarding Management of Chlorinated Solvents in Soils and Groundwater. Environmental Security Technology Certification Program (ESTCP, Project ER-200530), Department of Defense (DoD), Arlington, VA. [//www.enviro.wiki/images/c/cb/2008-Sale-Frequently_Asked_Questions_Regarding_Management_of_Chlorinated_Solvent_in_Soils_and_Groundwater.pdf Report.pdf]   Project Overview Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200530</ref>. If a chlorinated solvent site is mature, the contamination in the source area that was originally present as nonaqueous phase liquids (NAPL) has been redistributed and is now sequestered in a sorbed phase to aquifer solids or has diffused into non-transmissive portions of the aquifer matrix. Transfer of contaminants back into the more transmissive portions of the aquifer occurs by diffusion along a fixed path length, and the rate of transfer is controlled by the concentration of the contaminant remaining in the source material. Because the rate of transfer is proportional to the concentration of contaminant in the source material, attenuation of the source is a first-order process. These processes are discussed in more detail in [[Source Zone Modeling]].

Degradation processes are also usually first order. Abiotic reactions are almost always first order with respect to the concentration of the target chemical. Biodegradation reactions are zero order at high concentrations because the available enzymes are saturated with substrate, but are first order at lower concentrations that are typical of natural attenuation conditions in groundwater.

==Goals for MNA at Sites==

The information necessary to evaluate whether a site can be transitioned to MNA depends on the goal for MNA at the site. For many cleanup actions, the goal is to confine contamination within a waste management area where the contamination is left in place, in which case the cleanup goal applies to point-of-compliance wells that are outside the waste management area. For other cleanup actions, the entire site must be cleaned up, in which case the cleanup goal applies to any monitoring well on the site. The time by which the goal is to be attained is specified at CERCLA sites in the Record of Decision (the ROD). At RCRA sites, the time allowed for the cleanup to be attained may be specified in the permit.

==When the Goal Applies to Point-of-Compliance Wells==
Consider the following framework for evaluating a transition to MNA:

#Use a computer model to extract rate constants for the natural degradation of the contaminant that occurred in groundwater at the site before the active remedy was installed.
#Assume that the same rate constants will apply after the active remedy is no longer in operation. Note that this assumption may not be valid if the active remedy changes the geochemistry of the aquifer in the flow path to the point-of-compliance well.
#Calibrate a computer groundwater flow and transport model with the hydrogeological properties of the aquifer that pertain after the active remedy is no longer in operation, the concentration of contaminant after the active remedy, and the rate constants for natural degradation that are assumed to apply after the active remedy.
#Use the computer model to project the concentrations of the contaminant at the point-of-compliance well over time.
#If the concentrations at the point-of-compliance wells are predicted to be less than the goal before the specified date, that is a quantitative line of evidence in support of a transition to MNA.

There are several computer applications that are particularly useful to extract rate constants at a site from monitoring data that were collected before the active remedy was installed. For example, [https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS)]<ref name="Widdowson2008" />, [https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system BIOCHLOR]<ref name="Aziz2002" /> and [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC]<ref name="BioPIC2021" /> can be downloaded from the internet at no cost. Another recent example, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ TA2 Tool]<ref name="TATA2024" />, is discussed in detail later in this article.

[[File:Wilson1w2Fig2.png|thumb|left|400px| Figure 2. Example calibration of NAS to natural attenuation of total BTEX at a site (Figure 17 of NAS User’s Manual).]]
[[File:Wilson1w2Fig3.png|thumb|400px| Figure 3. The data input screen for BIOCHLOR before remediation with cis-1,2-Dichloroethene (DCE) and vinyl chloride (VC) source concentrations of 500 and 87 mg/L respectively at the source when the release first occurred.]]
[[File:Wilson1w2Fig4.png|thumb|left|400px| Figure 4. Output of the RUN CENTERLINE simulation in BIOCHLOR comparing the fit between the simulation and the field data for vinyl chloride before an active remedy was implemented]]
[[File:Wilson1w2Fig5.png|thumb|400px| Figure 5. Output of the RUN CENTERLINE simulation of conditions after an active remedy was implemented with a source concentration of 1.1 mg/L, projecting the concentration of vinyl chloride at a distance corresponding to a point-of-compliance well.]]
 
In NAS, the user inputs the hydrogeological data, the distance of wells along the flow path, and the concentrations of contaminants in the wells. The NAS application extracts rate constants and makes projections at the point-of-compliance. With NAS, it is possible to extract different rate constants for specific geochemical environments along the flow path.

Figure 2 provides an example calibration of NAS. The concentrations in the monitoring wells used to calibrate the model are compared to the simulation provided by the model. The values of the rate constants that are extracted from the field data are available in the “Output” tab under “Data and Results Table.”

Figure 3 depicts the input screen for BIOCHLOR. The user inputs the hydrogeological parameters, the first-order rate constants (1st Order Decay Coefficient), the distribution of the wells along the flow path, and the concentrations of contaminants in the wells. The model is set up for conditions that apply before the installation of the active remedy.

BIOCHLOR does not automatically fit the rate constants to the field data. Instead, the user examines the output of the model, and adjusts the rate constants until they provide the best fit between the model prediction and the monitoring data for wells at the site. This comparison is illustrated in Figure 4.

If the distance from the source well to the point-of-compliance well is set as the “Modeled Area Length” in Section 5 of the input screen, the “Run Centerline” output will provide the projected concentrations at that length. Assume the distance from the source well to the point-of-compliance well is 250 feet. The projected concentration in Figure 3 of vinyl chloride at a point-of-compliance well is 0.042 mg/L. If the regulatory goal were the federal drinking water maximum contaminant level (MCL)<ref>U. S. Environmental Protection Agency (USEPA), 2009. National Primary Drinking Water Regulations. EPA 816-F-09-004. [//www.enviro.wiki/images/a/ae/2009-USEPA-national_Primary_Drinking_Water_Regulations.pdf Report.pdf]</ref> of 0.002 mg/L, the projected concentration would exceed the goal, and MNA would not be adequate as a remedy.

For the sake of illustration, assume that an active remedy has been implemented, and the concentrations in the source well are 5.4 mg/L for DCE and 1.1 mg/L for vinyl chloride. To evaluate whether it is now appropriate to transition to MNA, BIOCHLOR could be calibrated with these concentrations to predict concentrations in the point-of-compliance well. (See Figure 5). In this example, the projected concentration at the point-of-compliance well does meet the goal.

Some active remedies are subject to rebound. If this is the case, the evaluation should begin at the point in time when it is clear that the trend in concentrations is downward.

A new EXCEL-based tool that does many of the same basic calculations as BIOCHLOR was recently developed as part of an update to the BioPIC<ref name="BioPIC2021" /> decision support software. This tool, the MNA Rate Constant Estimator, extracts rate constants from concentration versus distance data for a variety of different chemicals, including chlorinated ethenes (e.g., PCE and TCE), chlorinated ethanes (e.g., 1,1,1-TCA), and 1,4-dioxane. This tool was developed to run using current versions of EXCEL, whereas BIOCHLOR must be run using older versions of EXCEL that may be unavailable to many users. The MNA Rate Constant Estimator can be used to estimate degradation rate constants and/or predict plume footprints over time. Consequently, it is a useful addition to the BioPIC decision framework for understanding if MNA is appropriate remedy for a site, and it can also be helpful for estimating rate constants as part of a transition assessment.

==When the Goal Applies to All the Wells==
At sites where a concentration-based cleanup goal must be achieved at all wells, each well at the site is evaluated independently, and the rate constant that is applicable is the rate constant for attenuation over time in the well (''kpoint''). To evaluate whether the region in an aquifer that is sampled by a particular monitoring well is ready to transition to MNA, it is necessary to have monitoring data from a period of time before the remedy was implemented. This data is used to extract a value for ''kpoint'' in the aquifer under natural attenuation conditions. The evaluation of a transition to MNA will assume that the same value for ''kpoint'' will apply after the active remedy is complete. This assumption may not be appropriate if the active remedy caused a permanent change in the geochemistry of the aquifer. The assumption is usually appropriate for pump-and-treat remedies.

If ''kpoint'' before implementation of the active remedy describes the time course of natural attenuation after the active remedy is completed, the time required to attain the cleanup goal is predicted from the following:

:{|
| || || rowspan="2" |<big>''ln (''</big>
| style="border-style:solid; border-width: 0px 0px 1px 0px" |''Cgoal''
| rowspan="2" |<big>'')''</big>||                                                                       
|-
|'''Equation 2:'''        ||''<big>t =''    
| style="vertical-align:top;" |''Ccurrent''||
|-
| || || colspan="3" style="text-align:center; border-style:solid; border-width: 1px 0 0 0" |''<big>-k</big>point''||
|-
|where:
|-
| ||''Ccurrent''    || colspan="5" |is the current concentration after active remediation,
|-
| ||''Cgoal''|| colspan="5" |is the cleanup goal, and
|-
| ||''t''|| colspan="5" |is the time required for concentrations to attenuate from ''Ccurrent'' to ''Cgoal.''
|}

If the value of ''t'' estimated using Equation 2 is less than the difference between the current date and the date specified by the site stakeholders to attain the goal, that is evidence in support of a transition to MNA.

Some active remedies are subject to contaminant concentration rebound. If this is the case, the evaluation should use a value of ''C''current that is attained after the rebound has stabilized.

This approach depends on a robust value for ''kpoint''. It is worthwhile to do a sensitivity analysis on ''kpoint'' where the lower 95% or 90% confidence interval on ''kpoint'' is used in Equation 2 to see if that changes the outcome of the evaluation. The confidence intervals can be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Wilson<ref name="Wilson2011">Wilson, J.T. 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [//www.enviro.wiki/images/e/e3/Wilson-2011-An_Approach_for_Evaluating_Progress.pdf Report.pdf]</ref> provides detailed discussion of the use of linear regression to extract ''kpoint'' and confidence intervals on ''kpoint''. Wilson<ref name="Wilson2011" /> also discusses the use of goodness-of-fit tests to determine if there is evidence that a first-order rate equation is not the best fit to the monitoring data, and as a result the use of Equation 2 would not be appropriate. The TA2 Tool<ref name="TATA2024" /> also has the capability to calculate ''kpoint'' with a user-specified confidence interval, as described below.

At many sites, there is no specified date when the cleanup goal must be attained. In this situation, the monitoring data can be evaluated to determine if the current rate of attenuation under the active remedy is faster than the rate of natural attenuation before the active remedy was installed. The monitoring data can be examined to identify a time interval when the benefit of the active remedy has approached an asymptote. A second value of for ''kpoint'' can be extracted for that time interval. The two values for ''kpoint'' can be evaluated statistically to see if the current rate is faster at some appropriate level of confidence. If there is no statistical evidence that the rate of attenuation is faster, that determination can support a decision to transition to MNA.
[[File:Wilson1w2Fig6.png|thumb|400px| Figure 6. Example calibration of NAS to predict the reduced concentration at the source that is necessary to meet the remediation goal at a point-of-compliance well (Figure 19 of NAS User’s Manual).]]

==Extent of Treatment Necessary to Transition to MNA==
There are several computer applications that can predict the extent of treatment that must be achieved by the active remedy before it is worthwhile to evaluate the site for transition to MNA. For example, based on the distribution of contamination along the flow path, the NAS application will automatically predict a reduced concentration at the source well that will bring concentrations to the goal in the point-of-compliance well (Figure 6). A table that opens under the “DOS/TOS” tab provides the “Time of Equilibration” required to meet the goal at the reduced concentration. Modules in NAS allow the user to evaluate the effect of various pump-and-treat and source removal scenarios on the time required to attain the goal at the point-of-compliance well.

The [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018">Falta, R.W., Farhat, S.K., Newell, C.J. and Lynch, K., 2018. A Practical Approach for Modeling Matrix Diffusion Effects in REMChlor. SERDP/ESTCP Project ER-201426 [//www.enviro.wiki/images/0/0b/2018-Falta-REMChlor_Modeling_Matrix_Diffusion_Effects.pdf Report.pdf]   Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426</ref> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012">Falta, R.W., Ahsanuzzaman, A.N., Stacy, M.B., Earle, R.C. and Wilson, J.T., 2012. Remediation Evaluation Model for Fuel Hydrocarbons (REMFuel). Users Manual Version 1.0. U.S. Environmental Protection Agency. EPA/600/R-12/028. [//www.enviro.wiki/images/6/67/2012-Falta-REMFuel_Remediation_Evaluation-Model_for_Fuel_hydrocarbons_users_manual.PDF Report.pdf]   Website: https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel</ref> models are flexible screening tools that allow a simultaneous evaluation of the extent of treatment provided by (1) source removal, (2) ''in situ'' remediation of the contaminated groundwater, or (3) natural attenuation processes in three discrete intervals along the flow path and three discrete time periods. Both [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018" /> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012" /> can be downloaded from the internet at no cost. Liang ''et al.''<ref>Liang, H., Falta, R.W., Newell, C.J., Farhat, S.K., Rao, P.S. and Basu, N., 2010. Decision & Management Tools for DNAPL Sites: Optimization of Chlorinated Solvent Source and Plume Remediation Considering Uncertainty. SERDP/ESTCP Project ER-200704. [//www.enviro.wiki/images/c/ce/2010-Liang-Decision_and_Management_Tools_for_DNAPL_sites-ER-200704-FR.pdf Report.pdf]   Project Overview Website: https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200704/(language)/eng-US</ref> provide a modeling program that uses Monte Carlo simulations to evaluate the effects of the uncertainties in the modeling parameters on the predictions of REMChlor-MD.

==The Transition Assessment Teaching Assistant (TA2) Tool==
[[File:Wilson1w2Fig7.png|thumb|500px| Figure 7. Home Page for TA2 Tool. Users can click on buttons to access various modules that are designed to answer specific questions or research relevant topics.]]
[[File:Wilson1w2Fig8.png|thumb|500px| Figure 8. Example of an asymptote analysis using concentration versus time data in Tool 1 of the TA2 Tool. The source attenuation rate and corresponding remediation timeframe can be estimated for different monitoring periods.]]
A learning and decision-making tool was recently released as part of [https://serdp-estcp.mil/ Strategic Environmental Development and Research Program (SERDP)] Project [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview ER-201429] to help stakeholders gather information for the purposes of a site-specific transition assessment. This free software, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool]<ref name="TATA2024" />, was developed using the elements identified in the 2013 NRC report<ref name="NRC2013" /> as the critical learning objectives for end users.

The Tool is a web-based app that includes a collection of individual modules designed to answer specific questions or research relevant topics (Figure 7). The Tool has been developed as an R Shiny app (version 1.8.0)<ref> Chang, W., Cheng, J., Allaire, J., Sievert, C., Schloerke, B., Xie, Y., Allen, J., McPherson, J., Dipert, A., Borges, B., 2023. shiny: Web Application Framework for R. R package version 1.8.0, https://github.com/rstudio/shiny, https://shiny.posit.co/</ref>, which is an interactive platform using R programming to perform all quantitative functions. The user can then view the results in a simple interface that easily accommodates plots, charts, and various mapping features in a Web browser. The Tool is free and does not require the user to install R software.

The modules within the TA2 Tool include:

'''Five Quantitative Tools''' that focus on assessing asymptotic groundwater concentrations from monitoring data, evaluating plume stability, estimating remediation timeframes after a hypothetical source removal project, forecasting remediation performance if a technology is applied in the field, or projecting concentrations at downgradient points of compliance.

For example, Tool 1 in the TA2 Tool uses concentration versus time data from monitoring wells to estimate attenuation rate constants (''kpoint'') and evaluate if asymptotic conditions are present at particular locations or across the site. This helps to assess whether performance has plateaued at wells where a pump-and-treat system or other active treatment is in place. The user has the option to choose a “change point” within the monitoring record to determine if the attenuation rate has changed over time (e.g., once most of the accessible mass has been removed) (Figure 8). The user can either use visual interpretation to manually select the date when this apparent change occurred or have the date selected automatically using a binary segmentation protocol that is incorporated into the tool. The tool will calculate a rate for both the early period and a rate for the later period (after the change point), and then go through five different lines of evidence for asymptotic behavior (e.g., are the two rates of attenuation significantly different?). The user can then use the collective results as a technical justification demonstrating that the performance of the active remedy has plateaued as the first step in the transition assessment. The tool will also estimate the time to reach a user-specified cleanup goal if the overall attenuation rate (or the attenuation rate in the later period) were to continue.

Another module (Tool 5) focuses on evaluating sites where the concentration goal applies at a downgradient point of compliance, which is a key criterion for sites where MNA is being used as part of a risk-based strategy. The tool includes several different options to estimate a site-specific attenuation rate constant, including data from the pre-remediation period when natural attenuation processes were the sole means for reducing concentrations. Attenuation rate constants are then used to project the concentration versus distance from the contaminant source. Based on the predicted concentration at the downgradient point of compliance, the user can then see if the natural assimilative capacity along the aquifer flow path is sufficient to achieve the concentration goal in the absence of active treatment. For example, in the tab labeled “Use Pre-Remediation Rate Constant”, the logarithms of the concentrations from the period before active treatment began are plotted against the distance from the source well. The slope of the regression line is the rate constant for natural attenuation (including the contributions of degradation and dispersion). This rate constant can then be used to project the concentration moving downgradient from the well of concern after the end of active treatment. Similar approaches are provided within Tool 5 for using rate constants estimated from lab-based testing or derived from post-remediation data (after steady state has been reestablished).

'''Four Qualitative Tools''' provide information on matrix diffusion, enhanced attenuation options, geologic heterogeneity, and related research on transition assessments. Many of these modules are based on the current understanding of the role of matrix diffusion in influencing long-term concentration trends and remedial performance at contaminated groundwater sites. This includes summaries of different modeling options for better quantifying the effects of matrix diffusion. Sites impacted by matrix diffusion are generally challenging to treat using active remedies and thus are better candidates for less intensive management strategies that focus on reducing mass discharge rates, stabilizing the plume, and protecting potential downgradient receptors. As a result, matrix diffusion is critical to understanding and quantifying how natural attenuation processes are contributing to concentration trends.

'''One Summary Tool''' (Tool 10) compiles metrics from the other tools into a “Remediation Transition Assessment Index” (RTAI) and provides additional guidance on conducting site-specific transition assessments. The RTAI is a simple metric with a value from 1 to 5, where higher values reflect greater persistence of contamination due to matrix diffusion and other site-specific factors. An RTAI value is assigned to each of the results from the different tools that have been completed by the user. An RTAI of 5 suggests that the site is a strong candidate for transitioning to MNA or enhanced attenuation approaches, while a site with an RTAI value of 1 is a poor candidate. The user can assign an overall RTAI for the site based on the preponderance of evidence after reviewing the RTAI values generated by each tool, or calculate a site RTAI based on simple averaging, weighting, or other methods.

Tool 10 also contains a flowchart and a checklist for performing site-specific transition assessments that start with evaluating relevant bright line criteria, such as (1) can the concentration goals be met at the point of compliance by MNA; and (2) is the remediation timeframe for MNA reasonable and/or similar to the timeframe if source remediation were used. This checklist ensures that the user has gathered all relevant information that would be needed to support a technically rigorous site-specific Transition Assessment.

The TA2Tool provides a framework for remedial decision makers to evaluate different types of sites, including those where active treatment (e.g., pump and treat) is in use, as well as sites where future active source zone remediation is being considered. It also includes a description of enhanced MNA alternatives for sites where MNA alone may not be sufficient to control risk. As shown in Figure 8, the tool can be used to answer specific questions that have a primarily quantitative basis or to provide focused qualitative information for researching specific topics. Users can engage with just the modules that might be pertinent to assessment of an individual site, or they can go through all the modules to perform a more thorough, step-by-step analysis of the relevant issues for their site.

==Summary==
Tools and approaches are available that can be adapted to determine when a site is ready to transition from active remedy to MNA. However, these tools and approaches have not been applied for this purpose at a significant number of sites, and at the present time, they are not generally accepted by regulatory authorities. There is an opportunity to establish and implement a logical and consistent framework that can be widely implemented to evaluate sites for transition from active remedy to MNA.

==References==
<references />

==See Also==

*[http://dx.doi.org/10.1007/978-1-4614-6922-3 Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. In: Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward. SERDP ESTCP Environmental Remediation Technology, vol 7. Springer, New York, NY. pgs. 459-508. doi: 10.1007/978-1-4614-6922-3]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436/(language)/eng-US Kram, Mark, and Widdowson, Mark, 2008. Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation. ESTCP ER-200436]

Biodegradation - Reductive Processes

2026-05-07T17:05:44Z

Admin:

Microbial removal of [[wikipedia: Halogen| halogens]] from [[wikipedia: Organic compound | organic compounds]] by reductive processes forms the basis for many types of bioremediation technologies. The process was discovered within the last several decades and our understanding of how microbes perform this activity has improved significantly. Advances in our understanding of the microbiology of reductive dehalogenation have led to improvements in documenting natural attenuation and implementation of biostimulation and bioaugmentation. As a general rule, bioremediation is a lower cost approach to treatment of halogenated solvents than competing processes based on physical or chemical techniques (e.g., [[Chemical Oxidation (In Situ - ISCO) | chemical oxidation]]). For practitioners, a working knowledge of microbial reductive processes is essential to successful application at hazardous waste sites.

<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s)''':

*[[Bioremediation - Anaerobic]]
*[[Chemical Reduction (In Situ - ISCR)]]
*[[Chlorinated Solvents]]
*[[REMChlor - MD]]

'''Contributor(s):''' [[Dr. David L. Freedman]]

'''Key Resource(s)''':

*[https://doi.org/10.1002/jctb.1567 Enhanced Anaerobic Bioremediation of Chlorinated Solvents: Environmental Factors Influencing Microbial Activity and Their Relevance under Field Conditions]<ref name="Aulenta2006">Aulenta, F., Majone, M. and Tandoi, V., 2006. Enhanced anaerobic bioremediation of chlorinated solvents: environmental factors influencing microbial activity and their relevance under field conditions. Journal of Chemical Technology and Biotechnology, 81(9), 1463-1474. [https://doi.org/10.1002/jctb.1567 doi: 10.1002/jctb.1567]</ref>.
*[https://doi.org/10.1007/978-3-662-49875-0 Organohalide Respiring Bacteria]<ref name="Adrian2016">Adrian, L. and Löffler, F., 2016. Organohalide Respiring Bacteria. 495 pgs. ISBN: 978-3-662-49873-6. [https://doi.org/10.1007/978-3-662-49875-0 doi: 10.1007/978-3-662-49875-0]</ref>

==Introduction==
Organic compounds with one or more halogens attached are referred to as halogenated organics. The halogens include [[wikipedia:Chlorine |chlorine (Cl)]], [[wikipedia: Bromine | bromine (Br)]], [[wikipedia: Fluorine | fluorine (F)]], and [[wikipedia: Iodine | iodine (I)]]. For example, [[wikipedia: Ethylene | ethene]] (C2H4) is an organic compound. When three of the four hydrogen atoms on ethene are replaced with chlorine, the resulting compound is [[wikipedia: Trichloroethylene | trichloroethene]] (TCE; C2HCl3).

The proliferation of halogenated organic compounds in the environment is a consequence of their widespread use in industrial activities. A critical part of many manufacturing processes involves removal of oil and grease from metal, fabrics, and other commodities. Because “like dissolves like,” a common way to remove oil and grease is to soak products in a nonpolar solvent. Non-halogenated hydrocarbons serve this purpose; however, accumulation of hydrocarbon vapors creates the risk of an explosion. Organic chemists solved this problem by adding halogens to the hydrocarbons, rendering them non-flammable. Use of halogenated organic compounds grew dramatically after World War II. With increased use came increased releases to the environment. Initially, halogenated solvents were thought to be inert in the environment and hence not to pose any risk to humans or wildlife. Gradually, the risks associated with chronic human exposure to halogenated solvents were revealed and concern grew about their fate in the environment. Most of the compounds on the United Nations list of persistent organic pollutants (first developed at the Stockholm Convention on Persistent Organic Pollutants) are halogenated organic compounds.

==Dehalogenation==
Microbes possess the ability to remove halogens from halogenated organic compounds. Dehalogenation may occur via a variety of reactions, including oxidation, reduction, or hydrolysis. The halogens are released as halides, i.e., Cl-, Br-, F-, and I-. The process of removing a halogen from a halogenated organic compound by a reductive reaction is referred to as [[wikipedia: Reductive dechlorination | reductive dehalogenation]]. Reductive reactions are ones in which electrons are transferred to the carbon-halogen bond, thereby lowering the oxidation state of the parent compound (example reactions below).

One of the earliest studies (1982) to demonstrate that microbes are capable of removing halogens from halogenated organic compounds used halobenzoates (e.g., 3-chlorobenzoate)<ref>Suflita, J.M., Horowitz, A., Shelton, D.R. and Tiedje, J.M., 1982. Dehalogenation: a novel pathway for the anaerobic biodegradation of haloaromatic compounds. Science, 218(4577), 1115-1117. [https://doi.org/10.1126/science.218.4577.1115 doi: 10.1126/science.218.4577.1115]</ref>. Knowledge about microbial dehalogenation has since grown considerably and now forms the basis for bioremediation, a commonly applied strategy to treat halogenated organic compounds found in soils, groundwater, and wastewater.

==Bioremediation==
As a general rule, bioremediation is a lower cost approach to treatment of halogenated solvents than competing processes based on physical or chemical techniques (e.g., [[Chemical Oxidation (In Situ - ISCO) | chemical oxidation]]). The discovery that microbes are capable of replacing the chlorines on tetrachloroethene (PCE) and TCE, the two most frequently encountered organic contaminants at hazardous waste sites, opened the door to development of current bioremediation strategies. Notably, there was a pause in interest when it was initially believed that the dechlorination process stopped at vinyl chloride (VC), which is more toxic than PCE, TCE, and dichloroethene (DCE) isomers. It was subsequently determined that microbial reduction of VC to ethene occurs<ref>Freedman, D.L. and Gossett, J.M., 1989. Biological reductive dechlorination of tetrachloroethylene and trichloroethylene to ethylene under methanogenic conditions. Applied and Environmental Microbiology, 55(9), 2144-2151. [//www.enviro.wiki/images/9/96/Freedman-1989-Biological_reductive_dechlorination.pdf Report pdf]</ref>. Ethene is an acceptable endpoint since it poses no human health risks at the concentrations typically found in groundwater.

Although humans are responsible for a large influx of halogenated organics into the environment as a consequence of improper handling and disposal practices, it has also come to light that there are natural sources of halogenated organic compounds; nearly 5,000 have been catalogued to date<ref>Gribble, G. W., 2010. Naturally Occurring Organohalogen Compounds - A Comprehensive Update. SpringerWien: New York. [https://doi.org/10.1007/978-3-211-99323-1 doi: 10.1007/978-3-211-99323-1]</ref>. For example, marine algae produce chloromethane as part of a chemical defense system to dissuade predation. Consequently, it is not too surprising that natural processes exist to break down halogenated organic compounds, and those processes have been harnessed to help clean up the excessive amounts released to the environment as a consequence of human activity.

There are several types of reaction pathways that involve reduction and dehalogenation, including [[wikipedia:Hydrogenolysis |hydrogenolysis]] and dihaloelimination<ref>Vogel, T.M., Criddle, C.S. and McCarty, P.L., 1987. ES&T critical reviews: transformations of halogenated aliphatic compounds. Environmental Science & Technology, 21(8), 722-736. [http://dx.doi.org/10.1021/es00162a001 doi:10.1021/es00162a001]</ref>. Here, we detail each of these pathways and the conditions under which each is likely to be prevalent. It should be noted that many of the reactions are also possible via abiotic processes (e.g., via reaction with [[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR) |zerovalent iron (ZVI)]])<ref>Arnold, W.A. and Roberts, A.L., 2000. Pathways and kinetics of chlorinated ethylene and chlorinated acetylene reaction with Fe(0) particles. Environmental Science & Technology, 34(9), 1794-1805. [http://dx.doi.org/10.1021/es990884q doi:10.1021/es990884q]</ref>. The focus of this article is on biotic reductive processes.

==Hydrogenolysis==
[[wikipedia: Hydrogenolysis |Hydrogenolysis]] is the process by which a carbon—halogen bond is broken and hydrogen replaces the halogen substituent, resulting in release of a halide ion (R = organic compound, X = halide):
[[File:Freedman_BRP_EQ1.jpg|center|200 px|]]

The process requires an input of reducing power, represented in the above equation by 2e-, or two electron equivalents. Hydrogenolysis is the most frequently observed reductive pathway, and as such it is commonly referred to as reductive dehalogenation. Nevertheless, other pathways are also reductive and result in dehalogenation, so the more correct description of the above reaction is hydrogenolysis. This descriptor derives in part from the fact that hydrogen (H2) is often a source of the electron equivalents:
[[File:Freedman Article 1 Equation 1.PNG|150px|center|]]

Hydrogenolysis applies to any organohalide, yet it is most commonly associated with removal of chlorine from organic solvents and the term [[wikipedia: Reductive dechlorination | reductive dechlorination]] is often used synonymously (mostly amongst practitioners) to describe this reaction. As mentioned above, this is not quite correct, since other pathways are also reductive and result in removal of chlorine atoms, and the correct chemical mechanism is hydrogenolysis.

Hydrogenolysis applies to numerous categories of compounds; we highlight several of the major categories below. The process typically occurs via a respiratory process referred to as organohalide respiration (described below).

===Chlorinated Ethenes===
When applied to PCE, hydrogenolysis proceeds in 2e- steps through TCE, ''cis''- or ''trans''-1,2- DCE, VC, and ethene, which is also called ethylene (Fig. 1). ''cis''-DCE is the more frequently identified of the 1,2-DCE isomers. Nevertheless, ''trans''-DCE is predominant in some environments<ref>Griffin, B.M., Tiedje, J.M. and Löffler, F.E., 2004. Anaerobic microbial reductive dechlorination of tetrachloroethene to predominately trans-1, 2-dichloroethene. Environmental Science & Technology, 38(16), 4300-4303. [https://doi.org/10.1021/es035439g doi: 10.1021/es035439g]</ref>. 1,1-DCE is another possible dichloroethene isomer, but it is not typically formed during microbial TCE respiration. The presence of 1,1-DCE in the environment is most typically associated with prior contamination by 1,1,1-trichloroethane, which undergoes several types of transformation, including the abiotic process of dehydrohalogenation to 1,1-DCE.

In some locations, further reduction of ethene to ethane has been reported. This is not a dechlorination reaction, but it is important to mention because ethane may be the terminal product from hydrogenolysis of chlorinated ethenes. Monitoring ethane is recommended for establishing a complete assessment of the fate of chlorinated ethenes.

[[File:Freedman_BRP_Fig1.jpg|thumb|center|800 px|Figure 1. Stepwise reduction of PCE and TCE to ethene and ethane. cDCE = cis-1,2-dichloroethene; tDCE = trans-1,2-dichloroethene.]]
The oxidation state of carbon in an organic compound varies from -4 (e.g., in CH4) to +4 (e.g., in CO2). The lower the oxidation state, the easier it is for oxidation to occur, and vice versa.

The oxidation state of the carbon in PCE is +4. With each successive reduction step (via an input of 2e-), the oxidation state of the carbon decreases by 2, so that the carbon in TCE has an oxidation state of +2, DCE has 0, VC has -2, and ethene has -4. For this category of contaminants, the final step is critical from a remediation perspective, since VC is a known human carcinogen while ethene and athnae pose no human health risks at the concentrations typically found in groundwater.

===Chlorinated Ethanes===
Like chlorinated ethenes, chlorinated ethanes are reduced by hydrogenolysis. One of the most widely evaluated compounds is [[wikipedia:1,1,1-Trichloroethane|1,1,1-trichloroethane]], which undergoes sequential reduction to [[wikipedia: 1,1-Dichloroethane|1,1-dichloroethane]] and [[wikipedia: Chloroethane| chloroethane]]. Although further reduction to ethane is possible, it is a much slower reaction and chloroethane is typically regarded as the terminal product. This example serves to illustrate that hydrogenolysis does not always yield complete dechlorination.

Other commonly encountered chlorinated ethanes undergo hydrogenolysis, including reduction of [[wikipedia:1,2-Dichloroethane| 1,2-dichloroethane]] to chloroethane and [[wikipedia:1,2-Dichloropropane|1,2-dichloropropane]] to 1- or 2-chloropropane.

===Chlorinated Methanes===
[[wikipedia: Carbon tetrachloride|Carbon tetrachloride]] (tetrachloromethane) undergoes hydrogenolysis to [[wikipedia: Chloroform| chloroform]] (trichloromethane) and then methylene chloride (dichloromethane). These reactions are also catalyzed by reduced iron, which may be generated by iron-reducing bacteria.

Further reduction to chloromethane and methane is not commonly observed; other anaerobic biodegradation processes, such as organohalide fermentation, are more significant for dichloromethane and chloromethane.

===Chlorinated Aromatic Compounds===
Hydrogenolysis also occurs for chlorinated aromatic compounds, including chlorinated benzenes, [[wikipedia: Polychlorinated biphenyl| polychlorinated biphenyls (PCBs)]], chlorinated dioxins (e.g., 2,3,7,8-tetrachlorodibenzo-''p''-dioxin, or TCDD) and furans, and polychlorinated phenols (e.g. pentachlorophenol, or PCP). For these compounds, the pathways are more complicated, since reduction proceeds through multiple isomers, depending on which position on the ring that each specific chlorine atom is removed. Like chlorinated ethanes and methanes, hydrogenolysis of chlorinated aromatic compounds is rarely complete, and the rate of reduction often decreases with a decreasing number of chlorine-carbon bonds. An important exception has been identification of cultures that reduce chlorinated benzenes completely to benzene<ref>Fung, J.M., Weisenstein, B.P., Mack, E.E., Vidumsky, J.E., Ei, T.A. and Zinder, S.H., 2009. Reductive dehalogenation of dichlorobenzenes and monochlorobenzene to benzene in microcosms. Environmental Science & Technology, 43(7), 2302-2307. [https://doi.org/10.1021/es802131d doi: 10.1021/es802131d]</ref><ref>Nelson, J.L., Fung, J.M., Cadillo-Quiroz, H., Cheng, X. and Zinder, S.H., 2011. A role for ''Dehalobacter'' spp. in the reductive dehalogenation of dichlorobenzenes and monochlorobenzene. Environmental Science & Technology, 45(16), 6806-6813. [https://doi.org/10.1021/es200480k doi: 10.1021/es200480k]</ref>.

===Bromo- and Fluoro-Organic Compounds===
Hydrogenolysis of brominated organic compounds has been documented. Examples include reduction of 1,2-dibromoethane (ethylene dibromide) to bromoethane and reduction of polybrominated diphenyl ethers. Likewise, defluorination also occurs. For example, reduction of vinyl fluoride to ethene has been reported. Nevertheless, reductive defluorination is characterized by slow rates, if it occurs at all. This is consistent with the general expectation that when the rate-limiting step for a reaction is cleavage of the carbon halogen bond, the order of reactivity is:

<div align="center">C—I > C—Br > C—Cl > C—F<ref>Wackett, L.P., Logan, M.S., Blocki, F.A. and Bao-Li, C., 1992. A mechanistic perspective on bacterial metabolism of chlorinated methanes. Biodegradation, 3(1), 19-36. [https://doi.org/10.1007/bf00189633 doi: 10.1007/BF00189633]</ref></div>

For example, hydrogenolysis of trichlorofluoromethane (CFC-11) typically results in accumulation of dichloro- and chlorofluoro-methane; further hydrogenolysis occurs at a much slower rate, if at all. Microbial reductive defluorination of perflurooctanoic acid (PFOA; used in the manufacture of Teflon) has not been reported to any appreciable extent<ref>Liou, J.C., Szostek, B., DeRito, C.M. and Madsen, E.L., 2010. Investigating the biodegradability of perfluorooctanoic acid. Chemosphere, 80(2), 176-183. [http://dx.doi.org/10.1016/j.chemosphere.2010.03.009 doi: 10.1016/j.chemosphere.2010.03.009]</ref>.

==Dihaloelimination==
Dihaloelimination is the process by which two groups, e.g., a hydrogen and chlorine, are removed from adjacent carbon atoms, resulting in the formation of a double bond and release of two halide ions (X- = halide.):

[[File:Freedman A 1 Fig 3.PNG|center]]

Although less frequently encountered than hydrogenolysis, dihaloelimination is a critical pathway for several common groundwater contaminants, including reduction of 1,2-dichloroethane and 1,2-dibromoethane (more commonly referred to as ethylene dibromide, or EDB) to ethene<ref>Yu, R., Peethambaram, H.S., Falta, R.W., Verce, M.F., Henderson, J.K., Bagwell, C.E., Brigmon, R.L. and Freedman, D.L., 2013. Kinetics of 1,2-dichloroethane and 1,2-dibromoethane biodegradation in anaerobic enrichment cultures. Applied and Environmental Microbiology, 79(4), 1359-1367. [https://doi.org/10.1128/aem.02163-12 doi: 10.1128/AEM.02163-12]</ref>:

[[File:Freedman A 1 Fig 4.PNG|center|]]

In this example, the oxidation state of the carbon decreases from -2 in 1,2-dibromoethane to -4 in ethene, i.e., by 2 electrons. Thus, the process is both reductive and results in removal of halides. Other examples of dihaloelimination include reduction of 1- or 2-chloropropane to propene.

==Organohalide Respiration==
A variety of microbes have developed pathways to conserve the energy made available when breaking carbon-halogen bonds via reduction (i.e., hydrogenolysis and dihaloelimination), by using the halogenated organic compounds as terminal electron acceptors<ref name="Adrian2016" />. This has led to the notion that certain microbes are capable of “breathing” halogenated organics, analogous to respiration involving oxygen as the electron acceptor<ref name="McCarty1997">McCarty, P.L., 1997. Breathing with chlorinated solvents. Science, 276(5318), 1521-1522. [https://doi.org/10.1126/science.276.5318.1521 doi: 10.1126/science.276.5318.1521]</ref>. When halogenated organic compounds serve as terminal electron acceptors, the process is referred to as organohalide respiration. The discovery of this process adds to the extensive list of terminal electron acceptors that microbes are capable of exploiting. Notably, a few types of microbes are obligate halorespirers, meaning the only known terminal electron acceptors for the cells are halogenated organic compounds. Other types of microbes are facultative with respect to their use of halogenated organics as terminal electron acceptors.

Under some circumstances, microbes carry out reductive dehalogenation but are unable to conserve energy from the process. For example, several strains of microbes are able to use PCE, TCE and ''cis''-DCE as terminal electron acceptors, whereas reduction of VC to ethene is not a growth-linked respiratory process<ref>Maymó-Gatell, X., Anguish, T. and Zinder, S.H., 1999. Reductive dechlorination of chlorinated ethenes and 1, 2-dichloroethane by ''Dehalococcoides ethenogenes'' 195. Applied and Environmental Microbiology, 65(7), 3108-3113. [//www.enviro.wiki/images/2/22/Maymo-Gatell-1999-Reductive_dechlorination.pdf Report pdf]</ref><ref>Maymó-Gatell, X., Nijenhuis, I. and Zinder, S.H., 2001. Reductive dechlorination of cis-1,2-dichloroethene and vinyl chloride by ''Dehalococcoides ethenogenes''. Environmental Science & Technology, 35(3), 516-521. [https://doi.org/10.1021/es001285i doi: 10.1021/es001285i]</ref>. These cultures are able to reduce VC to ethene when growing with the other chlorinated ethenes as electron acceptors, but when provided with only VC, they are unable to grow. Under these circumstances, the transformation of VC to ethene is referred to as cometabolic, i.e., the transformation process is not linked to growth. In general, organohalide respiration occurs at a higher rate than reduction via cometabolism. Some microbes are capable of reducing VC to ethene via respiration, others are not.

Organohalide respiration appears to be a widely-distributed process in nature, even in some environments that have not previously been contaminated by human activity<ref>Krzmarzick, M.J., Crary, B.B., Harding, J.J., Oyerinde, O.O., Leri, A.C., Myneni, S.C. and Novak, P.J., 2012. Natural niche for organohalide-respiring ''Chloroflexi''. Applied and Environmental Microbiology, 78(2), 393-401. [https://doi.org/10.1128/aem.06510-11 doi: 10.1128/AEM.06510-11]</ref>. The widespread distribution of organohalide respiring microbes is likely related to the natural formation of halogenated compounds, which have been generated on the planet long before human activity increased the rate and amount of halogenated compounds released to the environment. Having the capacity to dehalogenate is essential in natural systems in which organohalide compounds are also synthesized.

The process of organohalide respiration is centered on reductive dehalogenases, an iron–sulfur and coronoid containing family of enzymes that break carbon-halogen bonds. Among the best characterized genes are the ones involved in reductive dehalogenation of PCE, TCE, ''cis''-DCE, and VC. Several microbes and enzymes are involved in each reduction step (Fig. 2).

[[File:Freedman_BRP_Fig2.jpg|700 px|thumb|center|Figure 2. Key genes in hydrogenolysis of chlorinated ethenes; M = metabolic, C = cometabolic]]

The identification of dehalogenases has progressed to the point that quantification of key genes (e.g., ''tceA'', ''bvcA'', ''vcrA'', and others) in environmental samples is now a routine part of assessing the capacity for reductive dehalogenation to occur in the environment.

[[File:Freedman Article 1 Figure 6.PNG|thumb|500 px|left|Figure 3. Schematic representation of a microbial cell carrying out organohalide respiration. Blue shape = the cell membrane; red oval = hydrogenase; yellow oval = electron carrier and proton translocation; orange oval = reductive dehalogenase; green shape = ATP synthase (modified from Jugder et al. (2016)<ref name="Jugder2016" />).]]
Figure 3 presents a simplified schematic for how energy may be conserved during organohalide respiration. Many details of the process still need to be resolved and likely vary among the growing list of organohalide respiring microbes<ref name="Jugder2016">Jugder, B.E., Ertan, H., Bohl, S., Lee, M., Marquis, C.P. and Manefield, M., 2016. Organohalide Respiring Bacteria and Reductive Dehalogenases: Key Tools in Organohalide Bioremediation. Frontiers in Microbiology, 7. [https://doi.org/10.3389/fmicb.2016.00249 doi: 10.3389/fmicb.2016.00249]</ref>. Reductive dehalogenases are a key component of the respiratory chain, which in the example shown culminates in development of a proton motive force and subsequent synthesis of adenosine triphosphate (ATP).

Among the various microbes capable of organohalide respiration, ''Dehalococcoides'' are the most frequently mentioned because of their capacity for complete reduction of chlorinated ethenes (i.e., PCE, TCE, DCEs, and VC) to ethene<ref>Löffler, F.E., Yan, J., Ritalahti, K.M., Adrian, L., Edwards, E.A., Konstantinidis, K.T., Müller, J.A., Fullerton, H., Zinder, S.H. and Spormann, A.M., 2013. Dehalococcoides mccartyi gen. nov., sp. nov., obligately organohalide-respiring anaerobic bacteria relevant to halogen cycling and bioremediation, belong to a novel bacterial class, Dehalococcoidia classis nov., order Dehalococcoidales ord. nov. and family Dehalococcoidaceae fam. nov., within the phylum Chloroflexi. International Journal of Systematic and Evolutionary Microbiology, 63(2), 625-635. [https://doi.org/10.1099/ijs.0.034926-0 doi: 10.1099/ijs.0.034926-0]</ref>. At this point, members of this genus are the only known that are capable of completely dechlorinating the chlorinated ethenes to ethene, and more specifically, the steps from ''cis''-DCE to VC, and VC to ethene. Their versatility extends to use of many other organohalides as terminal electron acceptors, including polychlorinated biphenyls, chlorinated ethanes, and chlorinated benzenes. ''Dehalococcoides'' use only hydrogen as an electron donor and acetate as a carbon source. Their limited electron donor use stands in contrast to other organohalide respiring microbes, which are able to use a variety of organic compounds as electron donors. Other key types of organohalide respiring microbes (that cannot generate ethene) include ''Dehalobacter'', ''Dehalogenimonas'', ''Desulfitobacterium'', ''Sulfurospirillum'', and a number of ''Deltaproteobacteria''<ref name="Adrian2016" /> 2. ''Dehalobacter'' includes microbes that respire chlorinated ethanes<ref>Sun, B., Griffin, B.M., Ayala-del-Rı́o, H.L., Hashsham, S.A. and Tiedje, J.M., 2002. Microbial dehalorespiration with 1, 1, 1-trichloroethane. Science, 298(5595), 1023-1025. [https://doi.org/10.1126/science.1074675 doi: 10.1126/science.1074675]</ref>, chlorinated ethenes<ref>Holliger, C., Hahn, D., Harmsen, H., Ludwig, W., Schumacher, W., Tindall, B., Vazquez, F., Weiss, N. and Zehnder, A.J., 1998. Dehalobacter restrictus gen. nov. and sp. nov., a strictly anaerobic bacterium that reductively dechlorinates tetra-and trichloroethene in an anaerobic respiration. Archives of Microbiology, 169(4), 313-321. [https://doi.org/10.1007/s002030050577 doi: 10.1007/s002030050577]</ref>, and chloroform<ref>Tang, S., Wang, P.H., Higgins, S.A., Löffler, F.E. and Edwards, E.A., 2016. Sister ''Dehalobacter'' Genomes reveal specialization in organohalide respiration and recent strain differentiation likely driven by chlorinated substrates. Frontiers in Microbiology, 7. [https://doi.org/10.3389/fmicb.2016.00100 doi: 10.3389/fmicb.2016.00100]</ref>.

==Environmental Conditions==
With few exceptions, reductive dehalogenation occurs under anoxic conditions<ref name="Adrian2016" />. With the exception of nitrate, reductive dehalogenation has been observed in the presence of other anaerobic terminal electron acceptors, including ferric iron and sulfate. The effect of iron and sulfate on the rate and extent of reductive dechlorination is a matter of some debate, with some observing that these compounds (or the reduced forms) are inhibitory (e.g., via competition for hydrogen or the toxicity of sulfide) while others have shown that iron reduction is beneficial to the process<ref name="Aulenta2006" /><ref>Wei, N. and Finneran, K.T., 2011. Influence of ferric iron on complete dechlorination of trichloroethylene (TCE) to ethene: Fe (III) reduction does not always inhibit complete dechlorination. Environmental Science & Technology, 45(17), 7422-7430. [https://doi.org/10.1021/es201501a doi 10.1021/es201501a]</ref>. An environment in which a community of anaerobes produces an excess of cobalamin (vitamin B12) is beneficial, since this coenzyme is an essential component of several dehalogenases<ref>Yan, J., Im, J., Yang, Y. and Löffler, F.E., 2013. Guided cobalamin biosynthesis supports ''Dehalococcoides mccartyi'' reductive dechlorination activity. Phil. Trans. R. Soc. B, 368(1616), 20120320. [https://doi.org/10.1098/rstb.2012.0320 doi: 10.1098/rstb.2012.0320]</ref>. Halogenated compounds can also be reduced in the presence of methanogens; however, methanogens compete for hydrogen as an electron donor and may at times limit the dechlorination reactions.

Circumneutral pH is considered to be optimum for complete reduction of chlorinated ethenes to ethene<ref>Robinson, C., Barry, D.A., McCarty, P.L., Gerhard, J.I. and Kouznetsova, I, 2009. pH control for enhanced reductive bioremediation of chlorinated solvent source zones. Science of the Total Environment, 407(16), 4560-4573. [https://doi.org/10.1016/j.scitotenv.2009.03.029 doi 10.1016/j.scitotenv.2009.03.029]</ref><ref>Vainberg, S., Condee, C.W. and Steffan, R.J., 2009. Large-scale production of bacterial consortia for remediation of chlorinated solvent-contaminated groundwater. Journal of Industrial Microbiology & Biotechnology, 36(9), 1189-1197. [https://doi.org/10.1007/s10295-009-0600-5 doi: 10.1007/s10295-009-0600-5]</ref>. However, organohalide respiring microbes other than ''Dehalococcoides'' tolerate lower pH levels (e.g., as low as 4). There is growing evidence to indicate that strains of ''Dehalococcoides'' exist that are also tolerant of pH levels below circumneutral. For example, the pH range for a commonly used bioaugmentation culture that includes ''Dehalococcoides'' is 5.8-6.3 ([http://siremlab.com/kb-1-kb-1-plus/ SIREM]). This is an important consideration for bioaugmentation in low pH aquifers, since there are significant challenges associates with adjusting groundwater pH.

==Significance==
Our understanding of the microbial processes that result in reductive removal of halogens from halogenated organic compounds has grown remarkably over the past three decades. The field has advanced from an assumption that halogenated organic compounds are non-biodegradable to our current understanding that not only do microbes perform dehalogenation reactions, but many do so via a growth-linked reductive, respiratory process (i.e. “breathing with chlorinated solvents”<ref name="McCarty1997" />). This underlying science has formed the basis for the practice of bioremediation, which has revolutionized the options available for cleaning up hazardous waste sites. Exciting new discoveries await that will open the door to biological treatment of emerging contaminants, as well as improvements in how to treat halogenated organics at complex sites where there are often mixtures of contaminants.

==References==

<references />

==See Also==

*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1167 Aerobic and Anaerobic Transformation of cis-DCE and VC: Steps for Reliable Remediation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1168 Characterization of the Aerobic Oxidation of cis-DCE and VC in Support of Bioremediation of Chloroethene-Contaminated Sites]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1169 Factors Affecting cis-DCE and VC Biological Transformation under Anaerobic Conditions]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1556 Characterization of Microbes Capable of Using Vinyl Chloride as a Sole Carbon and Energy Source by Anaerobic Oxidation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1557 Elucidation of the Mechanisms and Environmental Relevance of cis-Dichloroethene and Vinyl Chloride Biodegradation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1558 Microbial Dichloroethene and Vinyl Chloride Oxidation and the Fate of Ethene and Ethane Under Anoxic Conditions]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-199921 Push-Pull Tests for Evaluating the In-Situ Aerobic Treatment of Chlorinated Mixtures in Groundwater]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200516 Enhancing Natural Attenuation through Bioaugmentation with Aerobic Bacteria that Degrade cis-1,2-Dichloroethene]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201026/ER-201026 Incorporating Aerobic Processes into Remedies for Large Chlorinated Solvent Plumes]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200316 Enhanced Oxidative Bioremediation of cis-Dichloroethene and Vinyl Chloride Using Electron Shuttles]
*[https://www.coursera.org/learn/natural-attenuation-of-groundwater-contaminants/lecture/6UJTE/biodegradation-mechanisms-chlorinated-solvents-vs-hydrocarbons Online Lecture Course - Chlorinated Solvents Biodegradation]

Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions

2026-05-07T17:04:31Z

Admin:

The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective ''in situ'' approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.
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'''Related Article(s):'''

*[[Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| Monitored Natural Attenuation of Chlorinated Solvents]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
*[[Matrix Diffusion]]
*[[REMChlor - MD]]

'''Contributor(s):''' [[Dani Tran]], [[Dr. Charles Schaefer]], and [[Dr. Charles Werth]]

'''Key Resource(s):'''
*Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils<ref name="SchaeferEtAl2025"/>

==Introduction==
Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.

For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as [[REMChlor - MD]]<ref>Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]  [[Media: FaltaWang2017.pdf | Open Access Manuscript]]</ref><ref>Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. [https://doi.org/10.1016/j.jconhyd.2022.103986 doi: 10.1016/j.jconhyd.2022.103986]  [[Media: KulkarniEtAl2022.pdf | Open Access Manuscript]]</ref> to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.

==Recommended Approach==
[[File: TranFig1.png | thumb | 500 px | Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions. Circles are data from Schaefer ''et al.'', 2021<ref>Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2021. Abiotic dechlorination in the presence of ferrous minerals. Journal of Contaminant Hydrology, 241, 103839. [https://doi.org/10.1016/j.jconhyd.2021.103839 doi: 10.1016/j.jconhyd.2021.103839]  [[Media: SchaeferEtAl2021.pdf | Open Access Manuscript]]</ref>, filled squares from Schaefer ''et al.'', 2018<ref name="SchaeferEtAl2018"/>, and Schaefer ''et al.'', 2017<ref>Schaefer, C.E., Ho., Gurr, C., Berns, E., Werth, C., 2017. Abiotic dechlorination of chlorinated ethenes in natural clayey soils: impacts of mineralogy and temperature. Journal of Contaminant Hydrology, 206, pp. 10-17. [https://doi.org/10.1016/j.jconhyd.2017.09.007 doi: 10.1016/j.jconhyd.2017.09.007]  [[Media: SchaeferEtAl2017.pdf | Open Access Manuscript]]</ref>, and open squares from Schaefer ''et al.'', 2025<ref name="SchaeferEtAl2025"/>. ]]
[[File: TranFig2.png | thumb | 600 px | Figure 2: Flowchart diagram of field screening procedures]]
The recommended approach builds upon the methodology and findings of a recent study<ref name="SchaeferEtAl2025">Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. [https://doi.org/10.1111/gwmr.12709 doi: 10.1111/gwmr.12709]</ref>, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:
#Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
#Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
#Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
#Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)r) is estimated as shown in Equation 1:

::'''Equation 1:'''       <big>''Fe(II)r = DA + XRDpyr - XRDbiotite''</big>

where ''DA'' is the ferrous content from the dilute acid (1% HCl) extraction, ''XRDpyr'' is the pyrite content from XRD analysis, and ''XRDbiotite'' is the biotite content from XRD analysis<ref name="SchaeferEtAl2025"/>.

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1<ref name="SchaeferEtAl2018">Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. [https://doi.org/10.1021/acs.est.8b04108 doi: 10.1021/acs.est.8b04108]</ref><ref>Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. [https://doi.org/10.1016/j.jconhyd.2021.103889 doi: 10.1016/j.jconhyd.2021]  [[Media: BordenCha2021.pdf | Open Access Manuscript]]</ref>. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s ''r'' = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

If Fe(II)r is ≥ 100 mg/kg, a first-order dechlorination rate constant can be estimated and subsequently used within a contaminant fate and transport model. However, if acetylene is detected in the clay, even with Fe(II)r less than 100 mg/kg, then bench-scale testing using methods similar to those described in a recent study<ref name="SchaeferEtAl2025"/> is recommended, as such results would likely be inconsistent with those shown in Figure 1, suggesting some other mechanism might be involved, or that the system mineralogy might be more complex than anticipated. Even if Fe(II)r ≥ 100 mg/kg, confirmatory bench-scale testing may be conducted for additional verification and to refine estimation of the abiotic dechlorination rate constant.

==Summary and Recommendations==
The approach outlined above is intended to serve as a generalized guide for practitioners and site managers to cost-effectively determine the extent to which beneficial abiotic reductive dechlorination reactions are likely occurring in low permeability (e.g., clayey) zones. This approach may be contraindicated if co-contaminants are present. It is currently unclear whether other classes of potentially reactive chemicals, such as trinitrotoluene (TNT) or chlorinated ethanes, could interact competitively with PCE and TCE.

In addition, it remains unclear how other classes of compounds such as per- and polyfluoroalkyl substances (PFAS) may interact or sorb with ferrous minerals and potentially inhibit abiotic dechlorination reactions. Coupling these recommended activities with conventional site investigation tasks would provide an opportunity to perform many of the up-front screening activities with minimal additional project costs. It is important to note that the guidance proposed herein pertains to particularly low permeability media. Sites with complex or varying lithology, where the mineralogy and/or redox conditions may vary, might require evaluation of multiple samples to provide appropriate site-wide information.
 

==References==
<references />

==See Also==
*[https://serdp-estcp.mil/projects/details/a7e3f7b5-ed82-4591-adaa-6196ff33dd60 ESTCP Project ER20-5031 – In Situ Verification and Quantification of Naturally Occurring Dechlorination Rates in Clays: Demonstrating Processes that Mitigate Back-Diffusion and Plume Persistence]

REMChlor - MD

2026-05-07T17:04:06Z

Admin:

REMChlor-MD is a free toolkit available for download and is capable of simulating matrix diffusion in groundwater contaminant plumes. It is a significant upgrade from REMChlor 1.0<ref>Falta, R.W., 2008. Methodology for comparing source and plume remediation alternatives. Groundwater, 46(2), pp.272-285. [https://doi.org/10.1111/j.1745-6584.2007.00416.x doi: 10.1111/j.1745-6584.2007.00416.x]</ref>, which does not include matrix diffusion in the plume. REMChlor-MD is useful for planning-level approximations of contamination extent and duration at sites where matrix diffusion is important. REMChlor-MD employes a semi-analytical method for simulating mass transfer between high and low permeability zones that provides computationally accurate predictions of concentration distributions and mass discharge in the higher permeability portions of an aquifer. Model run times for REMChlor-MD are much shorter than traditional fine grid numerical models.

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'''Related Article(s)''':

*[[Dispersion and Diffusion]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Matrix Diffusion]]
*[[Plume Response Modeling]]
*[[Source Zone Modeling]]

'''Contributor(s):''' [[Dr. Ron Falta]] and [[Kien Pham]]

'''Key Resource(s)''':

*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426 REMChlor-MD Toolkit package]

==Matrix Diffusion of Dissolved Contaminants==
[[File:Falta1w2 Fig1.png|thumb|450px|Figure 1. Spread of CVOCs in the Subsurface]]
Dissolved contaminants, such as [[Chlorinated Solvents | chlorinated volatile organic compounds (CVOCs)]], are found in groundwater at many current and former industrial sites globally. Plumes of CVOCs dissolved in groundwater often originate when the chemicals enter the subsurface as dense non-aqueous phase liquids (DNAPL). Being heavier than water and immiscible, DNAPLs are particularly problematic because they can spread vertically into the aquifer, generating extensive contaminated groundwater plumes. In Figure 1, CVOCs in DNAPL form are shown in bold red, and the resulting dissolved groundwater plume is represented by the pink halo emanating from the DNAPLs. One of the most significant lessons learned about chlorinated solvents is that their groundwater plumes can persist even when the source DNAPLs have been depleted. Plume persistence in the absence of source materials has been attributed to a phenomenon called matrix diffusion. Matrix diffusion can affect any type of dissolved contaminant, particularly when dissolved concentrations are high relative to regulatory standards. In addition to CVOCs, matrix diffusion can occur with other dissolved contaminants including radionuclides, [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | per- and polyfluoroalkyl substances (PFAS)]], dissolved petroleum hydrocarbons such as benzene, toluene, ethylbenzene, and xylene (BTEX), and with solvent stabilizers such as [[1,4-Dioxane | 1,4-dioxane]].

The life cycle of a matrix diffusion site is separated into two periods. In the loading period, the [[Characterization Methods – Hydraulic Conductivity | low-permeability (low K)]] zones act as storage areas for the contaminant mass. Following clean-up actions in the transmissive (high K) zones, the concentration gradient is reversed. The unloading period begins when the contaminant mass previously stored in the low-permeability zones diffuses back out into the transmissive zones. In Figure 1, the blue arrows represent the forward and backward diffusion gradients into the surrounding materials. Because the pace of back diffusion can be significantly slower than the loading of the contaminant, the plume resulting from the back diffusing mass can last for decades to centuries<ref>Chapman, S.W. and Parker, B.L., 2005. Plume persistence due to aquitard back diffusion following dense nonaqueous phase liquid source removal or isolation. Water Resources Research, 41(12). [https://doi.org/10.1029/2005WR004224 doi: 10.1029/2005WR004224]</ref><ref>Parker, B.L., Chapman, S.W. and Guilbeault, M.A., 2008. Plume persistence caused by back diffusion from thin clay layers in a sand aquifer following TCE source-zone hydraulic isolation. Journal of Contaminant Hydrology, 102(1-2), pp.86-104. [https://doi.org/10.1016/j.jconhyd.2008.07.003 doi: 10.1016/j.jconhyd.2008.07.003]</ref>.

Given the prevalence and persistence of back diffusion plumes, modeling tools are needed that can accurately capture the dynamic progression of the contamination in both loading and unloading periods.

==Matrix Diffusion Modeling Options==
[[File:Falta1w2 Fig2.PNG|thumb|450px|Figure 2. A Typical Fine Grid Model Capable of Simulating Matrix Diffusion]]
There have been limited options available for simulating matrix diffusion effects at groundwater contamination sites. Available analytical models require simplifications that may overlook the unique complexities at actual remediation sites. Numerical models are often used to simulate solute transport, but require fine discretization to capture matrix diffusion concentration gradients, which occur at millimeters to centimeters scale. This very fine discretization (fine model grid) greatly increases the number of model cells required to simulate transport at field sites, greatly increasing computer model run times. Figure 2 shows a typical fine grid model containing almost 3 million gridblocks (cells).

Another type of numerical model of matrix diffusion is the dual-porosity model which is more computationally efficient because it uses a first order approximation to describe the mass transfer between the high K and low K zones. Dual-porosity models can be calibrated to match the effects of matrix diffusion on contaminant concentrations over short time periods. However, the first order mass transfer coefficient is time-dependent<ref>Guan, J., Molz, F.J., Zhou, Q., Liu, H.H. and Zheng, C., 2008. Behavior of the mass transfer coefficient during the MADE‐2 experiment: New insights. Water Resources Research, 44(2). [https://doi.org/10.1029/2007WR006120 doi: 10.1029/2007WR006120]</ref>, so models calibrated to early time data will not accurately simulate concentrations at a later time.

The REMChlor-MD model was developed under ESTCP project [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426 ER-201426] to provide a practical alternative to previous approaches for modeling matrix diffusion.

==REMChlor-MD’s Modeling Approach==
REMChlor-MD employs a semi-analytical method adapted from a heat diffusivity estimation strategy used in petroleum engineering and applied here to the problem of chemical diffusion with first order decay kinetics<ref>Vinsome, P.K.W. and Westerveld, J., 1980. A simple method for predicting cap and base rock heat losses in'thermal reservoir simulators. Journal of Canadian Petroleum Technology, 19(03). Pp 87-90 [https://doi.org/10.2118/80-03-04 doi: 10.2118/80-03-04]</ref><ref name="Falta2017">Falta, R.W. and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of contaminant hydrology, 197, pp.39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]</ref>. This semi-analytical method utilizes a fitting function to describe the concentration of the chemical in the low K zones in, or adjacent to, each gridblock:

::'''Equation 1:'''    <big>''Cl''</big>''(zl,t)<big> = (C</big>t + Δt<big> + pzl + qzl2) e</big>-zl / d''

{|
|-
|where:
|-
|''Cl''||represents the concentration in the low K material;
|-
|''Ct + Δt''||is the concentration in the high K zone during the next numerical time-step;
|-
|''zl''||is the distance into the low K zone from the high K / low K interface;
|-
|''p'' and ''q''    ||are fitting parameters that can be solved for by enforcing the diffusion differential equation at the interface and mass conservation in the matrix gridblock; and
|-
|''d''||is the concentration penetration depth, which is defined in the REMChlor-MD User’s Manual in Appendix 1.
|}

Using the trial function, the diffusive mass flux can be computed analytically, which is the key time-saving component of the method. The only variable that requires numerical simulation is the concentration in the high-permeability parts of the aquifer.

There are three key geometrical model inputs to the semi-analytical method that can be used to model the diffusion process:

*''Vf'' is the volume fraction of the high-permeability materials in a gridblock;
*''Amd'' is the interfacial area between the high-K and low-K materials in a gridblock; and
*''L'' is the characteristic maximum diffusion length.

Because the volume of low K material in the gridblock should equal the product of the interfacial area and the characteristic maximum diffusion length, the third parameter (L) can be calculated from the first two using the volume balance equation for a gridblock of volume V:

::'''Equation 2:'''    <big>''Amd L = (1 – Vf) V''</big>

This semi-analytical method has undergone thorough testing and proved to be accurate in numerous scenarios<ref name="Falta2017" /><ref name="Muskus2018">Muskus, N. and Falta, R.W., 2018. Semi-analytical method for matrix diffusion in heterogeneous and fractured systems with parent-daughter reactions. Journal of Contaminant Hydrology, 218, pp.94-109. [https://doi.org/10.1016/j.jconhyd.2018.10.002 doi: 10.1016/j.jconhyd.2018.10.002]</ref>. The following section shows common test scenarios where the method can be used.

[[File:Falta1w2 Fig3.png|thumb|left|Figure 3. Concentration profiles in fractures at 1, 49, 51 and 100 years.]]
[[File:Falta1w2 Fig4.png|thumb|right|Figure 4: Simulated mass discharge at downstream edge of fine-grid MT3DMS and REMChlor-MD models.]]
[[File:Falta1w2 Fig5.png|thumb|left|Figure 5: Top view (''xy'') of concentration contours computed with the fine-grid numerical model (top) and with REMChlor-MD (bottom) at 30 years.]]
[[File:Falta1w2 Fig6.png|thumb|right|Figure 6: Top view (''xy'') of concentration contours computed with the fine-grid numerical model (top) and with REMChlor-MD (bottom) at 130 years.]]

==Modeling Examples==

===Matrix Diffusion in Fractured Media===
Contaminants in fractured rock media are often the most challenging to remediate because of the small volume of individual fractures and the large surface area of the matrix. Networks of fractures can store a large mass of contaminants and potentially become a secondary source over time.

The semi-analytical method was tested in a parallel fractured system, and the results were compared against an exact analytical solution<ref>Sudicky, E.A. and Frind, E.O., 1982. Contaminant transport in fractured porous media: Analytical solutions for a system of parallel fractures. Water Resources Research, 18(6), pp.1634-1642. [https://doi.org/10.1029/WR018i006p01634 doi: 10.1029/WR018i006p01634]</ref>. In this example, the fracture spacing was set at 2 m, and the fracture aperture was 100 ''µm''. A known concentration of trichloroethene (TCE) was introduced into the fractures at the upstream end of the model, where its concentration was held constant for 50 years. Then the contaminant source was completely removed, and clean water was flushed through the fractures for another 50 years. Figure 3 shows the concentration profiles in the fractures calculated using the semi-analytical method over 100 years of simulation time compared to the exact analytical solution.

In the 1-year profile, TCE concentrations in the fractures decreased rapidly with distance, since the steep concentration gradient results in rapid mass transfer from the fracture into the low K matrix. Toward the end of the loading period at 49 years, TCE concentration declines more gradually with distance, since TCE has now diffused into the adjoining low K matrix, reducing the concentration gradient and associated mass flux. At 51 years, one year after source removal, there was a decrease in TCE concentration near the inlet, but almost no effect downstream in the fractures due to back diffusion of TCE from the matrix to the fractures. After 50 years of clean water flushing, back diffusing TCE was still acting as a secondary source, reaching a peak concentration of approximately 16 mg/L. The profiles show a typical fractured system prior to and after source removal with matrix diffusion effects preventing the system from reaching clean-up level.

The match between the analytical solution and the semi-analytical method was good, particularly during the loading period. In the unloading period, the semi-analytical method overestimated the peak concentration somewhat, but it still captured the long tailing of concentration almost exactly.

[[File:Falta1w2 Fig7.png|thumb|right|400px| link=https://www.enviro.wiki/images/c/c5/Falta1w2_Fig7.mp4 | [//www.enviro.wiki/images/c/c5/Falta1w2_Fig7.mp4 Figure 7: Video tutorial demonstrating use of REMChlor-MD]]]

===Matrix Diffusion in Heterogeneous Media===
One of the motivations for REMChlor-MD was to develop an alternative to a computationally expensive fine grid model for simulating matrix diffusion. To evaluate the ability of the semi-analytical method incorporated into REMChlor-MD to accurately simulate back-diffusion, a heterogeneous fine grid model was developed<ref name="Muskus2018" /> using MT3DMS. A geostatistical model generated multiple realizations of the subsurface’s heterogeneity using synthetic borehole data. Using the heterogeneous material distribution of sand and clay, a fine grid flow and transport model, consisting of almost 3 million gridblocks, was created. A TCE source was present near the upgradient end of the model for 30 years and then it was completely removed. Following source removal, clean water was flushed through the system for another 200 years.

For comparison, a REMChlor-MD model was developed with a much coarser grid (about 14,000 gridblocks) where flow and transport parameters were comparable to the fine grid model. The mass discharge at the outlet of the models was used as a comparison criterion between the two models. Figure 4 shows a comparison of the computed mass discharge versus time at the downstream edge of the model. The mass discharge predicted by the 3-million gridblock MT2DMS simulation is shown as the yellow dots. The green line shows the REMChlor-MD prediction based on the geostatistical properties of the high and low K zones (characteristic maximum diffusion length, L= 1.85m). The REMChlor-MD match to the fine-grid MT3DMS model was slightly improved by reducing the characteristic maximum diffusion length (L= 1.5m). Both of these results are similar to the 3-million gridblock MT3DMS simulation result.

Plan view concentrations from the fine grid model and the semi-analytical model were extracted to generate contours of the TCE plume in Figures 5 and 6 after 30 and 130 years, respectively. The top panels of these figures were generated with the fine grid MT3DMS model, whereas the bottom panels were generated with the REMChlor-MD model. The fine grid contour has jagged edges because small changes in concentration were captured at a higher spatial resolution. In contrast, the semi-analytical contour was smoother because the concentration changes were interpolated over larger gridblocks. However, the overall shape and contours from the two models are similar. On a high performance 2018 workstation, the run time for the fine-grid MT3DMS model was many hours, while the REMChlor-MD model completed its run in a little over one minute.

==REMChlor-MD User Interface==
The semi-analytical method was programmed in FORTRAN, and a graphical user interface (GUI) was built using Visual Basic in Excel® to facilitate data inputs and output processing<ref>Farhat, S. K., Newell, C. J., Falta, R. W., and Lynch, K. (2018). REMChlor-MD toolkit user’s manual. ER-201426 [//www.enviro.wiki/images/0/0b/2018-Falta-REMChlor_Modeling_Matrix_Diffusion_Effects.pdf Report.pdf]</ref>. The Visual Basic REMChlor-MD interface calls the FORTRAN executable using a dynamic link library, and then processes the FORTRAN output files to produce various graphs. A short video tutorial demonstrating data entry and some of the capabilities of REMChlor-MD is shown in Figure 7. Additional detail is provided in an online [https://www.serdp-estcp.org/Tools-and-Training/Webinar-Series/02-07-2019 webinar]. The toolkit package, including a user’s manual is available free of charge and can be downloaded here ([https://www.serdp-estcp.org/content/download/48566/462193/file/REMChlorMD_64bit.zip 64bit version]) and ([https://www.serdp-estcp.org/content/download/48565/462183/file/REMChlorMD_32bit.zip 32 bit version]). Readers are referred to the [https://www.serdp-estcp.org/content/download/48433/460814/file/ER-201426%20REMChlor-MD%20User's%20Manual.pdf user manual] for a complete explanation of the program, its input parameters, outputs, and examples.

==References==

<references />

==See Also==

REMChlor - MD

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Matrix Diffusion

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Monitored Natural Attenuation - Transitioning from Active Remedies

2026-05-07T16:59:00Z

Admin:

Many contaminated sites use active remedies such as pump-and-treat or ''in situ'' remediation to clean up impacted groundwater. Natural attenuation processes such as natural degradation or [[Dispersion and Diffusion | hydrodynamic dispersion]] also contribute to the cleanup. As remediation progresses, a point is often reached when the time required to reach the remedial objectives using the active remedy is roughly the same as the time required if the active remedy is shut down, and the continuing remediation of the site is provided by natural attenuation processes alone. From that point forward, the extra effort and expense of the active remedy provides no benefit over natural attenuation, and it may be appropriate to transition the site to [[Monitored Natural Attenuation (MNA)]]. This article deals with currently available tools and approaches that can be used to support a decision to transition from active remediation to MNA.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s)''':

*[[Alternative Endpoints]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Plume Response Modeling]]
*[[REMChlor - MD]]
*[[Source Zone Modeling]]

'''Contributor(s):'''

*[[Dr. John Wilson]]
*[[Dr. David Adamson, P.E.]]

'''Key Resource(s)''':

*[//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies]<ref name="Newell2002">Newell, C.J., Rifai, H.S., Wilson, J.T., Connor, J.A., Aziz, J.A., Suarez, M.P., 2002. Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies. 28p. EPA/540/S-02/500. [//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Report.pdf]</ref>

*[https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS) Version 2.3.3]<ref name="Widdowson2008">Widdowson, M.A., Mendez, E., Chapelle, F.H., Casey, C.C., 2008. Natural Attenuation Software (NAS) Version 2.3.3. Virginia Polytechnic Institute and State University, the United States Geological Survey, and the United States Naval Facilities Engineering Command. NAS webpage: https://www.nas.cee.vt.edu/index.php See also: https://toxics.usgs.gov/highlights/nas_2.2.0/index.html</ref>

*[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf BIOCHLOR Natural Attenuation Support System, Version 2.2]<ref name="Aziz2002">Aziz, C.E., Newell, C.J. and Gonzales, J.R., 2002. BIOCHLOR Natural Attenuation Decision Support System Version 2.2 User’s Manual Addendum. Groundwater Services, Inc., Houston, Texas for the Air Force Center for Environmental Excellence.[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf Report.pdf] Available at: https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system</ref>

*[https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC User's Guide and Tool Website]<ref name="BioPIC2021">Danko, A., Adamson, D., Newell, C., Wilson, J., Wilson, B., Freedman, D., Lebrón, C., 2021. Quick BioPIC User’s Guide, ESTCP Project ER-201730. [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 Project Website]   [//www.enviro.wiki/images/c/c9/ER-201730_BioPIC_User%27s_Guide.pdf User’s Guide]</ref>

*[https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool Website]<ref name="TATA2024">Adamson, D.T., Newell, C.J., Hort, H.M, Wilson, J.T., 2024. TA2: The SERDP Transition Assessment Teaching Assistant. Strategic Environmental Research and Development Program (SERDP) Project ER20-1429. [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview Project Website]  [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Online Tool]</ref>

==Introduction==
Many active remedies are effective at treating higher concentrations of contaminants, but as the contaminant concentrations decrease, the rate of cleanup may slow before the site reaches the cleanup goal. At some sites, the rate of cleanup may slow until it is not significantly different from the rate of cleanup provided by the natural attenuation processes that occur at the site. At other sites, the concentration of contaminants in water produced by a pumping system is below the cleanup goal, but the concentration in monitoring wells in the source area are still above the goal. At some sites, active treatment has stopped further expansion of the plume toward a receptor, and concentrations are declining over time throughout the plume, but back diffusion is sustaining concentrations in the plume that are above the cleanup goal.

In 2013, a significant National Research Council (NRC) report noted that despite years of effort and considerable investment, many sites “will require long-term management that could extend for decades or longer”<ref name="NRC2013">National Research Council (NRC), 2013. Alternatives for Managing the Nation's Complex Contaminated Groundwater Sites. Committee on Future Options for Management in the Nation's Subsurface Remediation Effort, Water Science, Technology Board, Division on Earth and Life Studies, NRC. National Academies Press, 422 pages, ISBN 978-0-309-27874-4 [https://doi.org/10.17226/14668 doi: 10.17226/14668]. [//www.enviro.wiki/images/4/48/NRC2013.pdf Report.pdf]</ref>. The authors of the report discussed the need for developments that can aid in “transition from active remediation to more passive strategies and provide more cost-effective and protective long-term management of complex sites”<ref name="NRC2013" />.

The United States Environmental Protection Agency<ref> U.S. Environmental Protection Agency (USEPA), 1999. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. OSWER Directive 9200.4-17P. 39pp.[//www.enviro.wiki/images/a/aa/1999_USEPA-_Use_of_monitored_natural_attenuation_at_superfund.pdf Report.pdf]</ref> allows the use of [[Monitored Natural Attenuation (MNA) | monitored natural attenuation (MNA)]] to attain the cleanup goals when the site-specific remediation objectives can be attained within a time frame that is reasonable compared to that offered by other more active methods. Many CERCLA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Summary of the Comprehensive Environmental Response, Compensation, and Liability Act (Superfund) https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act</ref> and RCRA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Resource Conservation and Recovery Act (RCRA) Laws and Regulations https://www.epa.gov/rcra</ref> sites take advantage of this policy. An active remedy is typically used initially to treat high concentrations of contaminants followed by MNA to treat the lower concentrations that remain.

Unfortunately, there is no well-established approach to determine when it is appropriate to discontinue the active remedy. The NRC report<ref name="NRC2013" /> emphasized the use of more rigorous evaluations of existing data to support these efforts. This can include a quantitative assessment of the performance of active remedies (e.g., evidence of asymptotic performance) as well as documenting site conditions that may be contributing to these performance limitations. Importantly, it also identifies alternative approaches for managing the site, which could include MNA if the natural attenuation processes can meaningfully contribute to the achievement of site cleanup objectives.

This article reviews available tools and approaches to evaluate a transition to MNA. The tools and approaches depend on calculations of rate constants for natural attenuation with distance in flowing groundwater or rate constants for attenuation over time in individual monitoring wells.

==Background on Rate Constants==
[[File:Wilson1w2Fig1.png|thumb|400px| Figure 1. Attenuation of Trichloroethene (TCE) over time in a monitoring well at a site in Michigan. The concentration vs. time rate constant is 0.326 per year and largely represents the rate of the attenuation of the source of contaminants in the aquifer.]]
At sites where a transition to MNA is being considered, a key step is estimating attenuation rate constants and understanding how they are extracted from monitoring data. A general formula to describe the rate of a chemical reaction is:

:{|
|-
|'''Equation 1:'''|| ||<big>''r = k [C]''</big>'' m''
|-
|where:
|-
| ||''r''    ||is the rate of the reaction,
|-
| ||''k''||is the rate constant,
|-
| ||''C''||is the concentration of the chemical undergoing the reaction, and
|-
|the exponent    ||''m''||is the order of the reaction.
|}

When the rate of the reaction is proportional to the concentration of the contaminant, the value of ''m'' is 1. Therefore, the reaction is described as a first-order reaction, and the rate constant is described as a first-order rate constant. In Equation 1, concentration could go up or down, but ''k'' is a constant of proportionality for the rate of increase in concentration. The rate constant for attenuation is the negative of ''k''. If the rate of degradation is a fixed value regardless of concentration, the value of ''m'' is 0, and degradation is a zero-order process.

Natural attenuation of concentrations over time in monitoring wells is frequently described by a first-order rate constant, and natural biological or abiotic degradation of contaminants in flowing groundwater is typically also described by a first-order rate constant. Figure 1 provides an example of monitoring data that is described by a first-order rate constant.

The rate constant for attenuation over time in a single well and the rate constant for attenuation with distance along a flow path in an aquifer describe different situations that are controlled by different processes. ''Attenuation over time'' in a well is largely controlled by the rate of attenuation of the source of contamination in the aquifer. ''Attenuation with distance'' along a flow path includes attenuation of concentrations in the source along with contributions from biological degradation processes, abiotic degradation processes and hydrodynamic dispersion of the contaminated groundwater into clean groundwater<ref name="Newell2002" />.

The first-order rate constant for attenuation over time in a single well is commonly referred to as ''kpoint''<ref name="Newell2002" />. A time series chart in Microsoft EXCEL of the concentrations of a contaminant (''y'' axis) on the date of sampling (''x'' axis) can be used to extract a value for ''kpoint''. Select the data, then insert an exponential trend line and display the equation on the chart. The value of ''kpoint'' can also be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Note that the rate constants extracted in EXCEL are constants for the rate of change, not the rate of attenuation. Take the negative of the rate of change to get ''kpoint''. In the example in Figure 1, the unit of time on the X axis is years, and the value of ''kpoint'' is 0.326 per year.

Attenuation versus distance rate coefficients describe a bulk attenuation rate including both degradation and non-destructive processes such as dispersion. To extract values for rate constants for degradation alone, it is necessary to calibrate a groundwater flow and transport model to the data at the site. The model is calibrated with values for the hydrogeological properties of the aquifer (effective porosity, hydraulic gradient, hydraulic conductivity, hydrodynamic dispersion and the organic carbon content of the aquifer matrix). After the hydrogeological properties of the aquifer are fixed in the model, the most appropriate values for the degradation rate constants are the values that produce the best fit between the contaminant concentrations that are predicted by the model and the contaminant monitoring data at the site.

There are a number of reasons why natural attenuation processes are better described as first-order relationship instead of zero-order or some other order. The attenuation over time in a monitoring well tracks the attenuation over time of the source of contamination that sustains the plume<ref name="Newell2002" />. Sites go through a lifecycle, and attenuation of sources at mature sites is often a first-order process<ref>Sale, T., Newell, C., Stroo, H., Hinchee, R. and Johnson, P., 2008. Frequently Asked Questions Regarding Management of Chlorinated Solvents in Soils and Groundwater. Environmental Security Technology Certification Program (ESTCP, Project ER-200530), Department of Defense (DoD), Arlington, VA. [//www.enviro.wiki/images/c/cb/2008-Sale-Frequently_Asked_Questions_Regarding_Management_of_Chlorinated_Solvent_in_Soils_and_Groundwater.pdf Report.pdf]   Project Overview Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200530</ref>. If a chlorinated solvent site is mature, the contamination in the source area that was originally present as nonaqueous phase liquids (NAPL) has been redistributed and is now sequestered in a sorbed phase to aquifer solids or has diffused into non-transmissive portions of the aquifer matrix. Transfer of contaminants back into the more transmissive portions of the aquifer occurs by diffusion along a fixed path length, and the rate of transfer is controlled by the concentration of the contaminant remaining in the source material. Because the rate of transfer is proportional to the concentration of contaminant in the source material, attenuation of the source is a first-order process. These processes are discussed in more detail in [[Source Zone Modeling]].

Degradation processes are also usually first order. Abiotic reactions are almost always first order with respect to the concentration of the target chemical. Biodegradation reactions are zero order at high concentrations because the available enzymes are saturated with substrate, but are first order at lower concentrations that are typical of natural attenuation conditions in groundwater.

==Goals for MNA at Sites==

The information necessary to evaluate whether a site can be transitioned to MNA depends on the goal for MNA at the site. For many cleanup actions, the goal is to confine contamination within a waste management area where the contamination is left in place, in which case the cleanup goal applies to point-of-compliance wells that are outside the waste management area. For other cleanup actions, the entire site must be cleaned up, in which case the cleanup goal applies to any monitoring well on the site. The time by which the goal is to be attained is specified at CERCLA sites in the Record of Decision (the ROD). At RCRA sites, the time allowed for the cleanup to be attained may be specified in the permit.

==When the Goal Applies to Point-of-Compliance Wells==
Consider the following framework for evaluating a transition to MNA:

#Use a computer model to extract rate constants for the natural degradation of the contaminant that occurred in groundwater at the site before the active remedy was installed.
#Assume that the same rate constants will apply after the active remedy is no longer in operation. Note that this assumption may not be valid if the active remedy changes the geochemistry of the aquifer in the flow path to the point-of-compliance well.
#Calibrate a computer groundwater flow and transport model with the hydrogeological properties of the aquifer that pertain after the active remedy is no longer in operation, the concentration of contaminant after the active remedy, and the rate constants for natural degradation that are assumed to apply after the active remedy.
#Use the computer model to project the concentrations of the contaminant at the point-of-compliance well over time.
#If the concentrations at the point-of-compliance wells are predicted to be less than the goal before the specified date, that is a quantitative line of evidence in support of a transition to MNA.

There are several computer applications that are particularly useful to extract rate constants at a site from monitoring data that were collected before the active remedy was installed. For example, [https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS)]<ref name="Widdowson2008" />, [https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system BIOCHLOR]<ref name="Aziz2002" /> and [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC]<ref name="BioPIC2021" /> can be downloaded from the internet at no cost. Another recent example, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ TA2 Tool]<ref name="TATA2024" />, is discussed in detail later in this article.

[[File:Wilson1w2Fig2.png|thumb|left|400px| Figure 2. Example calibration of NAS to natural attenuation of total BTEX at a site (Figure 17 of NAS User’s Manual).]]
[[File:Wilson1w2Fig3.png|thumb|400px| Figure 3. The data input screen for BIOCHLOR before remediation with cis-1,2-Dichloroethene (DCE) and vinyl chloride (VC) source concentrations of 500 and 87 mg/L respectively at the source when the release first occurred.]]
[[File:Wilson1w2Fig4.png|thumb|left|400px| Figure 4. Output of the RUN CENTERLINE simulation in BIOCHLOR comparing the fit between the simulation and the field data for vinyl chloride before an active remedy was implemented]]
[[File:Wilson1w2Fig5.png|thumb|400px| Figure 5. Output of the RUN CENTERLINE simulation of conditions after an active remedy was implemented with a source concentration of 1.1 mg/L, projecting the concentration of vinyl chloride at a distance corresponding to a point-of-compliance well.]]
 
In NAS, the user inputs the hydrogeological data, the distance of wells along the flow path, and the concentrations of contaminants in the wells. The NAS application extracts rate constants and makes projections at the point-of-compliance. With NAS, it is possible to extract different rate constants for specific geochemical environments along the flow path.

Figure 2 provides an example calibration of NAS. The concentrations in the monitoring wells used to calibrate the model are compared to the simulation provided by the model. The values of the rate constants that are extracted from the field data are available in the “Output” tab under “Data and Results Table.”

Figure 3 depicts the input screen for BIOCHLOR. The user inputs the hydrogeological parameters, the first-order rate constants (1st Order Decay Coefficient), the distribution of the wells along the flow path, and the concentrations of contaminants in the wells. The model is set up for conditions that apply before the installation of the active remedy.

BIOCHLOR does not automatically fit the rate constants to the field data. Instead, the user examines the output of the model, and adjusts the rate constants until they provide the best fit between the model prediction and the monitoring data for wells at the site. This comparison is illustrated in Figure 4.

If the distance from the source well to the point-of-compliance well is set as the “Modeled Area Length” in Section 5 of the input screen, the “Run Centerline” output will provide the projected concentrations at that length. Assume the distance from the source well to the point-of-compliance well is 250 feet. The projected concentration in Figure 3 of vinyl chloride at a point-of-compliance well is 0.042 mg/L. If the regulatory goal were the federal drinking water maximum contaminant level (MCL)<ref>U. S. Environmental Protection Agency (USEPA), 2009. National Primary Drinking Water Regulations. EPA 816-F-09-004. [//www.enviro.wiki/images/a/ae/2009-USEPA-national_Primary_Drinking_Water_Regulations.pdf Report.pdf]</ref> of 0.002 mg/L, the projected concentration would exceed the goal, and MNA would not be adequate as a remedy.

For the sake of illustration, assume that an active remedy has been implemented, and the concentrations in the source well are 5.4 mg/L for DCE and 1.1 mg/L for vinyl chloride. To evaluate whether it is now appropriate to transition to MNA, BIOCHLOR could be calibrated with these concentrations to predict concentrations in the point-of-compliance well. (See Figure 5). In this example, the projected concentration at the point-of-compliance well does meet the goal.

Some active remedies are subject to rebound. If this is the case, the evaluation should begin at the point in time when it is clear that the trend in concentrations is downward.

A new EXCEL-based tool that does many of the same basic calculations as BIOCHLOR was recently developed as part of an update to the BioPIC<ref name="BioPIC2021" /> decision support software. This tool, the MNA Rate Constant Estimator, extracts rate constants from concentration versus distance data for a variety of different chemicals, including chlorinated ethenes (e.g., PCE and TCE), chlorinated ethanes (e.g., 1,1,1-TCA), and 1,4-dioxane. This tool was developed to run using current versions of EXCEL, whereas BIOCHLOR must be run using older versions of EXCEL that may be unavailable to many users. The MNA Rate Constant Estimator can be used to estimate degradation rate constants and/or predict plume footprints over time. Consequently, it is a useful addition to the BioPIC decision framework for understanding if MNA is appropriate remedy for a site, and it can also be helpful for estimating rate constants as part of a transition assessment.

==When the Goal Applies to All the Wells==
At sites where a concentration-based cleanup goal must be achieved at all wells, each well at the site is evaluated independently, and the rate constant that is applicable is the rate constant for attenuation over time in the well (''kpoint''). To evaluate whether the region in an aquifer that is sampled by a particular monitoring well is ready to transition to MNA, it is necessary to have monitoring data from a period of time before the remedy was implemented. This data is used to extract a value for ''kpoint'' in the aquifer under natural attenuation conditions. The evaluation of a transition to MNA will assume that the same value for ''kpoint'' will apply after the active remedy is complete. This assumption may not be appropriate if the active remedy caused a permanent change in the geochemistry of the aquifer. The assumption is usually appropriate for pump-and-treat remedies.

If ''kpoint'' before implementation of the active remedy describes the time course of natural attenuation after the active remedy is completed, the time required to attain the cleanup goal is predicted from the following:

:{|
| || || rowspan="2" |<big>''ln (''</big>
| style="border-style:solid; border-width: 0px 0px 1px 0px" |''Cgoal''
| rowspan="2" |<big>'')''</big>||                                                                       
|-
|'''Equation 2:'''        ||''<big>t =''    
| style="vertical-align:top;" |''Ccurrent''||
|-
| || || colspan="3" style="text-align:center; border-style:solid; border-width: 1px 0 0 0" |''<big>-k</big>point''||
|-
|where:
|-
| ||''Ccurrent''    || colspan="5" |is the current concentration after active remediation,
|-
| ||''Cgoal''|| colspan="5" |is the cleanup goal, and
|-
| ||''t''|| colspan="5" |is the time required for concentrations to attenuate from ''Ccurrent'' to ''Cgoal.''
|}

If the value of ''t'' estimated using Equation 2 is less than the difference between the current date and the date specified by the site stakeholders to attain the goal, that is evidence in support of a transition to MNA.

Some active remedies are subject to contaminant concentration rebound. If this is the case, the evaluation should use a value of ''C''current that is attained after the rebound has stabilized.

This approach depends on a robust value for ''kpoint''. It is worthwhile to do a sensitivity analysis on ''kpoint'' where the lower 95% or 90% confidence interval on ''kpoint'' is used in Equation 2 to see if that changes the outcome of the evaluation. The confidence intervals can be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Wilson<ref name="Wilson2011">Wilson, J.T. 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [//www.enviro.wiki/images/e/e3/Wilson-2011-An_Approach_for_Evaluating_Progress.pdf Report.pdf]</ref> provides detailed discussion of the use of linear regression to extract ''kpoint'' and confidence intervals on ''kpoint''. Wilson<ref name="Wilson2011" /> also discusses the use of goodness-of-fit tests to determine if there is evidence that a first-order rate equation is not the best fit to the monitoring data, and as a result the use of Equation 2 would not be appropriate. The TA2 Tool<ref name="TATA2024" /> also has the capability to calculate ''kpoint'' with a user-specified confidence interval, as described below.

At many sites, there is no specified date when the cleanup goal must be attained. In this situation, the monitoring data can be evaluated to determine if the current rate of attenuation under the active remedy is faster than the rate of natural attenuation before the active remedy was installed. The monitoring data can be examined to identify a time interval when the benefit of the active remedy has approached an asymptote. A second value of for ''kpoint'' can be extracted for that time interval. The two values for ''kpoint'' can be evaluated statistically to see if the current rate is faster at some appropriate level of confidence. If there is no statistical evidence that the rate of attenuation is faster, that determination can support a decision to transition to MNA.
[[File:Wilson1w2Fig6.png|thumb|400px| Figure 6. Example calibration of NAS to predict the reduced concentration at the source that is necessary to meet the remediation goal at a point-of-compliance well (Figure 19 of NAS User’s Manual).]]

==Extent of Treatment Necessary to Transition to MNA==
There are several computer applications that can predict the extent of treatment that must be achieved by the active remedy before it is worthwhile to evaluate the site for transition to MNA. For example, based on the distribution of contamination along the flow path, the NAS application will automatically predict a reduced concentration at the source well that will bring concentrations to the goal in the point-of-compliance well (Figure 6). A table that opens under the “DOS/TOS” tab provides the “Time of Equilibration” required to meet the goal at the reduced concentration. Modules in NAS allow the user to evaluate the effect of various pump-and-treat and source removal scenarios on the time required to attain the goal at the point-of-compliance well.

The [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018">Falta, R.W., Farhat, S.K., Newell, C.J. and Lynch, K., 2018. A Practical Approach for Modeling Matrix Diffusion Effects in REMChlor. SERDP/ESTCP Project ER-201426 [//www.enviro.wiki/images/0/0b/2018-Falta-REMChlor_Modeling_Matrix_Diffusion_Effects.pdf Report.pdf]   Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426</ref> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012">Falta, R.W., Ahsanuzzaman, A.N., Stacy, M.B., Earle, R.C. and Wilson, J.T., 2012. Remediation Evaluation Model for Fuel Hydrocarbons (REMFuel). Users Manual Version 1.0. U.S. Environmental Protection Agency. EPA/600/R-12/028. [//www.enviro.wiki/images/6/67/2012-Falta-REMFuel_Remediation_Evaluation-Model_for_Fuel_hydrocarbons_users_manual.PDF Report.pdf]   Website: https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel</ref> models are flexible screening tools that allow a simultaneous evaluation of the extent of treatment provided by (1) source removal, (2) ''in situ'' remediation of the contaminated groundwater, or (3) natural attenuation processes in three discrete intervals along the flow path and three discrete time periods. Both [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018" /> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012" /> can be downloaded from the internet at no cost. Liang ''et al.''<ref>Liang, H., Falta, R.W., Newell, C.J., Farhat, S.K., Rao, P.S. and Basu, N., 2010. Decision & Management Tools for DNAPL Sites: Optimization of Chlorinated Solvent Source and Plume Remediation Considering Uncertainty. SERDP/ESTCP Project ER-200704. [//www.enviro.wiki/images/c/ce/2010-Liang-Decision_and_Management_Tools_for_DNAPL_sites-ER-200704-FR.pdf Report.pdf]   Project Overview Website: https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200704/(language)/eng-US</ref> provide a modeling program that uses Monte Carlo simulations to evaluate the effects of the uncertainties in the modeling parameters on the predictions of REMChlor-MD.

==The Transition Assessment Teaching Assistant (TA2) Tool==
[[File:Wilson1w2Fig7.png|thumb|500px| Figure 7. Home Page for TA2 Tool. Users can click on buttons to access various modules that are designed to answer specific questions or research relevant topics.]]
[[File:Wilson1w2Fig8.png|thumb|500px| Figure 8. Example of an asymptote analysis using concentration versus time data in Tool 1 of the TA2 Tool. The source attenuation rate and corresponding remediation timeframe can be estimated for different monitoring periods.]]
A learning and decision-making tool was recently released as part of [https://serdp-estcp.mil/ Strategic Environmental Development and Research Program (SERDP)] Project [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview ER-201429] to help stakeholders gather information for the purposes of a site-specific transition assessment. This free software, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool]<ref name="TATA2024" />, was developed using the elements identified in the 2013 NRC report<ref name="NRC2013" /> as the critical learning objectives for end users.

The Tool is a web-based app that includes a collection of individual modules designed to answer specific questions or research relevant topics (Figure 7). The Tool has been developed as an R Shiny app (version 1.8.0)<ref> Chang, W., Cheng, J., Allaire, J., Sievert, C., Schloerke, B., Xie, Y., Allen, J., McPherson, J., Dipert, A., Borges, B., 2023. shiny: Web Application Framework for R. R package version 1.8.0, https://github.com/rstudio/shiny, https://shiny.posit.co/</ref>, which is an interactive platform using R programming to perform all quantitative functions. The user can then view the results in a simple interface that easily accommodates plots, charts, and various mapping features in a Web browser. The Tool is free and does not require the user to install R software.

The modules within the TA2 Tool include:

'''Five Quantitative Tools''' that focus on assessing asymptotic groundwater concentrations from monitoring data, evaluating plume stability, estimating remediation timeframes after a hypothetical source removal project, forecasting remediation performance if a technology is applied in the field, or projecting concentrations at downgradient points of compliance.

For example, Tool 1 in the TA2 Tool uses concentration versus time data from monitoring wells to estimate attenuation rate constants (''kpoint'') and evaluate if asymptotic conditions are present at particular locations or across the site. This helps to assess whether performance has plateaued at wells where a pump-and-treat system or other active treatment is in place. The user has the option to choose a “change point” within the monitoring record to determine if the attenuation rate has changed over time (e.g., once most of the accessible mass has been removed) (Figure 8). The user can either use visual interpretation to manually select the date when this apparent change occurred or have the date selected automatically using a binary segmentation protocol that is incorporated into the tool. The tool will calculate a rate for both the early period and a rate for the later period (after the change point), and then go through five different lines of evidence for asymptotic behavior (e.g., are the two rates of attenuation significantly different?). The user can then use the collective results as a technical justification demonstrating that the performance of the active remedy has plateaued as the first step in the transition assessment. The tool will also estimate the time to reach a user-specified cleanup goal if the overall attenuation rate (or the attenuation rate in the later period) were to continue.

Another module (Tool 5) focuses on evaluating sites where the concentration goal applies at a downgradient point of compliance, which is a key criterion for sites where MNA is being used as part of a risk-based strategy. The tool includes several different options to estimate a site-specific attenuation rate constant, including data from the pre-remediation period when natural attenuation processes were the sole means for reducing concentrations. Attenuation rate constants are then used to project the concentration versus distance from the contaminant source. Based on the predicted concentration at the downgradient point of compliance, the user can then see if the natural assimilative capacity along the aquifer flow path is sufficient to achieve the concentration goal in the absence of active treatment. For example, in the tab labeled “Use Pre-Remediation Rate Constant”, the logarithms of the concentrations from the period before active treatment began are plotted against the distance from the source well. The slope of the regression line is the rate constant for natural attenuation (including the contributions of degradation and dispersion). This rate constant can then be used to project the concentration moving downgradient from the well of concern after the end of active treatment. Similar approaches are provided within Tool 5 for using rate constants estimated from lab-based testing or derived from post-remediation data (after steady state has been reestablished).

'''Four Qualitative Tools''' provide information on matrix diffusion, enhanced attenuation options, geologic heterogeneity, and related research on transition assessments. Many of these modules are based on the current understanding of the role of matrix diffusion in influencing long-term concentration trends and remedial performance at contaminated groundwater sites. This includes summaries of different modeling options for better quantifying the effects of matrix diffusion. Sites impacted by matrix diffusion are generally challenging to treat using active remedies and thus are better candidates for less intensive management strategies that focus on reducing mass discharge rates, stabilizing the plume, and protecting potential downgradient receptors. As a result, matrix diffusion is critical to understanding and quantifying how natural attenuation processes are contributing to concentration trends.

'''One Summary Tool''' (Tool 10) compiles metrics from the other tools into a “Remediation Transition Assessment Index” (RTAI) and provides additional guidance on conducting site-specific transition assessments. The RTAI is a simple metric with a value from 1 to 5, where higher values reflect greater persistence of contamination due to matrix diffusion and other site-specific factors. An RTAI value is assigned to each of the results from the different tools that have been completed by the user. An RTAI of 5 suggests that the site is a strong candidate for transitioning to MNA or enhanced attenuation approaches, while a site with an RTAI value of 1 is a poor candidate. The user can assign an overall RTAI for the site based on the preponderance of evidence after reviewing the RTAI values generated by each tool, or calculate a site RTAI based on simple averaging, weighting, or other methods.

Tool 10 also contains a flowchart and a checklist for performing site-specific transition assessments that start with evaluating relevant bright line criteria, such as (1) can the concentration goals be met at the point of compliance by MNA; and (2) is the remediation timeframe for MNA reasonable and/or similar to the timeframe if source remediation were used. This checklist ensures that the user has gathered all relevant information that would be needed to support a technically rigorous site-specific Transition Assessment.

The TA2Tool provides a framework for remedial decision makers to evaluate different types of sites, including those where active treatment (e.g., pump and treat) is in use, as well as sites where future active source zone remediation is being considered. It also includes a description of enhanced MNA alternatives for sites where MNA alone may not be sufficient to control risk. As shown in Figure 8, the tool can be used to answer specific questions that have a primarily quantitative basis or to provide focused qualitative information for researching specific topics. Users can engage with just the modules that might be pertinent to assessment of an individual site, or they can go through all the modules to perform a more thorough, step-by-step analysis of the relevant issues for their site.

==Summary==
Tools and approaches are available that can be adapted to determine when a site is ready to transition from active remedy to MNA. However, these tools and approaches have not been applied for this purpose at a significant number of sites, and at the present time, they are not generally accepted by regulatory authorities. There is an opportunity to establish and implement a logical and consistent framework that can be widely implemented to evaluate sites for transition from active remedy to MNA.

==References==
<references />

==See Also==

*[http://dx.doi.org/10.1007/978-1-4614-6922-3 Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. In: Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward. SERDP ESTCP Environmental Remediation Technology, vol 7. Springer, New York, NY. pgs. 459-508. doi: 10.1007/978-1-4614-6922-3]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436/(language)/eng-US Kram, Mark, and Widdowson, Mark, 2008. Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation. ESTCP ER-200436]

Monitored Natural Attenuation - Transitioning from Active Remedies

2026-05-07T16:58:45Z

Admin:

Many contaminated sites use active remedies such as pump-and-treat or ''in situ'' remediation to clean up impacted groundwater. Natural attenuation processes such as natural degradation or [[Dispersion and Diffusion | hydrodynamic dispersion]] also contribute to the cleanup. As remediation progresses, a point is often reached when the time required to reach the remedial objectives using the active remedy is roughly the same as the time required if the active remedy is shut down, and the continuing remediation of the site is provided by natural attenuation processes alone. From that point forward, the extra effort and expense of the active remedy provides no benefit over natural attenuation, and it may be appropriate to transition the site to [[Monitored Natural Attenuation (MNA)]]. This article deals with currently available tools and approaches that can be used to support a decision to transition from active remediation to MNA.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s)''':

*[[Alternative Endpoints]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Plume Response Modeling]]
*[[REMChlor - MD]]
*[[Source Zone Modeling]]

'''Contributor(s):'''

*[[Dr. John Wilson]]
*[[Dr. David Adamson, P.E.]]

'''Key Resource(s)''':

*[//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies]<ref name="Newell2002">Newell, C.J., Rifai, H.S., Wilson, J.T., Connor, J.A., Aziz, J.A., Suarez, M.P., 2002. Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies. 28p. EPA/540/S-02/500. [//www.enviro.wiki/images/1/10/2002-Newell-Calculation_and_Use_of_First-Order_Rate_Constants_for_Monitored_Natural_Attenuation_Studies.pdf Report.pdf]</ref>

*[https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS) Version 2.3.3]<ref name="Widdowson2008">Widdowson, M.A., Mendez, E., Chapelle, F.H., Casey, C.C., 2008. Natural Attenuation Software (NAS) Version 2.3.3. Virginia Polytechnic Institute and State University, the United States Geological Survey, and the United States Naval Facilities Engineering Command. NAS webpage: https://www.nas.cee.vt.edu/index.php See also: https://toxics.usgs.gov/highlights/nas_2.2.0/index.html</ref>

*[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf BIOCHLOR Natural Attenuation Support System, Version 2.2]<ref name="Aziz2002">Aziz, C.E., Newell, C.J. and Gonzales, J.R., 2002. BIOCHLOR Natural Attenuation Decision Support System Version 2.2 User’s Manual Addendum. Groundwater Services, Inc., Houston, Texas for the Air Force Center for Environmental Excellence.[//www.enviro.wiki/images/3/39/2002-Aziz-Biochlor_Natural_Attenuation_Decision_Support_System_Vs_2.2.pdf Report.pdf] Available at: https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system</ref>

*[https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC User's Guide and Tool Website]<ref name="BioPIC2021">Danko, A., Adamson, D., Newell, C., Wilson, J., Wilson, B., Freedman, D., Lebrón, C., 2021. Quick BioPIC User’s Guide, ESTCP Project ER-201730. [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 Project Website]   [//www.enviro.wiki/images/c/c9/ER-201730_BioPIC_User%27s_Guide.pdf User’s Guide]</ref>

*[https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool Website]<ref name="TATA2024">Adamson, D.T., Newell, C.J., Hort, H.M, Wilson, J.T., 2024. TA2: The SERDP Transition Assessment Teaching Assistant. Strategic Environmental Research and Development Program (SERDP) Project ER20-1429. [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview Project Website]  [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Online Tool]</ref>

==Introduction==
Many active remedies are effective at treating higher concentrations of contaminants, but as the contaminant concentrations decrease, the rate of cleanup may slow before the site reaches the cleanup goal. At some sites, the rate of cleanup may slow until it is not significantly different from the rate of cleanup provided by the natural attenuation processes that occur at the site. At other sites, the concentration of contaminants in water produced by a pumping system is below the cleanup goal, but the concentration in monitoring wells in the source area are still above the goal. At some sites, active treatment has stopped further expansion of the plume toward a receptor, and concentrations are declining over time throughout the plume, but back diffusion is sustaining concentrations in the plume that are above the cleanup goal.

In 2013, a significant National Research Council (NRC) report noted that despite years of effort and considerable investment, many sites “will require long-term management that could extend for decades or longer”<ref name="NRC2013">National Research Council (NRC), 2013. Alternatives for Managing the Nation's Complex Contaminated Groundwater Sites. Committee on Future Options for Management in the Nation's Subsurface Remediation Effort, Water Science, Technology Board, Division on Earth and Life Studies, NRC. National Academies Press, 422 pages, ISBN 978-0-309-27874-4 [https://doi.org/10.17226/14668 doi: 10.17226/14668]. [//www.enviro.wiki/images/4/48/NRC2013.pdf Report.pdf]</ref>. The authors of the report discussed the need for developments that can aid in “transition from active remediation to more passive strategies and provide more cost-effective and protective long-term management of complex sites”<ref name="NRC2013" />.

The United States Environmental Protection Agency<ref> U.S. Environmental Protection Agency (USEPA), 1999. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. OSWER Directive 9200.4-17P. 39pp.[//www.enviro.wiki/images/a/aa/1999_USEPA-_Use_of_monitored_natural_attenuation_at_superfund.pdf Report.pdf]</ref> allows the use of [[Monitored Natural Attenuation (MNA) | monitored natural attenuation (MNA)]] to attain the cleanup goals when the site-specific remediation objectives can be attained within a time frame that is reasonable compared to that offered by other more active methods. Many CERCLA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Summary of the Comprehensive Environmental Response, Compensation, and Liability Act (Superfund) https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act</ref> and RCRA<ref>U.S. Environmental Protection Agency (USEPA), 2019. Resource Conservation and Recovery Act (RCRA) Laws and Regulations https://www.epa.gov/rcra</ref> sites take advantage of this policy. An active remedy is typically used initially to treat high concentrations of contaminants followed by MNA to treat the lower concentrations that remain.

Unfortunately, there is no well-established approach to determine when it is appropriate to discontinue the active remedy. The NRC report<ref name="NRC2013" /> emphasized the use of more rigorous evaluations of existing data to support these efforts. This can include a quantitative assessment of the performance of active remedies (e.g., evidence of asymptotic performance) as well as documenting site conditions that may be contributing to these performance limitations. Importantly, it also identifies alternative approaches for managing the site, which could include MNA if the natural attenuation processes can meaningfully contribute to the achievement of site cleanup objectives.

This article reviews available tools and approaches to evaluate a transition to MNA. The tools and approaches depend on calculations of rate constants for natural attenuation with distance in flowing groundwater or rate constants for attenuation over time in individual monitoring wells.

==Background on Rate Constants==
[[File:Wilson1w2Fig1.png|thumb|400px| Figure 1. Attenuation of Trichloroethene (TCE) over time in a monitoring well at a site in Michigan. The concentration vs. time rate constant is 0.326 per year and largely represents the rate of the attenuation of the source of contaminants in the aquifer.]]
At sites where a transition to MNA is being considered, a key step is estimating attenuation rate constants and understanding how they are extracted from monitoring data. A general formula to describe the rate of a chemical reaction is:

:{|
|-
|'''Equation 1:'''|| ||<big>''r = k [C]''</big>'' m''
|-
|where:
|-
| ||''r''    ||is the rate of the reaction,
|-
| ||''k''||is the rate constant,
|-
| ||''C''||is the concentration of the chemical undergoing the reaction, and
|-
|the exponent    ||''m''||is the order of the reaction.
|}

When the rate of the reaction is proportional to the concentration of the contaminant, the value of ''m'' is 1. Therefore, the reaction is described as a first-order reaction, and the rate constant is described as a first-order rate constant. In Equation 1, concentration could go up or down, but ''k'' is a constant of proportionality for the rate of increase in concentration. The rate constant for attenuation is the negative of ''k''. If the rate of degradation is a fixed value regardless of concentration, the value of ''m'' is 0, and degradation is a zero-order process.

Natural attenuation of concentrations over time in monitoring wells is frequently described by a first-order rate constant, and natural biological or abiotic degradation of contaminants in flowing groundwater is typically also described by a first-order rate constant. Figure 1 provides an example of monitoring data that is described by a first-order rate constant.

The rate constant for attenuation over time in a single well and the rate constant for attenuation with distance along a flow path in an aquifer describe different situations that are controlled by different processes. ''Attenuation over time'' in a well is largely controlled by the rate of attenuation of the source of contamination in the aquifer. ''Attenuation with distance'' along a flow path includes attenuation of concentrations in the source along with contributions from biological degradation processes, abiotic degradation processes and hydrodynamic dispersion of the contaminated groundwater into clean groundwater<ref name="Newell2002" />.

The first-order rate constant for attenuation over time in a single well is commonly referred to as ''kpoint''<ref name="Newell2002" />. A time series chart in Microsoft EXCEL of the concentrations of a contaminant (''y'' axis) on the date of sampling (''x'' axis) can be used to extract a value for ''kpoint''. Select the data, then insert an exponential trend line and display the equation on the chart. The value of ''kpoint'' can also be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Note that the rate constants extracted in EXCEL are constants for the rate of change, not the rate of attenuation. Take the negative of the rate of change to get ''kpoint''. In the example in Figure 1, the unit of time on the X axis is years, and the value of ''kpoint'' is 0.326 per year.

Attenuation versus distance rate coefficients describe a bulk attenuation rate including both degradation and non-destructive processes such as dispersion. To extract values for rate constants for degradation alone, it is necessary to calibrate a groundwater flow and transport model to the data at the site. The model is calibrated with values for the hydrogeological properties of the aquifer (effective porosity, hydraulic gradient, hydraulic conductivity, hydrodynamic dispersion and the organic carbon content of the aquifer matrix). After the hydrogeological properties of the aquifer are fixed in the model, the most appropriate values for the degradation rate constants are the values that produce the best fit between the contaminant concentrations that are predicted by the model and the contaminant monitoring data at the site.

There are a number of reasons why natural attenuation processes are better described as first-order relationship instead of zero-order or some other order. The attenuation over time in a monitoring well tracks the attenuation over time of the source of contamination that sustains the plume<ref name="Newell2002" />. Sites go through a lifecycle, and attenuation of sources at mature sites is often a first-order process<ref>Sale, T., Newell, C., Stroo, H., Hinchee, R. and Johnson, P., 2008. Frequently Asked Questions Regarding Management of Chlorinated Solvents in Soils and Groundwater. Environmental Security Technology Certification Program (ESTCP, Project ER-200530), Department of Defense (DoD), Arlington, VA. [//www.enviro.wiki/images/c/cb/2008-Sale-Frequently_Asked_Questions_Regarding_Management_of_Chlorinated_Solvent_in_Soils_and_Groundwater.pdf Report.pdf]   Project Overview Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200530</ref>. If a chlorinated solvent site is mature, the contamination in the source area that was originally present as nonaqueous phase liquids (NAPL) has been redistributed and is now sequestered in a sorbed phase to aquifer solids or has diffused into non-transmissive portions of the aquifer matrix. Transfer of contaminants back into the more transmissive portions of the aquifer occurs by diffusion along a fixed path length, and the rate of transfer is controlled by the concentration of the contaminant remaining in the source material. Because the rate of transfer is proportional to the concentration of contaminant in the source material, attenuation of the source is a first-order process. These processes are discussed in more detail in [[Source Zone Modeling]].

Degradation processes are also usually first order. Abiotic reactions are almost always first order with respect to the concentration of the target chemical. Biodegradation reactions are zero order at high concentrations because the available enzymes are saturated with substrate, but are first order at lower concentrations that are typical of natural attenuation conditions in groundwater.

==Goals for MNA at Sites==

The information necessary to evaluate whether a site can be transitioned to MNA depends on the goal for MNA at the site. For many cleanup actions, the goal is to confine contamination within a waste management area where the contamination is left in place, in which case the cleanup goal applies to point-of-compliance wells that are outside the waste management area. For other cleanup actions, the entire site must be cleaned up, in which case the cleanup goal applies to any monitoring well on the site. The time by which the goal is to be attained is specified at CERCLA sites in the Record of Decision (the ROD). At RCRA sites, the time allowed for the cleanup to be attained may be specified in the permit.

==When the Goal Applies to Point-of-Compliance Wells==
Consider the following framework for evaluating a transition to MNA:

#Use a computer model to extract rate constants for the natural degradation of the contaminant that occurred in groundwater at the site before the active remedy was installed.
#Assume that the same rate constants will apply after the active remedy is no longer in operation. Note that this assumption may not be valid if the active remedy changes the geochemistry of the aquifer in the flow path to the point-of-compliance well.
#Calibrate a computer groundwater flow and transport model with the hydrogeological properties of the aquifer that pertain after the active remedy is no longer in operation, the concentration of contaminant after the active remedy, and the rate constants for natural degradation that are assumed to apply after the active remedy.
#Use the computer model to project the concentrations of the contaminant at the point-of-compliance well over time.
#If the concentrations at the point-of-compliance wells are predicted to be less than the goal before the specified date, that is a quantitative line of evidence in support of a transition to MNA.

There are several computer applications that are particularly useful to extract rate constants at a site from monitoring data that were collected before the active remedy was installed. For example, [https://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS)]<ref name="Widdowson2008" />, [https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system BIOCHLOR]<ref name="Aziz2002" /> and [https://serdp-estcp.mil/toolsandtraining/details/4bacf717-26a3-4a7a-a53d-bff9cf6aec77 BioPIC]<ref name="BioPIC2021" /> can be downloaded from the internet at no cost. Another recent example, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ TA2 Tool]<ref name="TATA2024" />, is discussed in detail later in this article.

[[File:Wilson1w2Fig2.png|thumb|left|400px| Figure 2. Example calibration of NAS to natural attenuation of total BTEX at a site (Figure 17 of NAS User’s Manual).]]
[[File:Wilson1w2Fig3.png|thumb|400px| Figure 3. The data input screen for BIOCHLOR before remediation with cis-1,2-Dichloroethene (DCE) and vinyl chloride (VC) source concentrations of 500 and 87 mg/L respectively at the source when the release first occurred.]]
[[File:Wilson1w2Fig4.png|thumb|left|400px| Figure 4. Output of the RUN CENTERLINE simulation in BIOCHLOR comparing the fit between the simulation and the field data for vinyl chloride before an active remedy was implemented]]
[[File:Wilson1w2Fig5.png|thumb|400px| Figure 5. Output of the RUN CENTERLINE simulation of conditions after an active remedy was implemented with a source concentration of 1.1 mg/L, projecting the concentration of vinyl chloride at a distance corresponding to a point-of-compliance well.]]
 
In NAS, the user inputs the hydrogeological data, the distance of wells along the flow path, and the concentrations of contaminants in the wells. The NAS application extracts rate constants and makes projections at the point-of-compliance. With NAS, it is possible to extract different rate constants for specific geochemical environments along the flow path.

Figure 2 provides an example calibration of NAS. The concentrations in the monitoring wells used to calibrate the model are compared to the simulation provided by the model. The values of the rate constants that are extracted from the field data are available in the “Output” tab under “Data and Results Table.”

Figure 3 depicts the input screen for BIOCHLOR. The user inputs the hydrogeological parameters, the first-order rate constants (1st Order Decay Coefficient), the distribution of the wells along the flow path, and the concentrations of contaminants in the wells. The model is set up for conditions that apply before the installation of the active remedy.

BIOCHLOR does not automatically fit the rate constants to the field data. Instead, the user examines the output of the model, and adjusts the rate constants until they provide the best fit between the model prediction and the monitoring data for wells at the site. This comparison is illustrated in Figure 4.

If the distance from the source well to the point-of-compliance well is set as the “Modeled Area Length” in Section 5 of the input screen, the “Run Centerline” output will provide the projected concentrations at that length. Assume the distance from the source well to the point-of-compliance well is 250 feet. The projected concentration in Figure 3 of vinyl chloride at a point-of-compliance well is 0.042 mg/L. If the regulatory goal were the federal drinking water maximum contaminant level (MCL)<ref>U. S. Environmental Protection Agency (USEPA), 2009. National Primary Drinking Water Regulations. EPA 816-F-09-004. [//www.enviro.wiki/images/a/ae/2009-USEPA-national_Primary_Drinking_Water_Regulations.pdf Report.pdf]</ref> of 0.002 mg/L, the projected concentration would exceed the goal, and MNA would not be adequate as a remedy.

For the sake of illustration, assume that an active remedy has been implemented, and the concentrations in the source well are 5.4 mg/L for DCE and 1.1 mg/L for vinyl chloride. To evaluate whether it is now appropriate to transition to MNA, BIOCHLOR could be calibrated with these concentrations to predict concentrations in the point-of-compliance well. (See Figure 5). In this example, the projected concentration at the point-of-compliance well does meet the goal.

Some active remedies are subject to rebound. If this is the case, the evaluation should begin at the point in time when it is clear that the trend in concentrations is downward.

A new EXCEL-based tool that does many of the same basic calculations as BIOCHLOR was recently developed as part of an update to the BioPIC<ref name="BioPIC2021" /> decision support software. This tool, the MNA Rate Constant Estimator, extracts rate constants from concentration versus distance data for a variety of different chemicals, including chlorinated ethenes (e.g., PCE and TCE), chlorinated ethanes (e.g., 1,1,1-TCA), and 1,4-dioxane. This tool was developed to run using current versions of EXCEL, whereas BIOCHLOR must be run using older versions of EXCEL that may be unavailable to many users. The MNA Rate Constant Estimator can be used to estimate degradation rate constants and/or predict plume footprints over time. Consequently, it is a useful addition to the BioPIC decision framework for understanding if MNA is appropriate remedy for a site, and it can also be helpful for estimating rate constants as part of a transition assessment.

==When the Goal Applies to All the Wells==
At sites where a concentration-based cleanup goal must be achieved at all wells, each well at the site is evaluated independently, and the rate constant that is applicable is the rate constant for attenuation over time in the well (''kpoint''). To evaluate whether the region in an aquifer that is sampled by a particular monitoring well is ready to transition to MNA, it is necessary to have monitoring data from a period of time before the remedy was implemented. This data is used to extract a value for ''kpoint'' in the aquifer under natural attenuation conditions. The evaluation of a transition to MNA will assume that the same value for ''kpoint'' will apply after the active remedy is complete. This assumption may not be appropriate if the active remedy caused a permanent change in the geochemistry of the aquifer. The assumption is usually appropriate for pump-and-treat remedies.

If ''kpoint'' before implementation of the active remedy describes the time course of natural attenuation after the active remedy is completed, the time required to attain the cleanup goal is predicted from the following:

:{|
| || || rowspan="2" |<big>''ln (''</big>
| style="border-style:solid; border-width: 0px 0px 1px 0px" |''Cgoal''
| rowspan="2" |<big>'')''</big>||                                                                       
|-
|'''Equation 2:'''        ||''<big>t =''    
| style="vertical-align:top;" |''Ccurrent''||
|-
| || || colspan="3" style="text-align:center; border-style:solid; border-width: 1px 0 0 0" |''<big>-k</big>point''||
|-
|where:
|-
| ||''Ccurrent''    || colspan="5" |is the current concentration after active remediation,
|-
| ||''Cgoal''|| colspan="5" |is the cleanup goal, and
|-
| ||''t''|| colspan="5" |is the time required for concentrations to attenuate from ''Ccurrent'' to ''Cgoal.''
|}

If the value of ''t'' estimated using Equation 2 is less than the difference between the current date and the date specified by the site stakeholders to attain the goal, that is evidence in support of a transition to MNA.

Some active remedies are subject to contaminant concentration rebound. If this is the case, the evaluation should use a value of ''C''current that is attained after the rebound has stabilized.

This approach depends on a robust value for ''kpoint''. It is worthwhile to do a sensitivity analysis on ''kpoint'' where the lower 95% or 90% confidence interval on ''kpoint'' is used in Equation 2 to see if that changes the outcome of the evaluation. The confidence intervals can be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Wilson<ref name="Wilson2011">Wilson, J.T. 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [//www.enviro.wiki/images/e/e3/Wilson-2011-An_Approach_for_Evaluating_Progress.pdf Report.pdf]</ref> provides detailed discussion of the use of linear regression to extract ''kpoint'' and confidence intervals on ''kpoint''. Wilson<ref name="Wilson2011" /> also discusses the use of goodness-of-fit tests to determine if there is evidence that a first-order rate equation is not the best fit to the monitoring data, and as a result the use of Equation 2 would not be appropriate. The TA2 Tool<ref name="TATA2024" /> also has the capability to calculate ''kpoint'' with a user-specified confidence interval, as described below.

At many sites, there is no specified date when the cleanup goal must be attained. In this situation, the monitoring data can be evaluated to determine if the current rate of attenuation under the active remedy is faster than the rate of natural attenuation before the active remedy was installed. The monitoring data can be examined to identify a time interval when the benefit of the active remedy has approached an asymptote. A second value of for ''kpoint'' can be extracted for that time interval. The two values for ''kpoint'' can be evaluated statistically to see if the current rate is faster at some appropriate level of confidence. If there is no statistical evidence that the rate of attenuation is faster, that determination can support a decision to transition to MNA.
[[File:Wilson1w2Fig6.png|thumb|400px| Figure 6. Example calibration of NAS to predict the reduced concentration at the source that is necessary to meet the remediation goal at a point-of-compliance well (Figure 19 of NAS User’s Manual).]]

==Extent of Treatment Necessary to Transition to MNA==
There are several computer applications that can predict the extent of treatment that must be achieved by the active remedy before it is worthwhile to evaluate the site for transition to MNA. For example, based on the distribution of contamination along the flow path, the NAS application will automatically predict a reduced concentration at the source well that will bring concentrations to the goal in the point-of-compliance well (Figure 6). A table that opens under the “DOS/TOS” tab provides the “Time of Equilibration” required to meet the goal at the reduced concentration. Modules in NAS allow the user to evaluate the effect of various pump-and-treat and source removal scenarios on the time required to attain the goal at the point-of-compliance well.

The [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018">Falta, R.W., Farhat, S.K., Newell, C.J. and Lynch, K., 2018. A Practical Approach for Modeling Matrix Diffusion Effects in REMChlor. SERDP/ESTCP Project ER-201426 [//www.enviro.wiki/images/0/0b/2018-Falta-REMChlor_Modeling_Matrix_Diffusion_Effects.pdf Report.pdf]   Website: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201426</ref> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012">Falta, R.W., Ahsanuzzaman, A.N., Stacy, M.B., Earle, R.C. and Wilson, J.T., 2012. Remediation Evaluation Model for Fuel Hydrocarbons (REMFuel). Users Manual Version 1.0. U.S. Environmental Protection Agency. EPA/600/R-12/028. [//www.enviro.wiki/images/6/67/2012-Falta-REMFuel_Remediation_Evaluation-Model_for_Fuel_hydrocarbons_users_manual.PDF Report.pdf]   Website: https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel</ref> models are flexible screening tools that allow a simultaneous evaluation of the extent of treatment provided by (1) source removal, (2) ''in situ'' remediation of the contaminated groundwater, or (3) natural attenuation processes in three discrete intervals along the flow path and three discrete time periods. Both [[REMChlor - MD | REMChlor-MD]]<ref name="Falta2018" /> and [https://www.epa.gov/water-research/remediation-evaluation-model-fuel-hydrocarbons-remfuel REMFuel]<ref name="Falta2012" /> can be downloaded from the internet at no cost. Liang ''et al.''<ref>Liang, H., Falta, R.W., Newell, C.J., Farhat, S.K., Rao, P.S. and Basu, N., 2010. Decision & Management Tools for DNAPL Sites: Optimization of Chlorinated Solvent Source and Plume Remediation Considering Uncertainty. SERDP/ESTCP Project ER-200704. [//www.enviro.wiki/images/c/ce/2010-Liang-Decision_and_Management_Tools_for_DNAPL_sites-ER-200704-FR.pdf Report.pdf]   Project Overview Website: https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200704/(language)/eng-US</ref> provide a modeling program that uses Monte Carlo simulations to evaluate the effects of the uncertainties in the modeling parameters on the predictions of REMChlor-MD.

==The Transition Assessment Teaching Assistant (TA2) Tool==
[[File:Wilson1w2Fig7.png|thumb|500px| Figure 7. Home Page for TA2 Tool. Users can click on buttons to access various modules that are designed to answer specific questions or research relevant topics.]]
[[File:Wilson1w2Fig8.png|thumb|500px| Figure 8. Example of an asymptote analysis using concentration versus time data in Tool 1 of the TA2 Tool. The source attenuation rate and corresponding remediation timeframe can be estimated for different monitoring periods.]]
A learning and decision-making tool was recently released as part of [https://serdp-estcp.mil/ Strategic Environmental Development and Research Program (SERDP)] Project [https://serdp-estcp.mil/projects/details/350cbc0b-893a-43a6-8a0c-c9c057bacac0/er20-1429-project-overview ER-201429] to help stakeholders gather information for the purposes of a site-specific transition assessment. This free software, the [https://gsi-environmental.shinyapps.io/SERDP_TA2_Tool/ Transition Assessment Teaching Assistant (TA2) Tool]<ref name="TATA2024" />, was developed using the elements identified in the 2013 NRC report<ref name="NRC2013" /> as the critical learning objectives for end users.

The Tool is a web-based app that includes a collection of individual modules designed to answer specific questions or research relevant topics (Figure 7). The Tool has been developed as an R Shiny app (version 1.8.0)<ref> Chang, W., Cheng, J., Allaire, J., Sievert, C., Schloerke, B., Xie, Y., Allen, J., McPherson, J., Dipert, A., Borges, B., 2023. shiny: Web Application Framework for R. R package version 1.8.0, https://github.com/rstudio/shiny, https://shiny.posit.co/</ref>, which is an interactive platform using R programming to perform all quantitative functions. The user can then view the results in a simple interface that easily accommodates plots, charts, and various mapping features in a Web browser. The Tool is free and does not require the user to install R software.

The modules within the TA2 Tool include:

'''Five Quantitative Tools''' that focus on assessing asymptotic groundwater concentrations from monitoring data, evaluating plume stability, estimating remediation timeframes after a hypothetical source removal project, forecasting remediation performance if a technology is applied in the field, or projecting concentrations at downgradient points of compliance.

For example, Tool 1 in the TA2 Tool uses concentration versus time data from monitoring wells to estimate attenuation rate constants (''kpoint'') and evaluate if asymptotic conditions are present at particular locations or across the site. This helps to assess whether performance has plateaued at wells where a pump-and-treat system or other active treatment is in place. The user has the option to choose a “change point” within the monitoring record to determine if the attenuation rate has changed over time (e.g., once most of the accessible mass has been removed) (Figure 8). The user can either use visual interpretation to manually select the date when this apparent change occurred or have the date selected automatically using a binary segmentation protocol that is incorporated into the tool. The tool will calculate a rate for both the early period and a rate for the later period (after the change point), and then go through five different lines of evidence for asymptotic behavior (e.g., are the two rates of attenuation significantly different?). The user can then use the collective results as a technical justification demonstrating that the performance of the active remedy has plateaued as the first step in the transition assessment. The tool will also estimate the time to reach a user-specified cleanup goal if the overall attenuation rate (or the attenuation rate in the later period) were to continue.

Another module (Tool 5) focuses on evaluating sites where the concentration goal applies at a downgradient point of compliance, which is a key criterion for sites where MNA is being used as part of a risk-based strategy. The tool includes several different options to estimate a site-specific attenuation rate constant, including data from the pre-remediation period when natural attenuation processes were the sole means for reducing concentrations. Attenuation rate constants are then used to project the concentration versus distance from the contaminant source. Based on the predicted concentration at the downgradient point of compliance, the user can then see if the natural assimilative capacity along the aquifer flow path is sufficient to achieve the concentration goal in the absence of active treatment. For example, in the tab labeled “Use Pre-Remediation Rate Constant”, the logarithms of the concentrations from the period before active treatment began are plotted against the distance from the source well. The slope of the regression line is the rate constant for natural attenuation (including the contributions of degradation and dispersion). This rate constant can then be used to project the concentration moving downgradient from the well of concern after the end of active treatment. Similar approaches are provided within Tool 5 for using rate constants estimated from lab-based testing or derived from post-remediation data (after steady state has been reestablished).

'''Four Qualitative Tools''' provide information on matrix diffusion, enhanced attenuation options, geologic heterogeneity, and related research on transition assessments. Many of these modules are based on the current understanding of the role of matrix diffusion in influencing long-term concentration trends and remedial performance at contaminated groundwater sites. This includes summaries of different modeling options for better quantifying the effects of matrix diffusion. Sites impacted by matrix diffusion are generally challenging to treat using active remedies and thus are better candidates for less intensive management strategies that focus on reducing mass discharge rates, stabilizing the plume, and protecting potential downgradient receptors. As a result, matrix diffusion is critical to understanding and quantifying how natural attenuation processes are contributing to concentration trends.

'''One Summary Tool''' (Tool 10) compiles metrics from the other tools into a “Remediation Transition Assessment Index” (RTAI) and provides additional guidance on conducting site-specific transition assessments. The RTAI is a simple metric with a value from 1 to 5, where higher values reflect greater persistence of contamination due to matrix diffusion and other site-specific factors. An RTAI value is assigned to each of the results from the different tools that have been completed by the user. An RTAI of 5 suggests that the site is a strong candidate for transitioning to MNA or enhanced attenuation approaches, while a site with an RTAI value of 1 is a poor candidate. The user can assign an overall RTAI for the site based on the preponderance of evidence after reviewing the RTAI values generated by each tool, or calculate a site RTAI based on simple averaging, weighting, or other methods.

Tool 10 also contains a flowchart and a checklist for performing site-specific transition assessments that start with evaluating relevant bright line criteria, such as (1) can the concentration goals be met at the point of compliance by MNA; and (2) is the remediation timeframe for MNA reasonable and/or similar to the timeframe if source remediation were used. This checklist ensures that the user has gathered all relevant information that would be needed to support a technically rigorous site-specific Transition Assessment.

The TA2Tool provides a framework for remedial decision makers to evaluate different types of sites, including those where active treatment (e.g., pump and treat) is in use, as well as sites where future active source zone remediation is being considered. It also includes a description of enhanced MNA alternatives for sites where MNA alone may not be sufficient to control risk. As shown in Figure 8, the tool can be used to answer specific questions that have a primarily quantitative basis or to provide focused qualitative information for researching specific topics. Users can engage with just the modules that might be pertinent to assessment of an individual site, or they can go through all the modules to perform a more thorough, step-by-step analysis of the relevant issues for their site.

==Summary==
Tools and approaches are available that can be adapted to determine when a site is ready to transition from active remedy to MNA. However, these tools and approaches have not been applied for this purpose at a significant number of sites, and at the present time, they are not generally accepted by regulatory authorities. There is an opportunity to establish and implement a logical and consistent framework that can be widely implemented to evaluate sites for transition from active remedy to MNA.

==References==
<references />

==See Also==

*[http://dx.doi.org/10.1007/978-1-4614-6922-3 Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. In: Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward. SERDP ESTCP Environmental Remediation Technology, vol 7. Springer, New York, NY. pgs. 459-508. doi: 10.1007/978-1-4614-6922-3]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436/(language)/eng-US Kram, Mark, and Widdowson, Mark, 2008. Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation. ESTCP ER-200436]

Monitored Natural Attenuation (MNA) of Chlorinated Solvents

2026-05-07T16:56:24Z

Admin:

[[Monitored Natural Attenuation (MNA)]] is a common remedy for contamination of [[Chlorinated Solvents |chlorinated solvents]] in groundwater. Chlorinated solvents are susceptible to many natural processes that can attenuate their concentrations in groundwater including biological degradation, abiotic degradation, sorption, dispersion, and volatilization. Typically, MNA is used for plumes with low dissolved concentrations or in peripheral areas of plumes away from areas with non-aqueous phase liquid (NAPL) or other materials that serve as the source of groundwater contamination.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''
*[[Chlorinated Solvents]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Monitored Natural Attenuation (MNA) of Fuels| Monitored Natural Attenuation of Fuels]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| Monitored Natural Attenuation of Metal and Metalloids]]
*[[Natural Source Zone Depletion (NSZD)]]

'''CONTRIBUTOR(S):''' [[Dr. John Wilson]] 

'''Key Resource(s):'''
*[[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Identification & Characterization Methods for Reactive Minerals Responsible for Natural Attenuation of Chlorinated Organic Compounds in Ground Water]]<ref name="HE2009">He, Y., Su, C., Wilson, J., Wilkin, R., Adair, C., Lee, T., Bradley, P. and Ferrey, M., 2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water. U.S. Environmental Protection Agency. [[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Report pdf]]</ref>

==Introduction==
[[Chlorinated Solvents |Chlorinated solvents]] and their transformation products are among the most abundant contaminants in groundwater. In 2006, the United States Geological Survey published results from a systematic survey of volatile organic chemicals in drinking water wells<ref>Zogorski, J.S., Carter, J.M., Ivahnenko, T., Lapham, W.W., Moran, M.J., Rowe, B.L., Squillace, P.J., Toccalino, P.L., 2006. The quality of our Nation’s waters - Volatile organic compounds in the nation’s ground water and drinking-water supply wells. US Geological Survey Circular, 1292, 101. [[Media:Zogorski-2006-_Volatile_organic_compounds_in_the_nations_ground_water_and_wells.pdf|Report pdf]]</ref> in the USA. Approximately 12% of wells contained detectable concentrations of tetrachloromethane ([[wikipedia: Chloroform | chloroform]]), 5% contained [[wikipedia: Tetrachloroethylene | tetrachloroethene (PCE)]], 4% contained [[wikipedia: Trichloroethylene | trichloroethene (TCE)]], 2% contained [[wikipedia: 1,1,1-Trichloroethane | 1,1,1-trichloroethane (1,1,1-TCA)]], and 2% contained [[wikipedia: 1,1-Dichloroethane | 1,1-dichloroethane (1,1-DCA)]].

[[Monitored Natural Attenuation (MNA) | Monitored Natural Attenuation (MNA)]] is one remedy that is available for contamination from chlorinated solvents in groundwater. Natural processes that can attenuate the concentrations of chlorinated solvents in groundwater include biological degradation, abiotic degradation, sorption, dispersion into ground adjacent to the contaminant plume, and volatilization to soil gas above the groundwater. At most sites where MNA has been selected as a remedy, or part of a remedy, the chlorinated solvents have been shown to be degrading in groundwater.

==Biodegradation==
The prospects for degradation of selected chlorinated solvents and their transformation products in groundwater are good (Table 1).

[[File:Wilson 3 Table1.JPG|thumbnail|600 px|left|Table 1. Summary of the prospects for degradation of selected chlorinated solvents and their transformation products in groundwater<ref>Lawrence, S.J., 2006. Description, properties, and degradation of selected volatile organic compounds detected in ground water--A review of selected literature (No. 2006-1338). [[Media:Lawrence-2006-Description_properties_degradation_of_VOCs.pdf|Report pdf]]</ref><ref name="HE2009"/>]]
Biodegradation can occur under both aerobic and anaerobic conditions. Under aerobic conditions, the chlorinated solvent can act as a source of food for the microorganisms (referred to as direct biodegradation in Table 1). Degradation can also be a fortuitous reaction that does not provide any benefit to the microorganisms. The fortuitous reaction is called a cometabolism or cooxidation. The fortuitous reaction is most commonly carried out by an oxygenase enzyme that is produced by the microorganisms in order to allow them to degrade some other compound.

When the chlorinated solvent is degraded as a food source, the population of active organisms and the rate of degradation will increase over time. If the degradation is fortuitous, the bacteria do not grow as a result of degrading the chlorinated solvent, and the rate constant does not increase over time.

The prospects for direct aerobic biodegradation of chlorinated alkenes depends on the extent of chlorination. PCE and TCE do not support growth under aerobic conditions, cis-dichloroethene<ref>Cox, E., 2012. Elucidation of the mechanisms and environmental relevance of cis-dichloroethene and vinyl chloride biodegradation. ER-1557. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1557/ER-1557 ER-1557]</ref> (c-DCE) can be degraded in aerobic groundwater, and vinyl chloride (VC) is readily degradable in many aerobic groundwaters.

Many samples of groundwater contain microorganisms that express oxygenase enzymes and can cometabolize PCE, TCE or dichloroethene (DCE)<ref>ITRC. 2011. Enzyme Activity Probes EMD Team Fact Sheet. [http://www.itrcweb.org/documents/team_emd/EAP_Fact_Sheet.pdf Fact Sheet]</ref>. However, the specific contribution of these organisms to MNA is not well understood<ref>Looney, B., 2010. Incorporating Aerobic Processes into Remedies for Large Chlorinated Solvent Plumes. ER-201026. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201026/ER-201026 ER-201026]</ref>, and studies are trying to define their contribution<ref>Wiedemeier, T.H., 2015. Providing Additional Support for MNA by Including Quantitative Lines of Evidence for Abiotic Degradation and Cometabolic Oxidation of Chlorinated Ethylenes. ER-201584. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201584/ER-201584 ER-201584]</ref>.

Under anaerobic conditions, the chlorinated solvents act as an electron acceptor. In such cases, electron donors may be in the form of naturally occurring, bioavailable organic carbon, or possibly from comingled plumes of petroleum hydrocarbons. The chlorinated solvents function in bacterial metabolism in the same fashion as oxygen functions in human metabolism. The chlorinated solvents are essentially something for the bacteria to breath in the absence of other electron acceptors such as oxygen, nitrate, or sulfate.

[[File:Wilson 3 Fig1.png|thumbnail|450 px|right|Figure 1. Degradation chlorinated alkenes to ethene.]]
In anaerobic groundwater, when conditions are favorable, chlorinated alkenes can undergo a sequential reductive dehalogenation where a chlorine atom is replaced with a hydrogen atom. Degradation proceeds from PCE to TCE, then to DCE, then to VC and finally to ethene (Fig. 1). The minimal geochemical conditions<ref>Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical protocol for evaluating natural attenuation of chlorinated solvents in ground water. EPA-600-R-98-128. [[Media:Wiedemeier-1998-Technical_Protocol_for_Evaluating_Natuaral_Attenuation.pdf|Report pdf]]</ref> that must be taken into account include pH, oxidation-reduction potential (ORP), dissolved oxygen (DO) concentration, total organic carbon (TOC) and competing electron acceptors including oxygen, nitrate, sulfate and ferric iron.

PCE and TCE can be used as an electron acceptor by a wide variety of bacteria<ref>Nyer, E.K., Payne, F., Sutherson, S., 2003. Discussion of environment vs. bacteria or let's play,‘name that bacteria’. Groundwater Monitoring & Remediation, 23(2), 32-48. [http://dx.doi.org/10.1111/j.1745-6592.2003.tb00665.x doi: 10.1111/j.1745-6592.2003.tb00665.x]</ref>. The bacteria can degrade PCE or TCE as far as DCE. The only organisms that can degrade DCE to VC and then degrade VC to the harmless end product ethene are stains of ''Dehalococcoides mccartyi''<ref>Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, Leeson, A., Ward, C.H. Springer, New York, NY. pgs. 39-88. ISBN: 978-1-4614-4114-4 ISBN 978-1-4614-4115-1. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>.

The degradation of chlorinated alkanes in anaerobic groundwater is more complicated (Fig. 2). Chlorinated alkanes can undergo a sequential reductive dehalogenation. In addition, they can undergo the loss of a hydrogen and a chlorine atom to form an alkene (a dehydrochlorination) or the loss of two chlorine atoms to form an alkene (a dichloroelimination).

Three reactions have been demonstrated for 1,1,1-TCA in groundwater (Fig. 2)<ref>Scheutz, C., Durant, N.D., Hansen, M.H., Bjerg, P.L., 2011. Natural and enhanced anaerobic degradation of 1,1,1-trichloroethane and its degradation products in the subsurface–a critical review. Water Research, 45(9), 2701-2723. [http://dx.doi.org/10.1016/j.watres.2011.02.027 doi:10.1016/j.watres.2011.02.027]</ref>. It can undergo an abiotic hydrolysis reaction to produce acetate, an abiotic dehydrochlorination to produce 1,1-DCE, and a biological reductive dechlorination reaction to 1,1-DCA and then chloroethane. In addition to being reduced to chloroethane, 1,1-DCA can undergo a dichloroelimination reaction<ref>Lollar, B.S., Hirschorn, S., Mundle, S.O., Grostern, A., Edwards, E.A., Lacrampe-Couloume, G., 2010. Insights into enzyme kinetics of chloroethane biodegradation using compound specific stable isotopes. Environmental Science & Technology, 44(19), 7498-7503. [http://dx.doi.org/10.1021/es101330r doi: 10.1021/es101330r]</ref> to produce ethene.

[[File:Wilson 3 Fig2.PNG|thumbnail|400 px|left|Figure 2. Degradation of Chlorinated alkanes to ethane.]]

The degradation of 1,1,2-TCA follows a similar pattern (Fig. 2). One strain of ''Desulfitobacterium'' has been shown to dechlorinate 1,1,2-TCA to 1,2-DCA and chloroethane<ref>Zhao, S., Ding, C., He, J., 2015. Detoxification of 1,1,2-trichloroethane to ethene by desulfitobacterium and identification of its functional reductase gene. PloS One, 10(4), p.e0119507. [http://dx.doi.org/10.1371/journal.pone.0119507 doi:10.1371/journal.pone.0119507]</ref> through a sequential reductive dehalogenation. Certain strains of ''Dehalogenimonas'' go through a dichloroelimination reaction<ref>Bowman, K.S., Nobre, M.F., da Costa, M.S., Rainey, F.A. and Moe, W.M., 2013. Dehalogenimonas alkenigignens sp. nov., a chlorinated-alkane-dehalogenating bacterium isolated from groundwater. International Journal of Systematic and Evolutionary Microbiology, 63(4), 1492-1498. [http://dx.doi.org/10.1099/ijs.0.045054-0 doi: 10.1099/ijs.0.045054-0]</ref> to dechlorinate 1,1,2-TCA to VC and 1,2-DCA to ethene. A strain of ''Dehalobacter'' can also dechlorinate 1,2-DCA to ethene<ref>Grostern, A., Edwards, E.A., 2009. Characterization of a Dehalobacter coculture that dechlorinates 1,2-dichloroethane to ethene and identification of the putative reductive dehalogenase gene. Applied and Environmental Microbiology, 75(9), 2684-2693. [http://dx.doi.org/10.1128/aem.02037-08 doi: 10.1128/AEM.02037-08]</ref>.

A quantitative framework (BioPIC)<ref name="Lebron2015">Lebron, C. A., Wiedemeier, T. H., Wilson, J.T., Löffler, F.E., Hinchee, R.E., Singletary, M.A., 2015. Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites. ER-201129. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129/ER-201129 ER-201129]</ref> is now available that allows an evaluation of the rate constant for anaerobic biological degradation of cDCE and VC based on the abundance of gene markers for ''Dehalococcoides mccartyi''. The relationships between the rate constants for degradation of the chlorinated alkanes and abundance of gene copies of ''Dehalobacter'', ''Dehalogenimonas'' and other active bacteria are still being explored.

==Abiotic Degradation==
Chlorinated solvents can chemically react with a number of iron minerals in aquifers<ref name="HE2009"/>. The most important of these are magnetite, iron mono-sulfide, and pyrite.

Iron sulfide minerals form as a consequence of sulfate reduction in groundwater. The sulfide produced from sulfate reduction will react with Iron (III) minerals to form iron mono-sulfide. Over time the iron mono-sulfide will react with excess sulfide to produce pyrite.

The reactions of the chlorinated alkanes with the iron sulfide minerals is a sequential reductive dechlorination. However, the reaction of iron sulfide minerals with chlorinated alkenes is more complex (Fig. 3). Reductive dechlorination and dichloroelimination can proceed at the same time.

[[File:Wilson 3 Fig3.png|thumbnail|250 px|right|Figure 3. Degradation of chlorinated alkenes carried out by iron sulfide minerals.]]

The U.S. EPA regulates the maximum concentration of contaminants that are allowed in water that is supplied as drinking water. These U.S. EPA regulations are referred to as the Maximum Contaminant Level<ref>U.S. Environmental Protection Agency, 2016. Table of Regulated Drinking Water Contaminants. [http://www.epa.gov/your-drinking-water/table-regulated-drinking-water-contaminants Table of Regulated Drinking Water]</ref> or MCL. There are MCLs for the transformation products of reductive dechlorination (the DCEs and VC) and these products will be included in the target list of analytes in any conventional monitoring program. The products of dichloroelimination do not have MCLs and are not usually on the target list of analytes for conventional monitoring.

If the major pathway of abiotic degradation is dichloroelimination, then conventional monitoring will fail to recognize the contribution of abiotic degradation on iron sulfide minerals. However, the stable isotopes of carbon in chlorinated solvents are strongly fractionated during abiotic degradation on iron sulfide minerals. [[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]]<ref>Hunkeler, D., Meckenstock, R. U., Sherwood Lollar, B., Schmidt, T.C., Wilson, J.T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148. [[Media:Hunkeler-2008-A_Guide.pdf|Report pdf]]</ref> can be a useful tool to recognize abiotic degradation of chlorinated alkenes on iron sulfide minerals.

Magnetite is often present in unconsolidated glacial aquifers and aquifers that form in sediments that are shed by uplands composed of granite or other igneous rocks. Magnetite reacts readily with the chlorinated alkenes. The actual chemical interactions on magnetite are not well understood (Fig. 4). The ultimate degradation products are oxidized organic compounds and carbon dioxide<ref>Darlington, R., Rectanus, H., 2015. Biogeochemical Transformation Handbook. TR-NAVFAC EXWC-EV-1601, 41 pgs. [[Media:Darlington-2015-Biogeochem_Transformation_Handbook.pdf|Report pdf]]</ref>.

[[File:Wilson 3 Fig4.png|thumbnail|300 px|left|Figure 4. Degradation of chlorinated alkenes carried out by magnetite.]]

==Footprints==
Most plumes have some contribution of anaerobic sequential reductive dechlorination. As a result, the primary contaminant and the transformation products of reductive dechlorination are present in the groundwater. The highest concentrations of the primary contaminant will be near the source of contamination, and the flow of groundwater carries the transformation products further downgradient from the source (Figure 5).

Many plumes of chlorinated solvents also have a contribution of abiotic degradation. As a result, the intermediate degradation products (such as DCE) do not accumulate to stoichiometric concentrations. There is an appearance that degradation of the cDCE has stalled, when in fact it is actively degrading, but not to vinyl chloride (Fig. 5). A quantitative framework<ref name="Lebron2015"/> is now available that allows an evaluation of the contribution of abiotic degradation on magnetite based on the magnetic susceptibility of the sediment, and the contribution of abiotic degradation on pyrite based on the extent of sulfate reduction and the geochemistry of the groundwater.
[[File:Wilson 3 Fig5.png|thumbnail|400 px|center|Figure 5. Comparison of a chlorinated alkenes plume undergoing biodegradation alone vs. biodegradation with abiotic degradation.]]

==Tools and Databases for Chlorinated Solvent MNA==
The Scenarios Evaluation Tool for Chlorinated Solvent MNA<ref>Truex, M.J., Newell, C.J., Looney, B.B, Vangelas, K., 2006. Scenarios evaluation tool for chlorinated solvent MNA. Savannah River National Laboratory, Aiken, South Carolina. WSRC-STI-2006-0096. [[Media:Truex-2006-Scenarios_Evaluation_Tool_for_Chlorinated_Solvent_MNA.pdf|Report pdf]]</ref> was designed to provide a structure where the MNA methods and decision logic are linked together in one of 13 different “scenarios” or site types. Based on site data (e.g. Table 2), one selects which of the 13 scenarios best fits their site or portion of a site. Then one goes to the description of that scenario to learn which attenuation reactions are likely to be active, how to design a MNA monitoring program, whether MNA will work, and other relevant factors.
[[File:Wilson 3 Table2.png|thumbnail|600 px|center|Table 2. Key elements of the scenarios tool for chlorinated solvent MNA.]]

A data mining study of MNA at 45 chlorinated solvent sites<ref>McGuire, T.M., Newell, C.J., Looney, B.B., Vangelas, K.M., 2003. Historical and retrospective survey of monitored natural attenuation: A line of inquiry supporting monitored natural attenuation and enhanced passive remediation of chlorinated solvents. Westinghouse Savannah River Company, Aiken, SC. [[Media:McGuire-2003-Historical_and_Retrospective_Survey_of_MNA.pdf|Report pdf]]</ref> provides some interesting information about plume sources, strength, and size (Fig. 6).

[[File:Wilson 3 Fig6.png|thumbnail|500 px|center|Figure 6. Plume characteristics evaluation of 45 chlorinated solvent sites.]]

The performance of MNA was evaluated<ref>McGuire, T., 2016. Development of an Expanded, High-Reliability Cost and Performance Database for In-Situ Remediation Technologies. ESTCP Project No. ER-201120. [https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201120/ER-201120 ER-201120]</ref> by comparing the change in concentrations of chlorinated organic compounds in wells in the source zone of plumes from the beginning to the end of an MNA monitoring period (Fig. 7).

[[File:Wilson 3 Fig7.png|thumbnail|500 px|center|Figure 7. Each dot represents an individual project, showing the geometric mean of the concentration at the beginning of the monitoring record (X-axis) and at the end of the monitoring record (Y-axis). The median duration of MNA monitoring for these 45 sites was 8.7 years and ranged from 4.1 to 15 years.]]

One study evaluated the change in source concentration over time at 23 chlorinated solvent sites by calculating concentration vs. time decay rates for source zone wells<ref>Newell, C.J., Cowie, I., McGuire, T.M., McNab Jr, W.W., 2006. Multiyear temporal changes in chlorinated solvent concentrations at 23 monitored natural attenuation sites. Journal of Environmental Engineering, American Society of Environmental Engineers, 132(6), 653-663. [http://dx.doi.org/10.1061/(asce)0733-9372(2006)132:6(653) doi: 10.1061/(asce)0733-9372(2006)132:6(653)]</ref>. The authors concluded, “If the median point decay rates from these sites are maintained over a 20 year period, the resulting reduction in concentration will be similar to the reported reduction in source zone concentrations achieved by active in situ source remediation technologies (typical project length: 1–2 years)".

As part of the development process for the chlorinated solvent natural attenuation model<ref>Aziz, C.E., Smith, A.P., Newell, C.J., Gonzales, J.R., 2000. BIOCHLOR Chlorinated solvent plume database report. Air Force Center for Environmental Excellence, Texas. [[Media:Aziz-2000-BIOCHLOR-plume-database.pdf|Report pdf]]</ref> BIOCHLOR, 24 chlorinated solvent plumes were studied in detail. Key findings included:

*TCE and c-DCE had median plume lengths of 1215 ft and 1205 ft, respectively.
*Chlorinated ethene plume lengths were moderately correlated with seepage velocity and source width (Fig. 8).
*First order decay rates ranged between 1 and 2 per year for the chlorinated ethane plumes.

[[File:Wilson 3 Fig8.png|thumbnail|900 px|center|Figure 8. Effect of estimated source size and groundwater seepage velocity on plume length.]]

==Summary==
MNA is an important remediation technology at some chlorinated solvent sites. There are numerous reactions, both biotic and abiotic, that can act on different chlorinated solvent compounds. Several tools and databases are available to help understand how chlorinated solvent plumes behave and to design and implement appropriate MNA programs.

==References==

<references/>

==See Also==
*[[Media:EPA-MNA-Chlorinated-Organics-Symposium.pdf|Proceedings of the Symposium on Natural Attenuation of Chlorinated Organics in Ground Water]]
*[[Media:AFCEE-Natural_Attenuation-Chlorinated_Solvents-1999.pdf|Natural Attenuation of Chlorinated Solvents Performance and Cost Results From Multiple Air Force Demonstration Sites]]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1348 Using Advanced Analysis Approaches to Complete Long-Term Evaluations of Natural Attenuation Processes on the Remediation of Dissolved Chlorinated Solvent Contamination]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1349/ER-1349 Integrated Protocol for Assessment of Long-Term Sustainability of Monitored Natural Attenuation of Chlorinated Solvent Plumes]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200019 Impact of Landfill Closure Designs on Long-Term Natural Attenuation of Chlorinated Hydrocarbons]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436 Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200708/ER-200708 Use of Enzyme Probes for Estimation of Trichloroethene Degradation Rates and Acceptance of Monitored Natural Attenuation ]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200824/ER-200824 Verification of Methods for Assessing the Sustainability of Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129 Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201211/ER-201211 Frequently Asked Questions about Monitored Natural Attenuation in the 21st Century]
*[https://www.coursera.org/learn/natural-attenuation-of-groundwater-contaminants/lecture/kBe2j/abiotic-degradation-principles Online Lecture Course - Abiotic Degradation]

Monitored Natural Attenuation (MNA) of Chlorinated Solvents

2026-05-07T16:56:03Z

Admin:

[[Monitored Natural Attenuation (MNA)]] is a common remedy for contamination of [[Chlorinated Solvents |chlorinated solvents]] in groundwater. Chlorinated solvents are susceptible to many natural processes that can attenuate their concentrations in groundwater including biological degradation, abiotic degradation, sorption, dispersion, and volatilization. Typically, MNA is used for plumes with low dissolved concentrations or in peripheral areas of plumes away from areas with non-aqueous phase liquid (NAPL) or other materials that serve as the source of groundwater contamination.
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'''Related Article(s):'''
*[[Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Monitored Natural Attenuation (MNA) of Fuels| Monitored Natural Attenuation of Fuels]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| Monitored Natural Attenuation of Metal and Metalloids]]
*[[Natural Source Zone Depletion (NSZD)]]

'''CONTRIBUTOR(S):''' [[Dr. John Wilson]] 

'''Key Resource(s):'''
*[[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Identification & Characterization Methods for Reactive Minerals Responsible for Natural Attenuation of Chlorinated Organic Compounds in Ground Water]]<ref name="HE2009">He, Y., Su, C., Wilson, J., Wilkin, R., Adair, C., Lee, T., Bradley, P. and Ferrey, M., 2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water. U.S. Environmental Protection Agency. [[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Report pdf]]</ref>

==Introduction==
[[Chlorinated Solvents |Chlorinated solvents]] and their transformation products are among the most abundant contaminants in groundwater. In 2006, the United States Geological Survey published results from a systematic survey of volatile organic chemicals in drinking water wells<ref>Zogorski, J.S., Carter, J.M., Ivahnenko, T., Lapham, W.W., Moran, M.J., Rowe, B.L., Squillace, P.J., Toccalino, P.L., 2006. The quality of our Nation’s waters - Volatile organic compounds in the nation’s ground water and drinking-water supply wells. US Geological Survey Circular, 1292, 101. [[Media:Zogorski-2006-_Volatile_organic_compounds_in_the_nations_ground_water_and_wells.pdf|Report pdf]]</ref> in the USA. Approximately 12% of wells contained detectable concentrations of tetrachloromethane ([[wikipedia: Chloroform | chloroform]]), 5% contained [[wikipedia: Tetrachloroethylene | tetrachloroethene (PCE)]], 4% contained [[wikipedia: Trichloroethylene | trichloroethene (TCE)]], 2% contained [[wikipedia: 1,1,1-Trichloroethane | 1,1,1-trichloroethane (1,1,1-TCA)]], and 2% contained [[wikipedia: 1,1-Dichloroethane | 1,1-dichloroethane (1,1-DCA)]].

[[Monitored Natural Attenuation (MNA) | Monitored Natural Attenuation (MNA)]] is one remedy that is available for contamination from chlorinated solvents in groundwater. Natural processes that can attenuate the concentrations of chlorinated solvents in groundwater include biological degradation, abiotic degradation, sorption, dispersion into ground adjacent to the contaminant plume, and volatilization to soil gas above the groundwater. At most sites where MNA has been selected as a remedy, or part of a remedy, the chlorinated solvents have been shown to be degrading in groundwater.

==Biodegradation==
The prospects for degradation of selected chlorinated solvents and their transformation products in groundwater are good (Table 1).

[[File:Wilson 3 Table1.JPG|thumbnail|600 px|left|Table 1. Summary of the prospects for degradation of selected chlorinated solvents and their transformation products in groundwater<ref>Lawrence, S.J., 2006. Description, properties, and degradation of selected volatile organic compounds detected in ground water--A review of selected literature (No. 2006-1338). [[Media:Lawrence-2006-Description_properties_degradation_of_VOCs.pdf|Report pdf]]</ref><ref name="HE2009"/>]]
Biodegradation can occur under both aerobic and anaerobic conditions. Under aerobic conditions, the chlorinated solvent can act as a source of food for the microorganisms (referred to as direct biodegradation in Table 1). Degradation can also be a fortuitous reaction that does not provide any benefit to the microorganisms. The fortuitous reaction is called a cometabolism or cooxidation. The fortuitous reaction is most commonly carried out by an oxygenase enzyme that is produced by the microorganisms in order to allow them to degrade some other compound.

When the chlorinated solvent is degraded as a food source, the population of active organisms and the rate of degradation will increase over time. If the degradation is fortuitous, the bacteria do not grow as a result of degrading the chlorinated solvent, and the rate constant does not increase over time.

The prospects for direct aerobic biodegradation of chlorinated alkenes depends on the extent of chlorination. PCE and TCE do not support growth under aerobic conditions, cis-dichloroethene<ref>Cox, E., 2012. Elucidation of the mechanisms and environmental relevance of cis-dichloroethene and vinyl chloride biodegradation. ER-1557. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1557/ER-1557 ER-1557]</ref> (c-DCE) can be degraded in aerobic groundwater, and vinyl chloride (VC) is readily degradable in many aerobic groundwaters.

Many samples of groundwater contain microorganisms that express oxygenase enzymes and can cometabolize PCE, TCE or dichloroethene (DCE)<ref>ITRC. 2011. Enzyme Activity Probes EMD Team Fact Sheet. [http://www.itrcweb.org/documents/team_emd/EAP_Fact_Sheet.pdf Fact Sheet]</ref>. However, the specific contribution of these organisms to MNA is not well understood<ref>Looney, B., 2010. Incorporating Aerobic Processes into Remedies for Large Chlorinated Solvent Plumes. ER-201026. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201026/ER-201026 ER-201026]</ref>, and studies are trying to define their contribution<ref>Wiedemeier, T.H., 2015. Providing Additional Support for MNA by Including Quantitative Lines of Evidence for Abiotic Degradation and Cometabolic Oxidation of Chlorinated Ethylenes. ER-201584. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201584/ER-201584 ER-201584]</ref>.

Under anaerobic conditions, the chlorinated solvents act as an electron acceptor. In such cases, electron donors may be in the form of naturally occurring, bioavailable organic carbon, or possibly from comingled plumes of petroleum hydrocarbons. The chlorinated solvents function in bacterial metabolism in the same fashion as oxygen functions in human metabolism. The chlorinated solvents are essentially something for the bacteria to breath in the absence of other electron acceptors such as oxygen, nitrate, or sulfate.

[[File:Wilson 3 Fig1.png|thumbnail|450 px|right|Figure 1. Degradation chlorinated alkenes to ethene.]]
In anaerobic groundwater, when conditions are favorable, chlorinated alkenes can undergo a sequential reductive dehalogenation where a chlorine atom is replaced with a hydrogen atom. Degradation proceeds from PCE to TCE, then to DCE, then to VC and finally to ethene (Fig. 1). The minimal geochemical conditions<ref>Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical protocol for evaluating natural attenuation of chlorinated solvents in ground water. EPA-600-R-98-128. [[Media:Wiedemeier-1998-Technical_Protocol_for_Evaluating_Natuaral_Attenuation.pdf|Report pdf]]</ref> that must be taken into account include pH, oxidation-reduction potential (ORP), dissolved oxygen (DO) concentration, total organic carbon (TOC) and competing electron acceptors including oxygen, nitrate, sulfate and ferric iron.

PCE and TCE can be used as an electron acceptor by a wide variety of bacteria<ref>Nyer, E.K., Payne, F., Sutherson, S., 2003. Discussion of environment vs. bacteria or let's play,‘name that bacteria’. Groundwater Monitoring & Remediation, 23(2), 32-48. [http://dx.doi.org/10.1111/j.1745-6592.2003.tb00665.x doi: 10.1111/j.1745-6592.2003.tb00665.x]</ref>. The bacteria can degrade PCE or TCE as far as DCE. The only organisms that can degrade DCE to VC and then degrade VC to the harmless end product ethene are stains of ''Dehalococcoides mccartyi''<ref>Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, Leeson, A., Ward, C.H. Springer, New York, NY. pgs. 39-88. ISBN: 978-1-4614-4114-4 ISBN 978-1-4614-4115-1. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>.

The degradation of chlorinated alkanes in anaerobic groundwater is more complicated (Fig. 2). Chlorinated alkanes can undergo a sequential reductive dehalogenation. In addition, they can undergo the loss of a hydrogen and a chlorine atom to form an alkene (a dehydrochlorination) or the loss of two chlorine atoms to form an alkene (a dichloroelimination).

Three reactions have been demonstrated for 1,1,1-TCA in groundwater (Fig. 2)<ref>Scheutz, C., Durant, N.D., Hansen, M.H., Bjerg, P.L., 2011. Natural and enhanced anaerobic degradation of 1,1,1-trichloroethane and its degradation products in the subsurface–a critical review. Water Research, 45(9), 2701-2723. [http://dx.doi.org/10.1016/j.watres.2011.02.027 doi:10.1016/j.watres.2011.02.027]</ref>. It can undergo an abiotic hydrolysis reaction to produce acetate, an abiotic dehydrochlorination to produce 1,1-DCE, and a biological reductive dechlorination reaction to 1,1-DCA and then chloroethane. In addition to being reduced to chloroethane, 1,1-DCA can undergo a dichloroelimination reaction<ref>Lollar, B.S., Hirschorn, S., Mundle, S.O., Grostern, A., Edwards, E.A., Lacrampe-Couloume, G., 2010. Insights into enzyme kinetics of chloroethane biodegradation using compound specific stable isotopes. Environmental Science & Technology, 44(19), 7498-7503. [http://dx.doi.org/10.1021/es101330r doi: 10.1021/es101330r]</ref> to produce ethene.

[[File:Wilson 3 Fig2.PNG|thumbnail|400 px|left|Figure 2. Degradation of Chlorinated alkanes to ethane.]]

The degradation of 1,1,2-TCA follows a similar pattern (Fig. 2). One strain of ''Desulfitobacterium'' has been shown to dechlorinate 1,1,2-TCA to 1,2-DCA and chloroethane<ref>Zhao, S., Ding, C., He, J., 2015. Detoxification of 1,1,2-trichloroethane to ethene by desulfitobacterium and identification of its functional reductase gene. PloS One, 10(4), p.e0119507. [http://dx.doi.org/10.1371/journal.pone.0119507 doi:10.1371/journal.pone.0119507]</ref> through a sequential reductive dehalogenation. Certain strains of ''Dehalogenimonas'' go through a dichloroelimination reaction<ref>Bowman, K.S., Nobre, M.F., da Costa, M.S., Rainey, F.A. and Moe, W.M., 2013. Dehalogenimonas alkenigignens sp. nov., a chlorinated-alkane-dehalogenating bacterium isolated from groundwater. International Journal of Systematic and Evolutionary Microbiology, 63(4), 1492-1498. [http://dx.doi.org/10.1099/ijs.0.045054-0 doi: 10.1099/ijs.0.045054-0]</ref> to dechlorinate 1,1,2-TCA to VC and 1,2-DCA to ethene. A strain of ''Dehalobacter'' can also dechlorinate 1,2-DCA to ethene<ref>Grostern, A., Edwards, E.A., 2009. Characterization of a Dehalobacter coculture that dechlorinates 1,2-dichloroethane to ethene and identification of the putative reductive dehalogenase gene. Applied and Environmental Microbiology, 75(9), 2684-2693. [http://dx.doi.org/10.1128/aem.02037-08 doi: 10.1128/AEM.02037-08]</ref>.

A quantitative framework (BioPIC)<ref name="Lebron2015">Lebron, C. A., Wiedemeier, T. H., Wilson, J.T., Löffler, F.E., Hinchee, R.E., Singletary, M.A., 2015. Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites. ER-201129. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129/ER-201129 ER-201129]</ref> is now available that allows an evaluation of the rate constant for anaerobic biological degradation of cDCE and VC based on the abundance of gene markers for ''Dehalococcoides mccartyi''. The relationships between the rate constants for degradation of the chlorinated alkanes and abundance of gene copies of ''Dehalobacter'', ''Dehalogenimonas'' and other active bacteria are still being explored.

==Abiotic Degradation==
Chlorinated solvents can chemically react with a number of iron minerals in aquifers<ref name="HE2009"/>. The most important of these are magnetite, iron mono-sulfide, and pyrite.

Iron sulfide minerals form as a consequence of sulfate reduction in groundwater. The sulfide produced from sulfate reduction will react with Iron (III) minerals to form iron mono-sulfide. Over time the iron mono-sulfide will react with excess sulfide to produce pyrite.

The reactions of the chlorinated alkanes with the iron sulfide minerals is a sequential reductive dechlorination. However, the reaction of iron sulfide minerals with chlorinated alkenes is more complex (Fig. 3). Reductive dechlorination and dichloroelimination can proceed at the same time.

[[File:Wilson 3 Fig3.png|thumbnail|250 px|right|Figure 3. Degradation of chlorinated alkenes carried out by iron sulfide minerals.]]

The U.S. EPA regulates the maximum concentration of contaminants that are allowed in water that is supplied as drinking water. These U.S. EPA regulations are referred to as the Maximum Contaminant Level<ref>U.S. Environmental Protection Agency, 2016. Table of Regulated Drinking Water Contaminants. [http://www.epa.gov/your-drinking-water/table-regulated-drinking-water-contaminants Table of Regulated Drinking Water]</ref> or MCL. There are MCLs for the transformation products of reductive dechlorination (the DCEs and VC) and these products will be included in the target list of analytes in any conventional monitoring program. The products of dichloroelimination do not have MCLs and are not usually on the target list of analytes for conventional monitoring.

If the major pathway of abiotic degradation is dichloroelimination, then conventional monitoring will fail to recognize the contribution of abiotic degradation on iron sulfide minerals. However, the stable isotopes of carbon in chlorinated solvents are strongly fractionated during abiotic degradation on iron sulfide minerals. [[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]]<ref>Hunkeler, D., Meckenstock, R. U., Sherwood Lollar, B., Schmidt, T.C., Wilson, J.T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148. [[Media:Hunkeler-2008-A_Guide.pdf|Report pdf]]</ref> can be a useful tool to recognize abiotic degradation of chlorinated alkenes on iron sulfide minerals.

Magnetite is often present in unconsolidated glacial aquifers and aquifers that form in sediments that are shed by uplands composed of granite or other igneous rocks. Magnetite reacts readily with the chlorinated alkenes. The actual chemical interactions on magnetite are not well understood (Fig. 4). The ultimate degradation products are oxidized organic compounds and carbon dioxide<ref>Darlington, R., Rectanus, H., 2015. Biogeochemical Transformation Handbook. TR-NAVFAC EXWC-EV-1601, 41 pgs. [[Media:Darlington-2015-Biogeochem_Transformation_Handbook.pdf|Report pdf]]</ref>.

[[File:Wilson 3 Fig4.png|thumbnail|300 px|left|Figure 4. Degradation of chlorinated alkenes carried out by magnetite.]]

==Footprints==
Most plumes have some contribution of anaerobic sequential reductive dechlorination. As a result, the primary contaminant and the transformation products of reductive dechlorination are present in the groundwater. The highest concentrations of the primary contaminant will be near the source of contamination, and the flow of groundwater carries the transformation products further downgradient from the source (Figure 5).

Many plumes of chlorinated solvents also have a contribution of abiotic degradation. As a result, the intermediate degradation products (such as DCE) do not accumulate to stoichiometric concentrations. There is an appearance that degradation of the cDCE has stalled, when in fact it is actively degrading, but not to vinyl chloride (Fig. 5). A quantitative framework<ref name="Lebron2015"/> is now available that allows an evaluation of the contribution of abiotic degradation on magnetite based on the magnetic susceptibility of the sediment, and the contribution of abiotic degradation on pyrite based on the extent of sulfate reduction and the geochemistry of the groundwater.
[[File:Wilson 3 Fig5.png|thumbnail|400 px|center|Figure 5. Comparison of a chlorinated alkenes plume undergoing biodegradation alone vs. biodegradation with abiotic degradation.]]

==Tools and Databases for Chlorinated Solvent MNA==
The Scenarios Evaluation Tool for Chlorinated Solvent MNA<ref>Truex, M.J., Newell, C.J., Looney, B.B, Vangelas, K., 2006. Scenarios evaluation tool for chlorinated solvent MNA. Savannah River National Laboratory, Aiken, South Carolina. WSRC-STI-2006-0096. [[Media:Truex-2006-Scenarios_Evaluation_Tool_for_Chlorinated_Solvent_MNA.pdf|Report pdf]]</ref> was designed to provide a structure where the MNA methods and decision logic are linked together in one of 13 different “scenarios” or site types. Based on site data (e.g. Table 2), one selects which of the 13 scenarios best fits their site or portion of a site. Then one goes to the description of that scenario to learn which attenuation reactions are likely to be active, how to design a MNA monitoring program, whether MNA will work, and other relevant factors.
[[File:Wilson 3 Table2.png|thumbnail|600 px|center|Table 2. Key elements of the scenarios tool for chlorinated solvent MNA.]]

A data mining study of MNA at 45 chlorinated solvent sites<ref>McGuire, T.M., Newell, C.J., Looney, B.B., Vangelas, K.M., 2003. Historical and retrospective survey of monitored natural attenuation: A line of inquiry supporting monitored natural attenuation and enhanced passive remediation of chlorinated solvents. Westinghouse Savannah River Company, Aiken, SC. [[Media:McGuire-2003-Historical_and_Retrospective_Survey_of_MNA.pdf|Report pdf]]</ref> provides some interesting information about plume sources, strength, and size (Fig. 6).

[[File:Wilson 3 Fig6.png|thumbnail|500 px|center|Figure 6. Plume characteristics evaluation of 45 chlorinated solvent sites.]]

The performance of MNA was evaluated<ref>McGuire, T., 2016. Development of an Expanded, High-Reliability Cost and Performance Database for In-Situ Remediation Technologies. ESTCP Project No. ER-201120. [https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201120/ER-201120 ER-201120]</ref> by comparing the change in concentrations of chlorinated organic compounds in wells in the source zone of plumes from the beginning to the end of an MNA monitoring period (Fig. 7).

[[File:Wilson 3 Fig7.png|thumbnail|500 px|center|Figure 7. Each dot represents an individual project, showing the geometric mean of the concentration at the beginning of the monitoring record (X-axis) and at the end of the monitoring record (Y-axis). The median duration of MNA monitoring for these 45 sites was 8.7 years and ranged from 4.1 to 15 years.]]

One study evaluated the change in source concentration over time at 23 chlorinated solvent sites by calculating concentration vs. time decay rates for source zone wells<ref>Newell, C.J., Cowie, I., McGuire, T.M., McNab Jr, W.W., 2006. Multiyear temporal changes in chlorinated solvent concentrations at 23 monitored natural attenuation sites. Journal of Environmental Engineering, American Society of Environmental Engineers, 132(6), 653-663. [http://dx.doi.org/10.1061/(asce)0733-9372(2006)132:6(653) doi: 10.1061/(asce)0733-9372(2006)132:6(653)]</ref>. The authors concluded, “If the median point decay rates from these sites are maintained over a 20 year period, the resulting reduction in concentration will be similar to the reported reduction in source zone concentrations achieved by active in situ source remediation technologies (typical project length: 1–2 years)".

As part of the development process for the chlorinated solvent natural attenuation model<ref>Aziz, C.E., Smith, A.P., Newell, C.J., Gonzales, J.R., 2000. BIOCHLOR Chlorinated solvent plume database report. Air Force Center for Environmental Excellence, Texas. [[Media:Aziz-2000-BIOCHLOR-plume-database.pdf|Report pdf]]</ref> BIOCHLOR, 24 chlorinated solvent plumes were studied in detail. Key findings included:

*TCE and c-DCE had median plume lengths of 1215 ft and 1205 ft, respectively.
*Chlorinated ethene plume lengths were moderately correlated with seepage velocity and source width (Fig. 8).
*First order decay rates ranged between 1 and 2 per year for the chlorinated ethane plumes.

[[File:Wilson 3 Fig8.png|thumbnail|900 px|center|Figure 8. Effect of estimated source size and groundwater seepage velocity on plume length.]]

==Summary==
MNA is an important remediation technology at some chlorinated solvent sites. There are numerous reactions, both biotic and abiotic, that can act on different chlorinated solvent compounds. Several tools and databases are available to help understand how chlorinated solvent plumes behave and to design and implement appropriate MNA programs.

==References==

<references/>

==See Also==
*[[Media:EPA-MNA-Chlorinated-Organics-Symposium.pdf|Proceedings of the Symposium on Natural Attenuation of Chlorinated Organics in Ground Water]]
*[[Media:AFCEE-Natural_Attenuation-Chlorinated_Solvents-1999.pdf|Natural Attenuation of Chlorinated Solvents Performance and Cost Results From Multiple Air Force Demonstration Sites]]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1348 Using Advanced Analysis Approaches to Complete Long-Term Evaluations of Natural Attenuation Processes on the Remediation of Dissolved Chlorinated Solvent Contamination]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1349/ER-1349 Integrated Protocol for Assessment of Long-Term Sustainability of Monitored Natural Attenuation of Chlorinated Solvent Plumes]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200019 Impact of Landfill Closure Designs on Long-Term Natural Attenuation of Chlorinated Hydrocarbons]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436 Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200708/ER-200708 Use of Enzyme Probes for Estimation of Trichloroethene Degradation Rates and Acceptance of Monitored Natural Attenuation ]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200824/ER-200824 Verification of Methods for Assessing the Sustainability of Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129 Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201211/ER-201211 Frequently Asked Questions about Monitored Natural Attenuation in the 21st Century]
*[https://www.coursera.org/learn/natural-attenuation-of-groundwater-contaminants/lecture/kBe2j/abiotic-degradation-principles Online Lecture Course - Abiotic Degradation]

Monitored Natural Attenuation (MNA) of Chlorinated Solvents

2026-05-07T16:55:37Z

Admin:

[[Monitored Natural Attenuation (MNA)]] is a common remedy for contamination of [[Chlorinated Solvents |chlorinated solvents]] in groundwater. Chlorinated solvents are susceptible to many natural processes that can attenuate their concentrations in groundwater including biological degradation, abiotic degradation, sorption, dispersion, and volatilization. Typically, MNA is used for plumes with low dissolved concentrations or in peripheral areas of plumes away from areas with non-aqueous phase liquid (NAPL) or other materials that serve as the source of groundwater contamination.
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'''Related Article(s):'''
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Monitored Natural Attenuation (MNA) of Fuels| Monitored Natural Attenuation of Fuels]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| Monitored Natural Attenuation of Metal and Metalloids]]
*[[Chlorinated Solvents]]
*[[Natural Source Zone Depletion (NSZD)]]

'''CONTRIBUTOR(S):''' [[Dr. John Wilson]] 

'''Key Resource(s):'''
*[[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Identification & Characterization Methods for Reactive Minerals Responsible for Natural Attenuation of Chlorinated Organic Compounds in Ground Water]]<ref name="HE2009">He, Y., Su, C., Wilson, J., Wilkin, R., Adair, C., Lee, T., Bradley, P. and Ferrey, M., 2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water. U.S. Environmental Protection Agency. [[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Report pdf]]</ref>

==Introduction==
[[Chlorinated Solvents |Chlorinated solvents]] and their transformation products are among the most abundant contaminants in groundwater. In 2006, the United States Geological Survey published results from a systematic survey of volatile organic chemicals in drinking water wells<ref>Zogorski, J.S., Carter, J.M., Ivahnenko, T., Lapham, W.W., Moran, M.J., Rowe, B.L., Squillace, P.J., Toccalino, P.L., 2006. The quality of our Nation’s waters - Volatile organic compounds in the nation’s ground water and drinking-water supply wells. US Geological Survey Circular, 1292, 101. [[Media:Zogorski-2006-_Volatile_organic_compounds_in_the_nations_ground_water_and_wells.pdf|Report pdf]]</ref> in the USA. Approximately 12% of wells contained detectable concentrations of tetrachloromethane ([[wikipedia: Chloroform | chloroform]]), 5% contained [[wikipedia: Tetrachloroethylene | tetrachloroethene (PCE)]], 4% contained [[wikipedia: Trichloroethylene | trichloroethene (TCE)]], 2% contained [[wikipedia: 1,1,1-Trichloroethane | 1,1,1-trichloroethane (1,1,1-TCA)]], and 2% contained [[wikipedia: 1,1-Dichloroethane | 1,1-dichloroethane (1,1-DCA)]].

[[Monitored Natural Attenuation (MNA) | Monitored Natural Attenuation (MNA)]] is one remedy that is available for contamination from chlorinated solvents in groundwater. Natural processes that can attenuate the concentrations of chlorinated solvents in groundwater include biological degradation, abiotic degradation, sorption, dispersion into ground adjacent to the contaminant plume, and volatilization to soil gas above the groundwater. At most sites where MNA has been selected as a remedy, or part of a remedy, the chlorinated solvents have been shown to be degrading in groundwater.

==Biodegradation==
The prospects for degradation of selected chlorinated solvents and their transformation products in groundwater are good (Table 1).

[[File:Wilson 3 Table1.JPG|thumbnail|600 px|left|Table 1. Summary of the prospects for degradation of selected chlorinated solvents and their transformation products in groundwater<ref>Lawrence, S.J., 2006. Description, properties, and degradation of selected volatile organic compounds detected in ground water--A review of selected literature (No. 2006-1338). [[Media:Lawrence-2006-Description_properties_degradation_of_VOCs.pdf|Report pdf]]</ref><ref name="HE2009"/>]]
Biodegradation can occur under both aerobic and anaerobic conditions. Under aerobic conditions, the chlorinated solvent can act as a source of food for the microorganisms (referred to as direct biodegradation in Table 1). Degradation can also be a fortuitous reaction that does not provide any benefit to the microorganisms. The fortuitous reaction is called a cometabolism or cooxidation. The fortuitous reaction is most commonly carried out by an oxygenase enzyme that is produced by the microorganisms in order to allow them to degrade some other compound.

When the chlorinated solvent is degraded as a food source, the population of active organisms and the rate of degradation will increase over time. If the degradation is fortuitous, the bacteria do not grow as a result of degrading the chlorinated solvent, and the rate constant does not increase over time.

The prospects for direct aerobic biodegradation of chlorinated alkenes depends on the extent of chlorination. PCE and TCE do not support growth under aerobic conditions, cis-dichloroethene<ref>Cox, E., 2012. Elucidation of the mechanisms and environmental relevance of cis-dichloroethene and vinyl chloride biodegradation. ER-1557. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1557/ER-1557 ER-1557]</ref> (c-DCE) can be degraded in aerobic groundwater, and vinyl chloride (VC) is readily degradable in many aerobic groundwaters.

Many samples of groundwater contain microorganisms that express oxygenase enzymes and can cometabolize PCE, TCE or dichloroethene (DCE)<ref>ITRC. 2011. Enzyme Activity Probes EMD Team Fact Sheet. [http://www.itrcweb.org/documents/team_emd/EAP_Fact_Sheet.pdf Fact Sheet]</ref>. However, the specific contribution of these organisms to MNA is not well understood<ref>Looney, B., 2010. Incorporating Aerobic Processes into Remedies for Large Chlorinated Solvent Plumes. ER-201026. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201026/ER-201026 ER-201026]</ref>, and studies are trying to define their contribution<ref>Wiedemeier, T.H., 2015. Providing Additional Support for MNA by Including Quantitative Lines of Evidence for Abiotic Degradation and Cometabolic Oxidation of Chlorinated Ethylenes. ER-201584. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201584/ER-201584 ER-201584]</ref>.

Under anaerobic conditions, the chlorinated solvents act as an electron acceptor. In such cases, electron donors may be in the form of naturally occurring, bioavailable organic carbon, or possibly from comingled plumes of petroleum hydrocarbons. The chlorinated solvents function in bacterial metabolism in the same fashion as oxygen functions in human metabolism. The chlorinated solvents are essentially something for the bacteria to breath in the absence of other electron acceptors such as oxygen, nitrate, or sulfate.

[[File:Wilson 3 Fig1.png|thumbnail|450 px|right|Figure 1. Degradation chlorinated alkenes to ethene.]]
In anaerobic groundwater, when conditions are favorable, chlorinated alkenes can undergo a sequential reductive dehalogenation where a chlorine atom is replaced with a hydrogen atom. Degradation proceeds from PCE to TCE, then to DCE, then to VC and finally to ethene (Fig. 1). The minimal geochemical conditions<ref>Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical protocol for evaluating natural attenuation of chlorinated solvents in ground water. EPA-600-R-98-128. [[Media:Wiedemeier-1998-Technical_Protocol_for_Evaluating_Natuaral_Attenuation.pdf|Report pdf]]</ref> that must be taken into account include pH, oxidation-reduction potential (ORP), dissolved oxygen (DO) concentration, total organic carbon (TOC) and competing electron acceptors including oxygen, nitrate, sulfate and ferric iron.

PCE and TCE can be used as an electron acceptor by a wide variety of bacteria<ref>Nyer, E.K., Payne, F., Sutherson, S., 2003. Discussion of environment vs. bacteria or let's play,‘name that bacteria’. Groundwater Monitoring & Remediation, 23(2), 32-48. [http://dx.doi.org/10.1111/j.1745-6592.2003.tb00665.x doi: 10.1111/j.1745-6592.2003.tb00665.x]</ref>. The bacteria can degrade PCE or TCE as far as DCE. The only organisms that can degrade DCE to VC and then degrade VC to the harmless end product ethene are stains of ''Dehalococcoides mccartyi''<ref>Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, Leeson, A., Ward, C.H. Springer, New York, NY. pgs. 39-88. ISBN: 978-1-4614-4114-4 ISBN 978-1-4614-4115-1. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>.

The degradation of chlorinated alkanes in anaerobic groundwater is more complicated (Fig. 2). Chlorinated alkanes can undergo a sequential reductive dehalogenation. In addition, they can undergo the loss of a hydrogen and a chlorine atom to form an alkene (a dehydrochlorination) or the loss of two chlorine atoms to form an alkene (a dichloroelimination).

Three reactions have been demonstrated for 1,1,1-TCA in groundwater (Fig. 2)<ref>Scheutz, C., Durant, N.D., Hansen, M.H., Bjerg, P.L., 2011. Natural and enhanced anaerobic degradation of 1,1,1-trichloroethane and its degradation products in the subsurface–a critical review. Water Research, 45(9), 2701-2723. [http://dx.doi.org/10.1016/j.watres.2011.02.027 doi:10.1016/j.watres.2011.02.027]</ref>. It can undergo an abiotic hydrolysis reaction to produce acetate, an abiotic dehydrochlorination to produce 1,1-DCE, and a biological reductive dechlorination reaction to 1,1-DCA and then chloroethane. In addition to being reduced to chloroethane, 1,1-DCA can undergo a dichloroelimination reaction<ref>Lollar, B.S., Hirschorn, S., Mundle, S.O., Grostern, A., Edwards, E.A., Lacrampe-Couloume, G., 2010. Insights into enzyme kinetics of chloroethane biodegradation using compound specific stable isotopes. Environmental Science & Technology, 44(19), 7498-7503. [http://dx.doi.org/10.1021/es101330r doi: 10.1021/es101330r]</ref> to produce ethene.

[[File:Wilson 3 Fig2.PNG|thumbnail|400 px|left|Figure 2. Degradation of Chlorinated alkanes to ethane.]]

The degradation of 1,1,2-TCA follows a similar pattern (Fig. 2). One strain of ''Desulfitobacterium'' has been shown to dechlorinate 1,1,2-TCA to 1,2-DCA and chloroethane<ref>Zhao, S., Ding, C., He, J., 2015. Detoxification of 1,1,2-trichloroethane to ethene by desulfitobacterium and identification of its functional reductase gene. PloS One, 10(4), p.e0119507. [http://dx.doi.org/10.1371/journal.pone.0119507 doi:10.1371/journal.pone.0119507]</ref> through a sequential reductive dehalogenation. Certain strains of ''Dehalogenimonas'' go through a dichloroelimination reaction<ref>Bowman, K.S., Nobre, M.F., da Costa, M.S., Rainey, F.A. and Moe, W.M., 2013. Dehalogenimonas alkenigignens sp. nov., a chlorinated-alkane-dehalogenating bacterium isolated from groundwater. International Journal of Systematic and Evolutionary Microbiology, 63(4), 1492-1498. [http://dx.doi.org/10.1099/ijs.0.045054-0 doi: 10.1099/ijs.0.045054-0]</ref> to dechlorinate 1,1,2-TCA to VC and 1,2-DCA to ethene. A strain of ''Dehalobacter'' can also dechlorinate 1,2-DCA to ethene<ref>Grostern, A., Edwards, E.A., 2009. Characterization of a Dehalobacter coculture that dechlorinates 1,2-dichloroethane to ethene and identification of the putative reductive dehalogenase gene. Applied and Environmental Microbiology, 75(9), 2684-2693. [http://dx.doi.org/10.1128/aem.02037-08 doi: 10.1128/AEM.02037-08]</ref>.

A quantitative framework (BioPIC)<ref name="Lebron2015">Lebron, C. A., Wiedemeier, T. H., Wilson, J.T., Löffler, F.E., Hinchee, R.E., Singletary, M.A., 2015. Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites. ER-201129. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129/ER-201129 ER-201129]</ref> is now available that allows an evaluation of the rate constant for anaerobic biological degradation of cDCE and VC based on the abundance of gene markers for ''Dehalococcoides mccartyi''. The relationships between the rate constants for degradation of the chlorinated alkanes and abundance of gene copies of ''Dehalobacter'', ''Dehalogenimonas'' and other active bacteria are still being explored.

==Abiotic Degradation==
Chlorinated solvents can chemically react with a number of iron minerals in aquifers<ref name="HE2009"/>. The most important of these are magnetite, iron mono-sulfide, and pyrite.

Iron sulfide minerals form as a consequence of sulfate reduction in groundwater. The sulfide produced from sulfate reduction will react with Iron (III) minerals to form iron mono-sulfide. Over time the iron mono-sulfide will react with excess sulfide to produce pyrite.

The reactions of the chlorinated alkanes with the iron sulfide minerals is a sequential reductive dechlorination. However, the reaction of iron sulfide minerals with chlorinated alkenes is more complex (Fig. 3). Reductive dechlorination and dichloroelimination can proceed at the same time.

[[File:Wilson 3 Fig3.png|thumbnail|250 px|right|Figure 3. Degradation of chlorinated alkenes carried out by iron sulfide minerals.]]

The U.S. EPA regulates the maximum concentration of contaminants that are allowed in water that is supplied as drinking water. These U.S. EPA regulations are referred to as the Maximum Contaminant Level<ref>U.S. Environmental Protection Agency, 2016. Table of Regulated Drinking Water Contaminants. [http://www.epa.gov/your-drinking-water/table-regulated-drinking-water-contaminants Table of Regulated Drinking Water]</ref> or MCL. There are MCLs for the transformation products of reductive dechlorination (the DCEs and VC) and these products will be included in the target list of analytes in any conventional monitoring program. The products of dichloroelimination do not have MCLs and are not usually on the target list of analytes for conventional monitoring.

If the major pathway of abiotic degradation is dichloroelimination, then conventional monitoring will fail to recognize the contribution of abiotic degradation on iron sulfide minerals. However, the stable isotopes of carbon in chlorinated solvents are strongly fractionated during abiotic degradation on iron sulfide minerals. [[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]]<ref>Hunkeler, D., Meckenstock, R. U., Sherwood Lollar, B., Schmidt, T.C., Wilson, J.T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148. [[Media:Hunkeler-2008-A_Guide.pdf|Report pdf]]</ref> can be a useful tool to recognize abiotic degradation of chlorinated alkenes on iron sulfide minerals.

Magnetite is often present in unconsolidated glacial aquifers and aquifers that form in sediments that are shed by uplands composed of granite or other igneous rocks. Magnetite reacts readily with the chlorinated alkenes. The actual chemical interactions on magnetite are not well understood (Fig. 4). The ultimate degradation products are oxidized organic compounds and carbon dioxide<ref>Darlington, R., Rectanus, H., 2015. Biogeochemical Transformation Handbook. TR-NAVFAC EXWC-EV-1601, 41 pgs. [[Media:Darlington-2015-Biogeochem_Transformation_Handbook.pdf|Report pdf]]</ref>.

[[File:Wilson 3 Fig4.png|thumbnail|300 px|left|Figure 4. Degradation of chlorinated alkenes carried out by magnetite.]]

==Footprints==
Most plumes have some contribution of anaerobic sequential reductive dechlorination. As a result, the primary contaminant and the transformation products of reductive dechlorination are present in the groundwater. The highest concentrations of the primary contaminant will be near the source of contamination, and the flow of groundwater carries the transformation products further downgradient from the source (Figure 5).

Many plumes of chlorinated solvents also have a contribution of abiotic degradation. As a result, the intermediate degradation products (such as DCE) do not accumulate to stoichiometric concentrations. There is an appearance that degradation of the cDCE has stalled, when in fact it is actively degrading, but not to vinyl chloride (Fig. 5). A quantitative framework<ref name="Lebron2015"/> is now available that allows an evaluation of the contribution of abiotic degradation on magnetite based on the magnetic susceptibility of the sediment, and the contribution of abiotic degradation on pyrite based on the extent of sulfate reduction and the geochemistry of the groundwater.
[[File:Wilson 3 Fig5.png|thumbnail|400 px|center|Figure 5. Comparison of a chlorinated alkenes plume undergoing biodegradation alone vs. biodegradation with abiotic degradation.]]

==Tools and Databases for Chlorinated Solvent MNA==
The Scenarios Evaluation Tool for Chlorinated Solvent MNA<ref>Truex, M.J., Newell, C.J., Looney, B.B, Vangelas, K., 2006. Scenarios evaluation tool for chlorinated solvent MNA. Savannah River National Laboratory, Aiken, South Carolina. WSRC-STI-2006-0096. [[Media:Truex-2006-Scenarios_Evaluation_Tool_for_Chlorinated_Solvent_MNA.pdf|Report pdf]]</ref> was designed to provide a structure where the MNA methods and decision logic are linked together in one of 13 different “scenarios” or site types. Based on site data (e.g. Table 2), one selects which of the 13 scenarios best fits their site or portion of a site. Then one goes to the description of that scenario to learn which attenuation reactions are likely to be active, how to design a MNA monitoring program, whether MNA will work, and other relevant factors.
[[File:Wilson 3 Table2.png|thumbnail|600 px|center|Table 2. Key elements of the scenarios tool for chlorinated solvent MNA.]]

A data mining study of MNA at 45 chlorinated solvent sites<ref>McGuire, T.M., Newell, C.J., Looney, B.B., Vangelas, K.M., 2003. Historical and retrospective survey of monitored natural attenuation: A line of inquiry supporting monitored natural attenuation and enhanced passive remediation of chlorinated solvents. Westinghouse Savannah River Company, Aiken, SC. [[Media:McGuire-2003-Historical_and_Retrospective_Survey_of_MNA.pdf|Report pdf]]</ref> provides some interesting information about plume sources, strength, and size (Fig. 6).

[[File:Wilson 3 Fig6.png|thumbnail|500 px|center|Figure 6. Plume characteristics evaluation of 45 chlorinated solvent sites.]]

The performance of MNA was evaluated<ref>McGuire, T., 2016. Development of an Expanded, High-Reliability Cost and Performance Database for In-Situ Remediation Technologies. ESTCP Project No. ER-201120. [https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201120/ER-201120 ER-201120]</ref> by comparing the change in concentrations of chlorinated organic compounds in wells in the source zone of plumes from the beginning to the end of an MNA monitoring period (Fig. 7).

[[File:Wilson 3 Fig7.png|thumbnail|500 px|center|Figure 7. Each dot represents an individual project, showing the geometric mean of the concentration at the beginning of the monitoring record (X-axis) and at the end of the monitoring record (Y-axis). The median duration of MNA monitoring for these 45 sites was 8.7 years and ranged from 4.1 to 15 years.]]

One study evaluated the change in source concentration over time at 23 chlorinated solvent sites by calculating concentration vs. time decay rates for source zone wells<ref>Newell, C.J., Cowie, I., McGuire, T.M., McNab Jr, W.W., 2006. Multiyear temporal changes in chlorinated solvent concentrations at 23 monitored natural attenuation sites. Journal of Environmental Engineering, American Society of Environmental Engineers, 132(6), 653-663. [http://dx.doi.org/10.1061/(asce)0733-9372(2006)132:6(653) doi: 10.1061/(asce)0733-9372(2006)132:6(653)]</ref>. The authors concluded, “If the median point decay rates from these sites are maintained over a 20 year period, the resulting reduction in concentration will be similar to the reported reduction in source zone concentrations achieved by active in situ source remediation technologies (typical project length: 1–2 years)".

As part of the development process for the chlorinated solvent natural attenuation model<ref>Aziz, C.E., Smith, A.P., Newell, C.J., Gonzales, J.R., 2000. BIOCHLOR Chlorinated solvent plume database report. Air Force Center for Environmental Excellence, Texas. [[Media:Aziz-2000-BIOCHLOR-plume-database.pdf|Report pdf]]</ref> BIOCHLOR, 24 chlorinated solvent plumes were studied in detail. Key findings included:

*TCE and c-DCE had median plume lengths of 1215 ft and 1205 ft, respectively.
*Chlorinated ethene plume lengths were moderately correlated with seepage velocity and source width (Fig. 8).
*First order decay rates ranged between 1 and 2 per year for the chlorinated ethane plumes.

[[File:Wilson 3 Fig8.png|thumbnail|900 px|center|Figure 8. Effect of estimated source size and groundwater seepage velocity on plume length.]]

==Summary==
MNA is an important remediation technology at some chlorinated solvent sites. There are numerous reactions, both biotic and abiotic, that can act on different chlorinated solvent compounds. Several tools and databases are available to help understand how chlorinated solvent plumes behave and to design and implement appropriate MNA programs.

==References==

<references/>

==See Also==
*[[Media:EPA-MNA-Chlorinated-Organics-Symposium.pdf|Proceedings of the Symposium on Natural Attenuation of Chlorinated Organics in Ground Water]]
*[[Media:AFCEE-Natural_Attenuation-Chlorinated_Solvents-1999.pdf|Natural Attenuation of Chlorinated Solvents Performance and Cost Results From Multiple Air Force Demonstration Sites]]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1348 Using Advanced Analysis Approaches to Complete Long-Term Evaluations of Natural Attenuation Processes on the Remediation of Dissolved Chlorinated Solvent Contamination]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1349/ER-1349 Integrated Protocol for Assessment of Long-Term Sustainability of Monitored Natural Attenuation of Chlorinated Solvent Plumes]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200019 Impact of Landfill Closure Designs on Long-Term Natural Attenuation of Chlorinated Hydrocarbons]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436 Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200708/ER-200708 Use of Enzyme Probes for Estimation of Trichloroethene Degradation Rates and Acceptance of Monitored Natural Attenuation ]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200824/ER-200824 Verification of Methods for Assessing the Sustainability of Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129 Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201211/ER-201211 Frequently Asked Questions about Monitored Natural Attenuation in the 21st Century]
*[https://www.coursera.org/learn/natural-attenuation-of-groundwater-contaminants/lecture/kBe2j/abiotic-degradation-principles Online Lecture Course - Abiotic Degradation]

Monitored Natural Attenuation (MNA) of Chlorinated Solvents

2026-05-07T16:55:24Z

Admin:

[[Monitored Natural Attenuation (MNA)]] is a common remedy for contamination of [[Chlorinated Solvents |chlorinated solvents]] in groundwater. Chlorinated solvents are susceptible to many natural processes that can attenuate their concentrations in groundwater including biological degradation, abiotic degradation, sorption, dispersion, and volatilization. Typically, MNA is used for plumes with low dissolved concentrations or in peripheral areas of plumes away from areas with non-aqueous phase liquid (NAPL) or other materials that serve as the source of groundwater contamination.
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'''Related Article(s):'''
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Monitored Natural Attenuation (MNA) of Fuels| Monitored Natural Attenuation of Fuels]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| Monitored Natural Attenuation (MNA) of Metal and Metalloids]]
*[[Chlorinated Solvents]]
*[[Natural Source Zone Depletion (NSZD)]]

'''CONTRIBUTOR(S):''' [[Dr. John Wilson]] 

'''Key Resource(s):'''
*[[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Identification & Characterization Methods for Reactive Minerals Responsible for Natural Attenuation of Chlorinated Organic Compounds in Ground Water]]<ref name="HE2009">He, Y., Su, C., Wilson, J., Wilkin, R., Adair, C., Lee, T., Bradley, P. and Ferrey, M., 2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water. U.S. Environmental Protection Agency. [[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Report pdf]]</ref>

==Introduction==
[[Chlorinated Solvents |Chlorinated solvents]] and their transformation products are among the most abundant contaminants in groundwater. In 2006, the United States Geological Survey published results from a systematic survey of volatile organic chemicals in drinking water wells<ref>Zogorski, J.S., Carter, J.M., Ivahnenko, T., Lapham, W.W., Moran, M.J., Rowe, B.L., Squillace, P.J., Toccalino, P.L., 2006. The quality of our Nation’s waters - Volatile organic compounds in the nation’s ground water and drinking-water supply wells. US Geological Survey Circular, 1292, 101. [[Media:Zogorski-2006-_Volatile_organic_compounds_in_the_nations_ground_water_and_wells.pdf|Report pdf]]</ref> in the USA. Approximately 12% of wells contained detectable concentrations of tetrachloromethane ([[wikipedia: Chloroform | chloroform]]), 5% contained [[wikipedia: Tetrachloroethylene | tetrachloroethene (PCE)]], 4% contained [[wikipedia: Trichloroethylene | trichloroethene (TCE)]], 2% contained [[wikipedia: 1,1,1-Trichloroethane | 1,1,1-trichloroethane (1,1,1-TCA)]], and 2% contained [[wikipedia: 1,1-Dichloroethane | 1,1-dichloroethane (1,1-DCA)]].

[[Monitored Natural Attenuation (MNA) | Monitored Natural Attenuation (MNA)]] is one remedy that is available for contamination from chlorinated solvents in groundwater. Natural processes that can attenuate the concentrations of chlorinated solvents in groundwater include biological degradation, abiotic degradation, sorption, dispersion into ground adjacent to the contaminant plume, and volatilization to soil gas above the groundwater. At most sites where MNA has been selected as a remedy, or part of a remedy, the chlorinated solvents have been shown to be degrading in groundwater.

==Biodegradation==
The prospects for degradation of selected chlorinated solvents and their transformation products in groundwater are good (Table 1).

[[File:Wilson 3 Table1.JPG|thumbnail|600 px|left|Table 1. Summary of the prospects for degradation of selected chlorinated solvents and their transformation products in groundwater<ref>Lawrence, S.J., 2006. Description, properties, and degradation of selected volatile organic compounds detected in ground water--A review of selected literature (No. 2006-1338). [[Media:Lawrence-2006-Description_properties_degradation_of_VOCs.pdf|Report pdf]]</ref><ref name="HE2009"/>]]
Biodegradation can occur under both aerobic and anaerobic conditions. Under aerobic conditions, the chlorinated solvent can act as a source of food for the microorganisms (referred to as direct biodegradation in Table 1). Degradation can also be a fortuitous reaction that does not provide any benefit to the microorganisms. The fortuitous reaction is called a cometabolism or cooxidation. The fortuitous reaction is most commonly carried out by an oxygenase enzyme that is produced by the microorganisms in order to allow them to degrade some other compound.

When the chlorinated solvent is degraded as a food source, the population of active organisms and the rate of degradation will increase over time. If the degradation is fortuitous, the bacteria do not grow as a result of degrading the chlorinated solvent, and the rate constant does not increase over time.

The prospects for direct aerobic biodegradation of chlorinated alkenes depends on the extent of chlorination. PCE and TCE do not support growth under aerobic conditions, cis-dichloroethene<ref>Cox, E., 2012. Elucidation of the mechanisms and environmental relevance of cis-dichloroethene and vinyl chloride biodegradation. ER-1557. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1557/ER-1557 ER-1557]</ref> (c-DCE) can be degraded in aerobic groundwater, and vinyl chloride (VC) is readily degradable in many aerobic groundwaters.

Many samples of groundwater contain microorganisms that express oxygenase enzymes and can cometabolize PCE, TCE or dichloroethene (DCE)<ref>ITRC. 2011. Enzyme Activity Probes EMD Team Fact Sheet. [http://www.itrcweb.org/documents/team_emd/EAP_Fact_Sheet.pdf Fact Sheet]</ref>. However, the specific contribution of these organisms to MNA is not well understood<ref>Looney, B., 2010. Incorporating Aerobic Processes into Remedies for Large Chlorinated Solvent Plumes. ER-201026. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201026/ER-201026 ER-201026]</ref>, and studies are trying to define their contribution<ref>Wiedemeier, T.H., 2015. Providing Additional Support for MNA by Including Quantitative Lines of Evidence for Abiotic Degradation and Cometabolic Oxidation of Chlorinated Ethylenes. ER-201584. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201584/ER-201584 ER-201584]</ref>.

Under anaerobic conditions, the chlorinated solvents act as an electron acceptor. In such cases, electron donors may be in the form of naturally occurring, bioavailable organic carbon, or possibly from comingled plumes of petroleum hydrocarbons. The chlorinated solvents function in bacterial metabolism in the same fashion as oxygen functions in human metabolism. The chlorinated solvents are essentially something for the bacteria to breath in the absence of other electron acceptors such as oxygen, nitrate, or sulfate.

[[File:Wilson 3 Fig1.png|thumbnail|450 px|right|Figure 1. Degradation chlorinated alkenes to ethene.]]
In anaerobic groundwater, when conditions are favorable, chlorinated alkenes can undergo a sequential reductive dehalogenation where a chlorine atom is replaced with a hydrogen atom. Degradation proceeds from PCE to TCE, then to DCE, then to VC and finally to ethene (Fig. 1). The minimal geochemical conditions<ref>Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical protocol for evaluating natural attenuation of chlorinated solvents in ground water. EPA-600-R-98-128. [[Media:Wiedemeier-1998-Technical_Protocol_for_Evaluating_Natuaral_Attenuation.pdf|Report pdf]]</ref> that must be taken into account include pH, oxidation-reduction potential (ORP), dissolved oxygen (DO) concentration, total organic carbon (TOC) and competing electron acceptors including oxygen, nitrate, sulfate and ferric iron.

PCE and TCE can be used as an electron acceptor by a wide variety of bacteria<ref>Nyer, E.K., Payne, F., Sutherson, S., 2003. Discussion of environment vs. bacteria or let's play,‘name that bacteria’. Groundwater Monitoring & Remediation, 23(2), 32-48. [http://dx.doi.org/10.1111/j.1745-6592.2003.tb00665.x doi: 10.1111/j.1745-6592.2003.tb00665.x]</ref>. The bacteria can degrade PCE or TCE as far as DCE. The only organisms that can degrade DCE to VC and then degrade VC to the harmless end product ethene are stains of ''Dehalococcoides mccartyi''<ref>Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, Leeson, A., Ward, C.H. Springer, New York, NY. pgs. 39-88. ISBN: 978-1-4614-4114-4 ISBN 978-1-4614-4115-1. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>.

The degradation of chlorinated alkanes in anaerobic groundwater is more complicated (Fig. 2). Chlorinated alkanes can undergo a sequential reductive dehalogenation. In addition, they can undergo the loss of a hydrogen and a chlorine atom to form an alkene (a dehydrochlorination) or the loss of two chlorine atoms to form an alkene (a dichloroelimination).

Three reactions have been demonstrated for 1,1,1-TCA in groundwater (Fig. 2)<ref>Scheutz, C., Durant, N.D., Hansen, M.H., Bjerg, P.L., 2011. Natural and enhanced anaerobic degradation of 1,1,1-trichloroethane and its degradation products in the subsurface–a critical review. Water Research, 45(9), 2701-2723. [http://dx.doi.org/10.1016/j.watres.2011.02.027 doi:10.1016/j.watres.2011.02.027]</ref>. It can undergo an abiotic hydrolysis reaction to produce acetate, an abiotic dehydrochlorination to produce 1,1-DCE, and a biological reductive dechlorination reaction to 1,1-DCA and then chloroethane. In addition to being reduced to chloroethane, 1,1-DCA can undergo a dichloroelimination reaction<ref>Lollar, B.S., Hirschorn, S., Mundle, S.O., Grostern, A., Edwards, E.A., Lacrampe-Couloume, G., 2010. Insights into enzyme kinetics of chloroethane biodegradation using compound specific stable isotopes. Environmental Science & Technology, 44(19), 7498-7503. [http://dx.doi.org/10.1021/es101330r doi: 10.1021/es101330r]</ref> to produce ethene.

[[File:Wilson 3 Fig2.PNG|thumbnail|400 px|left|Figure 2. Degradation of Chlorinated alkanes to ethane.]]

The degradation of 1,1,2-TCA follows a similar pattern (Fig. 2). One strain of ''Desulfitobacterium'' has been shown to dechlorinate 1,1,2-TCA to 1,2-DCA and chloroethane<ref>Zhao, S., Ding, C., He, J., 2015. Detoxification of 1,1,2-trichloroethane to ethene by desulfitobacterium and identification of its functional reductase gene. PloS One, 10(4), p.e0119507. [http://dx.doi.org/10.1371/journal.pone.0119507 doi:10.1371/journal.pone.0119507]</ref> through a sequential reductive dehalogenation. Certain strains of ''Dehalogenimonas'' go through a dichloroelimination reaction<ref>Bowman, K.S., Nobre, M.F., da Costa, M.S., Rainey, F.A. and Moe, W.M., 2013. Dehalogenimonas alkenigignens sp. nov., a chlorinated-alkane-dehalogenating bacterium isolated from groundwater. International Journal of Systematic and Evolutionary Microbiology, 63(4), 1492-1498. [http://dx.doi.org/10.1099/ijs.0.045054-0 doi: 10.1099/ijs.0.045054-0]</ref> to dechlorinate 1,1,2-TCA to VC and 1,2-DCA to ethene. A strain of ''Dehalobacter'' can also dechlorinate 1,2-DCA to ethene<ref>Grostern, A., Edwards, E.A., 2009. Characterization of a Dehalobacter coculture that dechlorinates 1,2-dichloroethane to ethene and identification of the putative reductive dehalogenase gene. Applied and Environmental Microbiology, 75(9), 2684-2693. [http://dx.doi.org/10.1128/aem.02037-08 doi: 10.1128/AEM.02037-08]</ref>.

A quantitative framework (BioPIC)<ref name="Lebron2015">Lebron, C. A., Wiedemeier, T. H., Wilson, J.T., Löffler, F.E., Hinchee, R.E., Singletary, M.A., 2015. Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites. ER-201129. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129/ER-201129 ER-201129]</ref> is now available that allows an evaluation of the rate constant for anaerobic biological degradation of cDCE and VC based on the abundance of gene markers for ''Dehalococcoides mccartyi''. The relationships between the rate constants for degradation of the chlorinated alkanes and abundance of gene copies of ''Dehalobacter'', ''Dehalogenimonas'' and other active bacteria are still being explored.

==Abiotic Degradation==
Chlorinated solvents can chemically react with a number of iron minerals in aquifers<ref name="HE2009"/>. The most important of these are magnetite, iron mono-sulfide, and pyrite.

Iron sulfide minerals form as a consequence of sulfate reduction in groundwater. The sulfide produced from sulfate reduction will react with Iron (III) minerals to form iron mono-sulfide. Over time the iron mono-sulfide will react with excess sulfide to produce pyrite.

The reactions of the chlorinated alkanes with the iron sulfide minerals is a sequential reductive dechlorination. However, the reaction of iron sulfide minerals with chlorinated alkenes is more complex (Fig. 3). Reductive dechlorination and dichloroelimination can proceed at the same time.

[[File:Wilson 3 Fig3.png|thumbnail|250 px|right|Figure 3. Degradation of chlorinated alkenes carried out by iron sulfide minerals.]]

The U.S. EPA regulates the maximum concentration of contaminants that are allowed in water that is supplied as drinking water. These U.S. EPA regulations are referred to as the Maximum Contaminant Level<ref>U.S. Environmental Protection Agency, 2016. Table of Regulated Drinking Water Contaminants. [http://www.epa.gov/your-drinking-water/table-regulated-drinking-water-contaminants Table of Regulated Drinking Water]</ref> or MCL. There are MCLs for the transformation products of reductive dechlorination (the DCEs and VC) and these products will be included in the target list of analytes in any conventional monitoring program. The products of dichloroelimination do not have MCLs and are not usually on the target list of analytes for conventional monitoring.

If the major pathway of abiotic degradation is dichloroelimination, then conventional monitoring will fail to recognize the contribution of abiotic degradation on iron sulfide minerals. However, the stable isotopes of carbon in chlorinated solvents are strongly fractionated during abiotic degradation on iron sulfide minerals. [[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]]<ref>Hunkeler, D., Meckenstock, R. U., Sherwood Lollar, B., Schmidt, T.C., Wilson, J.T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148. [[Media:Hunkeler-2008-A_Guide.pdf|Report pdf]]</ref> can be a useful tool to recognize abiotic degradation of chlorinated alkenes on iron sulfide minerals.

Magnetite is often present in unconsolidated glacial aquifers and aquifers that form in sediments that are shed by uplands composed of granite or other igneous rocks. Magnetite reacts readily with the chlorinated alkenes. The actual chemical interactions on magnetite are not well understood (Fig. 4). The ultimate degradation products are oxidized organic compounds and carbon dioxide<ref>Darlington, R., Rectanus, H., 2015. Biogeochemical Transformation Handbook. TR-NAVFAC EXWC-EV-1601, 41 pgs. [[Media:Darlington-2015-Biogeochem_Transformation_Handbook.pdf|Report pdf]]</ref>.

[[File:Wilson 3 Fig4.png|thumbnail|300 px|left|Figure 4. Degradation of chlorinated alkenes carried out by magnetite.]]

==Footprints==
Most plumes have some contribution of anaerobic sequential reductive dechlorination. As a result, the primary contaminant and the transformation products of reductive dechlorination are present in the groundwater. The highest concentrations of the primary contaminant will be near the source of contamination, and the flow of groundwater carries the transformation products further downgradient from the source (Figure 5).

Many plumes of chlorinated solvents also have a contribution of abiotic degradation. As a result, the intermediate degradation products (such as DCE) do not accumulate to stoichiometric concentrations. There is an appearance that degradation of the cDCE has stalled, when in fact it is actively degrading, but not to vinyl chloride (Fig. 5). A quantitative framework<ref name="Lebron2015"/> is now available that allows an evaluation of the contribution of abiotic degradation on magnetite based on the magnetic susceptibility of the sediment, and the contribution of abiotic degradation on pyrite based on the extent of sulfate reduction and the geochemistry of the groundwater.
[[File:Wilson 3 Fig5.png|thumbnail|400 px|center|Figure 5. Comparison of a chlorinated alkenes plume undergoing biodegradation alone vs. biodegradation with abiotic degradation.]]

==Tools and Databases for Chlorinated Solvent MNA==
The Scenarios Evaluation Tool for Chlorinated Solvent MNA<ref>Truex, M.J., Newell, C.J., Looney, B.B, Vangelas, K., 2006. Scenarios evaluation tool for chlorinated solvent MNA. Savannah River National Laboratory, Aiken, South Carolina. WSRC-STI-2006-0096. [[Media:Truex-2006-Scenarios_Evaluation_Tool_for_Chlorinated_Solvent_MNA.pdf|Report pdf]]</ref> was designed to provide a structure where the MNA methods and decision logic are linked together in one of 13 different “scenarios” or site types. Based on site data (e.g. Table 2), one selects which of the 13 scenarios best fits their site or portion of a site. Then one goes to the description of that scenario to learn which attenuation reactions are likely to be active, how to design a MNA monitoring program, whether MNA will work, and other relevant factors.
[[File:Wilson 3 Table2.png|thumbnail|600 px|center|Table 2. Key elements of the scenarios tool for chlorinated solvent MNA.]]

A data mining study of MNA at 45 chlorinated solvent sites<ref>McGuire, T.M., Newell, C.J., Looney, B.B., Vangelas, K.M., 2003. Historical and retrospective survey of monitored natural attenuation: A line of inquiry supporting monitored natural attenuation and enhanced passive remediation of chlorinated solvents. Westinghouse Savannah River Company, Aiken, SC. [[Media:McGuire-2003-Historical_and_Retrospective_Survey_of_MNA.pdf|Report pdf]]</ref> provides some interesting information about plume sources, strength, and size (Fig. 6).

[[File:Wilson 3 Fig6.png|thumbnail|500 px|center|Figure 6. Plume characteristics evaluation of 45 chlorinated solvent sites.]]

The performance of MNA was evaluated<ref>McGuire, T., 2016. Development of an Expanded, High-Reliability Cost and Performance Database for In-Situ Remediation Technologies. ESTCP Project No. ER-201120. [https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201120/ER-201120 ER-201120]</ref> by comparing the change in concentrations of chlorinated organic compounds in wells in the source zone of plumes from the beginning to the end of an MNA monitoring period (Fig. 7).

[[File:Wilson 3 Fig7.png|thumbnail|500 px|center|Figure 7. Each dot represents an individual project, showing the geometric mean of the concentration at the beginning of the monitoring record (X-axis) and at the end of the monitoring record (Y-axis). The median duration of MNA monitoring for these 45 sites was 8.7 years and ranged from 4.1 to 15 years.]]

One study evaluated the change in source concentration over time at 23 chlorinated solvent sites by calculating concentration vs. time decay rates for source zone wells<ref>Newell, C.J., Cowie, I., McGuire, T.M., McNab Jr, W.W., 2006. Multiyear temporal changes in chlorinated solvent concentrations at 23 monitored natural attenuation sites. Journal of Environmental Engineering, American Society of Environmental Engineers, 132(6), 653-663. [http://dx.doi.org/10.1061/(asce)0733-9372(2006)132:6(653) doi: 10.1061/(asce)0733-9372(2006)132:6(653)]</ref>. The authors concluded, “If the median point decay rates from these sites are maintained over a 20 year period, the resulting reduction in concentration will be similar to the reported reduction in source zone concentrations achieved by active in situ source remediation technologies (typical project length: 1–2 years)".

As part of the development process for the chlorinated solvent natural attenuation model<ref>Aziz, C.E., Smith, A.P., Newell, C.J., Gonzales, J.R., 2000. BIOCHLOR Chlorinated solvent plume database report. Air Force Center for Environmental Excellence, Texas. [[Media:Aziz-2000-BIOCHLOR-plume-database.pdf|Report pdf]]</ref> BIOCHLOR, 24 chlorinated solvent plumes were studied in detail. Key findings included:

*TCE and c-DCE had median plume lengths of 1215 ft and 1205 ft, respectively.
*Chlorinated ethene plume lengths were moderately correlated with seepage velocity and source width (Fig. 8).
*First order decay rates ranged between 1 and 2 per year for the chlorinated ethane plumes.

[[File:Wilson 3 Fig8.png|thumbnail|900 px|center|Figure 8. Effect of estimated source size and groundwater seepage velocity on plume length.]]

==Summary==
MNA is an important remediation technology at some chlorinated solvent sites. There are numerous reactions, both biotic and abiotic, that can act on different chlorinated solvent compounds. Several tools and databases are available to help understand how chlorinated solvent plumes behave and to design and implement appropriate MNA programs.

==References==

<references/>

==See Also==
*[[Media:EPA-MNA-Chlorinated-Organics-Symposium.pdf|Proceedings of the Symposium on Natural Attenuation of Chlorinated Organics in Ground Water]]
*[[Media:AFCEE-Natural_Attenuation-Chlorinated_Solvents-1999.pdf|Natural Attenuation of Chlorinated Solvents Performance and Cost Results From Multiple Air Force Demonstration Sites]]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1348 Using Advanced Analysis Approaches to Complete Long-Term Evaluations of Natural Attenuation Processes on the Remediation of Dissolved Chlorinated Solvent Contamination]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-1349/ER-1349 Integrated Protocol for Assessment of Long-Term Sustainability of Monitored Natural Attenuation of Chlorinated Solvent Plumes]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200019 Impact of Landfill Closure Designs on Long-Term Natural Attenuation of Chlorinated Hydrocarbons]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436 Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200708/ER-200708 Use of Enzyme Probes for Estimation of Trichloroethene Degradation Rates and Acceptance of Monitored Natural Attenuation ]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200824/ER-200824 Verification of Methods for Assessing the Sustainability of Monitored Natural Attenuation]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129 Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201211/ER-201211 Frequently Asked Questions about Monitored Natural Attenuation in the 21st Century]
*[https://www.coursera.org/learn/natural-attenuation-of-groundwater-contaminants/lecture/kBe2j/abiotic-degradation-principles Online Lecture Course - Abiotic Degradation]

Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions

2026-05-07T16:52:23Z

Admin:

The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective ''in situ'' approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA)| Monitored Natural Attenuation]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| Monitored Natural Attenuation of Chlorinated Solvents]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
*[[Matrix Diffusion]]
*[[REMChlor - MD]]

'''Contributor(s):''' [[Dani Tran]], [[Dr. Charles Schaefer]], and [[Dr. Charles Werth]]

'''Key Resource(s):'''
*Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils<ref name="SchaeferEtAl2025"/>

==Introduction==
Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.

For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as [[REMChlor - MD]]<ref>Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]  [[Media: FaltaWang2017.pdf | Open Access Manuscript]]</ref><ref>Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. [https://doi.org/10.1016/j.jconhyd.2022.103986 doi: 10.1016/j.jconhyd.2022.103986]  [[Media: KulkarniEtAl2022.pdf | Open Access Manuscript]]</ref> to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.

==Recommended Approach==
[[File: TranFig1.png | thumb | 500 px | Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions. Circles are data from Schaefer ''et al.'', 2021<ref>Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2021. Abiotic dechlorination in the presence of ferrous minerals. Journal of Contaminant Hydrology, 241, 103839. [https://doi.org/10.1016/j.jconhyd.2021.103839 doi: 10.1016/j.jconhyd.2021.103839]  [[Media: SchaeferEtAl2021.pdf | Open Access Manuscript]]</ref>, filled squares from Schaefer ''et al.'', 2018<ref name="SchaeferEtAl2018"/>, and Schaefer ''et al.'', 2017<ref>Schaefer, C.E., Ho., Gurr, C., Berns, E., Werth, C., 2017. Abiotic dechlorination of chlorinated ethenes in natural clayey soils: impacts of mineralogy and temperature. Journal of Contaminant Hydrology, 206, pp. 10-17. [https://doi.org/10.1016/j.jconhyd.2017.09.007 doi: 10.1016/j.jconhyd.2017.09.007]  [[Media: SchaeferEtAl2017.pdf | Open Access Manuscript]]</ref>, and open squares from Schaefer ''et al.'', 2025<ref name="SchaeferEtAl2025"/>. ]]
[[File: TranFig2.png | thumb | 600 px | Figure 2: Flowchart diagram of field screening procedures]]
The recommended approach builds upon the methodology and findings of a recent study<ref name="SchaeferEtAl2025">Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. [https://doi.org/10.1111/gwmr.12709 doi: 10.1111/gwmr.12709]</ref>, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:
#Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
#Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
#Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
#Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)r) is estimated as shown in Equation 1:

::'''Equation 1:'''       <big>''Fe(II)r = DA + XRDpyr - XRDbiotite''</big>

where ''DA'' is the ferrous content from the dilute acid (1% HCl) extraction, ''XRDpyr'' is the pyrite content from XRD analysis, and ''XRDbiotite'' is the biotite content from XRD analysis<ref name="SchaeferEtAl2025"/>.

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1<ref name="SchaeferEtAl2018">Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. [https://doi.org/10.1021/acs.est.8b04108 doi: 10.1021/acs.est.8b04108]</ref><ref>Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. [https://doi.org/10.1016/j.jconhyd.2021.103889 doi: 10.1016/j.jconhyd.2021]  [[Media: BordenCha2021.pdf | Open Access Manuscript]]</ref>. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s ''r'' = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

If Fe(II)r is ≥ 100 mg/kg, a first-order dechlorination rate constant can be estimated and subsequently used within a contaminant fate and transport model. However, if acetylene is detected in the clay, even with Fe(II)r less than 100 mg/kg, then bench-scale testing using methods similar to those described in a recent study<ref name="SchaeferEtAl2025"/> is recommended, as such results would likely be inconsistent with those shown in Figure 1, suggesting some other mechanism might be involved, or that the system mineralogy might be more complex than anticipated. Even if Fe(II)r ≥ 100 mg/kg, confirmatory bench-scale testing may be conducted for additional verification and to refine estimation of the abiotic dechlorination rate constant.

==Summary and Recommendations==
The approach outlined above is intended to serve as a generalized guide for practitioners and site managers to cost-effectively determine the extent to which beneficial abiotic reductive dechlorination reactions are likely occurring in low permeability (e.g., clayey) zones. This approach may be contraindicated if co-contaminants are present. It is currently unclear whether other classes of potentially reactive chemicals, such as trinitrotoluene (TNT) or chlorinated ethanes, could interact competitively with PCE and TCE.

In addition, it remains unclear how other classes of compounds such as per- and polyfluoroalkyl substances (PFAS) may interact or sorb with ferrous minerals and potentially inhibit abiotic dechlorination reactions. Coupling these recommended activities with conventional site investigation tasks would provide an opportunity to perform many of the up-front screening activities with minimal additional project costs. It is important to note that the guidance proposed herein pertains to particularly low permeability media. Sites with complex or varying lithology, where the mineralogy and/or redox conditions may vary, might require evaluation of multiple samples to provide appropriate site-wide information.
 

==References==
<references />

==See Also==
*[https://serdp-estcp.mil/projects/details/a7e3f7b5-ed82-4591-adaa-6196ff33dd60 ESTCP Project ER20-5031 – In Situ Verification and Quantification of Naturally Occurring Dechlorination Rates in Clays: Demonstrating Processes that Mitigate Back-Diffusion and Plume Persistence]

Chlorinated Solvents

2026-05-07T16:51:07Z

Admin:

Chlorinated solvents, including chlorinated volatile organic compounds (CVOC or CVOCs), are chemical compounds containing chlorine that have been widely used in various industries. They are divided in three groups (methanes, ethanes, ethenes) based on their structures, and include common groundwater contaminants such as carbon tetrachloride (CT), perchloroethene (PCE), trichloroethene (TCE), and vinyl chloride (VC). Chlorinated solvents tend to be colorless liquids at room temperatures, heavier than water, volatile, sparingly soluble, and moderately hydrophobic.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
'''Related Article(s):'''

*[[Biodegradation - Cometabolic]]
*[[Biodegradation - Reductive Processes]]
*[[Bioremediation - Anaerobic]]
*[[Bioremediation - Anaerobic Design Considerations]]
*[[Chemical Oxidation (In Situ - ISCO)]]
*[[Chemical Reduction (In Situ - ISCR)]]
*[[Design Tool - Base Addition for ERD]]
*[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation| Emulsified Vegetable Oil for Anaerobic Bioremediation]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Low pH Inhibition of Reductive Dechlorination]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| Monitored Natural Attenuation of Chlorinated Solvents]]
*[[pH Buffering in Aquifers]]
*[[Remediation Performance Assessment at Chlorinated Solvent Sites]]
*[[Soil & Groundwater Contaminants]]
*[[Thermal Remediation]]

'''Contributor(s): ''' [[Dr. Bilgen Yuncu, P.E.]] and [[M. Tony Lieberman]]

'''Key Resource(s):'''

*[http://dx.doi.org/10.1007/978-1-4419-1401-9_2 Chlorinated Solvent Chemistry: Structures, Nomenclature and Properties]<ref name="CS2010">Cwiertny, D.M., Scherer, M.M., 2010. Chlorinated solvent chemistry: structures, nomenclature and properties. In In situ remediation of chlorinated solvent plumes. Springer New York. pgs. 29-37. [http://dx.doi.org/10.1007/978-1-4419-1401-9_2 doi:10.1007/978-1-4419-1401-9_2]</ref>

==Introduction==

Chlorinated solvents are a large family of organic solvents that contain chlorine atoms in their molecular structure. They were first produced in Germany in the 1800s, and widespread use in the United States (U.S.) began after World War II. In the period of 1940-1980, the U.S. produced about 2 billion pounds of chlorinated solvents each year<ref name="PC 1996"> Pankow, J.F., Cherry, J.A., 1996. Dense Chlorinated Solvents and Other DNAPLs in Groundwater, Waterloo Press, Portland, OR. ISBN 0964801418</ref>. Chlorinated solvents, including [[wikipedia:Carbon_tetrachloride|carbon tetrachloride (CT)]], [[wikipedia:1,1,1-Trichloroethane|1,1,1-trichloroethane (TCA)]], [[wikipedia:Tetrachloroethylene|perchloroethene or tetrachloroethene (PCE)]] and [[wikipedia:Trichloroethylene|trichloroethene (TCE)]] have been among the most widely used cleaning and degreasing solvents in the U.S<ref>Doherty, R.E., 2000. A history of the production and use of carbon tetrachloride, tetrachloroethylene, trichloroethylene and 1, 1, 1-trichloroethane in the United States: Part 1--historical background; carbon tetrachloride and tetrachloroethylene. Environmental Forensics, 1(2), 69-81. [http://dx.doi.org/10.1006/enfo.2000.0010 doi:10.1006/enfo.2000.0010]</ref>. They also have been used in a wide variety of other purposes such as adhesives, chemical intermediates, clothes, pharmaceuticals, pesticides, and textile processing.

==Physical & Chemical Properties==

Chlorinated solvents are organic compounds generally constructed of a simple hydrocarbon chain (typically one to three carbon atoms in length). They can be divided into three categories based on their structural characteristics: chlorinated methanes, chlorinated ethanes and chlorinated ethenes.

Chlorinated methanes represent the most structurally simple solvent class and consist of a single carbon center (known as a methyl carbon) to which as many as four chlorine atoms are bonded. From the perspective of groundwater contamination, perhaps the most well-known chlorinated methanes are [[wikipedia:carbon tetrachloride|carbon tetrachloride (CT)]] or [[wikipedia:tetrachloromethane|tetrachloromethane]], [[wikipedia:trichloromethane|trichloromethane]] (commonly known as [[wikipedia:chloroform|chloroform (CF)]]), [[wikipedia:dichloromethane|dichloromethane (DCM)]], or [[wikipedia:methylene chloride|methylene chloride (MC)]] and [[wikipedia:chloromethane|chloromethane (CM)]], or [[wikipedia:methyl chloride|methyl chloride]].

Chlorinated ethanes consist of two carbon centers joined by a single [[wikipedia:Covalent_bond|covalent bond]]. The most frequently encountered groundwater pollutants of this class include [[wikipedia:1,1,1-trichloroethane|1,1,1-trichloroethane (1,1,1-TCA)]] and [[wikipedia:1,2-dichloroethane|1,2-dichloroethane]].

Chlorinated ethenes (also referred to as chlorinated ethylenes) also possess two carbon centers, but unlike chlorinated ethanes, these carbon atoms are joined by a carbon-carbon double bond. Chlorinated ethenes that are important groundwater contaminants include [[wikipedia:tetrachloroethene|tetrachloroethene]], or [[wikipedia:perchloroethene|perchloroethene (PCE)]], [[wikipedia:trichloroethene|trichloroethene (TCE)]], [[wikipedia:dichloroethene|dichloroethene (DCE)]]) (DCE, mainly two geometric isomers cis-1,2-dichloroethene and trans-1,2-dichloroethene), and [[wikipedia:vinyl chloride|vinyl chloride (VC)]].

Nomenclature and structure of selected compounds from each solvent class as well as some physical and chemical properties of most widely used chlorinated solvents are listed in Table 1.


{| class="mw-collapsible wikitable" style="margin: auto; color:black; background-color:white; width: 100%;"
|+Table 1. Nomenclature, Structure, Chemical and Physical Properties of Most Widely Used Chlorinated Solvents<ref name="CS2010" />.
|- style="color:white; background-color:#476b6b; text-align:center;"
|IUPAC Name
|Common Name
|Acronym
|Molecular Formula
|Chemical Structure
|Formula Weight
|Density (ρ)(g/mL)
|Aqueous Solubility (mg/L)
|Vapor Pressure (ρ0)(kPa)
|Henry's Law Constanta
|Log Kow
|MCLb (mg/L)
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Methanes
|-
| style="color:black; background-color:#d1e0e0;" |tetrachloromethane
| style="text-align:center;" |carbon tetrachloride
| style="text-align:center;" |CT
| style="text-align:center;" |CCl4
|
[[File:Tetrachloromethane.png|center|70 px|frameless]]
| style="text-align:center;" |153.8
| style="text-align:center;" |1.59
| style="text-align:center;" |800
| style="text-align:center;" |20.5
| style="text-align:center;" |28.9
| style="text-align:center;" |2.64
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |trichloromethane
| style="text-align:center;" |chloroform
| style="text-align:center;" |CF
| style="text-align:center;" |CHCl3
|
[[File:Trichloromethane.png|center|70px|frameless]]
| style="text-align:center;" |119.4
| style="text-align:center;" |1.49
| style="text-align:center;" |8,200
| style="text-align:center;" |26.2
| style="text-align:center;" |3.8
| style="text-align:center;" |1.97
| style="text-align:center;" |0.080c
|-
| style="color:black; background-color:#d1e0e0;" |dichloromethane
| style="text-align:center;" |methylene chloride
| style="text-align:center;" |DCM
| style="text-align:center;" |CH2Cl2
|
[[File:Dichloromethane.png|center|70px|frameless]]
| style="text-align:center;" |84.9
| style="text-align:center;" |1.33
| style="text-align:center;" |13,200
| style="text-align:center;" |55.3
| style="text-align:center;" |1.7
| style="text-align:center;" |1.25
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |chloromethane
| style="text-align:center;" |methyl chloride
| style="text-align:center;" |CM
| style="text-align:center;" |CH3Cl
|
[[File:Chloromethane.png|center|70px|frameless]]
| style="text-align:center;" |50.5
| style="text-align:center;" |0.92
| style="text-align:center;" |5,235
| style="text-align:center;" |570
| style="text-align:center;" |9.6
| style="text-align:center;" |0.91
| style="text-align:center;" |NRd
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Ethanes
|-
| style="color:black; background-color:#d1e0e0;" |hexachloroethane
| style="text-align:center;" |perchloroethane
| style="text-align:center;" |HCA
| style="text-align:center;" |C2Cl6
|
[[File:Hexachloroethane.png|center|70px|frameless]]
| style="text-align:center;" |236.7
| style="text-align:center;" |2.09
| style="text-align:center;" |50
| style="text-align:center;" |0.05e
| style="text-align:center;" | -
| style="text-align:center;" |3.93
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |pentachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |PCA
| style="text-align:center;" |C2HCl5
|
[[File:Pentachloroethane.png|center|70px|frameless]]
| style="text-align:center;" |202.3
| style="text-align:center;" |1.68
| style="text-align:center;" |500
| style="text-align:center;" |0.6
| style="text-align:center;" |2.5
| style="text-align:center;" |2.89
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,1,2-tetrachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,1,2-TeCA
| style="text-align:center;" |C2H2Cl4
|
[[File:1,1,1,2-Tetrachloroethane.png|72px|frameless|center]]
| style="text-align:center;" |167.9
| style="text-align:center;" |1.54
| style="text-align:center;" |1,100
| style="text-align:center;" |1.6
| style="text-align:center;" |2.4
| style="text-align:center;" | -
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,2,2-tetrachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,2,2-TeCA
| style="text-align:center;" |C2H2Cl4
|
[[File:1,1,2,2-Tetrachloroethane.png|72px|frameless|center]]
| style="text-align:center;" |167.9
| style="text-align:center;" |1.60
| style="text-align:center;" |2,962
| style="text-align:center;" |0.8
| style="text-align:center;" |0.44
| style="text-align:center;" |2.39
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,2-trichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,2-TCA
| style="text-align:center;" |C2H3Cl3
|
[[File:1,1,2-Trichloroethane.svg.png|72px|frameless|center]]
| style="text-align:center;" |133.4
| style="text-align:center;" |1.44
| style="text-align:center;" |4,394
| style="text-align:center;" |3.22
| style="text-align:center;" |0.96
| style="text-align:center;" |2.38
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |1,1,1-trichloroethane
| style="text-align:center;" |methyl chloroform
| style="text-align:center;" |1,1,1-TCA
| style="text-align:center;" |C2H3Cl3
|
[[File:1,1,1-trichloroethane.png|center|70px|frameless]]
| style="text-align:center;" |133.4
| style="text-align:center;" |1.35
| style="text-align:center;" |1,495
| style="text-align:center;" |16.5
| style="text-align:center;" |14.5
| style="text-align:center;" |2.49
| style="text-align:center;" |0.20
|-
| style="color:black; background-color:#d1e0e0;" |1,2-dichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,2-DCA
| style="text-align:center;" |C2H4Cl2
|
[[File:1,2-dichloroethane.png|center|70px|frameless]]
| style="text-align:center;" |99.0
| style="text-align:center;" |1.25
| style="text-align:center;" |8,606
| style="text-align:center;" |10.5
| style="text-align:center;" |1.2
| style="text-align:center;" |1.48
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |1,1-dichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1-DCA
| style="text-align:center;" |C2H4Cl2
|
[[File:1,1-Dichloroethane 2.svg.png|72px|frameless|center]]
| style="text-align:center;" |99.0
| style="text-align:center;" |1.17
| style="text-align:center;" |4,676
| style="text-align:center;" |30.3
| style="text-align:center;" |6.2
| style="text-align:center;" |1.79
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |chloroethane
| style="text-align:center;" | -
| style="text-align:center;" |CA
| style="text-align:center;" |C2H5Cl
|
[[File:Chloroethane.png|center|70px|frameless]]
| style="text-align:center;" |64.5
| style="text-align:center;" |0.92
| style="text-align:center;" |5,700
| style="text-align:center;" |16.0
| style="text-align:center;" |1.8
| style="text-align:center;" |1.43
| style="text-align:center;" |NR
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Ethenes
|-
| style="color:black; background-color:#d1e0e0;" |tetrachloroethene
| style="text-align:center;" |perchloroethene
| style="text-align:center;" |PCE
| style="text-align:center;" |C2Cl4
|
[[File:Tetrachloroethene.png|center|70px|frameless]]
| style="text-align:center;" |165.8
| style="text-align:center;" |1.63
| style="text-align:center;" |150
| style="text-align:center;" |2.4
| style="text-align:center;" |26.3
| style="text-align:center;" |2.88
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |trichloroethene
| style="text-align:center;" | -
| style="text-align:center;" |TCE
| style="text-align:center;" |C2HCl3
|
[[File:Trichloroethene.png|72px|frameless|center]]
| style="text-align:center;" |131.4
| style="text-align:center;" |1.46
| style="text-align:center;" |1,100
| style="text-align:center;" |9.9
| style="text-align:center;" |11.7
| style="text-align:center;" |2.53
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |cis-1,2-dichloroethene
| style="text-align:center;" |cis-dichloroethene
| style="text-align:center;" |cis-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:Cis-1,2-dichloroethene.png|center|70px|frameless]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.28
| style="text-align:center;" |3,500
| style="text-align:center;" |27.1
| style="text-align:center;" |7.4
| style="text-align:center;" |1.86
| style="text-align:center;" |0.07
|-
| style="color:black; background-color:#d1e0e0;" |trans-1,2-dichloroethene
| style="text-align:center;" |trans-dichloroethene
| style="text-align:center;" |trans-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:Trans-1,2-dichloroethene.png|72px|frameless|center]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.26
| style="text-align:center;" |6,260
| style="text-align:center;" |44.4
| style="text-align:center;" |6.8
| style="text-align:center;" |1.93
| style="text-align:center;" |0.1
|-
| style="color:black; background-color:#d1e0e0;" |1,1-dichloroethene
| style="text-align:center;" |vinylidene chloride
| style="text-align:center;" |1,1-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:1,1-Dichloroethene.svg.png|72px|frameless|center]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.22
| style="text-align:center;" |3,344
| style="text-align:center;" |80.5
| style="text-align:center;" |23.0
| style="text-align:center;" |2.13
| style="text-align:center;" |0.007
|-
| style="color:black; background-color:#d1e0e0;" |chloroethene
| style="text-align:center;" |vinyl chloride
| style="text-align:center;" |VC
| style="text-align:center;" |C2H3Cl
|
[[File:Chloroethene.png|center|70px|frameless]]
| style="text-align:center;" |62.5
| style="text-align:center;" |0.91
| style="text-align:center;" |2,763
| style="text-align:center;" |355
| style="text-align:center;" |79.2
| style="text-align:center;" |1.38
| style="text-align:center;" |0.002
|-
| colspan="12" style="color:black; background-color:#d1e0e0;" |Notes:
atm = atmosphere; g = gram; Kow = octanol/water partitioning coefficient; Koc -- soil organic carbon/water partitioning coefficient; L = liter; MCL = maximum contaminant level; mg = milligram; mL = milliliter; mol = mole.

aHenry's Law Constant (KH)(x10-3 atm ・ m3/mol)

bSource: http://water.epa.gov/drink/contaminants/#List

cMCL for total trihalomethanes is defined as the summed concentration of chloroform, bromoform (CHBr3),bromodichloromethane (CHBrCl2), and dibromochloromethane (CHBr2Cl). http://water.epa.gov/drink/contaminants/basicinformation/disinfectionbyproducts.cfm

dNR : Not regulated.

eReported vapor pressure for solid-phase hexachloroethane.

|-
|}

Chlorinated solvents and many of their transformation products are colorless liquids at room temperature. They are heavier than water with densities greater than 1 gram per cubic centimeter (g/cm3) which means they can penetrate deeply into an aquifer. They are relatively volatile compounds with relatively high [[wikipedia:Henry’s Law|Henry’s Law]] constants(KH), a measure of the strength of partitioning from water into air). Generally, when KH for a compound exceeds 0.2 atmosphere/mole fraction (atm/M), they can readily be removed from water by air stripping it. Most chlorinated solvents can be classified as sparingly soluble in water, with aqueous solubilities generally on the order of 10s to 100s of mg/L. As the number of chlorine atoms on a compound increases, the solubility decreases. Because of their relatively low solubilities, chlorinated solvents dissolve slowly in groundwater. Another consequence of their limited solubility is their tendency to occur in the subsurface as a separate immiscible liquid phase which, because of its density compared to water, tends to sink in groundwater. Under these conditions, these are referred to as [[wikipedia:DNAPL|dense non-aqueous phase liquid (DNAPL)]]. Although chlorinated solvents are not very soluble in water, their solubility is typically orders of magnitude greater than their established [http://water.epa.gov/drink/contaminants/#Organic drinking water standards].

Chlorinated solvents can be considered moderately hydrophobic which can be determined by their [[wikipedia:Partition coefficient|octanol-water partition coefficient]]s (Kow, a measure of the tendency of a substance to prefer an organic or oily phase rather than an aqueous phase). Log Kow values less than 3 indicate that the compound does not sorb strongly to aquifer solids, but can be removed readily by activated carbon. On the other hand, compounds with log Kow less than 2, such as VC, generally are not removed well by activated carbon either<ref name="CS2010" />.

==References==

<references />

==See Also==

*[https://serdp-estcp.org/Tools-and-Training/Environmental-Restoration/DNAPL-Source-Zones/Frequently-Asked-Questions-Regarding-Management-of-Chlorinated-Solvents-in-Soils-and-Groundwater FAQ Regarding Management of Chlorinated Solvents in Soil and Groundwater]
*[//www.enviro.wiki/images/6/6b/AFCEE_Protocol_2007_chlorinated_solvents.pdf Protocol for In Situ Bioremediation of Chlorinated Solvents Using Edible Oil]

Monitored Natural Attenuation (MNA)

2026-05-07T16:49:39Z

Admin:

Monitored Natural Attenuation (MNA) is an important, common groundwater remediation technology used for treating some dissolved groundwater contaminants. MNA relies on natural attenuation processes to achieve site-specific remediation objectives within a reasonable time frame compared to more active approaches. While MNA has primarily focused on managing plumes with low residual contamination, there is an growing movement to also apply it to source zones via [[ Natural Source Zone Depletion (NSZD) | natural source zone depletion (NSZD)]]. [[Long-Term Monitoring (LTM) | Long-term monitoring]] is required to determine if the concentrations of target contaminants are behaving as predicted. 
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
'''Related Article(s):'''

*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| Monitored Natural Attenuation of Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| Monitored Natural Attenuation of Metals and Metalloids]]
*[[Monitored Natural Attenuation (MNA) of Fuels|Monitored Natural Attenuation of Petroleum Hydrocarbons and Fuel Components]]
*[[Natural Source Zone Depletion (NSZD)]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]

'''Contributor(s):''' [[Dr. John Wilson]]

'''Key Resource(s):'''
*[[Media:EPA-1999-Use_of_MNA_at_Superfund%2C_RCRA_and_UST_sites.pdf|Use of monitored natural attenuation at superfund, RCRA corrective action, and underground storage tank sites]]<ref name="EPA1999"> U.S. Environmental Protection Agency, 1999. Use of monitored natural attenuation at superfund, RCRA corrective action, and underground storage tank sites. [[Media:EPA-1999-Use_of_MNA_at_Superfund%2C_RCRA_and_UST_sites.pdf|Report.pdf]]</ref>

==Introduction==
A number of natural processes can attenuate the concentrations of contaminants in groundwater including biological degradation, abiotic degradation, sorption, dispersion into groundwater adjacent to the contaminant plume, and volatilization to soil gas above the groundwater. As the concentration declines, it may reach a point where it is no longer considered hazardous. If the natural processes that attenuate the concentrations of a particular hazardous chemical can meet the cleanup goals for a site, the processes can provide the basis for a cleanup technology. The United States Environmental Protection Agency (U.S. EPA), defines Monitored Natural Attenuation (MNA) as ''“the reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods”''<ref name="EPA1999"/>.

The concentration at which a contaminant is no longer hazardous is defined by U.S. EPA and state regulations. The U.S. EPA regulates the maximum concentration of contaminants that are allowed in water that is supplied as drinking water. These U.S. EPA regulations are referred to as the Maximum Contaminant Level (MCL)<ref> U.S. Environmental Protection Agency (USEPA), 2016. Table of Regulated Drinking Water Contaminants.[http://www.epa.gov/your-drinking-water/table-regulated-drinking-water-contaminants Table of Regulated Drinking Water]</ref>. Often, the MCL is selected as the cleanup goal for MNA. However, other goals<ref> Deeb, R., Hawley, E., Kell, L. and O'Laskey, R., 2011. Assessing alternative endpoints for groundwater remediation at contaminated sites. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200832 ER-200832]</ref> are occasionally selected.

To accept MNA as remedial technology on the same basis as engineered remedial technologies, it is necessary to characterize the distribution of contamination at a site, characterize the [[ Advection and Groundwater Flow | flow of groundwater]], understand the processes that contribute to natural attenuation and use this information to build a conceptual model of the site. Sometimes the site conceptual model is used to organize an analytical model of the transport and fate<ref> VirginiaTech, United States Geological Survey (USGS), and Naval Facilites Engineering Command (NAVFAC). 2016. Natural Attenuation Software (NAS). [http://www.nas.cee.vt.edu/index.php Software]</ref> of the contaminants in groundwater. The forecasts of the transport and fate model are compared to the cleanup goals for the site to determine if natural attenuation is an appropriate remedy. If natural attenuation is selected as a remedy, the site is monitored over time to ensure that the attenuation of the contaminant proceeds as anticipated. The entire package of site characterization<ref> Pivetz, B.E., Abshire, D., Brandon, W., Mangion,S., Roberts, B., Stuart, B., Vanderpool, L., Wilson, B., Acree, S.D., 2012. Framework for Site Characterization for Monitored Natural Attenuation of Volatile Organic Compounds in Ground Water. EPA 600-R-12-712, 89 pgs. [[Media:Pivetz-2012-Framework_for_Site_Char_for_MNA.pdf|Report pdf]]</ref>, a site conceptual model, and monitoring<ref> Pope, D.F., Acree, S.D., Levine, H., Mangion, S., Van Ee, J., Hurt, K., Wilson, B. and Burden, D.S., 2004. Performance monitoring of MNA remedies for VOCs in ground water. US Environmental Protection Agency, National Risk Management Research Laboratory. [[Media:Pope-2012-Performance_Monitoring_of_MNA_Remedies.pdf|Report pdf]]</ref> are necessary components of MNA as a formal remedy for any site selected.

The U.S. EPA considers three lines of evidence<ref name= "EPA1999"/> before MNA can be accepted as the remedy for a site:
*Historical groundwater and/or soil chemistry data that demonstrate a clear and meaningful trend of decreasing contaminant mass and/or concentration over time at appropriate monitoring or sampling points.
*Hydrogeologic and geochemical data that can be used to demonstrate indirectly the type(s) of natural attenuation processes active at the site, and the rate at which such processes will reduce contaminant concentrations to required levels.
*Data from field or microcosm studies (conducted in or with actual contaminated site media) which directly demonstrate the occurrence of a particular natural attenuation process at the site and its ability to degrade the contaminants of concern (typically used to demonstrate biological degradation processes only).

At most sites, U.S. EPA requires the first two lines of evidence. The third line of evidence is reserved for contaminants that are not well understood.

MNA is often used as a remedy, or part of a remedy, where contaminants have been demonstrated to be degrading or sequestered in groundwater. A number of technical protocols have been developed to guide the application of MNA for particular contaminants, including [[Monitored Natural Attenuation (MNA) of Fuels|fuel hydrocarbons]]<ref>Wiedemeier, T.H., Wilson, J.T., Kampbell, D.H., Miller, R.N., Hansen, J.E., 1999. Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater. Volume I. [[Media:Wiedemeier-1999-technical_Protocol_for_implementing_Intrinsic_remediation.pdf|Report pdf]]</ref>, [[Monitored Natural Attenuation (MNA) of Chlorinated Solvents|chlorinated solvents]]<ref> Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water. EPA-600-R-98-128. [[Media:Wiedemeier-1998-Technical_Protocol_for_Evaluating_Natuaral_Attenuation.pdf|Report pdf]]</ref>, methyl ''tert''-butyl ether (MTBE<ref>Wilson, J.T., Kaiser, P.M., Adair, C., 2005. Monitored Natural Attenuation of MTBE as a Risk Management Option at Leaking Underground Storage Tank Sites EPA/600/R-04/1790. [[Media:Wilson-2005-MNA_of_MTBE.pdf|Report pdf]]</ref>), inorganics , metals , radionuclides<ref> Truex, M., Brady, P., Newell, C.J., Rysz, M., Denham, M., Vangelas, K. 2011. The Scenarios Approach to Attenuation-Based Remedies for Inorganic and Radionuclide Contaminants. Savannah-River National Laboratory U.S. Department of Energy. [[Media:TRUEX-2011-Scenarios_Approach_to_Attenuation-Based_Remedies.pdf|Report pdf]]</ref>, and explosives<ref> Pennington, J.C., Zakikhani, M., Harrelson, D., 1999. Monitored Natural Attenuation of Explosives in Groundwater. ESTCP Completion Report ER-199518. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-199518 ER-199518]</ref><ref> Borden, R.C., Knox, S.L., Lieberman, M.T., Ogles, D., 2014. Perchlorate natural attenuation in a riparian zone. Journal of Environmental Science and Health, Part A, Toxic/Hazardous Substances and Environmental Engineering, 49(10), 1100-1109. [http://dx.doi.org/10.1080/10934529.2014.897145 doi: 10.1080/10934529.2014.897145]</ref>. These protocols were developed from 1999 to 2010, in the same time period when U.S. EPA developed its policy guidance. Since that time, there have been significant advances<ref name="Adamson2014"> Adamson, D., Newell, C., 2014. Frequently Asked Questions about Monitored Natural Attenuation in the 21st Century. ER-201211. Environmental Security and Technology Certification Program, Arlington, Virginia. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201211 ER-201211]</ref> in our understanding of the processes that degrade contaminants in groundwater.

==Abiotic Process==
Abiotic processes<ref> Darlington, R., Rectanus, H. 2015. Biogeochemical Transformation Handbook. TR-NAVFAC EXWC-EV-1601, 41 pgs. [[Media:Darlington-2015-Biogeochem_Transformation_Handbook.pdf|Report pdf]]</ref> can contribute to natural attenuation of certain contaminants such as chlorinated solvents. For example, chlorinated alkenes can react with naturally occurring magnetite or other iron minerals in aquifer materials<ref>He, Y., Su, C., Wilson, J., Wilkin, R., Adair, C., Lee, T., Bradley, P., Ferrey, M., 2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water. US Environmental Protection Agency. [[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Report pdf]]</ref>. The rate constants are generally slow, but abiotic degradation can be important if the travel time of the contamination to the point of compliance is long.

==Tools for Assessing Monitored Natural Attenuation==
# '''Statistical Tools to Evaluate Trends'''. Computer programs such as MAROS<ref>Aziz, J.J., Ling, M., Rifai, H.S., Newell, C.J., Gonzales, J.R., 2003. MAROS: A decision support system for optimizing monitoring plans. Ground Water, 41(3), 355-367. [http://dx.doi.org/10.1111/j.1745-6584.2003.tb02605.x doi: 10.1111/j.1745-6584.2003.tb02605.x]</ref><ref> Aziz, J.J., Newell, C.J., Rifai, H.S., Ling, M., Gonzales, J.R., 2000. Monitoring and Remediation Optimization System (MAROS): Software User’s Guide. [[Media:Aziz-2000-Monitoring_and_Remed._Opt._Syst._Guide.pdf|Report pdf]]</ref> and the Mann-Kendall Toolkit<ref>Connor, J., Farhat, S. K., Vanderford, M. V., Newell, C. J., 2012. GSI Mann-Kendall Toolkit. [http://www.gsi-net.com/en/software/free-software/gsi-mann-kendall-toolkit.html Mann Kendall Toolkit]</ref> can be used to help confirm trends in groundwater data used as a line of evidence for MNA. 
# '''[[Molecular Biological Tools - MBTs|Molecular Biological Tools (MBTs)]]'''. MBTs are used to identify and characterize the bacteria that carry out critical steps in the biodegradation of the contaminants in groundwater. In the case of chlorinated solvents tetrachloroethene (PCE) and trichloroethene (TCE), a key bacterium is ''Dehalococcoides mccartyi''<ref name ="Löffle2013">Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, A. Leeson, C.H. Ward, Springer, New York, NY. pgs. 39-88. ISBN: 978-1-4614-4114-4. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>. In anaerobic groundwater, chlorinated alkenes can undergo a sequential reductive dehalogenation from PCE, to TCE, to dichloroethene (DCE) and then to vinyl chloride (VC) and finally to ethane. Anaerobic microbial communities that contain ''Dehalococcoides'' can degrade PCE and TCE all the way to harmless end products. The abundance of ''Dehalococcoides'' cells in groundwater can be determined by an assay based on the polymerase chain reaction<ref>Lebron, C.A., Petrovskis, E., Loffler, F., Henn, K., 2011. Application of Nucleic Acid-Based Tools for Monitoring Monitored Natural Attenuation (MNA), Biostimulation and Bioaugmentation at Chlorinated Solvent Sites (No. NFESC-CR-11-028-ENV). ER-200518. Naval Facilities Engineering Command Port Hueneme CA Engineering Service Center. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200518/ER-200518 ER-200518]</ref>. Other assays can determine the abundance of reductase genes<ref name ="Löffle2013"/> that code for enzymes that can carry out specific steps in the dechlorination pathway. Similar assays are available to determine the abundance of ''Dehalobacter, Dehalogenimonas, and Desulfitobacterium'' strains that degrade chlorinated alkanes, and MBT assays are available for several of their reductase genes. A great variety of bacteria degrade petroleum hydrocarbons. Bacteria that degrade hydrocarbons using oxygen initiate degradation with an oxygenase enzyme, and [[Quantitative Polymerase Chain Reaction (qPCR) | qPCR]] assays are available for a variety of oxygenase enzymes<ref>Baldwin, B.R., Nakatsu, C.H., Nies, L., 2008. Enumeration of aromatic oxygenase genes to evaluate monitored natural attenuation at gasoline-contaminated sites. Water Research, 42(3), 723-731. [http://dx.doi.org/10.1016/j.watres.2007.07.052 doi:10.1016/j.watres.2007.07.052]</ref>. The bacteria that degrade hydrocarbons under anaerobic conditions are particularly important for natural attenuation, and there are qPCR assays for the enzymes that initiate degradation under anaerobic conditions<ref> da Silva, M.L.B., Corseuil, H.X., 2012. Groundwater microbial analysis to assess enhanced BTEX biodegradation by nitrate injection at a gasohol-contaminated site. International Biodeterioration & Biodegradation, 67, 21-27. [http://dx.doi.org/10.1016/j.ibiod.2011.11.005 doi:10.1016/j.ibiod.2011.11.005]</ref>. See an entire article on MBTs here: [[Molecular Biological Tools - MBTs]] 
# '''[[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]]'''. CSIA can unequivocally demonstrate that a compound has degraded in groundwater. It is difficult to document the degradation of a compound in groundwater if the only information available is an apparent attenuation in concentrations along a flow path in the plume. There is always a possibility that a downgradient well is askew of the true flow path, and the attenuation is caused by dilution and not degradation. CSIA determines the ratio of stable isotopes in a compound. As a compound degrades, molecules with lighter isotopes degrade faster. As degradation progresses, the material that has not degraded becomes enriched in the heavier stable isotope. At many sites, degradation of the compound can be recognized and documented from a change in the ratio of isotopes<ref>Hunkeler, D., Meckenstock, R.U., Sherwood Lollar, B., Schmidt, T.C., Wilson, J.T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148, 2008. [[Media:Hunkeler-2008-A_Guide.pdf|Report pdf]]</ref>. At some sites, it is possible to use CSIA and reactive transport modeling<ref> Kuder, T., Philp, P., van Breukelen, B., Thouement, H., Vanderford, M., Newell, C. 2014. Integrated Stable Isotope-Reactive Transport Model Approach for Assessment of Chlorinated Solvent Degradation. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-201029/ER-201029 ER-201029]</ref> to evaluate the plausibility of alternate degradation pathways, and to estimate the extent of degradation. See an entire article on CSIA here: [[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]] 
# '''Computer Models'''. Groundwater fate and transport computer models are often used to evaluate how attenuation processes can control the migration of a plume. Public domain software is available that can incorporate terms for advective flow of groundwater, [[ Dispersion and Diffusion | dispersion]] (and more recently diffusion) of contaminations in groundwater, and biotic or abiotic reactions. Examples of commonly used models include analytical models REMChlor<ref name= "Falta2007">Falta, R.W., Stacy, M.B., Ahsanuzzaman, A.N.M., Wang, M., Earle, R., 2007. REMChlor remediation evaluation model for chlorinated solvents user’s manual Version 1.0. Cent. for subsurface model. support, US Environ. Prot. Agency, Ada, Okla.[https://www.epa.gov/water-research/remediation-evaluation-model-chlorinated-solvents-remchlor User's Manual v1.0]</ref> and REMFuel<ref name="Falta2007"/> , and the numerical models MODFLOW/RT3D<ref>2005. MODFLOW and Related Programs [http://water.usgs.gov/ogw/modflow Modflow]</ref>, MODFLOW/MT3DMS, and the Natural Attenuation Software (NAS<ref> Widdowson, M.A., Mendez III, E., Chapelle, F.H., Casey, C.C., 2005. Natural Attenuation Software (NAS) User’s Manual Version 2. [[Media:Widdowson2005-NAS_Users_Guide.pdf|Report pdf]]</ref>).
[[File:Wilson 1 Fig1a.JPG|375px|thumbnail|right|Figure 1a. Evolution of a plume when the plume and source do not attenuate.]]
[[File:Wilson 1 Fig1b.JPG|375px|thumbnail|right|Figure 1b. Evolution of a plume when the source and concentrations in groundwater both attenuate.]]
[[File:Wilson 1 Fig1c.JPG|375px|thumbnail|right|Figure 1c. Evolution of a plume when the source attenuates faster than the plume.]]

==Source Area Considerations==
In most plumes, the time frame that is required for natural attenuation to reach a cleanup goal across the entire plume is not controlled by the rate of attenuation in the groundwater. In many plumes, a source of contamination, such as residual oily phase material (non-aqueous phase liquid [NAPL]), contaminated soils, and matrix diffusion sources, provides a continuous supply of new contamination to the groundwater.

As a result, the lifecycle of the source<ref>Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward, C. H. Springer New York. pgs. 459-508. [http://dx.doi.org/10.1007/978-1-4614-6922-3 doi: 10.1007/978-1-4614-6922-3]</ref> largely controls the lifecycle of contamination in groundwater. As a consequence, at many sites, some attempt is made to actively remediate the source of contamination. In almost every instance, active remediation is successful in reducing the concentration of the contamination, but fails to reduce the concentration to the cleanup goal. The final remedy is a pragmatic combination of active source remediation and MNA. Transport and fate models<ref> Widdowson, M., Chapelle, F., Casey, C., Kram, M., 2008. Estimating Cleanup Times Associated With Combining Source-Area Remediation With Monitored Natural Attenuation. ER-200436 [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436 ER-200436]</ref> can be used to evaluate the benefits from source remediation on the size and lifecycle of the plume of contaminated ground water. The models can estimate the reduction in concentration at the source that is necessary to pull a plume back behind a point of compliance and the time that is required for the plume to recede behind the point of compliance.

==Regulatory Considerations==
If a site is regulated under the Resource Conservation and Recovery Act (RCRA)<ref>[https://www.epa.gov/rcra US EPA RCRA Laws & Regulations]</ref>, the usual goal is for the contaminants to attenuate to acceptable concentrations before groundwater can migrate off-site and impact receptors. Under this MNA approach, the groundwater must reach a cleanup goal before it reaches a point of compliance. For this implementation, a quantitative framework (BioPIC)<ref>Lebron, C. A., Wiedemeier, T. H., Wilson, J.T., Löffler, F.E., Hinchee, R.E., Singletary, M.A., 2015. Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites. ER-201129. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129/ER-201129 ER-201129]</ref> is now available that integrates new discoveries on degradation processes into the U.S. EPA’s approach to evaluate MNA.

When a site is regulated under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA or Superfund)<ref>[https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act US EPA CERCLA Act]</ref>, there is often an additional requirement that all the contamination must reach the cleanup goal by a specified date. The performance of a remedy at a Superfund site is reviewed on a five-year cycle. A framework<ref> Wilson, J.T., 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [[Media:Wilson-2011-An_Approach_for_Evaluating_Progress.pdf|Report pdf]]</ref> is available to review long-term monitoring data to determine whether the attenuation within the review cycle is adequate to meet the cleanup goal by the specified date.

In the USA, the individual states have provided regulations to supplement the U.S. EPA guidance. Examples include general guidance on MNA provided by California<ref>California Regional Water Quality Control Board, 2014. Workshop - Monitored Natural Attenuation. Barstow, California, September 10 & 11. [[Media:MNA_Workshop-2014_California_Water_Boards.pdf|Report pdf]]</ref>, Minnesota<ref>Minnesota Pollution Control Agency. Natural Attenuation of Groundwater. [https://www.pca.state.mn.us/water/natural-attenuation-groundwater Natural Attenuation of Groundwater]</ref>, New Jersey<ref> New Jersey Department of Environmental Protection - Site Remediation Program. 2012. Monitored Natural Attenuation Technical Guidance. [[Media:NJDEP-SRP-2012-MNA_Technical_Guidance_v_1_0.pdf|Report pdf]]</ref> , Ohio<ref>Ohio Environmental Protection Agency - Division of Environmental Response and Revitalization, 2001. Remedial Response Program Fact Sheet. Remediation Using Monitored Natural Attenuation.[[Media:OhioEPA-2001-Division_of_Envl_Response_and_Revitalization_fact_sheet.pdf|Report pdf]]</ref>, and Texas<ref> Texas Commission on Environmental Quality - Remediation Division, 2010. Monitored Natural Attenuation Demonstrations under TRRP. RG-366/TRRP-33. [[Media:TRRP-TCEQ-2010-Regulatory_Guidance-RG-366-TRRP-33.pdf|Report pdf]]</ref>. In addition, California<ref>California State Water Resources Control Board. 2012. Low-threat Underground Storage Tank Case Closure Policy. [[Media:CA-SWB-2012-Low-threat_UST_Case_Closure_Policy.pdf|Report pdf]]</ref>, Minnesota<ref> Minnesota Pollution Control Agency, 2005. Assessment of Natural Biogradation at Petroleum Release Sites. Guidance Document 4-03. [[Media:MINN-PCA-2005-Assessment_of_Natural_Biogradation_at_Petroleum_Rel_Sites.pdf|Report pdf]]</ref>, Washington State<ref> Washington State Department of Ecology, 2005. Guidance on Remediation of Petroleum-Contaminated Ground Water by Natural Attenuation. Publication Number 05-09-091 (Version 1.0). [[Media:WASH-ECOL-2005-Guidance_on_Remediation_of_Petroleum_Contaminated_GW.pdf|Report pdf]]</ref>, and Wisconsin<ref>Wisconsin Department of Natural Resources, 2014. Guidance on Natural Attenuation For Petroleum Releases. Remediation and Redevelopment Program. RR-614. [[Media:WIS-DNR-2014-Guidance_on_Natural_Attenuation_for_Petroleum_Releases.pdf|Report pdf]]</ref> provide guidance on petroleum releases. Minnesota<ref> Minnesota Pollution Control Agency Site Remediation Section. 2006. Guidelines Natural Attenuation of Chlorinated Solvents in Ground Water. [[Media:MINN-PCA-2006-Guidelines_Natural_Attenuation_of_Chlorinated_Solvents_in_GW.pdf|Report pdf]]</ref> and Wisconsin<ref> Wisconsin Department of Natural Resources, 2014. Understanding Chlorinated Hydrocarbon Behavior in Groundwater: Guidance on the Investigation, Assessment and Limitations of Monitored Natural Attenuation. RR-699. [[Media:WIS-DNR-2014-Understanding_Chlorinated_Hydrocarbon_Behavior_In_GW.pdf|Report pdf]]</ref> provide guidance on chlorinated solvents.

==Additional Information==
Additional information on MNA is available on web pages that are maintained by the United State Environmental Protection Agency<ref>U.S. Environmental Protection Agency, 2016. Natural Attenuation Overview. Technology Innovation and Field Services Division. [https://clu-in.org/techfocus/default.focus/sec/Natural_Attenuation/cat/Overview Natural Attenuation Overview]</ref>, the United States Geological Survey<ref> Natural Attenuation Definitions. 2015. United States Geological Survey. </ref>, Department of Energy, and the Interstate Technology Regulatory Council<ref>ITRC, 2008. Enhanced attenuation of chlorinated organics (EACO): A decision framework for site transition. [[Media:ITRC-2008-EACO_Framework_General.pdf|Report pdf]]</ref>. In addition, ESTCP has published “Frequently Asked Questions Regarding MNA in Groundwater” which provides a recent summary overview of key approaches, technologies, and best practices for applying MNA<ref name="Adamson2014"/>.

==References==

<references/>

==See Also==
*[[Long-Term Monitoring (LTM)]]
*[[Media:AFCEE_Long_Term_Monitoring_Protocol_2000.pdf|Designing Monitoring Programs to Effectively Evaluate the Performance of Natural Attenuation]]
*[[Media:ER-201032_Final_Report.pdf|Determining Source Attenuation History to Support Closure by Natural Attenuation]]
*[[Media:Role-of-DHC-Organism-Natural-Attenuation-Chlorinated-Ethylenes.pdf|Evaluation of the Role of Dehalococcoides Organisms in the Natural Attenuation of Chlorinated Ethylenes in Ground Water]]
*[[Media:Natatt_Cr.pdf|EPA Ground Water Issue: Natural Attenuation of Hexavalent Chromium in Groundwater and Soils]]
*[[Media:Parsons_MNA-Altus.pdf|Remediation by Natural Attenuation Treatability Study at Altus Air Force Base]]
*[[Media:Mnatoolbox.pdf|Site Screening and Technical Guidance for Monitored Natural Attenuation at DOE Sites]]
*[https://www.enviro.wiki/images/3/33/mna1198.pdf Technical Guidelines for Evaluating Monitored Natural Attenuation of Petroleum Hydrocarbons and Chlorinated Solvents in Groundwater at Naval and Marine Corps Facilities]
*[[Media:MNA-Guidance-2015.pdf|Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites]]
*[https://www.serdp-estcp.org/index.php/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-199518/ER-199518/(language)/eng-US Monitored Natural Attenuation of Explosives in Groundwater]

Chlorinated Solvents

2026-05-07T16:48:32Z

Admin:

Chlorinated solvents, including chlorinated volatile organic compounds (CVOC or CVOCs), are chemical compounds containing chlorine that have been widely used in various industries. They are divided in three groups (methanes, ethanes, ethenes) based on their structures, and include common groundwater contaminants such as carbon tetrachloride (CT), perchloroethene (PCE), trichloroethene (TCE), and vinyl chloride (VC). Chlorinated solvents tend to be colorless liquids at room temperatures, heavier than water, volatile, sparingly soluble, and moderately hydrophobic.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
'''Related Article(s):'''

*[[Biodegradation - Cometabolic]]
*[[Biodegradation - Reductive Processes]]
*[[Bioremediation - Anaerobic]]
*[[Bioremediation - Anaerobic Design Considerations]]
*[[Chemical Oxidation (In Situ - ISCO)]]
*[[Chemical Reduction (In Situ - ISCR)]]
*[[Design Tool - Base Addition for ERD]]
*[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Low pH Inhibition of Reductive Dechlorination]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| Monitored Natural Attenuation of Chlorinated Solvents]]
*[[pH Buffering in Aquifers]]
*[[Remediation Performance Assessment at Chlorinated Solvent Sites]]
*[[Soil & Groundwater Contaminants]]
*[[Thermal Remediation]]

'''Contributor(s): ''' [[Dr. Bilgen Yuncu, P.E.]] and [[M. Tony Lieberman]]

'''Key Resource(s):'''

*[http://dx.doi.org/10.1007/978-1-4419-1401-9_2 Chlorinated Solvent Chemistry: Structures, Nomenclature and Properties]<ref name="CS2010">Cwiertny, D.M., Scherer, M.M., 2010. Chlorinated solvent chemistry: structures, nomenclature and properties. In In situ remediation of chlorinated solvent plumes. Springer New York. pgs. 29-37. [http://dx.doi.org/10.1007/978-1-4419-1401-9_2 doi:10.1007/978-1-4419-1401-9_2]</ref>

==Introduction==

Chlorinated solvents are a large family of organic solvents that contain chlorine atoms in their molecular structure. They were first produced in Germany in the 1800s, and widespread use in the United States (U.S.) began after World War II. In the period of 1940-1980, the U.S. produced about 2 billion pounds of chlorinated solvents each year<ref name="PC 1996"> Pankow, J.F., Cherry, J.A., 1996. Dense Chlorinated Solvents and Other DNAPLs in Groundwater, Waterloo Press, Portland, OR. ISBN 0964801418</ref>. Chlorinated solvents, including [[wikipedia:Carbon_tetrachloride|carbon tetrachloride (CT)]], [[wikipedia:1,1,1-Trichloroethane|1,1,1-trichloroethane (TCA)]], [[wikipedia:Tetrachloroethylene|perchloroethene or tetrachloroethene (PCE)]] and [[wikipedia:Trichloroethylene|trichloroethene (TCE)]] have been among the most widely used cleaning and degreasing solvents in the U.S<ref>Doherty, R.E., 2000. A history of the production and use of carbon tetrachloride, tetrachloroethylene, trichloroethylene and 1, 1, 1-trichloroethane in the United States: Part 1--historical background; carbon tetrachloride and tetrachloroethylene. Environmental Forensics, 1(2), 69-81. [http://dx.doi.org/10.1006/enfo.2000.0010 doi:10.1006/enfo.2000.0010]</ref>. They also have been used in a wide variety of other purposes such as adhesives, chemical intermediates, clothes, pharmaceuticals, pesticides, and textile processing.

==Physical & Chemical Properties==

Chlorinated solvents are organic compounds generally constructed of a simple hydrocarbon chain (typically one to three carbon atoms in length). They can be divided into three categories based on their structural characteristics: chlorinated methanes, chlorinated ethanes and chlorinated ethenes.

Chlorinated methanes represent the most structurally simple solvent class and consist of a single carbon center (known as a methyl carbon) to which as many as four chlorine atoms are bonded. From the perspective of groundwater contamination, perhaps the most well-known chlorinated methanes are [[wikipedia:carbon tetrachloride|carbon tetrachloride (CT)]] or [[wikipedia:tetrachloromethane|tetrachloromethane]], [[wikipedia:trichloromethane|trichloromethane]] (commonly known as [[wikipedia:chloroform|chloroform (CF)]]), [[wikipedia:dichloromethane|dichloromethane (DCM)]], or [[wikipedia:methylene chloride|methylene chloride (MC)]] and [[wikipedia:chloromethane|chloromethane (CM)]], or [[wikipedia:methyl chloride|methyl chloride]].

Chlorinated ethanes consist of two carbon centers joined by a single [[wikipedia:Covalent_bond|covalent bond]]. The most frequently encountered groundwater pollutants of this class include [[wikipedia:1,1,1-trichloroethane|1,1,1-trichloroethane (1,1,1-TCA)]] and [[wikipedia:1,2-dichloroethane|1,2-dichloroethane]].

Chlorinated ethenes (also referred to as chlorinated ethylenes) also possess two carbon centers, but unlike chlorinated ethanes, these carbon atoms are joined by a carbon-carbon double bond. Chlorinated ethenes that are important groundwater contaminants include [[wikipedia:tetrachloroethene|tetrachloroethene]], or [[wikipedia:perchloroethene|perchloroethene (PCE)]], [[wikipedia:trichloroethene|trichloroethene (TCE)]], [[wikipedia:dichloroethene|dichloroethene (DCE)]]) (DCE, mainly two geometric isomers cis-1,2-dichloroethene and trans-1,2-dichloroethene), and [[wikipedia:vinyl chloride|vinyl chloride (VC)]].

Nomenclature and structure of selected compounds from each solvent class as well as some physical and chemical properties of most widely used chlorinated solvents are listed in Table 1.


{| class="mw-collapsible wikitable" style="margin: auto; color:black; background-color:white; width: 100%;"
|+Table 1. Nomenclature, Structure, Chemical and Physical Properties of Most Widely Used Chlorinated Solvents<ref name="CS2010" />.
|- style="color:white; background-color:#476b6b; text-align:center;"
|IUPAC Name
|Common Name
|Acronym
|Molecular Formula
|Chemical Structure
|Formula Weight
|Density (ρ)(g/mL)
|Aqueous Solubility (mg/L)
|Vapor Pressure (ρ0)(kPa)
|Henry's Law Constanta
|Log Kow
|MCLb (mg/L)
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Methanes
|-
| style="color:black; background-color:#d1e0e0;" |tetrachloromethane
| style="text-align:center;" |carbon tetrachloride
| style="text-align:center;" |CT
| style="text-align:center;" |CCl4
|
[[File:Tetrachloromethane.png|center|70 px|frameless]]
| style="text-align:center;" |153.8
| style="text-align:center;" |1.59
| style="text-align:center;" |800
| style="text-align:center;" |20.5
| style="text-align:center;" |28.9
| style="text-align:center;" |2.64
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |trichloromethane
| style="text-align:center;" |chloroform
| style="text-align:center;" |CF
| style="text-align:center;" |CHCl3
|
[[File:Trichloromethane.png|center|70px|frameless]]
| style="text-align:center;" |119.4
| style="text-align:center;" |1.49
| style="text-align:center;" |8,200
| style="text-align:center;" |26.2
| style="text-align:center;" |3.8
| style="text-align:center;" |1.97
| style="text-align:center;" |0.080c
|-
| style="color:black; background-color:#d1e0e0;" |dichloromethane
| style="text-align:center;" |methylene chloride
| style="text-align:center;" |DCM
| style="text-align:center;" |CH2Cl2
|
[[File:Dichloromethane.png|center|70px|frameless]]
| style="text-align:center;" |84.9
| style="text-align:center;" |1.33
| style="text-align:center;" |13,200
| style="text-align:center;" |55.3
| style="text-align:center;" |1.7
| style="text-align:center;" |1.25
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |chloromethane
| style="text-align:center;" |methyl chloride
| style="text-align:center;" |CM
| style="text-align:center;" |CH3Cl
|
[[File:Chloromethane.png|center|70px|frameless]]
| style="text-align:center;" |50.5
| style="text-align:center;" |0.92
| style="text-align:center;" |5,235
| style="text-align:center;" |570
| style="text-align:center;" |9.6
| style="text-align:center;" |0.91
| style="text-align:center;" |NRd
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Ethanes
|-
| style="color:black; background-color:#d1e0e0;" |hexachloroethane
| style="text-align:center;" |perchloroethane
| style="text-align:center;" |HCA
| style="text-align:center;" |C2Cl6
|
[[File:Hexachloroethane.png|center|70px|frameless]]
| style="text-align:center;" |236.7
| style="text-align:center;" |2.09
| style="text-align:center;" |50
| style="text-align:center;" |0.05e
| style="text-align:center;" | -
| style="text-align:center;" |3.93
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |pentachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |PCA
| style="text-align:center;" |C2HCl5
|
[[File:Pentachloroethane.png|center|70px|frameless]]
| style="text-align:center;" |202.3
| style="text-align:center;" |1.68
| style="text-align:center;" |500
| style="text-align:center;" |0.6
| style="text-align:center;" |2.5
| style="text-align:center;" |2.89
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,1,2-tetrachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,1,2-TeCA
| style="text-align:center;" |C2H2Cl4
|
[[File:1,1,1,2-Tetrachloroethane.png|72px|frameless|center]]
| style="text-align:center;" |167.9
| style="text-align:center;" |1.54
| style="text-align:center;" |1,100
| style="text-align:center;" |1.6
| style="text-align:center;" |2.4
| style="text-align:center;" | -
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,2,2-tetrachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,2,2-TeCA
| style="text-align:center;" |C2H2Cl4
|
[[File:1,1,2,2-Tetrachloroethane.png|72px|frameless|center]]
| style="text-align:center;" |167.9
| style="text-align:center;" |1.60
| style="text-align:center;" |2,962
| style="text-align:center;" |0.8
| style="text-align:center;" |0.44
| style="text-align:center;" |2.39
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,2-trichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,2-TCA
| style="text-align:center;" |C2H3Cl3
|
[[File:1,1,2-Trichloroethane.svg.png|72px|frameless|center]]
| style="text-align:center;" |133.4
| style="text-align:center;" |1.44
| style="text-align:center;" |4,394
| style="text-align:center;" |3.22
| style="text-align:center;" |0.96
| style="text-align:center;" |2.38
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |1,1,1-trichloroethane
| style="text-align:center;" |methyl chloroform
| style="text-align:center;" |1,1,1-TCA
| style="text-align:center;" |C2H3Cl3
|
[[File:1,1,1-trichloroethane.png|center|70px|frameless]]
| style="text-align:center;" |133.4
| style="text-align:center;" |1.35
| style="text-align:center;" |1,495
| style="text-align:center;" |16.5
| style="text-align:center;" |14.5
| style="text-align:center;" |2.49
| style="text-align:center;" |0.20
|-
| style="color:black; background-color:#d1e0e0;" |1,2-dichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,2-DCA
| style="text-align:center;" |C2H4Cl2
|
[[File:1,2-dichloroethane.png|center|70px|frameless]]
| style="text-align:center;" |99.0
| style="text-align:center;" |1.25
| style="text-align:center;" |8,606
| style="text-align:center;" |10.5
| style="text-align:center;" |1.2
| style="text-align:center;" |1.48
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |1,1-dichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1-DCA
| style="text-align:center;" |C2H4Cl2
|
[[File:1,1-Dichloroethane 2.svg.png|72px|frameless|center]]
| style="text-align:center;" |99.0
| style="text-align:center;" |1.17
| style="text-align:center;" |4,676
| style="text-align:center;" |30.3
| style="text-align:center;" |6.2
| style="text-align:center;" |1.79
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |chloroethane
| style="text-align:center;" | -
| style="text-align:center;" |CA
| style="text-align:center;" |C2H5Cl
|
[[File:Chloroethane.png|center|70px|frameless]]
| style="text-align:center;" |64.5
| style="text-align:center;" |0.92
| style="text-align:center;" |5,700
| style="text-align:center;" |16.0
| style="text-align:center;" |1.8
| style="text-align:center;" |1.43
| style="text-align:center;" |NR
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Ethenes
|-
| style="color:black; background-color:#d1e0e0;" |tetrachloroethene
| style="text-align:center;" |perchloroethene
| style="text-align:center;" |PCE
| style="text-align:center;" |C2Cl4
|
[[File:Tetrachloroethene.png|center|70px|frameless]]
| style="text-align:center;" |165.8
| style="text-align:center;" |1.63
| style="text-align:center;" |150
| style="text-align:center;" |2.4
| style="text-align:center;" |26.3
| style="text-align:center;" |2.88
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |trichloroethene
| style="text-align:center;" | -
| style="text-align:center;" |TCE
| style="text-align:center;" |C2HCl3
|
[[File:Trichloroethene.png|72px|frameless|center]]
| style="text-align:center;" |131.4
| style="text-align:center;" |1.46
| style="text-align:center;" |1,100
| style="text-align:center;" |9.9
| style="text-align:center;" |11.7
| style="text-align:center;" |2.53
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |cis-1,2-dichloroethene
| style="text-align:center;" |cis-dichloroethene
| style="text-align:center;" |cis-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:Cis-1,2-dichloroethene.png|center|70px|frameless]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.28
| style="text-align:center;" |3,500
| style="text-align:center;" |27.1
| style="text-align:center;" |7.4
| style="text-align:center;" |1.86
| style="text-align:center;" |0.07
|-
| style="color:black; background-color:#d1e0e0;" |trans-1,2-dichloroethene
| style="text-align:center;" |trans-dichloroethene
| style="text-align:center;" |trans-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:Trans-1,2-dichloroethene.png|72px|frameless|center]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.26
| style="text-align:center;" |6,260
| style="text-align:center;" |44.4
| style="text-align:center;" |6.8
| style="text-align:center;" |1.93
| style="text-align:center;" |0.1
|-
| style="color:black; background-color:#d1e0e0;" |1,1-dichloroethene
| style="text-align:center;" |vinylidene chloride
| style="text-align:center;" |1,1-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:1,1-Dichloroethene.svg.png|72px|frameless|center]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.22
| style="text-align:center;" |3,344
| style="text-align:center;" |80.5
| style="text-align:center;" |23.0
| style="text-align:center;" |2.13
| style="text-align:center;" |0.007
|-
| style="color:black; background-color:#d1e0e0;" |chloroethene
| style="text-align:center;" |vinyl chloride
| style="text-align:center;" |VC
| style="text-align:center;" |C2H3Cl
|
[[File:Chloroethene.png|center|70px|frameless]]
| style="text-align:center;" |62.5
| style="text-align:center;" |0.91
| style="text-align:center;" |2,763
| style="text-align:center;" |355
| style="text-align:center;" |79.2
| style="text-align:center;" |1.38
| style="text-align:center;" |0.002
|-
| colspan="12" style="color:black; background-color:#d1e0e0;" |Notes:
atm = atmosphere; g = gram; Kow = octanol/water partitioning coefficient; Koc -- soil organic carbon/water partitioning coefficient; L = liter; MCL = maximum contaminant level; mg = milligram; mL = milliliter; mol = mole.

aHenry's Law Constant (KH)(x10-3 atm ・ m3/mol)

bSource: http://water.epa.gov/drink/contaminants/#List

cMCL for total trihalomethanes is defined as the summed concentration of chloroform, bromoform (CHBr3),bromodichloromethane (CHBrCl2), and dibromochloromethane (CHBr2Cl). http://water.epa.gov/drink/contaminants/basicinformation/disinfectionbyproducts.cfm

dNR : Not regulated.

eReported vapor pressure for solid-phase hexachloroethane.

|-
|}

Chlorinated solvents and many of their transformation products are colorless liquids at room temperature. They are heavier than water with densities greater than 1 gram per cubic centimeter (g/cm3) which means they can penetrate deeply into an aquifer. They are relatively volatile compounds with relatively high [[wikipedia:Henry’s Law|Henry’s Law]] constants(KH), a measure of the strength of partitioning from water into air). Generally, when KH for a compound exceeds 0.2 atmosphere/mole fraction (atm/M), they can readily be removed from water by air stripping it. Most chlorinated solvents can be classified as sparingly soluble in water, with aqueous solubilities generally on the order of 10s to 100s of mg/L. As the number of chlorine atoms on a compound increases, the solubility decreases. Because of their relatively low solubilities, chlorinated solvents dissolve slowly in groundwater. Another consequence of their limited solubility is their tendency to occur in the subsurface as a separate immiscible liquid phase which, because of its density compared to water, tends to sink in groundwater. Under these conditions, these are referred to as [[wikipedia:DNAPL|dense non-aqueous phase liquid (DNAPL)]]. Although chlorinated solvents are not very soluble in water, their solubility is typically orders of magnitude greater than their established [http://water.epa.gov/drink/contaminants/#Organic drinking water standards].

Chlorinated solvents can be considered moderately hydrophobic which can be determined by their [[wikipedia:Partition coefficient|octanol-water partition coefficient]]s (Kow, a measure of the tendency of a substance to prefer an organic or oily phase rather than an aqueous phase). Log Kow values less than 3 indicate that the compound does not sorb strongly to aquifer solids, but can be removed readily by activated carbon. On the other hand, compounds with log Kow less than 2, such as VC, generally are not removed well by activated carbon either<ref name="CS2010" />.

==References==

<references />

==See Also==

*[https://serdp-estcp.org/Tools-and-Training/Environmental-Restoration/DNAPL-Source-Zones/Frequently-Asked-Questions-Regarding-Management-of-Chlorinated-Solvents-in-Soils-and-Groundwater FAQ Regarding Management of Chlorinated Solvents in Soil and Groundwater]
*[//www.enviro.wiki/images/6/6b/AFCEE_Protocol_2007_chlorinated_solvents.pdf Protocol for In Situ Bioremediation of Chlorinated Solvents Using Edible Oil]

Monitored Natural Attenuation (MNA)

2026-05-07T16:42:20Z

Admin:

Monitored Natural Attenuation (MNA) is an important, common groundwater remediation technology used for treating some dissolved groundwater contaminants. MNA relies on natural attenuation processes to achieve site-specific remediation objectives within a reasonable time frame compared to more active approaches. While MNA has primarily focused on managing plumes with low residual contamination, there is an growing movement to also apply it to source zones via [[ Natural Source Zone Depletion (NSZD) | natural source zone depletion (NSZD)]]. [[Long-Term Monitoring (LTM) | Long-term monitoring]] is required to determine if the concentrations of target contaminants are behaving as predicted. 
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'''Related Article(s):'''

*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| MNA of Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| MNA of Metals and Metalloids]]
*[[Monitored Natural Attenuation (MNA) of Fuels|MNA of Petroleum Hydrocarbons and Fuel Components]]
*[[Natural Source Zone Depletion (NSZD)]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]

'''Contributor(s):''' [[Dr. John Wilson]]

'''Key Resource(s):'''
*[[Media:EPA-1999-Use_of_MNA_at_Superfund%2C_RCRA_and_UST_sites.pdf|Use of monitored natural attenuation at superfund, RCRA corrective action, and underground storage tank sites]]<ref name="EPA1999"> U.S. Environmental Protection Agency, 1999. Use of monitored natural attenuation at superfund, RCRA corrective action, and underground storage tank sites. [[Media:EPA-1999-Use_of_MNA_at_Superfund%2C_RCRA_and_UST_sites.pdf|Report.pdf]]</ref>

==Introduction==
A number of natural processes can attenuate the concentrations of contaminants in groundwater including biological degradation, abiotic degradation, sorption, dispersion into groundwater adjacent to the contaminant plume, and volatilization to soil gas above the groundwater. As the concentration declines, it may reach a point where it is no longer considered hazardous. If the natural processes that attenuate the concentrations of a particular hazardous chemical can meet the cleanup goals for a site, the processes can provide the basis for a cleanup technology. The United States Environmental Protection Agency (U.S. EPA), defines Monitored Natural Attenuation (MNA) as ''“the reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods”''<ref name="EPA1999"/>.

The concentration at which a contaminant is no longer hazardous is defined by U.S. EPA and state regulations. The U.S. EPA regulates the maximum concentration of contaminants that are allowed in water that is supplied as drinking water. These U.S. EPA regulations are referred to as the Maximum Contaminant Level (MCL)<ref> U.S. Environmental Protection Agency (USEPA), 2016. Table of Regulated Drinking Water Contaminants.[http://www.epa.gov/your-drinking-water/table-regulated-drinking-water-contaminants Table of Regulated Drinking Water]</ref>. Often, the MCL is selected as the cleanup goal for MNA. However, other goals<ref> Deeb, R., Hawley, E., Kell, L. and O'Laskey, R., 2011. Assessing alternative endpoints for groundwater remediation at contaminated sites. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200832 ER-200832]</ref> are occasionally selected.

To accept MNA as remedial technology on the same basis as engineered remedial technologies, it is necessary to characterize the distribution of contamination at a site, characterize the [[ Advection and Groundwater Flow | flow of groundwater]], understand the processes that contribute to natural attenuation and use this information to build a conceptual model of the site. Sometimes the site conceptual model is used to organize an analytical model of the transport and fate<ref> VirginiaTech, United States Geological Survey (USGS), and Naval Facilites Engineering Command (NAVFAC). 2016. Natural Attenuation Software (NAS). [http://www.nas.cee.vt.edu/index.php Software]</ref> of the contaminants in groundwater. The forecasts of the transport and fate model are compared to the cleanup goals for the site to determine if natural attenuation is an appropriate remedy. If natural attenuation is selected as a remedy, the site is monitored over time to ensure that the attenuation of the contaminant proceeds as anticipated. The entire package of site characterization<ref> Pivetz, B.E., Abshire, D., Brandon, W., Mangion,S., Roberts, B., Stuart, B., Vanderpool, L., Wilson, B., Acree, S.D., 2012. Framework for Site Characterization for Monitored Natural Attenuation of Volatile Organic Compounds in Ground Water. EPA 600-R-12-712, 89 pgs. [[Media:Pivetz-2012-Framework_for_Site_Char_for_MNA.pdf|Report pdf]]</ref>, a site conceptual model, and monitoring<ref> Pope, D.F., Acree, S.D., Levine, H., Mangion, S., Van Ee, J., Hurt, K., Wilson, B. and Burden, D.S., 2004. Performance monitoring of MNA remedies for VOCs in ground water. US Environmental Protection Agency, National Risk Management Research Laboratory. [[Media:Pope-2012-Performance_Monitoring_of_MNA_Remedies.pdf|Report pdf]]</ref> are necessary components of MNA as a formal remedy for any site selected.

The U.S. EPA considers three lines of evidence<ref name= "EPA1999"/> before MNA can be accepted as the remedy for a site:
*Historical groundwater and/or soil chemistry data that demonstrate a clear and meaningful trend of decreasing contaminant mass and/or concentration over time at appropriate monitoring or sampling points.
*Hydrogeologic and geochemical data that can be used to demonstrate indirectly the type(s) of natural attenuation processes active at the site, and the rate at which such processes will reduce contaminant concentrations to required levels.
*Data from field or microcosm studies (conducted in or with actual contaminated site media) which directly demonstrate the occurrence of a particular natural attenuation process at the site and its ability to degrade the contaminants of concern (typically used to demonstrate biological degradation processes only).

At most sites, U.S. EPA requires the first two lines of evidence. The third line of evidence is reserved for contaminants that are not well understood.

MNA is often used as a remedy, or part of a remedy, where contaminants have been demonstrated to be degrading or sequestered in groundwater. A number of technical protocols have been developed to guide the application of MNA for particular contaminants, including [[Monitored Natural Attenuation (MNA) of Fuels|fuel hydrocarbons]]<ref>Wiedemeier, T.H., Wilson, J.T., Kampbell, D.H., Miller, R.N., Hansen, J.E., 1999. Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater. Volume I. [[Media:Wiedemeier-1999-technical_Protocol_for_implementing_Intrinsic_remediation.pdf|Report pdf]]</ref>, [[Monitored Natural Attenuation (MNA) of Chlorinated Solvents|chlorinated solvents]]<ref> Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water. EPA-600-R-98-128. [[Media:Wiedemeier-1998-Technical_Protocol_for_Evaluating_Natuaral_Attenuation.pdf|Report pdf]]</ref>, methyl ''tert''-butyl ether (MTBE<ref>Wilson, J.T., Kaiser, P.M., Adair, C., 2005. Monitored Natural Attenuation of MTBE as a Risk Management Option at Leaking Underground Storage Tank Sites EPA/600/R-04/1790. [[Media:Wilson-2005-MNA_of_MTBE.pdf|Report pdf]]</ref>), inorganics , metals , radionuclides<ref> Truex, M., Brady, P., Newell, C.J., Rysz, M., Denham, M., Vangelas, K. 2011. The Scenarios Approach to Attenuation-Based Remedies for Inorganic and Radionuclide Contaminants. Savannah-River National Laboratory U.S. Department of Energy. [[Media:TRUEX-2011-Scenarios_Approach_to_Attenuation-Based_Remedies.pdf|Report pdf]]</ref>, and explosives<ref> Pennington, J.C., Zakikhani, M., Harrelson, D., 1999. Monitored Natural Attenuation of Explosives in Groundwater. ESTCP Completion Report ER-199518. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-199518 ER-199518]</ref><ref> Borden, R.C., Knox, S.L., Lieberman, M.T., Ogles, D., 2014. Perchlorate natural attenuation in a riparian zone. Journal of Environmental Science and Health, Part A, Toxic/Hazardous Substances and Environmental Engineering, 49(10), 1100-1109. [http://dx.doi.org/10.1080/10934529.2014.897145 doi: 10.1080/10934529.2014.897145]</ref>. These protocols were developed from 1999 to 2010, in the same time period when U.S. EPA developed its policy guidance. Since that time, there have been significant advances<ref name="Adamson2014"> Adamson, D., Newell, C., 2014. Frequently Asked Questions about Monitored Natural Attenuation in the 21st Century. ER-201211. Environmental Security and Technology Certification Program, Arlington, Virginia. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201211 ER-201211]</ref> in our understanding of the processes that degrade contaminants in groundwater.

==Abiotic Process==
Abiotic processes<ref> Darlington, R., Rectanus, H. 2015. Biogeochemical Transformation Handbook. TR-NAVFAC EXWC-EV-1601, 41 pgs. [[Media:Darlington-2015-Biogeochem_Transformation_Handbook.pdf|Report pdf]]</ref> can contribute to natural attenuation of certain contaminants such as chlorinated solvents. For example, chlorinated alkenes can react with naturally occurring magnetite or other iron minerals in aquifer materials<ref>He, Y., Su, C., Wilson, J., Wilkin, R., Adair, C., Lee, T., Bradley, P., Ferrey, M., 2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water. US Environmental Protection Agency. [[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Report pdf]]</ref>. The rate constants are generally slow, but abiotic degradation can be important if the travel time of the contamination to the point of compliance is long.

==Tools for Assessing Monitored Natural Attenuation==
# '''Statistical Tools to Evaluate Trends'''. Computer programs such as MAROS<ref>Aziz, J.J., Ling, M., Rifai, H.S., Newell, C.J., Gonzales, J.R., 2003. MAROS: A decision support system for optimizing monitoring plans. Ground Water, 41(3), 355-367. [http://dx.doi.org/10.1111/j.1745-6584.2003.tb02605.x doi: 10.1111/j.1745-6584.2003.tb02605.x]</ref><ref> Aziz, J.J., Newell, C.J., Rifai, H.S., Ling, M., Gonzales, J.R., 2000. Monitoring and Remediation Optimization System (MAROS): Software User’s Guide. [[Media:Aziz-2000-Monitoring_and_Remed._Opt._Syst._Guide.pdf|Report pdf]]</ref> and the Mann-Kendall Toolkit<ref>Connor, J., Farhat, S. K., Vanderford, M. V., Newell, C. J., 2012. GSI Mann-Kendall Toolkit. [http://www.gsi-net.com/en/software/free-software/gsi-mann-kendall-toolkit.html Mann Kendall Toolkit]</ref> can be used to help confirm trends in groundwater data used as a line of evidence for MNA. 
# '''[[Molecular Biological Tools - MBTs|Molecular Biological Tools (MBTs)]]'''. MBTs are used to identify and characterize the bacteria that carry out critical steps in the biodegradation of the contaminants in groundwater. In the case of chlorinated solvents tetrachloroethene (PCE) and trichloroethene (TCE), a key bacterium is ''Dehalococcoides mccartyi''<ref name ="Löffle2013">Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, A. Leeson, C.H. Ward, Springer, New York, NY. pgs. 39-88. ISBN: 978-1-4614-4114-4. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>. In anaerobic groundwater, chlorinated alkenes can undergo a sequential reductive dehalogenation from PCE, to TCE, to dichloroethene (DCE) and then to vinyl chloride (VC) and finally to ethane. Anaerobic microbial communities that contain ''Dehalococcoides'' can degrade PCE and TCE all the way to harmless end products. The abundance of ''Dehalococcoides'' cells in groundwater can be determined by an assay based on the polymerase chain reaction<ref>Lebron, C.A., Petrovskis, E., Loffler, F., Henn, K., 2011. Application of Nucleic Acid-Based Tools for Monitoring Monitored Natural Attenuation (MNA), Biostimulation and Bioaugmentation at Chlorinated Solvent Sites (No. NFESC-CR-11-028-ENV). ER-200518. Naval Facilities Engineering Command Port Hueneme CA Engineering Service Center. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200518/ER-200518 ER-200518]</ref>. Other assays can determine the abundance of reductase genes<ref name ="Löffle2013"/> that code for enzymes that can carry out specific steps in the dechlorination pathway. Similar assays are available to determine the abundance of ''Dehalobacter, Dehalogenimonas, and Desulfitobacterium'' strains that degrade chlorinated alkanes, and MBT assays are available for several of their reductase genes. A great variety of bacteria degrade petroleum hydrocarbons. Bacteria that degrade hydrocarbons using oxygen initiate degradation with an oxygenase enzyme, and [[Quantitative Polymerase Chain Reaction (qPCR) | qPCR]] assays are available for a variety of oxygenase enzymes<ref>Baldwin, B.R., Nakatsu, C.H., Nies, L., 2008. Enumeration of aromatic oxygenase genes to evaluate monitored natural attenuation at gasoline-contaminated sites. Water Research, 42(3), 723-731. [http://dx.doi.org/10.1016/j.watres.2007.07.052 doi:10.1016/j.watres.2007.07.052]</ref>. The bacteria that degrade hydrocarbons under anaerobic conditions are particularly important for natural attenuation, and there are qPCR assays for the enzymes that initiate degradation under anaerobic conditions<ref> da Silva, M.L.B., Corseuil, H.X., 2012. Groundwater microbial analysis to assess enhanced BTEX biodegradation by nitrate injection at a gasohol-contaminated site. International Biodeterioration & Biodegradation, 67, 21-27. [http://dx.doi.org/10.1016/j.ibiod.2011.11.005 doi:10.1016/j.ibiod.2011.11.005]</ref>. See an entire article on MBTs here: [[Molecular Biological Tools - MBTs]] 
# '''[[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]]'''. CSIA can unequivocally demonstrate that a compound has degraded in groundwater. It is difficult to document the degradation of a compound in groundwater if the only information available is an apparent attenuation in concentrations along a flow path in the plume. There is always a possibility that a downgradient well is askew of the true flow path, and the attenuation is caused by dilution and not degradation. CSIA determines the ratio of stable isotopes in a compound. As a compound degrades, molecules with lighter isotopes degrade faster. As degradation progresses, the material that has not degraded becomes enriched in the heavier stable isotope. At many sites, degradation of the compound can be recognized and documented from a change in the ratio of isotopes<ref>Hunkeler, D., Meckenstock, R.U., Sherwood Lollar, B., Schmidt, T.C., Wilson, J.T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148, 2008. [[Media:Hunkeler-2008-A_Guide.pdf|Report pdf]]</ref>. At some sites, it is possible to use CSIA and reactive transport modeling<ref> Kuder, T., Philp, P., van Breukelen, B., Thouement, H., Vanderford, M., Newell, C. 2014. Integrated Stable Isotope-Reactive Transport Model Approach for Assessment of Chlorinated Solvent Degradation. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-201029/ER-201029 ER-201029]</ref> to evaluate the plausibility of alternate degradation pathways, and to estimate the extent of degradation. See an entire article on CSIA here: [[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]] 
# '''Computer Models'''. Groundwater fate and transport computer models are often used to evaluate how attenuation processes can control the migration of a plume. Public domain software is available that can incorporate terms for advective flow of groundwater, [[ Dispersion and Diffusion | dispersion]] (and more recently diffusion) of contaminations in groundwater, and biotic or abiotic reactions. Examples of commonly used models include analytical models REMChlor<ref name= "Falta2007">Falta, R.W., Stacy, M.B., Ahsanuzzaman, A.N.M., Wang, M., Earle, R., 2007. REMChlor remediation evaluation model for chlorinated solvents user’s manual Version 1.0. Cent. for subsurface model. support, US Environ. Prot. Agency, Ada, Okla.[https://www.epa.gov/water-research/remediation-evaluation-model-chlorinated-solvents-remchlor User's Manual v1.0]</ref> and REMFuel<ref name="Falta2007"/> , and the numerical models MODFLOW/RT3D<ref>2005. MODFLOW and Related Programs [http://water.usgs.gov/ogw/modflow Modflow]</ref>, MODFLOW/MT3DMS, and the Natural Attenuation Software (NAS<ref> Widdowson, M.A., Mendez III, E., Chapelle, F.H., Casey, C.C., 2005. Natural Attenuation Software (NAS) User’s Manual Version 2. [[Media:Widdowson2005-NAS_Users_Guide.pdf|Report pdf]]</ref>).
[[File:Wilson 1 Fig1a.JPG|375px|thumbnail|right|Figure 1a. Evolution of a plume when the plume and source do not attenuate.]]
[[File:Wilson 1 Fig1b.JPG|375px|thumbnail|right|Figure 1b. Evolution of a plume when the source and concentrations in groundwater both attenuate.]]
[[File:Wilson 1 Fig1c.JPG|375px|thumbnail|right|Figure 1c. Evolution of a plume when the source attenuates faster than the plume.]]

==Source Area Considerations==
In most plumes, the time frame that is required for natural attenuation to reach a cleanup goal across the entire plume is not controlled by the rate of attenuation in the groundwater. In many plumes, a source of contamination, such as residual oily phase material (non-aqueous phase liquid [NAPL]), contaminated soils, and matrix diffusion sources, provides a continuous supply of new contamination to the groundwater.

As a result, the lifecycle of the source<ref>Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward, C. H. Springer New York. pgs. 459-508. [http://dx.doi.org/10.1007/978-1-4614-6922-3 doi: 10.1007/978-1-4614-6922-3]</ref> largely controls the lifecycle of contamination in groundwater. As a consequence, at many sites, some attempt is made to actively remediate the source of contamination. In almost every instance, active remediation is successful in reducing the concentration of the contamination, but fails to reduce the concentration to the cleanup goal. The final remedy is a pragmatic combination of active source remediation and MNA. Transport and fate models<ref> Widdowson, M., Chapelle, F., Casey, C., Kram, M., 2008. Estimating Cleanup Times Associated With Combining Source-Area Remediation With Monitored Natural Attenuation. ER-200436 [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436 ER-200436]</ref> can be used to evaluate the benefits from source remediation on the size and lifecycle of the plume of contaminated ground water. The models can estimate the reduction in concentration at the source that is necessary to pull a plume back behind a point of compliance and the time that is required for the plume to recede behind the point of compliance.

==Regulatory Considerations==
If a site is regulated under the Resource Conservation and Recovery Act (RCRA)<ref>[https://www.epa.gov/rcra US EPA RCRA Laws & Regulations]</ref>, the usual goal is for the contaminants to attenuate to acceptable concentrations before groundwater can migrate off-site and impact receptors. Under this MNA approach, the groundwater must reach a cleanup goal before it reaches a point of compliance. For this implementation, a quantitative framework (BioPIC)<ref>Lebron, C. A., Wiedemeier, T. H., Wilson, J.T., Löffler, F.E., Hinchee, R.E., Singletary, M.A., 2015. Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites. ER-201129. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129/ER-201129 ER-201129]</ref> is now available that integrates new discoveries on degradation processes into the U.S. EPA’s approach to evaluate MNA.

When a site is regulated under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA or Superfund)<ref>[https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act US EPA CERCLA Act]</ref>, there is often an additional requirement that all the contamination must reach the cleanup goal by a specified date. The performance of a remedy at a Superfund site is reviewed on a five-year cycle. A framework<ref> Wilson, J.T., 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [[Media:Wilson-2011-An_Approach_for_Evaluating_Progress.pdf|Report pdf]]</ref> is available to review long-term monitoring data to determine whether the attenuation within the review cycle is adequate to meet the cleanup goal by the specified date.

In the USA, the individual states have provided regulations to supplement the U.S. EPA guidance. Examples include general guidance on MNA provided by California<ref>California Regional Water Quality Control Board, 2014. Workshop - Monitored Natural Attenuation. Barstow, California, September 10 & 11. [[Media:MNA_Workshop-2014_California_Water_Boards.pdf|Report pdf]]</ref>, Minnesota<ref>Minnesota Pollution Control Agency. Natural Attenuation of Groundwater. [https://www.pca.state.mn.us/water/natural-attenuation-groundwater Natural Attenuation of Groundwater]</ref>, New Jersey<ref> New Jersey Department of Environmental Protection - Site Remediation Program. 2012. Monitored Natural Attenuation Technical Guidance. [[Media:NJDEP-SRP-2012-MNA_Technical_Guidance_v_1_0.pdf|Report pdf]]</ref> , Ohio<ref>Ohio Environmental Protection Agency - Division of Environmental Response and Revitalization, 2001. Remedial Response Program Fact Sheet. Remediation Using Monitored Natural Attenuation.[[Media:OhioEPA-2001-Division_of_Envl_Response_and_Revitalization_fact_sheet.pdf|Report pdf]]</ref>, and Texas<ref> Texas Commission on Environmental Quality - Remediation Division, 2010. Monitored Natural Attenuation Demonstrations under TRRP. RG-366/TRRP-33. [[Media:TRRP-TCEQ-2010-Regulatory_Guidance-RG-366-TRRP-33.pdf|Report pdf]]</ref>. In addition, California<ref>California State Water Resources Control Board. 2012. Low-threat Underground Storage Tank Case Closure Policy. [[Media:CA-SWB-2012-Low-threat_UST_Case_Closure_Policy.pdf|Report pdf]]</ref>, Minnesota<ref> Minnesota Pollution Control Agency, 2005. Assessment of Natural Biogradation at Petroleum Release Sites. Guidance Document 4-03. [[Media:MINN-PCA-2005-Assessment_of_Natural_Biogradation_at_Petroleum_Rel_Sites.pdf|Report pdf]]</ref>, Washington State<ref> Washington State Department of Ecology, 2005. Guidance on Remediation of Petroleum-Contaminated Ground Water by Natural Attenuation. Publication Number 05-09-091 (Version 1.0). [[Media:WASH-ECOL-2005-Guidance_on_Remediation_of_Petroleum_Contaminated_GW.pdf|Report pdf]]</ref>, and Wisconsin<ref>Wisconsin Department of Natural Resources, 2014. Guidance on Natural Attenuation For Petroleum Releases. Remediation and Redevelopment Program. RR-614. [[Media:WIS-DNR-2014-Guidance_on_Natural_Attenuation_for_Petroleum_Releases.pdf|Report pdf]]</ref> provide guidance on petroleum releases. Minnesota<ref> Minnesota Pollution Control Agency Site Remediation Section. 2006. Guidelines Natural Attenuation of Chlorinated Solvents in Ground Water. [[Media:MINN-PCA-2006-Guidelines_Natural_Attenuation_of_Chlorinated_Solvents_in_GW.pdf|Report pdf]]</ref> and Wisconsin<ref> Wisconsin Department of Natural Resources, 2014. Understanding Chlorinated Hydrocarbon Behavior in Groundwater: Guidance on the Investigation, Assessment and Limitations of Monitored Natural Attenuation. RR-699. [[Media:WIS-DNR-2014-Understanding_Chlorinated_Hydrocarbon_Behavior_In_GW.pdf|Report pdf]]</ref> provide guidance on chlorinated solvents.

==Additional Information==
Additional information on MNA is available on web pages that are maintained by the United State Environmental Protection Agency<ref>U.S. Environmental Protection Agency, 2016. Natural Attenuation Overview. Technology Innovation and Field Services Division. [https://clu-in.org/techfocus/default.focus/sec/Natural_Attenuation/cat/Overview Natural Attenuation Overview]</ref>, the United States Geological Survey<ref> Natural Attenuation Definitions. 2015. United States Geological Survey. </ref>, Department of Energy, and the Interstate Technology Regulatory Council<ref>ITRC, 2008. Enhanced attenuation of chlorinated organics (EACO): A decision framework for site transition. [[Media:ITRC-2008-EACO_Framework_General.pdf|Report pdf]]</ref>. In addition, ESTCP has published “Frequently Asked Questions Regarding MNA in Groundwater” which provides a recent summary overview of key approaches, technologies, and best practices for applying MNA<ref name="Adamson2014"/>.

==References==

<references/>

==See Also==
*[[Long-Term Monitoring (LTM)]]
*[[Media:AFCEE_Long_Term_Monitoring_Protocol_2000.pdf|Designing Monitoring Programs to Effectively Evaluate the Performance of Natural Attenuation]]
*[[Media:ER-201032_Final_Report.pdf|Determining Source Attenuation History to Support Closure by Natural Attenuation]]
*[[Media:Role-of-DHC-Organism-Natural-Attenuation-Chlorinated-Ethylenes.pdf|Evaluation of the Role of Dehalococcoides Organisms in the Natural Attenuation of Chlorinated Ethylenes in Ground Water]]
*[[Media:Natatt_Cr.pdf|EPA Ground Water Issue: Natural Attenuation of Hexavalent Chromium in Groundwater and Soils]]
*[[Media:Parsons_MNA-Altus.pdf|Remediation by Natural Attenuation Treatability Study at Altus Air Force Base]]
*[[Media:Mnatoolbox.pdf|Site Screening and Technical Guidance for Monitored Natural Attenuation at DOE Sites]]
*[https://www.enviro.wiki/images/3/33/mna1198.pdf Technical Guidelines for Evaluating Monitored Natural Attenuation of Petroleum Hydrocarbons and Chlorinated Solvents in Groundwater at Naval and Marine Corps Facilities]
*[[Media:MNA-Guidance-2015.pdf|Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites]]
*[https://www.serdp-estcp.org/index.php/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-199518/ER-199518/(language)/eng-US Monitored Natural Attenuation of Explosives in Groundwater]

Monitored Natural Attenuation (MNA)

2026-05-07T16:41:24Z

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Monitored Natural Attenuation (MNA) is an important, common groundwater remediation technology used for treating some dissolved groundwater contaminants. MNA relies on natural attenuation processes to achieve site-specific remediation objectives within a reasonable time frame compared to more active approaches. While MNA has primarily focused on managing plumes with low residual contamination, there is an growing movement to also apply it to source zones via [[ Natural Source Zone Depletion (NSZD) | natural source zone depletion (NSZD)]]. [[Long-Term Monitoring (LTM) | Long-term monitoring]] is required to determine if the concentrations of target contaminants are behaving as predicted. 
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'''Related Article(s):'''

*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| MNA of Chlorinated Solvents]]
**[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| MNA of Metals and Metalloids]]
*[[Monitored Natural Attenuation (MNA) of Fuels|MNA of Petroleum Hydrocarbons and Fuel Components]]
*[[Natural Source Zone Depletion (NSZD)]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]

'''Contributor(s):''' [[Dr. John Wilson]]

'''Key Resource(s):'''
*[[Media:EPA-1999-Use_of_MNA_at_Superfund%2C_RCRA_and_UST_sites.pdf|Use of monitored natural attenuation at superfund, RCRA corrective action, and underground storage tank sites]]<ref name="EPA1999"> U.S. Environmental Protection Agency, 1999. Use of monitored natural attenuation at superfund, RCRA corrective action, and underground storage tank sites. [[Media:EPA-1999-Use_of_MNA_at_Superfund%2C_RCRA_and_UST_sites.pdf|Report.pdf]]</ref>

==Introduction==
A number of natural processes can attenuate the concentrations of contaminants in groundwater including biological degradation, abiotic degradation, sorption, dispersion into groundwater adjacent to the contaminant plume, and volatilization to soil gas above the groundwater. As the concentration declines, it may reach a point where it is no longer considered hazardous. If the natural processes that attenuate the concentrations of a particular hazardous chemical can meet the cleanup goals for a site, the processes can provide the basis for a cleanup technology. The United States Environmental Protection Agency (U.S. EPA), defines Monitored Natural Attenuation (MNA) as ''“the reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods”''<ref name="EPA1999"/>.

The concentration at which a contaminant is no longer hazardous is defined by U.S. EPA and state regulations. The U.S. EPA regulates the maximum concentration of contaminants that are allowed in water that is supplied as drinking water. These U.S. EPA regulations are referred to as the Maximum Contaminant Level (MCL)<ref> U.S. Environmental Protection Agency (USEPA), 2016. Table of Regulated Drinking Water Contaminants.[http://www.epa.gov/your-drinking-water/table-regulated-drinking-water-contaminants Table of Regulated Drinking Water]</ref>. Often, the MCL is selected as the cleanup goal for MNA. However, other goals<ref> Deeb, R., Hawley, E., Kell, L. and O'Laskey, R., 2011. Assessing alternative endpoints for groundwater remediation at contaminated sites. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200832 ER-200832]</ref> are occasionally selected.

To accept MNA as remedial technology on the same basis as engineered remedial technologies, it is necessary to characterize the distribution of contamination at a site, characterize the [[ Advection and Groundwater Flow | flow of groundwater]], understand the processes that contribute to natural attenuation and use this information to build a conceptual model of the site. Sometimes the site conceptual model is used to organize an analytical model of the transport and fate<ref> VirginiaTech, United States Geological Survey (USGS), and Naval Facilites Engineering Command (NAVFAC). 2016. Natural Attenuation Software (NAS). [http://www.nas.cee.vt.edu/index.php Software]</ref> of the contaminants in groundwater. The forecasts of the transport and fate model are compared to the cleanup goals for the site to determine if natural attenuation is an appropriate remedy. If natural attenuation is selected as a remedy, the site is monitored over time to ensure that the attenuation of the contaminant proceeds as anticipated. The entire package of site characterization<ref> Pivetz, B.E., Abshire, D., Brandon, W., Mangion,S., Roberts, B., Stuart, B., Vanderpool, L., Wilson, B., Acree, S.D., 2012. Framework for Site Characterization for Monitored Natural Attenuation of Volatile Organic Compounds in Ground Water. EPA 600-R-12-712, 89 pgs. [[Media:Pivetz-2012-Framework_for_Site_Char_for_MNA.pdf|Report pdf]]</ref>, a site conceptual model, and monitoring<ref> Pope, D.F., Acree, S.D., Levine, H., Mangion, S., Van Ee, J., Hurt, K., Wilson, B. and Burden, D.S., 2004. Performance monitoring of MNA remedies for VOCs in ground water. US Environmental Protection Agency, National Risk Management Research Laboratory. [[Media:Pope-2012-Performance_Monitoring_of_MNA_Remedies.pdf|Report pdf]]</ref> are necessary components of MNA as a formal remedy for any site selected.

The U.S. EPA considers three lines of evidence<ref name= "EPA1999"/> before MNA can be accepted as the remedy for a site:
*Historical groundwater and/or soil chemistry data that demonstrate a clear and meaningful trend of decreasing contaminant mass and/or concentration over time at appropriate monitoring or sampling points.
*Hydrogeologic and geochemical data that can be used to demonstrate indirectly the type(s) of natural attenuation processes active at the site, and the rate at which such processes will reduce contaminant concentrations to required levels.
*Data from field or microcosm studies (conducted in or with actual contaminated site media) which directly demonstrate the occurrence of a particular natural attenuation process at the site and its ability to degrade the contaminants of concern (typically used to demonstrate biological degradation processes only).

At most sites, U.S. EPA requires the first two lines of evidence. The third line of evidence is reserved for contaminants that are not well understood.

MNA is often used as a remedy, or part of a remedy, where contaminants have been demonstrated to be degrading or sequestered in groundwater. A number of technical protocols have been developed to guide the application of MNA for particular contaminants, including [[Monitored Natural Attenuation (MNA) of Fuels|fuel hydrocarbons]]<ref>Wiedemeier, T.H., Wilson, J.T., Kampbell, D.H., Miller, R.N., Hansen, J.E., 1999. Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater. Volume I. [[Media:Wiedemeier-1999-technical_Protocol_for_implementing_Intrinsic_remediation.pdf|Report pdf]]</ref>, [[Monitored Natural Attenuation (MNA) of Chlorinated Solvents|chlorinated solvents]]<ref> Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water. EPA-600-R-98-128. [[Media:Wiedemeier-1998-Technical_Protocol_for_Evaluating_Natuaral_Attenuation.pdf|Report pdf]]</ref>, methyl ''tert''-butyl ether (MTBE<ref>Wilson, J.T., Kaiser, P.M., Adair, C., 2005. Monitored Natural Attenuation of MTBE as a Risk Management Option at Leaking Underground Storage Tank Sites EPA/600/R-04/1790. [[Media:Wilson-2005-MNA_of_MTBE.pdf|Report pdf]]</ref>), inorganics , metals , radionuclides<ref> Truex, M., Brady, P., Newell, C.J., Rysz, M., Denham, M., Vangelas, K. 2011. The Scenarios Approach to Attenuation-Based Remedies for Inorganic and Radionuclide Contaminants. Savannah-River National Laboratory U.S. Department of Energy. [[Media:TRUEX-2011-Scenarios_Approach_to_Attenuation-Based_Remedies.pdf|Report pdf]]</ref>, and explosives<ref> Pennington, J.C., Zakikhani, M., Harrelson, D., 1999. Monitored Natural Attenuation of Explosives in Groundwater. ESTCP Completion Report ER-199518. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-199518 ER-199518]</ref><ref> Borden, R.C., Knox, S.L., Lieberman, M.T., Ogles, D., 2014. Perchlorate natural attenuation in a riparian zone. Journal of Environmental Science and Health, Part A, Toxic/Hazardous Substances and Environmental Engineering, 49(10), 1100-1109. [http://dx.doi.org/10.1080/10934529.2014.897145 doi: 10.1080/10934529.2014.897145]</ref>. These protocols were developed from 1999 to 2010, in the same time period when U.S. EPA developed its policy guidance. Since that time, there have been significant advances<ref name="Adamson2014"> Adamson, D., Newell, C., 2014. Frequently Asked Questions about Monitored Natural Attenuation in the 21st Century. ER-201211. Environmental Security and Technology Certification Program, Arlington, Virginia. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201211 ER-201211]</ref> in our understanding of the processes that degrade contaminants in groundwater.

==Abiotic Process==
Abiotic processes<ref> Darlington, R., Rectanus, H. 2015. Biogeochemical Transformation Handbook. TR-NAVFAC EXWC-EV-1601, 41 pgs. [[Media:Darlington-2015-Biogeochem_Transformation_Handbook.pdf|Report pdf]]</ref> can contribute to natural attenuation of certain contaminants such as chlorinated solvents. For example, chlorinated alkenes can react with naturally occurring magnetite or other iron minerals in aquifer materials<ref>He, Y., Su, C., Wilson, J., Wilkin, R., Adair, C., Lee, T., Bradley, P., Ferrey, M., 2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water. US Environmental Protection Agency. [[Media:He-2009-Identification_and_characterization_methods_for_reactive_minerals_.pdf|Report pdf]]</ref>. The rate constants are generally slow, but abiotic degradation can be important if the travel time of the contamination to the point of compliance is long.

==Tools for Assessing Monitored Natural Attenuation==
# '''Statistical Tools to Evaluate Trends'''. Computer programs such as MAROS<ref>Aziz, J.J., Ling, M., Rifai, H.S., Newell, C.J., Gonzales, J.R., 2003. MAROS: A decision support system for optimizing monitoring plans. Ground Water, 41(3), 355-367. [http://dx.doi.org/10.1111/j.1745-6584.2003.tb02605.x doi: 10.1111/j.1745-6584.2003.tb02605.x]</ref><ref> Aziz, J.J., Newell, C.J., Rifai, H.S., Ling, M., Gonzales, J.R., 2000. Monitoring and Remediation Optimization System (MAROS): Software User’s Guide. [[Media:Aziz-2000-Monitoring_and_Remed._Opt._Syst._Guide.pdf|Report pdf]]</ref> and the Mann-Kendall Toolkit<ref>Connor, J., Farhat, S. K., Vanderford, M. V., Newell, C. J., 2012. GSI Mann-Kendall Toolkit. [http://www.gsi-net.com/en/software/free-software/gsi-mann-kendall-toolkit.html Mann Kendall Toolkit]</ref> can be used to help confirm trends in groundwater data used as a line of evidence for MNA. 
# '''[[Molecular Biological Tools - MBTs|Molecular Biological Tools (MBTs)]]'''. MBTs are used to identify and characterize the bacteria that carry out critical steps in the biodegradation of the contaminants in groundwater. In the case of chlorinated solvents tetrachloroethene (PCE) and trichloroethene (TCE), a key bacterium is ''Dehalococcoides mccartyi''<ref name ="Löffle2013">Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, A. Leeson, C.H. Ward, Springer, New York, NY. pgs. 39-88. ISBN: 978-1-4614-4114-4. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>. In anaerobic groundwater, chlorinated alkenes can undergo a sequential reductive dehalogenation from PCE, to TCE, to dichloroethene (DCE) and then to vinyl chloride (VC) and finally to ethane. Anaerobic microbial communities that contain ''Dehalococcoides'' can degrade PCE and TCE all the way to harmless end products. The abundance of ''Dehalococcoides'' cells in groundwater can be determined by an assay based on the polymerase chain reaction<ref>Lebron, C.A., Petrovskis, E., Loffler, F., Henn, K., 2011. Application of Nucleic Acid-Based Tools for Monitoring Monitored Natural Attenuation (MNA), Biostimulation and Bioaugmentation at Chlorinated Solvent Sites (No. NFESC-CR-11-028-ENV). ER-200518. Naval Facilities Engineering Command Port Hueneme CA Engineering Service Center. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200518/ER-200518 ER-200518]</ref>. Other assays can determine the abundance of reductase genes<ref name ="Löffle2013"/> that code for enzymes that can carry out specific steps in the dechlorination pathway. Similar assays are available to determine the abundance of ''Dehalobacter, Dehalogenimonas, and Desulfitobacterium'' strains that degrade chlorinated alkanes, and MBT assays are available for several of their reductase genes. A great variety of bacteria degrade petroleum hydrocarbons. Bacteria that degrade hydrocarbons using oxygen initiate degradation with an oxygenase enzyme, and [[Quantitative Polymerase Chain Reaction (qPCR) | qPCR]] assays are available for a variety of oxygenase enzymes<ref>Baldwin, B.R., Nakatsu, C.H., Nies, L., 2008. Enumeration of aromatic oxygenase genes to evaluate monitored natural attenuation at gasoline-contaminated sites. Water Research, 42(3), 723-731. [http://dx.doi.org/10.1016/j.watres.2007.07.052 doi:10.1016/j.watres.2007.07.052]</ref>. The bacteria that degrade hydrocarbons under anaerobic conditions are particularly important for natural attenuation, and there are qPCR assays for the enzymes that initiate degradation under anaerobic conditions<ref> da Silva, M.L.B., Corseuil, H.X., 2012. Groundwater microbial analysis to assess enhanced BTEX biodegradation by nitrate injection at a gasohol-contaminated site. International Biodeterioration & Biodegradation, 67, 21-27. [http://dx.doi.org/10.1016/j.ibiod.2011.11.005 doi:10.1016/j.ibiod.2011.11.005]</ref>. See an entire article on MBTs here: [[Molecular Biological Tools - MBTs]] 
# '''[[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]]'''. CSIA can unequivocally demonstrate that a compound has degraded in groundwater. It is difficult to document the degradation of a compound in groundwater if the only information available is an apparent attenuation in concentrations along a flow path in the plume. There is always a possibility that a downgradient well is askew of the true flow path, and the attenuation is caused by dilution and not degradation. CSIA determines the ratio of stable isotopes in a compound. As a compound degrades, molecules with lighter isotopes degrade faster. As degradation progresses, the material that has not degraded becomes enriched in the heavier stable isotope. At many sites, degradation of the compound can be recognized and documented from a change in the ratio of isotopes<ref>Hunkeler, D., Meckenstock, R.U., Sherwood Lollar, B., Schmidt, T.C., Wilson, J.T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148, 2008. [[Media:Hunkeler-2008-A_Guide.pdf|Report pdf]]</ref>. At some sites, it is possible to use CSIA and reactive transport modeling<ref> Kuder, T., Philp, P., van Breukelen, B., Thouement, H., Vanderford, M., Newell, C. 2014. Integrated Stable Isotope-Reactive Transport Model Approach for Assessment of Chlorinated Solvent Degradation. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-201029/ER-201029 ER-201029]</ref> to evaluate the plausibility of alternate degradation pathways, and to estimate the extent of degradation. See an entire article on CSIA here: [[Compound Specific Isotope Analysis (CSIA) | Compound Specific Isotope Analysis (CSIA)]] 
# '''Computer Models'''. Groundwater fate and transport computer models are often used to evaluate how attenuation processes can control the migration of a plume. Public domain software is available that can incorporate terms for advective flow of groundwater, [[ Dispersion and Diffusion | dispersion]] (and more recently diffusion) of contaminations in groundwater, and biotic or abiotic reactions. Examples of commonly used models include analytical models REMChlor<ref name= "Falta2007">Falta, R.W., Stacy, M.B., Ahsanuzzaman, A.N.M., Wang, M., Earle, R., 2007. REMChlor remediation evaluation model for chlorinated solvents user’s manual Version 1.0. Cent. for subsurface model. support, US Environ. Prot. Agency, Ada, Okla.[https://www.epa.gov/water-research/remediation-evaluation-model-chlorinated-solvents-remchlor User's Manual v1.0]</ref> and REMFuel<ref name="Falta2007"/> , and the numerical models MODFLOW/RT3D<ref>2005. MODFLOW and Related Programs [http://water.usgs.gov/ogw/modflow Modflow]</ref>, MODFLOW/MT3DMS, and the Natural Attenuation Software (NAS<ref> Widdowson, M.A., Mendez III, E., Chapelle, F.H., Casey, C.C., 2005. Natural Attenuation Software (NAS) User’s Manual Version 2. [[Media:Widdowson2005-NAS_Users_Guide.pdf|Report pdf]]</ref>).
[[File:Wilson 1 Fig1a.JPG|375px|thumbnail|right|Figure 1a. Evolution of a plume when the plume and source do not attenuate.]]
[[File:Wilson 1 Fig1b.JPG|375px|thumbnail|right|Figure 1b. Evolution of a plume when the source and concentrations in groundwater both attenuate.]]
[[File:Wilson 1 Fig1c.JPG|375px|thumbnail|right|Figure 1c. Evolution of a plume when the source attenuates faster than the plume.]]

==Source Area Considerations==
In most plumes, the time frame that is required for natural attenuation to reach a cleanup goal across the entire plume is not controlled by the rate of attenuation in the groundwater. In many plumes, a source of contamination, such as residual oily phase material (non-aqueous phase liquid [NAPL]), contaminated soils, and matrix diffusion sources, provides a continuous supply of new contamination to the groundwater.

As a result, the lifecycle of the source<ref>Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward, C. H. Springer New York. pgs. 459-508. [http://dx.doi.org/10.1007/978-1-4614-6922-3 doi: 10.1007/978-1-4614-6922-3]</ref> largely controls the lifecycle of contamination in groundwater. As a consequence, at many sites, some attempt is made to actively remediate the source of contamination. In almost every instance, active remediation is successful in reducing the concentration of the contamination, but fails to reduce the concentration to the cleanup goal. The final remedy is a pragmatic combination of active source remediation and MNA. Transport and fate models<ref> Widdowson, M., Chapelle, F., Casey, C., Kram, M., 2008. Estimating Cleanup Times Associated With Combining Source-Area Remediation With Monitored Natural Attenuation. ER-200436 [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436 ER-200436]</ref> can be used to evaluate the benefits from source remediation on the size and lifecycle of the plume of contaminated ground water. The models can estimate the reduction in concentration at the source that is necessary to pull a plume back behind a point of compliance and the time that is required for the plume to recede behind the point of compliance.

==Regulatory Considerations==
If a site is regulated under the Resource Conservation and Recovery Act (RCRA)<ref>[https://www.epa.gov/rcra US EPA RCRA Laws & Regulations]</ref>, the usual goal is for the contaminants to attenuate to acceptable concentrations before groundwater can migrate off-site and impact receptors. Under this MNA approach, the groundwater must reach a cleanup goal before it reaches a point of compliance. For this implementation, a quantitative framework (BioPIC)<ref>Lebron, C. A., Wiedemeier, T. H., Wilson, J.T., Löffler, F.E., Hinchee, R.E., Singletary, M.A., 2015. Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites. ER-201129. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201129/ER-201129 ER-201129]</ref> is now available that integrates new discoveries on degradation processes into the U.S. EPA’s approach to evaluate MNA.

When a site is regulated under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA or Superfund)<ref>[https://www.epa.gov/laws-regulations/summary-comprehensive-environmental-response-compensation-and-liability-act US EPA CERCLA Act]</ref>, there is often an additional requirement that all the contamination must reach the cleanup goal by a specified date. The performance of a remedy at a Superfund site is reviewed on a five-year cycle. A framework<ref> Wilson, J.T., 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. [[Media:Wilson-2011-An_Approach_for_Evaluating_Progress.pdf|Report pdf]]</ref> is available to review long-term monitoring data to determine whether the attenuation within the review cycle is adequate to meet the cleanup goal by the specified date.

In the USA, the individual states have provided regulations to supplement the U.S. EPA guidance. Examples include general guidance on MNA provided by California<ref>California Regional Water Quality Control Board, 2014. Workshop - Monitored Natural Attenuation. Barstow, California, September 10 & 11. [[Media:MNA_Workshop-2014_California_Water_Boards.pdf|Report pdf]]</ref>, Minnesota<ref>Minnesota Pollution Control Agency. Natural Attenuation of Groundwater. [https://www.pca.state.mn.us/water/natural-attenuation-groundwater Natural Attenuation of Groundwater]</ref>, New Jersey<ref> New Jersey Department of Environmental Protection - Site Remediation Program. 2012. Monitored Natural Attenuation Technical Guidance. [[Media:NJDEP-SRP-2012-MNA_Technical_Guidance_v_1_0.pdf|Report pdf]]</ref> , Ohio<ref>Ohio Environmental Protection Agency - Division of Environmental Response and Revitalization, 2001. Remedial Response Program Fact Sheet. Remediation Using Monitored Natural Attenuation.[[Media:OhioEPA-2001-Division_of_Envl_Response_and_Revitalization_fact_sheet.pdf|Report pdf]]</ref>, and Texas<ref> Texas Commission on Environmental Quality - Remediation Division, 2010. Monitored Natural Attenuation Demonstrations under TRRP. RG-366/TRRP-33. [[Media:TRRP-TCEQ-2010-Regulatory_Guidance-RG-366-TRRP-33.pdf|Report pdf]]</ref>. In addition, California<ref>California State Water Resources Control Board. 2012. Low-threat Underground Storage Tank Case Closure Policy. [[Media:CA-SWB-2012-Low-threat_UST_Case_Closure_Policy.pdf|Report pdf]]</ref>, Minnesota<ref> Minnesota Pollution Control Agency, 2005. Assessment of Natural Biogradation at Petroleum Release Sites. Guidance Document 4-03. [[Media:MINN-PCA-2005-Assessment_of_Natural_Biogradation_at_Petroleum_Rel_Sites.pdf|Report pdf]]</ref>, Washington State<ref> Washington State Department of Ecology, 2005. Guidance on Remediation of Petroleum-Contaminated Ground Water by Natural Attenuation. Publication Number 05-09-091 (Version 1.0). [[Media:WASH-ECOL-2005-Guidance_on_Remediation_of_Petroleum_Contaminated_GW.pdf|Report pdf]]</ref>, and Wisconsin<ref>Wisconsin Department of Natural Resources, 2014. Guidance on Natural Attenuation For Petroleum Releases. Remediation and Redevelopment Program. RR-614. [[Media:WIS-DNR-2014-Guidance_on_Natural_Attenuation_for_Petroleum_Releases.pdf|Report pdf]]</ref> provide guidance on petroleum releases. Minnesota<ref> Minnesota Pollution Control Agency Site Remediation Section. 2006. Guidelines Natural Attenuation of Chlorinated Solvents in Ground Water. [[Media:MINN-PCA-2006-Guidelines_Natural_Attenuation_of_Chlorinated_Solvents_in_GW.pdf|Report pdf]]</ref> and Wisconsin<ref> Wisconsin Department of Natural Resources, 2014. Understanding Chlorinated Hydrocarbon Behavior in Groundwater: Guidance on the Investigation, Assessment and Limitations of Monitored Natural Attenuation. RR-699. [[Media:WIS-DNR-2014-Understanding_Chlorinated_Hydrocarbon_Behavior_In_GW.pdf|Report pdf]]</ref> provide guidance on chlorinated solvents.

==Additional Information==
Additional information on MNA is available on web pages that are maintained by the United State Environmental Protection Agency<ref>U.S. Environmental Protection Agency, 2016. Natural Attenuation Overview. Technology Innovation and Field Services Division. [https://clu-in.org/techfocus/default.focus/sec/Natural_Attenuation/cat/Overview Natural Attenuation Overview]</ref>, the United States Geological Survey<ref> Natural Attenuation Definitions. 2015. United States Geological Survey. </ref>, Department of Energy, and the Interstate Technology Regulatory Council<ref>ITRC, 2008. Enhanced attenuation of chlorinated organics (EACO): A decision framework for site transition. [[Media:ITRC-2008-EACO_Framework_General.pdf|Report pdf]]</ref>. In addition, ESTCP has published “Frequently Asked Questions Regarding MNA in Groundwater” which provides a recent summary overview of key approaches, technologies, and best practices for applying MNA<ref name="Adamson2014"/>.

==References==

<references/>

==See Also==
*[[Long-Term Monitoring (LTM)]]
*[[Media:AFCEE_Long_Term_Monitoring_Protocol_2000.pdf|Designing Monitoring Programs to Effectively Evaluate the Performance of Natural Attenuation]]
*[[Media:ER-201032_Final_Report.pdf|Determining Source Attenuation History to Support Closure by Natural Attenuation]]
*[[Media:Role-of-DHC-Organism-Natural-Attenuation-Chlorinated-Ethylenes.pdf|Evaluation of the Role of Dehalococcoides Organisms in the Natural Attenuation of Chlorinated Ethylenes in Ground Water]]
*[[Media:Natatt_Cr.pdf|EPA Ground Water Issue: Natural Attenuation of Hexavalent Chromium in Groundwater and Soils]]
*[[Media:Parsons_MNA-Altus.pdf|Remediation by Natural Attenuation Treatability Study at Altus Air Force Base]]
*[[Media:Mnatoolbox.pdf|Site Screening and Technical Guidance for Monitored Natural Attenuation at DOE Sites]]
*[https://www.enviro.wiki/images/3/33/mna1198.pdf Technical Guidelines for Evaluating Monitored Natural Attenuation of Petroleum Hydrocarbons and Chlorinated Solvents in Groundwater at Naval and Marine Corps Facilities]
*[[Media:MNA-Guidance-2015.pdf|Use of Monitored Natural Attenuation for Inorganic Contaminants in Groundwater at Superfund Sites]]
*[https://www.serdp-estcp.org/index.php/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-199518/ER-199518/(language)/eng-US Monitored Natural Attenuation of Explosives in Groundwater]

Chlorinated Solvents

2026-05-07T16:39:52Z

Admin:

Chlorinated solvents, including chlorinated volatile organic compounds (CVOC or CVOCs), are chemical compounds containing chlorine that have been widely used in various industries. They are divided in three groups (methanes, ethanes, ethenes) based on their structures, and include common groundwater contaminants such as carbon tetrachloride (CT), perchloroethene (PCE), trichloroethene (TCE), and vinyl chloride (VC). Chlorinated solvents tend to be colorless liquids at room temperatures, heavier than water, volatile, sparingly soluble, and moderately hydrophobic.
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'''Related Article(s):'''

*[[Biodegradation - Cometabolic]]
*[[Biodegradation - Reductive Processes]]
*[[Bioremediation - Anaerobic]]
*[[Bioremediation - Anaerobic Design Considerations]]
*[[Chemical Oxidation (In Situ - ISCO)]]
*[[Chemical Reduction (In Situ - ISCR)]]
*[[Design Tool - Base Addition for ERD]]
*[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Low pH Inhibition of Reductive Dechlorination]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]
*[[pH Buffering in Aquifers]]
*[[Remediation Performance Assessment at Chlorinated Solvent Sites]]
*[[Soil & Groundwater Contaminants]]
*[[Thermal Remediation]]

'''Contributor(s): ''' [[Dr. Bilgen Yuncu, P.E.]] and [[M. Tony Lieberman]]

'''Key Resource(s):'''

*[http://dx.doi.org/10.1007/978-1-4419-1401-9_2 Chlorinated Solvent Chemistry: Structures, Nomenclature and Properties]<ref name="CS2010">Cwiertny, D.M., Scherer, M.M., 2010. Chlorinated solvent chemistry: structures, nomenclature and properties. In In situ remediation of chlorinated solvent plumes. Springer New York. pgs. 29-37. [http://dx.doi.org/10.1007/978-1-4419-1401-9_2 doi:10.1007/978-1-4419-1401-9_2]</ref>

==Introduction==

Chlorinated solvents are a large family of organic solvents that contain chlorine atoms in their molecular structure. They were first produced in Germany in the 1800s, and widespread use in the United States (U.S.) began after World War II. In the period of 1940-1980, the U.S. produced about 2 billion pounds of chlorinated solvents each year<ref name="PC 1996"> Pankow, J.F., Cherry, J.A., 1996. Dense Chlorinated Solvents and Other DNAPLs in Groundwater, Waterloo Press, Portland, OR. ISBN 0964801418</ref>. Chlorinated solvents, including [[wikipedia:Carbon_tetrachloride|carbon tetrachloride (CT)]], [[wikipedia:1,1,1-Trichloroethane|1,1,1-trichloroethane (TCA)]], [[wikipedia:Tetrachloroethylene|perchloroethene or tetrachloroethene (PCE)]] and [[wikipedia:Trichloroethylene|trichloroethene (TCE)]] have been among the most widely used cleaning and degreasing solvents in the U.S<ref>Doherty, R.E., 2000. A history of the production and use of carbon tetrachloride, tetrachloroethylene, trichloroethylene and 1, 1, 1-trichloroethane in the United States: Part 1--historical background; carbon tetrachloride and tetrachloroethylene. Environmental Forensics, 1(2), 69-81. [http://dx.doi.org/10.1006/enfo.2000.0010 doi:10.1006/enfo.2000.0010]</ref>. They also have been used in a wide variety of other purposes such as adhesives, chemical intermediates, clothes, pharmaceuticals, pesticides, and textile processing.

==Physical & Chemical Properties==

Chlorinated solvents are organic compounds generally constructed of a simple hydrocarbon chain (typically one to three carbon atoms in length). They can be divided into three categories based on their structural characteristics: chlorinated methanes, chlorinated ethanes and chlorinated ethenes.

Chlorinated methanes represent the most structurally simple solvent class and consist of a single carbon center (known as a methyl carbon) to which as many as four chlorine atoms are bonded. From the perspective of groundwater contamination, perhaps the most well-known chlorinated methanes are [[wikipedia:carbon tetrachloride|carbon tetrachloride (CT)]] or [[wikipedia:tetrachloromethane|tetrachloromethane]], [[wikipedia:trichloromethane|trichloromethane]] (commonly known as [[wikipedia:chloroform|chloroform (CF)]]), [[wikipedia:dichloromethane|dichloromethane (DCM)]], or [[wikipedia:methylene chloride|methylene chloride (MC)]] and [[wikipedia:chloromethane|chloromethane (CM)]], or [[wikipedia:methyl chloride|methyl chloride]].

Chlorinated ethanes consist of two carbon centers joined by a single [[wikipedia:Covalent_bond|covalent bond]]. The most frequently encountered groundwater pollutants of this class include [[wikipedia:1,1,1-trichloroethane|1,1,1-trichloroethane (1,1,1-TCA)]] and [[wikipedia:1,2-dichloroethane|1,2-dichloroethane]].

Chlorinated ethenes (also referred to as chlorinated ethylenes) also possess two carbon centers, but unlike chlorinated ethanes, these carbon atoms are joined by a carbon-carbon double bond. Chlorinated ethenes that are important groundwater contaminants include [[wikipedia:tetrachloroethene|tetrachloroethene]], or [[wikipedia:perchloroethene|perchloroethene (PCE)]], [[wikipedia:trichloroethene|trichloroethene (TCE)]], [[wikipedia:dichloroethene|dichloroethene (DCE)]]) (DCE, mainly two geometric isomers cis-1,2-dichloroethene and trans-1,2-dichloroethene), and [[wikipedia:vinyl chloride|vinyl chloride (VC)]].

Nomenclature and structure of selected compounds from each solvent class as well as some physical and chemical properties of most widely used chlorinated solvents are listed in Table 1.


{| class="mw-collapsible wikitable" style="margin: auto; color:black; background-color:white; width: 100%;"
|+Table 1. Nomenclature, Structure, Chemical and Physical Properties of Most Widely Used Chlorinated Solvents<ref name="CS2010" />.
|- style="color:white; background-color:#476b6b; text-align:center;"
|IUPAC Name
|Common Name
|Acronym
|Molecular Formula
|Chemical Structure
|Formula Weight
|Density (ρ)(g/mL)
|Aqueous Solubility (mg/L)
|Vapor Pressure (ρ0)(kPa)
|Henry's Law Constanta
|Log Kow
|MCLb (mg/L)
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Methanes
|-
| style="color:black; background-color:#d1e0e0;" |tetrachloromethane
| style="text-align:center;" |carbon tetrachloride
| style="text-align:center;" |CT
| style="text-align:center;" |CCl4
|
[[File:Tetrachloromethane.png|center|70 px|frameless]]
| style="text-align:center;" |153.8
| style="text-align:center;" |1.59
| style="text-align:center;" |800
| style="text-align:center;" |20.5
| style="text-align:center;" |28.9
| style="text-align:center;" |2.64
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |trichloromethane
| style="text-align:center;" |chloroform
| style="text-align:center;" |CF
| style="text-align:center;" |CHCl3
|
[[File:Trichloromethane.png|center|70px|frameless]]
| style="text-align:center;" |119.4
| style="text-align:center;" |1.49
| style="text-align:center;" |8,200
| style="text-align:center;" |26.2
| style="text-align:center;" |3.8
| style="text-align:center;" |1.97
| style="text-align:center;" |0.080c
|-
| style="color:black; background-color:#d1e0e0;" |dichloromethane
| style="text-align:center;" |methylene chloride
| style="text-align:center;" |DCM
| style="text-align:center;" |CH2Cl2
|
[[File:Dichloromethane.png|center|70px|frameless]]
| style="text-align:center;" |84.9
| style="text-align:center;" |1.33
| style="text-align:center;" |13,200
| style="text-align:center;" |55.3
| style="text-align:center;" |1.7
| style="text-align:center;" |1.25
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |chloromethane
| style="text-align:center;" |methyl chloride
| style="text-align:center;" |CM
| style="text-align:center;" |CH3Cl
|
[[File:Chloromethane.png|center|70px|frameless]]
| style="text-align:center;" |50.5
| style="text-align:center;" |0.92
| style="text-align:center;" |5,235
| style="text-align:center;" |570
| style="text-align:center;" |9.6
| style="text-align:center;" |0.91
| style="text-align:center;" |NRd
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Ethanes
|-
| style="color:black; background-color:#d1e0e0;" |hexachloroethane
| style="text-align:center;" |perchloroethane
| style="text-align:center;" |HCA
| style="text-align:center;" |C2Cl6
|
[[File:Hexachloroethane.png|center|70px|frameless]]
| style="text-align:center;" |236.7
| style="text-align:center;" |2.09
| style="text-align:center;" |50
| style="text-align:center;" |0.05e
| style="text-align:center;" | -
| style="text-align:center;" |3.93
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |pentachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |PCA
| style="text-align:center;" |C2HCl5
|
[[File:Pentachloroethane.png|center|70px|frameless]]
| style="text-align:center;" |202.3
| style="text-align:center;" |1.68
| style="text-align:center;" |500
| style="text-align:center;" |0.6
| style="text-align:center;" |2.5
| style="text-align:center;" |2.89
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,1,2-tetrachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,1,2-TeCA
| style="text-align:center;" |C2H2Cl4
|
[[File:1,1,1,2-Tetrachloroethane.png|72px|frameless|center]]
| style="text-align:center;" |167.9
| style="text-align:center;" |1.54
| style="text-align:center;" |1,100
| style="text-align:center;" |1.6
| style="text-align:center;" |2.4
| style="text-align:center;" | -
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,2,2-tetrachloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,2,2-TeCA
| style="text-align:center;" |C2H2Cl4
|
[[File:1,1,2,2-Tetrachloroethane.png|72px|frameless|center]]
| style="text-align:center;" |167.9
| style="text-align:center;" |1.60
| style="text-align:center;" |2,962
| style="text-align:center;" |0.8
| style="text-align:center;" |0.44
| style="text-align:center;" |2.39
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |1,1,2-trichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1,2-TCA
| style="text-align:center;" |C2H3Cl3
|
[[File:1,1,2-Trichloroethane.svg.png|72px|frameless|center]]
| style="text-align:center;" |133.4
| style="text-align:center;" |1.44
| style="text-align:center;" |4,394
| style="text-align:center;" |3.22
| style="text-align:center;" |0.96
| style="text-align:center;" |2.38
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |1,1,1-trichloroethane
| style="text-align:center;" |methyl chloroform
| style="text-align:center;" |1,1,1-TCA
| style="text-align:center;" |C2H3Cl3
|
[[File:1,1,1-trichloroethane.png|center|70px|frameless]]
| style="text-align:center;" |133.4
| style="text-align:center;" |1.35
| style="text-align:center;" |1,495
| style="text-align:center;" |16.5
| style="text-align:center;" |14.5
| style="text-align:center;" |2.49
| style="text-align:center;" |0.20
|-
| style="color:black; background-color:#d1e0e0;" |1,2-dichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,2-DCA
| style="text-align:center;" |C2H4Cl2
|
[[File:1,2-dichloroethane.png|center|70px|frameless]]
| style="text-align:center;" |99.0
| style="text-align:center;" |1.25
| style="text-align:center;" |8,606
| style="text-align:center;" |10.5
| style="text-align:center;" |1.2
| style="text-align:center;" |1.48
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |1,1-dichloroethane
| style="text-align:center;" | -
| style="text-align:center;" |1,1-DCA
| style="text-align:center;" |C2H4Cl2
|
[[File:1,1-Dichloroethane 2.svg.png|72px|frameless|center]]
| style="text-align:center;" |99.0
| style="text-align:center;" |1.17
| style="text-align:center;" |4,676
| style="text-align:center;" |30.3
| style="text-align:center;" |6.2
| style="text-align:center;" |1.79
| style="text-align:center;" |NR
|-
| style="color:black; background-color:#d1e0e0;" |chloroethane
| style="text-align:center;" | -
| style="text-align:center;" |CA
| style="text-align:center;" |C2H5Cl
|
[[File:Chloroethane.png|center|70px|frameless]]
| style="text-align:center;" |64.5
| style="text-align:center;" |0.92
| style="text-align:center;" |5,700
| style="text-align:center;" |16.0
| style="text-align:center;" |1.8
| style="text-align:center;" |1.43
| style="text-align:center;" |NR
|-
| colspan="12" style="color:black; background-color:#94b8b8;" |Chlorinated Ethenes
|-
| style="color:black; background-color:#d1e0e0;" |tetrachloroethene
| style="text-align:center;" |perchloroethene
| style="text-align:center;" |PCE
| style="text-align:center;" |C2Cl4
|
[[File:Tetrachloroethene.png|center|70px|frameless]]
| style="text-align:center;" |165.8
| style="text-align:center;" |1.63
| style="text-align:center;" |150
| style="text-align:center;" |2.4
| style="text-align:center;" |26.3
| style="text-align:center;" |2.88
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |trichloroethene
| style="text-align:center;" | -
| style="text-align:center;" |TCE
| style="text-align:center;" |C2HCl3
|
[[File:Trichloroethene.png|72px|frameless|center]]
| style="text-align:center;" |131.4
| style="text-align:center;" |1.46
| style="text-align:center;" |1,100
| style="text-align:center;" |9.9
| style="text-align:center;" |11.7
| style="text-align:center;" |2.53
| style="text-align:center;" |0.005
|-
| style="color:black; background-color:#d1e0e0;" |cis-1,2-dichloroethene
| style="text-align:center;" |cis-dichloroethene
| style="text-align:center;" |cis-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:Cis-1,2-dichloroethene.png|center|70px|frameless]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.28
| style="text-align:center;" |3,500
| style="text-align:center;" |27.1
| style="text-align:center;" |7.4
| style="text-align:center;" |1.86
| style="text-align:center;" |0.07
|-
| style="color:black; background-color:#d1e0e0;" |trans-1,2-dichloroethene
| style="text-align:center;" |trans-dichloroethene
| style="text-align:center;" |trans-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:Trans-1,2-dichloroethene.png|72px|frameless|center]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.26
| style="text-align:center;" |6,260
| style="text-align:center;" |44.4
| style="text-align:center;" |6.8
| style="text-align:center;" |1.93
| style="text-align:center;" |0.1
|-
| style="color:black; background-color:#d1e0e0;" |1,1-dichloroethene
| style="text-align:center;" |vinylidene chloride
| style="text-align:center;" |1,1-DCE
| style="text-align:center;" |C2H2Cl2
|
[[File:1,1-Dichloroethene.svg.png|72px|frameless|center]]
| style="text-align:center;" |96.9
| style="text-align:center;" |1.22
| style="text-align:center;" |3,344
| style="text-align:center;" |80.5
| style="text-align:center;" |23.0
| style="text-align:center;" |2.13
| style="text-align:center;" |0.007
|-
| style="color:black; background-color:#d1e0e0;" |chloroethene
| style="text-align:center;" |vinyl chloride
| style="text-align:center;" |VC
| style="text-align:center;" |C2H3Cl
|
[[File:Chloroethene.png|center|70px|frameless]]
| style="text-align:center;" |62.5
| style="text-align:center;" |0.91
| style="text-align:center;" |2,763
| style="text-align:center;" |355
| style="text-align:center;" |79.2
| style="text-align:center;" |1.38
| style="text-align:center;" |0.002
|-
| colspan="12" style="color:black; background-color:#d1e0e0;" |Notes:
atm = atmosphere; g = gram; Kow = octanol/water partitioning coefficient; Koc -- soil organic carbon/water partitioning coefficient; L = liter; MCL = maximum contaminant level; mg = milligram; mL = milliliter; mol = mole.

aHenry's Law Constant (KH)(x10-3 atm ・ m3/mol)

bSource: http://water.epa.gov/drink/contaminants/#List

cMCL for total trihalomethanes is defined as the summed concentration of chloroform, bromoform (CHBr3),bromodichloromethane (CHBrCl2), and dibromochloromethane (CHBr2Cl). http://water.epa.gov/drink/contaminants/basicinformation/disinfectionbyproducts.cfm

dNR : Not regulated.

eReported vapor pressure for solid-phase hexachloroethane.

|-
|}

Chlorinated solvents and many of their transformation products are colorless liquids at room temperature. They are heavier than water with densities greater than 1 gram per cubic centimeter (g/cm3) which means they can penetrate deeply into an aquifer. They are relatively volatile compounds with relatively high [[wikipedia:Henry’s Law|Henry’s Law]] constants(KH), a measure of the strength of partitioning from water into air). Generally, when KH for a compound exceeds 0.2 atmosphere/mole fraction (atm/M), they can readily be removed from water by air stripping it. Most chlorinated solvents can be classified as sparingly soluble in water, with aqueous solubilities generally on the order of 10s to 100s of mg/L. As the number of chlorine atoms on a compound increases, the solubility decreases. Because of their relatively low solubilities, chlorinated solvents dissolve slowly in groundwater. Another consequence of their limited solubility is their tendency to occur in the subsurface as a separate immiscible liquid phase which, because of its density compared to water, tends to sink in groundwater. Under these conditions, these are referred to as [[wikipedia:DNAPL|dense non-aqueous phase liquid (DNAPL)]]. Although chlorinated solvents are not very soluble in water, their solubility is typically orders of magnitude greater than their established [http://water.epa.gov/drink/contaminants/#Organic drinking water standards].

Chlorinated solvents can be considered moderately hydrophobic which can be determined by their [[wikipedia:Partition coefficient|octanol-water partition coefficient]]s (Kow, a measure of the tendency of a substance to prefer an organic or oily phase rather than an aqueous phase). Log Kow values less than 3 indicate that the compound does not sorb strongly to aquifer solids, but can be removed readily by activated carbon. On the other hand, compounds with log Kow less than 2, such as VC, generally are not removed well by activated carbon either<ref name="CS2010" />.

==References==

<references />

==See Also==

*[https://serdp-estcp.org/Tools-and-Training/Environmental-Restoration/DNAPL-Source-Zones/Frequently-Asked-Questions-Regarding-Management-of-Chlorinated-Solvents-in-Soils-and-Groundwater FAQ Regarding Management of Chlorinated Solvents in Soil and Groundwater]
*[//www.enviro.wiki/images/6/6b/AFCEE_Protocol_2007_chlorinated_solvents.pdf Protocol for In Situ Bioremediation of Chlorinated Solvents Using Edible Oil]

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[[File:WH Picture1.JPG|thumb|center|x350px|link=Matrix Diffusion|Molecular diffusion slowly transports solutes into clay-rich, lower permeability zones]]
[[File:WH Picture2.JPG|thumb|center|x350px|link=Subgrade Biogeochemical Reactor (SBGR)|Typical subgrade biogeochemical reactor (SBGR) layout. The SBGR is an in situ remediation technology for treatment of contaminated source areas and groundwater plume hot spots ]]
[[File:WH Picture3.JPG|thumb|center|x350px|link=Direct Push Logging|An Hydraulic Profiling Tool (HPT) log with electrical conductivity (EC) on left, injection pressure in middle, and flow rate on the right]]
[[File:WH Picture4.JPG|thumb|center|x350px|link=PH Buffering in Aquifers|Diagram of mineral surface exchanging hydrogen ions with varying pH. The surface of most aquifer minerals carries an electrical charge that varies with pH]]
[[File:WH Picture5.JPG|thumb|center|x350px|link=Biodegradation - Hydrocarbons|Comparison of the longitudinal redox zonation concept (A) and the plume fringe concept (B). Both concepts describe the spatial distribution of electron acceptors and respiration processes in a hydrocarbon contaminant plume]]
[[File:WH Picture6.JPG|thumb|center|x350px|link=Direct Push Logging|Schematic of an Hydraulic Profiling Tool (HPT) probe. HPT were developed to better understand formation permeability and the distribution of permeable and low permeability zones in unconsolidated formations]]
[[File:WH Picture7.JPG|thumb|center|x350px|link=Chemical Oxidation Design Considerations(In Situ - ISCO)|In situ chemical oxidation using (a) direct-push injection probes or (b) well-to-well flushing to delivery oxidants (shown in blue) into a target treatment zone of groundwater contaminated by dense nonaqueous phase liquid compounds (shown in red)]]
[[File:WH Picture8.JPG|thumb|center|x350px|link=Geophysical Methods - Case_Studies|High-resolution 3D cross-borehole electrical imaging of contaminated fractured rock at the former Naval Air Warfare Center in New Jersey. Cross-borehole resistivity tomography imaging is a geophysical technique that can be used for site characterization and monitoring by observing variations in the electrical properties of subsurface materials]]
[[File:WH Picture9.JPG|thumb|center|x350px|link=Stable_Isotope_Probing_(SIP)|Stable isotope probing (SIP) in use: Loading, deployment and recovery of Bio-Trap® passive sampler with 13C-labeled benzene. Stable isotope probing (SIP) is used to conclusively determine whether in situ biodegradation of a contaminant is occurring]]
[[File:WH Picture10.JPG|thumb|center|x350px|link=1,2,3-Trichloropropane|Summary of anticipated, primary reaction pathways for degradation of 1,2,3-Trichloropropane (TCP). TCP is a man-made chemical that was used in the past primarily as a solvent and extractive agent, a paint and varnish remover, and as a cleaning and degreasing agent]]
[[File:WH Picture11.JPG|thumb|center|x350px|link=Monitored Natural Attenuation (MNA) of Fuels|Distribution of BTEX plume lengths from 604 hydrocarbon sites. Monitored Natural Attenuation (MNA) is one of the most commonly used remediation approaches for groundwater contaminated with petroleum hydrocarbons (PHCs) and certain fuel additives such as fuel oxygenates or lead scavengers]]
[[File:WH Picture12.JPG|thumb|center|x350px|link=Groundwater Sampling - No-Purge/Passive|No-purge and passive sampling methods eliminate the pre-purging step for groundwater sample collection and represent alternatives to conventional sampling methods that rely on low-flow purging of a well prior to collection. The Snap SamplerTM is an example of a passive grab sampler]]
[[File:WH Picture13.JPG|thumb|center|x350px|link=Natural Source Zone Depletion (NSZD)|Conceptualization of Vapor Transport-related Natural Source Zone Depletion (NSZD) processes at a Petroleum Release Site]]
[[File:WH Picture14.JPG|thumb|center|x350px|link=Soil Vapor Extraction (SVE)|Conceptual diagram of basic Soil Vapor Extraction (SVE) system for vadose zone remediation. (SVE) is a common and typically effective physical treatment process for remediation of volatile contaminants in vadose zone (unsaturated) soils]]
[[File:WH Picture15.JPG|thumb|center|x350px|link=Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation|Emulsified Vegetable Oil (EVO) mixed in field during early pilot test. EVO is commonly added as a slowly fermentable substrate to stimulate the in situ anaerobic bioremediation of chlorinated solvents, explosives, perchlorate, chromate, and other contaminants]]
[[File:WH Picture16.JPG|thumb|center|x350px|link=Vapor_Intrusion_(VI)|Key elements of vapor intrusion pathways]]
[[File:WH Picture17.JPG|thumb|center|x350px|link=Sorption_of_Organic_Contaminants|Batch reactor experiments to generate points on a sorption isotherm]]
[[File:WH Picture18.JPG|thumb|center|x350px|link=Metagenomics|Results for metagenomic analysis of a groundwater sample obtained from a site impacted with petroleum hydrocarbons]]
[[File:WH Picture19.JPG|thumb|center|x350px|link=Perchlorate|Perchlorate releases and drinking water detections]]
[[File:WH Picture20.JPG|thumb|center|x350px|link=Mass_Flux_and_Mass_Discharge|Data input screen for ESTCP Mass Flux Toolkit]]
[[File:WH Picture21.JPG|thumb|center|x350px|link=Bioremediation_-_Anaerobic_Design_Considerations|Amendment addition for biobarrier]]
[[File:WH Picture22.JPG|thumb|center|x350px|link=Thermal Conduction Heating (TCH)|Thermal Remediation - Desorption schematic]]
[[File:WH_Picture23.jpg|thumb|center|x350px|link=Contaminated_Sediments_-_Introduction |Key exposure pathways for human health risk from contaminated sediments]]
[[File:WH_Picture24.jpg|thumb|center|x350px|link=Perfluoroalkyl_and_Polyfluoroalkyl_Substances_(PFAS)| The PFAS family of compounds]]

</slideshow>
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| style="padding:2px;" |<h2 id="mp-tfa-h2_2" style="margin:3px; background:#cef2e0; font-family:inherit; font-size:120%; font-weight:bold; border:1px solid #a3bfb1; text-align:center; color:#000; padding:0.2em 0.4em;">Table of Contents </h2>
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'''[[Transport & Attenuation Processes | Attenuation & Transport Processes]]'''

*[[Biodegradation - 1,4-Dioxane]]
*[[Biodegradation - Cometabolic]]
*[[Biodegradation - Hydrocarbons]]
*[[Biodegradation - Reductive Processes]]
*[[Groundwater Flow and Solute Transport]]
*[[Matrix Diffusion]]
*[[Metals and Metalloids - Mobility in Groundwater | Mobility of Metals and Metalloids]]
*[[pH Buffering in Aquifers]]
*[[Sorption of Organic Contaminants]]
*[[Vapor Intrusion (VI)]]
**[[Vapor Intrusion - Separation Distances from Petroleum Sources]]
**[[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways|Vapor Intrusion - Sewers and Utility Tunnels as Preferential Pathways]]
**[[Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)|Vapor Intrusion - Assessing VI Impacts in Neighborhoods with Groundwater Contaminated CVOCs]]

'''[[Characterization, Assessment & Monitoring]]'''

*[[Characterization Methods – Hydraulic Conductivity]]
*[[Compound Specific Isotope Analysis (CSIA)|Compound Specific Isotope Analysis (CSIA)]]
*[[Direct Push (DP) Technology]]
**[[Direct Push Logging |Direct Push Logging]]
**[[Direct Push Sampling |Direct Push Sampling]]
*[[Geophysical Methods | Geophysical Methods]]
**[[Geophysical Methods - Case Studies |Case Studies]]
**[[Hydrogeophysical Methods for Characterization and Monitoring of Groundwater-Surface Water Exchanges]]
*[[Groundwater Sampling - No-Purge/Passive]]
*[[Long-Term Monitoring (LTM)|Long-Term Monitoring (LTM)]]
**[[Long-Term Monitoring (LTM) - Data Analysis |LTM Data Analysis]]
**[[Long-Term Monitoring (LTM) - Data Variability |LTM Data Variability]]
*[[Molecular Biological Tools - MBTs |Molecular Biological Tools (MBTs)]]
**[[Metagenomics]]
**[[Proteomics and Proteogenomics]]
**[[Quantitative Polymerase Chain Reaction (qPCR)]]
**[[Stable Isotope Probing (SIP)]]
*[[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill |Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
*[[OPTically-based In-situ Characterization System (OPTICS)]]

'''[[Coastal and Estuarine Ecology]]'''

*[[Phytoplankton (Algae) Blooms]]

'''[[Contaminated Sediments - Introduction | Contaminated Sediments]]'''

*[[Contaminated Sediment Risk Assessment]]
*[[In Situ Toxicity Identification Evaluation (iTIE) | In Situ Toxicity Identification Evaluation]]
*[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
*[[Mercury in Sediments]]
*[[Passive Sampling of Sediments]]
**[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]
*[[Sediment Capping]]

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'''[[Light Non-Aqueous Phase Liquids (LNAPLs)]]'''

*[[LNAPL Conceptual Site Models]]
*[[LNAPL Remediation Technologies]]
*[[NAPL Mobility]]

'''[[Munitions Constituents]]'''

*[[Munitions Constituents - Abiotic Reduction|Abiotic Reduction]]
*[[Munitions Constituents - Alkaline Degradation|Alkaline Degradation]]
**[[Pyrogenic Carbonaceous Matter Enhanced Alkaline Hydrolysis]]
*[[Munitions Constituents - Composting|Composting]]
*[[Munitions Constituents - Deposition |Deposition]]
*[[Munitions Constituents - Dissolution |Dissolution]]
*[[Munitions Constituents - Electrochemical Treatment|Electrochemical Treatment]]
*[[Metal(loid)s - Small Arms Ranges]]
*[[Passive Sampling of Munitions Constituents|Passive Sampling]]
*[[Munitions Constituents – Photolysis |Photolysis]]
*[[Remediation of Stormwater Runoff Contaminated by Munition Constituents |Remediation of Stormwater Runoff ]]
*[[Munitions Constituents – Sample Extraction and Analytical Techniques|Sample Extraction and Analytical Techniques]]
*[[Munitions Constituents - Soil Sampling |Soil Sampling]]
*[[Munitions Constituents - Sorption |Sorption]]
*[[Munitions Constituents - IM Toxicology |Toxicology]]
*[[Munitions Constituents- TREECS™ Fate and Risk Modeling|TREECS™]]

'''[[Monitored Natural Attenuation (MNA)]]'''

*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents| MNA of Chlorinated Solvents]]
**[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Monitored Natural Attenuation (MNA) of Fuels| MNA of Fuels]]
*[[Monitored Natural Attenuation (MNA) of Metal and Metalloids| MNA of Metals and Metalloids]]
*[[Natural Source Zone Depletion (NSZD)]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies| Transitioning from Active Remedies]]

'''[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]'''

*[[Hydrothermal Alkaline Treatment (HALT)]]
*[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
*[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]
*[[PFAS Ex Situ Water Treatment]]
**[[PFAS Treatment by Anion Exchange]]
*[[PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)]]
*[[PFAS Soil Remediation Technologies]]
*[[PFAS Sources]]
*[[PFAS Toxicology and Risk Assessment]]
*[[PFAS Transport and Fate]]
*[[PFAS Treatment by Electrical Discharge Plasma]]
*[[Photoactivated Reductive Defluorination - PFAS Destruction | Photoactivated Reductive Defluorination]]
*[[Reverse Osmosis and Nanofiltration Membrane Filtration Systems for PFAS Removal]]
*[[Thermal Conduction Heating for Treatment of PFAS-Impacted Soil]]
*[[Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)| Transition of Aqueous Film Forming Foam Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances]]

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'''[[Regulatory Issues and Site Management]]'''

*[[Alternative Endpoints]]
*[[Mass Flux and Mass Discharge]]
*[[Plume Response Modeling]]
*[[REMChlor - MD | REMChlor-MD]]
*[[Source Zone Modeling]]
*[[Sustainable Remediation]]

'''[[Remediation Technologies]]'''
*[[Amendment Distribution in Low Conductivity Materials]]
*[[Bioremediation - Anaerobic|Anaerobic Bioremediation]]
**[[Bioremediation - Anaerobic Design Considerations | Design Considerations]]
**[[Design Tool - Base Addition for ERD]]
**[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
**[[Low pH Inhibition of Reductive Dechlorination]]
**[[Bioremediation - Anaerobic Secondary Water Quality Impacts | Secondary Water Quality Impacts]]
*[[Chemical Oxidation (In Situ - ISCO) | In Situ Chemical Oxidation (ISCO)]]
**[[Chemical Oxidation Design Considerations(In Situ - ISCO) | Design Considerations]]
**[[Chemical Oxidation Oxidant Selection (In Situ - ISCO) | Oxidant Selection]]
*[[Chemical Reduction (In Situ - ISCR) | In Situ Chemical Reduction (ISCR)]]
**[[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR) | Zero-Valent Iron (ZVI)]]
**[[Zerovalent Iron Permeable Reactive Barriers]]
*[[In Situ Groundwater Treatment with Activated Carbon]]
*[[Injection Techniques for Liquid Amendments]]
*[[Injection Techniques - Viscosity Modification]]
*[[Landfarming]]
*[[Metal and Metalloids - Remediation | Remediation of Metals and Metalloids]]
*[[Remediation Performance Assessment at Chlorinated Solvent Sites]]
*[[Soil Vapor Extraction (SVE)]]
*[[Stream Restoration]]
*[[Subgrade Biogeochemical Reactor (SBGR)]]
*[[Supercritical Water Oxidation (SCWO)]]
*[[Thermal Remediation]]
**[[Thermal Remediation - Combined Remedies | Combined Remedies]]
**[[Thermal Remediation - Electrical Resistance Heating | Electrical Resistance Heating (ERH)]]
**[[Thermal Remediation - Smoldering | Smoldering]]
**[[Thermal Remediation - Steam | Steam Enhanced Extraction (SEE)]]
**[[Thermal Conduction Heating (TCH)]]

'''[[Soil & Groundwater Contaminants]]'''

*[[1,2,3-Trichloropropane]]
*[[1,4-Dioxane]]
*[[Chlorinated Solvents]]
*[[Metal and Metalloid Contaminants|Metals and Metalloids]]
*[[N-nitrosodimethylamine (NDMA)]]
*[[Perchlorate|Perchlorate]]
*[[Petroleum Hydrocarbons (PHCs)]]
*[[Polycyclic Aromatic Hydrocarbons (PAHs)]]
|}
|}
|}

Polycyclic Aromatic Hydrocarbons (PAHs)

2026-04-28T20:19:40Z

Admin:

Polycyclic aromatic hydrocarbons (PAHs) are a class of organic compounds that consist solely of carbon and hydrogen atoms in aromatic ring structures. Sixteen PAHs are regulated by the U.S. Environmental Protection Agency (USEPA) based on their potential human and ecological health effects. These compounds can be naturally occurring (e.g., forest fires) or anthropogenic (e.g., coal gasification, automobile exhaust). [[Remediation Technologies | Remedial techniques]] are available for addressing PAH-contaminated soil, groundwater, and surface waters. However, such efforts must carefully consider the hydrophobic nature of PAHs, effects of PAH weathering in soil/sediment, and the poor biodegradability of high-molecular weight PAHs. Bioavailability of PAHs is also a key consideration for health and ecological risk assessment and selection of remedial techniques. Here, we review the physical and chemical properties of PAHs, their toxicity and rationale as priority pollutants, remedial options, and risk assessment considerations.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
'''Related Article(s):'''

*[[Biodegradation - Hydrocarbons]]
*[[Monitored Natural Attenuation (MNA) of Fuels]]
*[[Petroleum Hydrocarbons (PHCs)]]
*[[Remediation Technologies]]

'''Contributor(s):''' [[Dr. Stephen Richardson]]

'''Key Resource(s):'''

*[http://www.cambridge.org/us/academic/subjects/medicine/oncology/polycyclic-aromatic-hydrocarbons-chemistry-and-carcinogenicity Polycyclic aromatic hydrocarbons chemistry and carcinogenicity]<ref name="Harvey1991">Harvey, R.G., 1991. Polycyclic aromatic hydrocarbons: chemistry and carcinogenicity. Cambridge University Press: Cambridge, 396 pgs. ISBN 978-0521292047</ref>
*[//www.enviro.wiki/images/2/25/USEPA-1993-Provisional_Guidance_for_quantitative_Risk_Assessment....pdf Provisional guidance for quantitative risk assessment of polycyclic aromatic hydrocarbons]<ref name="USEPA1993">U.S. Environmental Protection Agency, 1993. Provisional guidance for quantitative risk assessment of polycyclic aromatic hydrocarbons. EPA 600-R-93-089. [//www.enviro.wiki/images/2/25/USEPA-1993-Provisional_Guidance_for_quantitative_Risk_Assessment....pdf Report pdf]</ref>

==Introduction==
[[wikipedia: Polycyclic aromatic hydrocarbon | Polycyclic aromatic hydrocarbons (PAHs)]], also known as polyaromatic hydrocarbons, are a class of hundreds of organic compounds that consist of two or more aromatic rings fused in linear, angular, or clustered arrangements<ref name="Harvey1991" />. PAHs are ubiquitous in the environment, predominantly formed by the incomplete combustion of organic materials from both natural sources (e.g., forest fires, volcanic events), and anthropogenic activities (e.g., coal gasification, automobile exhaust, incinerators, coke production, cooking, tobacco smoke)<ref name="Harvey1991" />. The USEPA has listed 16 PAHs as [https://www.epa.gov/eg/toxic-and-priority-pollutants-under-clean-water-act 'priority pollutants'] in aquatic and terrestrial ecosystems<ref>Keith, L. and Telliard, W., 1979. ES&T special report: priority pollutants: I-a perspective view. Environmental Science & Technology, 13(4), 416-423. [http://dx.doi.org/10.1021/es60152a601 doi: 10.1021/es60152a601]</ref>. Seven of these PAHs may cause cancer in humans<ref name="USEPA1993" />, and benzo[a]pyrene is considered the highest cancer risk amongst the 16 PAHs<ref name="USEPA1993" /><ref name="LaGoy1994">LaGoy, P.K., Quirk, T.C., 1994. Establishing generic remediation goals for the polycyclic aromatic hydrocarbons: critical issues. Environmental Health Perspectives, 102(4), 348-352. [//www.enviro.wiki/images/c/c0/LaGoy-1994-Establishing_generic_remediation_goals_for_the_plycyclic_aromatic_hydrocarbons.pdf Report pdf]</ref>.

==Physical and Chemical Properties==
PAHs are [[wikipedia: Hydrophobe | hydrophobic]] and do not readily dissolve in water or volatilize to the atmosphere (with the exception of [[wikipedia: Naphthalene | naphthalene]], which was once used in 'moth balls'). The chemical stability, low water solubility, and high sorption capacity of PAHs contribute greatly to their persistence in the environment<ref>Kanaly, R.A., Harayama, S., 2000. Biodegradation of high-molecular-weight polycyclic aromatic hydrocarbons by bacteria. Journal of Bacteriology, 182(8), 2059-2067. [http://dx.doi.org/10.1128/jb.182.8.2059-2067.2000 doi: 10.1128/JB.182.8.2059-2067.2000]</ref><ref>Van Hamme, J.D., Singh, A., Ward, O.P., 2003. Recent advances in petroleum microbiology. Microbiology and Molecular Biology Reviews, 67(4), 503-549. [http://dx.doi.org/10.1128/mmbr.67.4.503-549.2003 doi: 10.1128/MMBR.67.4.503-549.2003]</ref>. PAHs can be divided into two categories: '''(1) low molecular weight PAHs''' composed of less than four aromatic rings (e.g., naphthalene, acenaphthene, fluorene, phenanthrene), and '''(2) high molecular weight PAHs''' composed of four or more rings (e.g., pyrene, chrysene, benzo[a]pyrene, dibenz[a,h]anthracene). High molecular weight PAHs are generally less water soluble, have lower vapor pressures and Henry’s constants, and partition more readily into organic matter than low molecular weight PAHs. Selected physical and chemical properties of the 16 USEPA-regulated PAHs are presented in Table 1.

[[File:Richardson-Article 1-Table 1.JPG|thumbnail|600px|center|Table 1. Chemical structures and selected properties of the 16 USEPA priority pollutant PAHsab. aabbreviations: MW = molecular weight (g/mol); Cwsat = aqueous solubility (mg/L); p* = vapor pressure (mm Hg); Kow = octanol-water partitioning coefficient; Koc = organic carbon partitioning coefficient; TEF = toxic equivalency factor; PLHS = Priority List of Hazardous Substances. bAll data are from<ref>LaGrega, M.D., Buckingham, P.L., Evans, J.C., 2001. Hazardous waste management: 2nd edition. McGraw-Hill, Boston. ISBN 1577666933.</ref> unless otherwise noted; cdata from<ref name="NRC2003" />; ddata from<ref>Mackay, D., Shiu, W.Y., Ma, K.C., 1997. Illustrated handbook of physical-chemical properties of environmental fate for organic chemicals.</ref>.]]

==Toxicity==
The most important property driving PAH remediation is their toxicity (or carcinogenicity). PAHs ranked 9th on the 2015 Agency for Toxic Substances and Disease Registry (ATSDR) Priority List of Hazardous Substances (PLHS) based on their toxicity, frequency of occurrence at USEPA National Priorities List (i.e. Superfund) sites, and potential for human exposure<ref>ATSDR, 2015. Comprehensive environmental response, compensation, and liability act (CERCLA) priority list of hazardous substances. [http://www.atsdr.cdc.gov/spl List]</ref>. Individually, all 16 regulated PAHs are included on the ATSDR PLHS, with six ranked in the top 100; most notably, benzo[a]pyrene at 8th (Table 1).

Regulatory guidelines or site-specific cleanup goals for soil commonly account for PAH toxicity by assigning toxic equivalency factors (TEFs) to individual PAHs (Table 1), normalized to benzo[a]pyrene toxicity. Multiplying the measured concentration of each PAH by its respective TEF yields an equivalent concentration of benzo[a]pyrene for the PAH mixture, called a benzo[a]pyrene equivalent<ref name="LaGoy1994" />. These adjusted values often serve as soil remediation goals at PAH-contaminated sites. For drinking water, the USEPA has established a maximum contaminant level for benzo[a]pyrene of 0.2 µg/L.

==PAH Bioavailability==
Bioavailability is an important concept for PAH remediation and risk assessment in soil and sediments. With respect to bioremediation, bioavailability refers to the contaminant fraction that can be effectively accessed by contaminant-degrading microbial communities<ref name="NRC2003">National Research Council (US). Committee on Bioavailability of Contaminants in Soils and Sediments, 2003. Bioavailability of contaminants in soils and sediments: Processes, tools, and applications. Washington, DC: National Academies Press., 432 pgs. [http://dx.doi.org/10.17226/10523 doi: 10.17226/10523]</ref><ref>Ortega-Calvo, J.J., Harmsen, J., Parsons, J.R., Semple, K.T., Aitken, M.D., Ajao, C., Eadsforth, C., Galay-Burgos, M., Naidu, R., Oliver, R., Peijnenburg, W.J., Römbke, J., Streck, G., Versonnen, B. 2015. From bioavailability science to regulation of organic chemicals. Environmental Science & Technology, 49(17), 10255-10264. [http://dx.doi.org/10.1021/acs.est.5b02412 doi: 10.1021/acs.est.5b02412]</ref>. Bioavailability and degradation of PAHs in natural soils and sediment are generally two-phased, with an initial phase of rapid PAH removal followed by a longer period of limited PAH reduction. During the initial phase, bioavailability of PAHs is high and degradation rates may be limited by reaction rate (e.g., microbial uptake rather than mass transfer from soil particles). As bioavailable PAHs are removed, mass transfer mechanisms (desorption and diffusion) become controlling factors for the rate of PAH degradation<ref>Bosma, T.N., Middeldorp, P.J., Schraa, G., Zehnder, A.J., 1997. Mass transfer limitation of biotransformation: quantifying bioavailability. Environmental Science & Technology, 31(1), 248-252. [http://dx.doi.org/10.1021/es960383u doi: 10.1021/es960383u]</ref>.
Physical, chemical, and biological remedial methods have been developed to either increase PAH bioavailability (e.g., mixing, surfactants, cosolvents), or decrease PAH bioavailability (e.g., biostabilization, sediment capping, solidification), depending on the treatment design and site cleanup goals<ref>Ehlers, L.J., Luthy, R.G., 2003. Contaminant bioavailability in soil and sediment. Environmental Science & Technology, 37, 295A-302A. [http://dx.doi.org/10.1021/es032524f doi: 10.1021/es032524f]</ref>. The former generally incorporate a mass removal step (e.g., enhanced biodegradation, aqueous phase extraction) to minimize risk. The latter strategies reduce risk by creating a barrier between the contaminations and surrounding receptors.

==Remediation of PAHs==
Anthropogenic sources of PAHs vary widely, including former manufactured gas plants, petroleum fuel spills, coal- and gas-fired power plants, and industrial incinerators, as well as are present in a range of environments (e.g., harbor sediments, marine waters), often proximate to past or current industrial operations. The fate and transport of PAHs in these environments is controlled by PAH hydrophobicity, rates of dissolution, physicochemical properties of the soil, and the source phase. In sediments, for example, PAHs sorb to natural organic matter and are present in oils, tars, residues, and other nonaqueous phase liquids (NAPLs) deposited from industrial activities. These phases act as long-term sources of PAHs to the water column through dissolution processes and can greatly influence overall PAH transport, degradation, and bioavailability<ref>Wick, A.F., Haus, N.W., Sukkariyah, B.F., Haering, K.C., Daniels, W.L., 2011. Remediation of PAH-contaminated soils and sediments: a literature review. Virginia Polytechnic Institute and State University, USA. [//www.enviro.wiki/images/7/7f/Wick-2011-Virginia_Tech_PAH_Remediation_Lit_Review.pdf Report pdf]</ref>.
A variety of ''ex situ'' and ''in situ'' remediation methods have been used to address PAH contaminated soils, groundwater, and surface waters. The most common ''ex situ'' practices include landfill disposal, incineration, [[Thermal Remediation | thermal desorption]], and soil washing<ref>U.S. Environmental Protection Agency, 2004. Cleaning up the nation's waste sites: markets and technology trends. EPA 542-R-04-015. [//www.enviro.wiki/images/8/88/USEPA-2004-Cleaning_Up_the_Nations_Waste_Sites.pdf Report pdf]</ref>. Generally, these methods are expensive and can be cost-prohibitive for sites with large footprints, significant depth of contamination, and existing infrastructure. Alternatively, ''in situ'' treatments such as chemical oxidation, solvent and surfactant flushing, and bioremediation are available, although they are used to a lesser extent than ''ex situ'' methods. These remediation methods are generally less expensive, but require longer treatment times to meet regulatory criteria.

Physical treatments such as in situ stabilization and solidification prevent or minimize the release (or leaching) of PAHs from contaminated soils and sediments by using binding agents (e.g., cement, asphalt, fly ash, and clay) to limit water infiltration and bind PAHs into less mobile forms. The addition of granular activated carbon to contaminated sediments has also been tested as a means to strongly bind available PAHs and reduce the ecological risks to overlying surface waters<ref>Luthy, R.G., Zimmerman, J.R., McLeod, P.B., Zare, R.N., Mahajan, T., Ghosh, U., Bridges, T.S., Millward, R.N., Talley, J.W., 2004. In situ stabilization of persistent organic contaminants in Marine Sediments. Strategic Environmental Research and Development Program, Arlington, Virginia. SERDP Project ER-1207. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Sediments/ER-1207 ER-1207]</ref><ref>Luthy, R.G., Zimmerman, J.R., McLeod, P.B., Zare, R.N., Mahajan, T., Ghosh, U., Bridges, T.S., Millward, R.N., Talley, J.W., 2014. Demonstration of in situ treatment with reactive amendments for contaminated sediments in active DoD harbors. Project ER-201131. Strategic Environmental Research and Development Program, Arlington, Virginia. [https://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Sediments/ER-201131/ER-201131 ER-201131]</ref>.

Chemical treatments for PAHs include Fenton’s reagent, hydrogen peroxide, activated persulfate, and ozone<ref name="Huling2006">Huling, S. G., Pivetz, B. E., 2006. In-situ chemical oxidation (No. EPA/600/R-06/072). Environmental Protection Agency Washington DC Office of Water. [//www.enviro.wiki/images/7/79/Huling-EPA-ISCO.pdf Report pdf]</ref>. These methods generate very reactive free radicals (e.g., hydroxyl radical, sulfate radical, ozone radical) and other reactive species (e.g., persulfate anion, peroxides), capable of attacking the aromatic structure of PAHs. Advantages of in situ chemical oxidation over conventional remediation methods include reasonable treatment times, reactivity with a broad range of PAHs, and destruction of contaminants in situ. However, the use of chemical oxidants is complicated by oxidation of non-target species such as soil organic matter and limited control of oxidant delivery in heterogeneous media<ref name="Huling2006" />.
Biodegradation of PAHs can occur both aerobically and anaerobically in the subsurface. Numerous aerobic bacterial species and fungi are capable of transforming two-, three-, and four-ring PAHs to non-toxic end products such as water and carbon dioxide (i.e., PAH mineralization) and partially degrading five- and six-ring PAHs to intermediate compounds<ref>Cerniglia, C.E., 1992. Biodegradation of polycyclic aromatic hydrocarbons. Biodegradation, 3(2-3), 351-368. [http://dx.doi.org/10.1007/bf00129093 doi: 10.1007/BF00129093]</ref>. Under anaerobic conditions, degradation of two- and three-ring PAHs has been documented under nitrate<ref>Mihelcic, J.R., Luthy, R.G., 1988. Degradation of polycyclic aromatic hydrocarbon compounds under various redox conditions in soil-water systems. Applied and Environmental Microbiology, 54(5), 1182-1187. [http://aem.asm.org/content/54/5/1182.short Journal Article Page]</ref><ref>McNally, D.L., Mihelcic, J.R., Lueking, D.R., 1998. Biodegradation of three-and four-ring polycyclic aromatic hydrocarbons under aerobic and denitrifying conditions. Environmental Science & Technology, 32(17), 2633-2639. [http://dx.doi.org/10.1021/es980006c doi: 10.1021/es980006c]</ref>, iron<ref>Anderson, R.T., Lovley, D.R., 1999. Naphthalene and benzene degradation under Fe(III)-reducing conditions in petroleum-contaminated aquifers. Bioremediation Journal, 3(2), 121-135. [http://dx.doi.org/10.1080/10889869991219271 doi: 10.1080/10889869991219271]</ref>, and sulfate-reducing conditions<ref>Coates, J.D., Anderson, R.T., Woodward, J.C., Phillips, E.J., Lovley, D.R., 1996. Anaerobic hydrocarbon degradation in petroleum-contaminated harbor sediments under sulfate-reducing and artificially imposed iron-reducing conditions. Environmental Science & Technology, 30(9), 2784-2789. [http://dx.doi.org/10.1021/es9600441 doi: 10.1021/es9600441]</ref><ref>Coates, J.D., Anderson, R.T., Lovley, D.R., 1996. Oxidation of polycyclic aromatic hydrocarbons under sulfate-reducing conditions. Applied and Environmental Microbiology, 62(3), 1099-1101. [http://aem.asm.org/content/62/3/1099.short Journal Article]</ref>. However, rates of anaerobic PAH biodegradation are generally much lower (several orders of magnitude) than aerobic metabolism. Since many contaminated sites are oxygen- and nutrient-limited, a variety of biostimulation methods (e.g., can be composting, landfarming, biosparging, peroxide injection) can be used to deliver oxygen/nutrients into groundwater and soil/sediments to stimulate aerobic degradation of PAHs<ref>Mueller, J.G., Chapman, P.J., Pritchard, P.H., 1989. Creosote-contaminated sites. Their potential for bioremediation. Environmental Science & Technology, 23(10), 1197-1201. [http://dx.doi.org/10.1021/es00068a003 doi: 10.1021/es00068a003]</ref><ref>Johnson, C.R., Scow, K.M., 1999. Effect of nitrogen and phosphorus addition on phenanthrene biodegradation in four soils. Biodegradation, 10(1), 43-50. [http://dx.doi.org/10.1023/a:1008359606545 doi: 10.1023/A:1008359606545]</ref><ref>Breedveld, G.D., Sparrevik, M., 2000. Nutrient-limited biodegradation of PAH in various soil strata at a creosote contaminated site. Biodegradation, 11(6), pp.391-399. [http://dx.doi.org/10.1023/a:1011695023196 doi: 10.1023/A:1011695023196]</ref><ref>Carmichael, L.M., Pfaender, F.K., 1997. The effect of inorganic and organic supplements on the microbial degradation of phenanthrene and pyrene in soils. Biodegradation, 8(1), 1-13. [http://dx.doi.org/10.1023/a:1008258720649 doi: 10.1023/A:1008258720649]</ref>.

==References==

<references />

==See Also==

Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions

2026-04-28T20:19:15Z

Admin:

The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective ''in situ'' approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA)]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
*[[Matrix Diffusion]]
*[[REMChlor - MD]]

'''Contributor(s):''' Dani Tran, [[Dr. Charles Schaefer]], Dr. Charles Werth

'''Key Resource(s):'''
*Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils<ref name="SchaeferEtAl2025"/>

==Introduction==
Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.

For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as [[REMChlor - MD]]<ref>Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]  [[Media: FaltaWang2017.pdf | Open Access Manuscript]]</ref><ref>Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. [https://doi.org/10.1016/j.jconhyd.2022.103986 doi: 10.1016/j.jconhyd.2022.103986]  [[Media: KulkarniEtAl2022.pdf | Open Access Manuscript]]</ref> to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.

==Recommended Approach==
[[File: TranFig1.png | thumb | 500 px | Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions. Circles are data from Schaefer ''et al.'', 2021<ref>Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2021. Abiotic dechlorination in the presence of ferrous minerals. Journal of Contaminant Hydrology, 241, 103839. [https://doi.org/10.1016/j.jconhyd.2021.103839 doi: 10.1016/j.jconhyd.2021.103839]  [[Media: SchaeferEtAl2021.pdf | Open Access Manuscript]]</ref>, filled squares from Schaefer ''et al.'', 2018<ref name="SchaeferEtAl2018"/>, and Schaefer ''et al.'', 2017<ref>Schaefer, C.E., Ho., Gurr, C., Berns, E., Werth, C., 2017. Abiotic dechlorination of chlorinated ethenes in natural clayey soils: impacts of mineralogy and temperature. Journal of Contaminant Hydrology, 206, pp. 10-17. [https://doi.org/10.1016/j.jconhyd.2017.09.007 doi: 10.1016/j.jconhyd.2017.09.007]  [[Media: SchaeferEtAl2017.pdf | Open Access Manuscript]]</ref>, and open squares from Schaefer ''et al.'', 2025<ref name="SchaeferEtAl2025"/>. ]]
[[File: TranFig2.png | thumb | 600 px | Figure 2: Flowchart diagram of field screening procedures]]
The recommended approach builds upon the methodology and findings of a recent study<ref name="SchaeferEtAl2025">Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. [https://doi.org/10.1111/gwmr.12709 doi: 10.1111/gwmr.12709]</ref>, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:
#Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
#Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
#Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
#Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)r) is estimated as shown in Equation 1:

::'''Equation 1:'''       <big>''Fe(II)r = DA + XRDpyr - XRDbiotite''</big>

where ''DA'' is the ferrous content from the dilute acid (1% HCl) extraction, ''XRDpyr'' is the pyrite content from XRD analysis, and ''XRDbiotite'' is the biotite content from XRD analysis<ref name="SchaeferEtAl2025"/>.

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1<ref name="SchaeferEtAl2018">Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. [https://doi.org/10.1021/acs.est.8b04108 doi: 10.1021/acs.est.8b04108]</ref><ref>Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. [https://doi.org/10.1016/j.jconhyd.2021.103889 doi: 10.1016/j.jconhyd.2021]  [[Media: BordenCha2021.pdf | Open Access Manuscript]]</ref>. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s ''r'' = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

If Fe(II)r is ≥ 100 mg/kg, a first-order dechlorination rate constant can be estimated and subsequently used within a contaminant fate and transport model. However, if acetylene is detected in the clay, even with Fe(II)r less than 100 mg/kg, then bench-scale testing using methods similar to those described in a recent study<ref name="SchaeferEtAl2025"/> is recommended, as such results would likely be inconsistent with those shown in Figure 1, suggesting some other mechanism might be involved, or that the system mineralogy might be more complex than anticipated. Even if Fe(II)r ≥ 100 mg/kg, confirmatory bench-scale testing may be conducted for additional verification and to refine estimation of the abiotic dechlorination rate constant.

==Summary and Recommendations==
The approach outlined above is intended to serve as a generalized guide for practitioners and site managers to cost-effectively determine the extent to which beneficial abiotic reductive dechlorination reactions are likely occurring in low permeability (e.g., clayey) zones. This approach may be contraindicated if co-contaminants are present. It is currently unclear whether other classes of potentially reactive chemicals, such as trinitrotoluene (TNT) or chlorinated ethanes, could interact competitively with PCE and TCE.

In addition, it remains unclear how other classes of compounds such as per- and polyfluoroalkyl substances (PFAS) may interact or sorb with ferrous minerals and potentially inhibit abiotic dechlorination reactions. Coupling these recommended activities with conventional site investigation tasks would provide an opportunity to perform many of the up-front screening activities with minimal additional project costs. It is important to note that the guidance proposed herein pertains to particularly low permeability media. Sites with complex or varying lithology, where the mineralogy and/or redox conditions may vary, might require evaluation of multiple samples to provide appropriate site-wide information.
 

==References==
<references />

==See Also==
*[https://serdp-estcp.mil/projects/details/a7e3f7b5-ed82-4591-adaa-6196ff33dd60 ESTCP Project ER20-5031 – In Situ Verification and Quantification of Naturally Occurring Dechlorination Rates in Clays: Demonstrating Processes that Mitigate Back-Diffusion and Plume Persistence]

Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions

2026-04-28T20:18:23Z

Admin:

The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective ''in situ'' approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA)]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
*[[Matrix Diffusion]]
*[[REMChlor - MD]]

'''Contributor(s):''' Dani Tran, [[Dr. Charles Schaefer]], Dr. Charles Werth

'''Key Resource (s):'''
*Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils<ref name="SchaeferEtAl2025"/>

==Introduction==
Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.

For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as [[REMChlor - MD]]<ref>Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]  [[Media: FaltaWang2017.pdf | Open Access Manuscript]]</ref><ref>Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. [https://doi.org/10.1016/j.jconhyd.2022.103986 doi: 10.1016/j.jconhyd.2022.103986]  [[Media: KulkarniEtAl2022.pdf | Open Access Manuscript]]</ref> to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.

==Recommended Approach==
[[File: TranFig1.png | thumb | 500 px | Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions. Circles are data from Schaefer ''et al.'', 2021<ref>Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2021. Abiotic dechlorination in the presence of ferrous minerals. Journal of Contaminant Hydrology, 241, 103839. [https://doi.org/10.1016/j.jconhyd.2021.103839 doi: 10.1016/j.jconhyd.2021.103839]  [[Media: SchaeferEtAl2021.pdf | Open Access Manuscript]]</ref>, filled squares from Schaefer ''et al.'', 2018<ref name="SchaeferEtAl2018"/>, and Schaefer ''et al.'', 2017<ref>Schaefer, C.E., Ho., Gurr, C., Berns, E., Werth, C., 2017. Abiotic dechlorination of chlorinated ethenes in natural clayey soils: impacts of mineralogy and temperature. Journal of Contaminant Hydrology, 206, pp. 10-17. [https://doi.org/10.1016/j.jconhyd.2017.09.007 doi: 10.1016/j.jconhyd.2017.09.007]  [[Media: SchaeferEtAl2017.pdf | Open Access Manuscript]]</ref>, and open squares from Schaefer ''et al.'', 2025<ref name="SchaeferEtAl2025"/>. ]]
[[File: TranFig2.png | thumb | 600 px | Figure 2: Flowchart diagram of field screening procedures]]
The recommended approach builds upon the methodology and findings of a recent study<ref name="SchaeferEtAl2025">Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. [https://doi.org/10.1111/gwmr.12709 doi: 10.1111/gwmr.12709]</ref>, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:
#Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
#Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
#Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
#Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)r) is estimated as shown in Equation 1:

::'''Equation 1:'''       <big>''Fe(II)r = DA + XRDpyr - XRDbiotite''</big>

where ''DA'' is the ferrous content from the dilute acid (1% HCl) extraction, ''XRDpyr'' is the pyrite content from XRD analysis, and ''XRDbiotite'' is the biotite content from XRD analysis<ref name="SchaeferEtAl2025"/>.

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1<ref name="SchaeferEtAl2018">Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. [https://doi.org/10.1021/acs.est.8b04108 doi: 10.1021/acs.est.8b04108]</ref><ref>Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. [https://doi.org/10.1016/j.jconhyd.2021.103889 doi: 10.1016/j.jconhyd.2021]  [[Media: BordenCha2021.pdf | Open Access Manuscript]]</ref>. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s ''r'' = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

If Fe(II)r is ≥ 100 mg/kg, a first-order dechlorination rate constant can be estimated and subsequently used within a contaminant fate and transport model. However, if acetylene is detected in the clay, even with Fe(II)r less than 100 mg/kg, then bench-scale testing using methods similar to those described in a recent study<ref name="SchaeferEtAl2025"/> is recommended, as such results would likely be inconsistent with those shown in Figure 1, suggesting some other mechanism might be involved, or that the system mineralogy might be more complex than anticipated. Even if Fe(II)r ≥ 100 mg/kg, confirmatory bench-scale testing may be conducted for additional verification and to refine estimation of the abiotic dechlorination rate constant.

==Summary and Recommendations==
The approach outlined above is intended to serve as a generalized guide for practitioners and site managers to cost-effectively determine the extent to which beneficial abiotic reductive dechlorination reactions are likely occurring in low permeability (e.g., clayey) zones. This approach may be contraindicated if co-contaminants are present. It is currently unclear whether other classes of potentially reactive chemicals, such as trinitrotoluene (TNT) or chlorinated ethanes, could interact competitively with PCE and TCE.

In addition, it remains unclear how other classes of compounds such as per- and polyfluoroalkyl substances (PFAS) may interact or sorb with ferrous minerals and potentially inhibit abiotic dechlorination reactions. Coupling these recommended activities with conventional site investigation tasks would provide an opportunity to perform many of the up-front screening activities with minimal additional project costs. It is important to note that the guidance proposed herein pertains to particularly low permeability media. Sites with complex or varying lithology, where the mineralogy and/or redox conditions may vary, might require evaluation of multiple samples to provide appropriate site-wide information.
 

==References==
<references />

==See Also==
*[https://serdp-estcp.mil/projects/details/a7e3f7b5-ed82-4591-adaa-6196ff33dd60 ESTCP Project ER20-5031 – In Situ Verification and Quantification of Naturally Occurring Dechlorination Rates in Clays: Demonstrating Processes that Mitigate Back-Diffusion and Plume Persistence]

Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions

2026-04-28T20:17:57Z

Admin:

The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective ''in situ'' approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Chlorinated Solvents]]
*[[Monitored Natural Attenuation (MNA)]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
*[[Matrix Diffusion]]
*[[REMChlor - MD]]

'''Contributors:''' Dani Tran, [[Dr. Charles Schaefer]], Dr. Charles Werth

'''Key Resource:'''
*Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils<ref name="SchaeferEtAl2025"/>

==Introduction==
Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.

For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as [[REMChlor - MD]]<ref>Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]  [[Media: FaltaWang2017.pdf | Open Access Manuscript]]</ref><ref>Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. [https://doi.org/10.1016/j.jconhyd.2022.103986 doi: 10.1016/j.jconhyd.2022.103986]  [[Media: KulkarniEtAl2022.pdf | Open Access Manuscript]]</ref> to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.

==Recommended Approach==
[[File: TranFig1.png | thumb | 500 px | Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions. Circles are data from Schaefer ''et al.'', 2021<ref>Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2021. Abiotic dechlorination in the presence of ferrous minerals. Journal of Contaminant Hydrology, 241, 103839. [https://doi.org/10.1016/j.jconhyd.2021.103839 doi: 10.1016/j.jconhyd.2021.103839]  [[Media: SchaeferEtAl2021.pdf | Open Access Manuscript]]</ref>, filled squares from Schaefer ''et al.'', 2018<ref name="SchaeferEtAl2018"/>, and Schaefer ''et al.'', 2017<ref>Schaefer, C.E., Ho., Gurr, C., Berns, E., Werth, C., 2017. Abiotic dechlorination of chlorinated ethenes in natural clayey soils: impacts of mineralogy and temperature. Journal of Contaminant Hydrology, 206, pp. 10-17. [https://doi.org/10.1016/j.jconhyd.2017.09.007 doi: 10.1016/j.jconhyd.2017.09.007]  [[Media: SchaeferEtAl2017.pdf | Open Access Manuscript]]</ref>, and open squares from Schaefer ''et al.'', 2025<ref name="SchaeferEtAl2025"/>. ]]
[[File: TranFig2.png | thumb | 600 px | Figure 2: Flowchart diagram of field screening procedures]]
The recommended approach builds upon the methodology and findings of a recent study<ref name="SchaeferEtAl2025">Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. [https://doi.org/10.1111/gwmr.12709 doi: 10.1111/gwmr.12709]</ref>, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:
#Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
#Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
#Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
#Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)r) is estimated as shown in Equation 1:

::'''Equation 1:'''       <big>''Fe(II)r = DA + XRDpyr - XRDbiotite''</big>

where ''DA'' is the ferrous content from the dilute acid (1% HCl) extraction, ''XRDpyr'' is the pyrite content from XRD analysis, and ''XRDbiotite'' is the biotite content from XRD analysis<ref name="SchaeferEtAl2025"/>.

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1<ref name="SchaeferEtAl2018">Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. [https://doi.org/10.1021/acs.est.8b04108 doi: 10.1021/acs.est.8b04108]</ref><ref>Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. [https://doi.org/10.1016/j.jconhyd.2021.103889 doi: 10.1016/j.jconhyd.2021]  [[Media: BordenCha2021.pdf | Open Access Manuscript]]</ref>. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s ''r'' = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

If Fe(II)r is ≥ 100 mg/kg, a first-order dechlorination rate constant can be estimated and subsequently used within a contaminant fate and transport model. However, if acetylene is detected in the clay, even with Fe(II)r less than 100 mg/kg, then bench-scale testing using methods similar to those described in a recent study<ref name="SchaeferEtAl2025"/> is recommended, as such results would likely be inconsistent with those shown in Figure 1, suggesting some other mechanism might be involved, or that the system mineralogy might be more complex than anticipated. Even if Fe(II)r ≥ 100 mg/kg, confirmatory bench-scale testing may be conducted for additional verification and to refine estimation of the abiotic dechlorination rate constant.

==Summary and Recommendations==
The approach outlined above is intended to serve as a generalized guide for practitioners and site managers to cost-effectively determine the extent to which beneficial abiotic reductive dechlorination reactions are likely occurring in low permeability (e.g., clayey) zones. This approach may be contraindicated if co-contaminants are present. It is currently unclear whether other classes of potentially reactive chemicals, such as trinitrotoluene (TNT) or chlorinated ethanes, could interact competitively with PCE and TCE.

In addition, it remains unclear how other classes of compounds such as per- and polyfluoroalkyl substances (PFAS) may interact or sorb with ferrous minerals and potentially inhibit abiotic dechlorination reactions. Coupling these recommended activities with conventional site investigation tasks would provide an opportunity to perform many of the up-front screening activities with minimal additional project costs. It is important to note that the guidance proposed herein pertains to particularly low permeability media. Sites with complex or varying lithology, where the mineralogy and/or redox conditions may vary, might require evaluation of multiple samples to provide appropriate site-wide information.
 

==References==
<references />

==See Also==
*[https://serdp-estcp.mil/projects/details/a7e3f7b5-ed82-4591-adaa-6196ff33dd60 ESTCP Project ER20-5031 – In Situ Verification and Quantification of Naturally Occurring Dechlorination Rates in Clays: Demonstrating Processes that Mitigate Back-Diffusion and Plume Persistence]

Dr. Charles Schaefer

2026-04-28T20:15:17Z

Admin:

==Work and Contact Information==

EMPLOYER:
:CDM Smith 
:110 Fieldcrest Avenue
:6th Floor 
:Edison, NJ 08837

EMAIL: [mailto:schaeferce@cdmsmith.com schaeferce@cdmsmith.com]

WEBPAGE: https://www.cdmsmith.com/en/experts/charles-schaefer

==About the Contributor==
Charles Schaefer is a chemical engineer with over 25 years of years of experience in laboratory and field evaluations of contaminant transport in subsurface systems and engineered water systems. Dr. Schaefer is the director of CDM Smith’s Research and Testing laboratory located in Bellevue, WA

==Article Contributions==
*[[Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions]]
*[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]

__NOTOC__

[[Category: Contributors|Schaefer]]

Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions

2026-04-28T20:14:28Z

Admin:

The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective ''in situ'' approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

'''Related Article(s):'''

*[[Monitored Natural Attenuation (MNA)]]
*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]
*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
*[[Matrix Diffusion]]
*[[REMChlor - MD]]

'''Contributors:''' Dani Tran, [[Dr. Charles Schaefer]], Dr. Charles Werth

'''Key Resource:'''
*Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils<ref name="SchaeferEtAl2025"/>

==Introduction==
Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.

For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as [[REMChlor - MD]]<ref>Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]  [[Media: FaltaWang2017.pdf | Open Access Manuscript]]</ref><ref>Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. [https://doi.org/10.1016/j.jconhyd.2022.103986 doi: 10.1016/j.jconhyd.2022.103986]  [[Media: KulkarniEtAl2022.pdf | Open Access Manuscript]]</ref> to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.

==Recommended Approach==
[[File: TranFig1.png | thumb | 500 px | Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions. Circles are data from Schaefer ''et al.'', 2021<ref>Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2021. Abiotic dechlorination in the presence of ferrous minerals. Journal of Contaminant Hydrology, 241, 103839. [https://doi.org/10.1016/j.jconhyd.2021.103839 doi: 10.1016/j.jconhyd.2021.103839]  [[Media: SchaeferEtAl2021.pdf | Open Access Manuscript]]</ref>, filled squares from Schaefer ''et al.'', 2018<ref name="SchaeferEtAl2018"/>, and Schaefer ''et al.'', 2017<ref>Schaefer, C.E., Ho., Gurr, C., Berns, E., Werth, C., 2017. Abiotic dechlorination of chlorinated ethenes in natural clayey soils: impacts of mineralogy and temperature. Journal of Contaminant Hydrology, 206, pp. 10-17. [https://doi.org/10.1016/j.jconhyd.2017.09.007 doi: 10.1016/j.jconhyd.2017.09.007]  [[Media: SchaeferEtAl2017.pdf | Open Access Manuscript]]</ref>, and open squares from Schaefer ''et al.'', 2025<ref name="SchaeferEtAl2025"/>. ]]
[[File: TranFig2.png | thumb | 600 px | Figure 2: Flowchart diagram of field screening procedures]]
The recommended approach builds upon the methodology and findings of a recent study<ref name="SchaeferEtAl2025">Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. [https://doi.org/10.1111/gwmr.12709 doi: 10.1111/gwmr.12709]</ref>, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:
#Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
#Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
#Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
#Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)r) is estimated as shown in Equation 1:

::'''Equation 1:'''       <big>''Fe(II)r = DA + XRDpyr - XRDbiotite''</big>

where ''DA'' is the ferrous content from the dilute acid (1% HCl) extraction, ''XRDpyr'' is the pyrite content from XRD analysis, and ''XRDbiotite'' is the biotite content from XRD analysis<ref name="SchaeferEtAl2025"/>.

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1<ref name="SchaeferEtAl2018">Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. [https://doi.org/10.1021/acs.est.8b04108 doi: 10.1021/acs.est.8b04108]</ref><ref>Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. [https://doi.org/10.1016/j.jconhyd.2021.103889 doi: 10.1016/j.jconhyd.2021]  [[Media: BordenCha2021.pdf | Open Access Manuscript]]</ref>. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s ''r'' = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

If Fe(II)r is ≥ 100 mg/kg, a first-order dechlorination rate constant can be estimated and subsequently used within a contaminant fate and transport model. However, if acetylene is detected in the clay, even with Fe(II)r less than 100 mg/kg, then bench-scale testing using methods similar to those described in a recent study<ref name="SchaeferEtAl2025"/> is recommended, as such results would likely be inconsistent with those shown in Figure 1, suggesting some other mechanism might be involved, or that the system mineralogy might be more complex than anticipated. Even if Fe(II)r ≥ 100 mg/kg, confirmatory bench-scale testing may be conducted for additional verification and to refine estimation of the abiotic dechlorination rate constant.

==Summary and Recommendations==
The approach outlined above is intended to serve as a generalized guide for practitioners and site managers to cost-effectively determine the extent to which beneficial abiotic reductive dechlorination reactions are likely occurring in low permeability (e.g., clayey) zones. This approach may be contraindicated if co-contaminants are present. It is currently unclear whether other classes of potentially reactive chemicals, such as trinitrotoluene (TNT) or chlorinated ethanes, could interact competitively with PCE and TCE.

In addition, it remains unclear how other classes of compounds such as per- and polyfluoroalkyl substances (PFAS) may interact or sorb with ferrous minerals and potentially inhibit abiotic dechlorination reactions. Coupling these recommended activities with conventional site investigation tasks would provide an opportunity to perform many of the up-front screening activities with minimal additional project costs. It is important to note that the guidance proposed herein pertains to particularly low permeability media. Sites with complex or varying lithology, where the mineralogy and/or redox conditions may vary, might require evaluation of multiple samples to provide appropriate site-wide information.
 

==References==
<references />

==See Also==
*[https://serdp-estcp.mil/projects/details/a7e3f7b5-ed82-4591-adaa-6196ff33dd60 ESTCP Project ER20-5031 – In Situ Verification and Quantification of Naturally Occurring Dechlorination Rates in Clays: Demonstrating Processes that Mitigate Back-Diffusion and Plume Persistence]

Dr. Stephen Richardson

2026-04-28T20:10:32Z

Admin:

==Work and Contact Information==
EMPLOYER:
:GSI Environmental
:9600 Great Hills Trail, Suite 350E
:Austin, TX 78759

EMAIL: [mailto:sdrichardson@gsi-net.com sdrichardson@gsi-net.com]

WEBPAGE: http://www.gsi-net.com/en/people/employees/stephen-d-richardson-ph-d-p-e.html

==About the Contributor==
Dr. Stephen Richardson is a Vice President and Principal Engineer with GSI Environmental in Austin, Texas. Stephen specializes in the application of innovative strategies to treat conventional and emerging contaminants in soil, groundwater, and surface water at a wide range of contaminated sites. He has served as a Principal Investigator on several DoD-sponsored research projects on cometabolic biodegradation of 1,4-Dioxane, innovative approaches for treatment of chlorinated solvents in low permeability zones, anaerobic bioremediation of DNAPL, and treatment of per- and polyfluoroalkyl substances. Stephen has authored more than 15 peer-reviewed journal articles on ''in situ'' bioremediation, PFAS remediation, chemical oxidation, cosolvent flushing, decentralized water treatment, contaminant bioavailability, and water chemistry in areas of oil and gas development. Stephen is a Licensed Professional Engineer in Texas, Louisiana, North Carolina, and Alberta, Canada and holds a doctoral degree in environmental engineering from the University of North Carolina at Chapel Hill, a master’s degree from Louisiana State University, and a bachelor’s degree from the University of Waterloo.

==Article Contributions==
*[[Polycyclic Aromatic Hydrocarbons (PAHs)]]
*[[Amendment Distribution in Low Conductivity Materials]]
*[[PFAS Treatment by Electrical Discharge Plasma]]

__NOTOC__

[[Category: Contributors|Richardson]]

MediaWiki:Sidebar

2026-04-04T00:48:35Z

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Articles

2026-03-13T21:31:29Z

Admin:

{| class="wikitable sortable"
|-
!Title!!First Author!!Linking Phrases
|-
|[[Groundwater Sampling - No-Purge/Passive]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||passive sampling, no purge sampling, grab samplers, diffusion samplers, sorptive samplers
|-
|[[ Long-Term Monitoring (LTM)]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||long-term monitoring, LTM, LTM objectives, LTM programs, LTM challenges
|-
|[[PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)]]
|[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]
|PFAS, MNA, natural attenuation
|-
|[[Sorption of Organic Contaminants]]||[[Richelle Allen-King|Allen-King, Richelle]]||
|-
|[[PFAS Transport and Fate]]
|[[Dr. Richard Anderson|Anderson, Richard, Ph.D.]]
|PFAS, fate and transport
|-
|[[Mass Flux and Mass Discharge]]||[[Dr. Michael Annable, P.E. |Annable, Michael, Ph.D., P.E.]]||source reduction
|-
|[[PFAS Toxicology and Risk Assessment]]
|[[Jennifer Arblaster|Arblaster, Jennifer]]
|PFAS, toxicology, risk assessment
|-
|[[Metal(loid)s - Small Arms Ranges]]|| Dr. Amanda Barker |[[Dr. Amanda Barker|Barker, Amanda, Ph.D.]]
|
|-
|[[Munitions Constituents – Photolysis|Munitions Constituents - Photolysis]]
|[[Dr. Warren Kadoya|Kadoya, Warren, Ph.D.]]
|
|-
|[[Munitions Constituents - Soil Sampling]]
|[[Dr. Samuel Beal|Beal, Samuel, Ph.D.]]
|
|-
|[[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways|Vapor Intrusion - Sewers and Utility Tunnels as Preferential Pathways]]
|[[Lila Beckley|Beckley, Lila]]
|vapor intrusion
|-
|[[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
|[[Dr. Barbara Bekins|Bekins, Barbara, Ph.D.]]||
|-
|[[Bioremediation - Anaerobic Secondary Water Quality Impacts]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||secondary impacts, water quality (in regards to anaerobic conditions)
|-
|[[Design Tool - Base Addition for ERD]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||aquifer acidity, base addition
|-
|[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
|[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||
|-
|[[Low pH Inhibition of Reductive Dechlorination]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||low pH inhibition
|-
|[[In Situ Toxicity Identification Evaluation (iTIE)]]||Burton, Allen, P.E.||toxicity evaluation
|-
|[[OPTically-based In-situ Characterization System (OPTICS)]]
|[[Dr. Grace Chang|Chang, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents - Electrochemical Treatment]]
|[[Dr. Brian P. Chaplin|Chaplin, Brian, Ph.D.]]
|munitions constituents remediation
|-
|[[PFAS Sources]]
|[[Dr. Dora Chiang|Chiang, Dora, Ph.D.]]
|
|-
|[[Biodegradation - Cometabolic]]||[[Dr. Kung-Hui (Bella) Chu |Chu, Kung-Hui (Bella), Ph.D]]||cometabolic biodegradation
|-
|[[Munitions Constituents - Composting]]
|[[Harry Craig|Craig, Harry]]
|
|-
|[[Chemical Oxidation (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||ISCO, chemical oxidation
|-
|[[Chemical Oxidation Oxidant Selection (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||chemical oxidant, oxidant (in regards to ISCO)
|-
|[[Chemical Oxidation Design Considerations(In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||screening, design, implementation, oxidant delivery (in regards to ISCO)
|-
|[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]||[[Dr. Rula Deeb |Deeb, Rula, Ph.D.]]||PFAS, perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS)
|-
|[[Metal and Metalloid Contaminants]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal contaminant(s), metalloid contaminant(s), metal(s), metalloid(s),
|-
|[[Metals and Metalloids - Mobility in Groundwater]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal mobility, aqueous speciation, adsorption, precipitation, colloidal transport (in regards to metals)
|-
|[[Monitored Natural Attenuation (MNA) of Metal and Metalloids]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||MNA, attenuation of metal(s), natural attenuation processes, attenuation (in regards to metals)
|-
|[[Metal and Metalloids - Remediation]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||remediation, in situ technologies, contaminant removal (in regards to metals)
|-
|[[pH Buffering in Aquifers]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||ph buffer, natural pH buffer, engineered pH buffer
|-
|[[Munitions Constituents - Sorption]]||[[Dr. Katerina Dontsova |Dontsova, Katerina, Ph.D.]]||energetics, sorption
|-
|[[Biodegradation - Hydrocarbons]]||[[Dr. Elizabeth Edwards |Edwards, Elizabeth, Ph.D.]]||hydrocarbon, biodegradation
|-
|[[Source Zone Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||source zone modeling
|-
|[[Plume Response Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||plume response modeling
|-
|[[REMChlor - MD]]
|[[Dr. Ron Falta|Falta, Ron, Ph.D.]]
|
|-
|[[In Situ Groundwater Treatment with Activated Carbon]]
|[[Dr. Dimin Fan|Fan, Dimin, Ph.D.]]
|
|-
|[[LNAPL Remediation Technologies]]||[[Dr. Shahla Farhat |Farhat, Shahla, Ph.D.]]||
|-
|[[Sustainable Remediation]]||[[Paul Favara |Favara, Paul]]||sustainable remediation, social, economic and environmental impacts
|-
|[[Munitions Constituents]]||[[Dr. Kevin Finneran |Finneran, Kevin, Ph.D.]]||Explosives, energetics, insensitive munitions
|-
|[[Biodegradation - Reductive Processes]]||[[Dr. David Freedman |Freedman, David, Ph.D.]]||biotic reduction, biotic reductive processes, hydrogenolysis, dihaloelimination, coupling, organohalide respiration
|-
|[[Remediation of Stormwater Runoff Contaminated by Munition Constituents|Munitions Constituents - Remediation of Stormwater Runoff]]||Fuller, Mark, Ph.D.||energetics, insensitive munitions, stormwater runoff
|-
|[[Subgrade Biogeochemical Reactor (SBGR)]]||[[Jeff Gamlin |Gamlin, Jeff]]||SBGR, subgrade biogeochemical reactor, bioreactor
|-
|[[Thermal Remediation - Smoldering]]||[[Dr. Jason Gerhard |Gerhard, Jason, Ph.D.]]||smouldering remediation, self-sustaining treatment for active remediation, STAR
|-
|[[Contaminated Sediments - Introduction]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[Contaminated Sediment Risk Assessment]]
|[[Richard Wenning|Wenning, Richard]]
|
|-
|[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[PFAS Ex Situ Water Treatment]]
|[[Dr. Scott Grieco |Grieco, Scott, Ph.D.]]
|
|-
|[[Stream Restoration]]
|[[Dr. Natalie Griffiths|Griffiths, Natalie, Ph.D.]]
|
|-
|[[Passive Sampling of Sediments]]
|[[Dr. Philip M. Gschwend|Gschwend, Philip]]
|
|-
|[[Phytoplankton (Algae) Blooms]]
|[[Dr. Nathan Hall|Hall, Nathan]]
|
|-
|[[PFAS Soil Remediation Technologies]]||[[James_Hatton |Hatton, Jim]]||PFAS, Soil source zones
|-
|[[Proteomics and Proteogenomics]]
|[[Dr. Kate Kucharzyk|Kucharzyk, Kate, Ph.D.]]
|
|-
|[[N-nitrosodimethylamine (NDMA)]]
|[[Paul Hatzinger|Hatzinger, Paul, Ph.D.]]
|
|-
|[[Alternative Endpoints]]||[[Elisabeth Hawley |Hawley, Elisabeth]]||management of complex sites||
|-
|[[Thermal Remediation]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, in situ thermal
|-
|[[Thermal Remediation - Steam]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Steam Enhanced Extraction
|-
|[[Thermal Remediation - Electrical Resistance Heating]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Electrical Resistance Heating
|-
|[[Thermal Conduction Heating (TCH)]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal desorption
|-
|[[Thermal Remediation - Combined Remedies]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation
|-
|[[Thermal Conduction Heating for Treatment of PFAS-Impacted Soil]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, PFAS
|-
|[[Predicting Species Responses to Climate Change with Population Models]]
|[[Dr. Brian Hudgens|Hudgens, Brian, Ph.D.]]
|climate change
|-
|[[Infrastructure Resilience]]
|[[Dr. John Hummel|Hummel, John, Ph.D.]]
|
|-
|[[Munitions Constituents- TREECS™ Fate and Risk Modeling|Munitions Constituents - TREECS™ Fate and Risk Modeling]]||[[Dr. Billy E. Johnson |Johnson, Billy, Ph.D.]]||munitions constituents fate and transport modeling, TREECS
|-
|[[Munitions Constituents - Alkaline Degradation]]
|[[Jared Johnson|Johnson, Jared]]
|
|-
|[[Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)|Vapor Intrusion - Assessing VI Impacts in Neighborhoods with Groundwater Contaminated CVOCs]]
|[[Dr. Paul C. Johnson|Johnson, Paul, Ph.D.]]
|vapor intrusion
|-
|[[Munitions Constituents - IM Toxicology]]||-----||insensitive explosives, insensitive munitions, IMX-101, IMX
|-
|[[Landfarming]]
|[[Dr. Roopa Kamath|Kamath, Roopa, Ph.D.]]
|
|-
|[[NAPL Mobility]]
|[[Andrew Kirkman|Kirkman, Andrew]]
|
|-
|[[Climate Change Primer]]
|[[Dr. Rao Kotamarthi|Kotamarthi, Rao, Ph.D.]]
|
|-
|[[Perchlorate]]||[[Thomas Krug |Krug, Thomas]]||perchlorate
|-
|[[Injection Techniques for Liquid Amendments]]||[[Thomas Krug |Krug, Thomas]]||amendment injection
|-
|[[Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)]]
|[[Dr. Johnsie Ray Lang|Lang, Johnsie Ray, Ph.D.]]||PFAS
|
|-
|[[Characterization Methods – Hydraulic Conductivity]]
|[[Dr. Gaisheng Liu|Liu, Gaisheng, Ph.D.]]
|
|-
|[[Compound Specific Isotope Analysis (CSIA)]]||[[Dr. Barbara Sherwood Lollar, F.R.S.C. |Lollar, Barbara S., FRSC]]||Compound Specific Isotope Analysis (CSIA)
|-
|[[Passive Sampling of Munitions Constituents|Munitions Constituents - Passive Sampling]]
|[[Dr. Guilherme Lotufo|Lotufo, Guilerme, Ph.D.]]
|
|-
|[[Vapor Intrusion (VI)]]||[[Chris Lutes |Lutes, Chris]]||vapor intrusion
|-
|[[Bioremediation - Anaerobic]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||anaerobic bioremediation
|-
|[[Bioremediation - Anaerobic Design Considerations]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||
|-
|[[Biodegradation - 1,4-Dioxane]]
|[[Dr. Shaily Mahendra|Mahendra, Shaily, Ph.D.]]
|
|-
|[[Direct Push (DP) Technology]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push, DP, DP machines, DP technology
|-
|[[Direct Push Sampling]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push sampling, soil sampling, groundwater sampling, well installation, soil vapor sampling (in regards to DP)
|-
|[[Direct Push Logging]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push logging, Cone Penetration Testing, CPT, Electrical Conductivity, EC, Hydraulic Profiling Tool, HPT, Membrane Interface Probe, MIP, Optical Imaging Profiler, OIP
|-
|[[Downscaled High Resolution Datasets for Climate Change Projections]]
|[[Dr. Rao Kotamarthi|Kotamarthi, Rao, Ph.D.]]
|
|-
|[[Remediation Performance Assessment at Chlorinated Solvent Sites]]||[[Travis McGuire|McGuire, Travis]]||multi-site studies
|-
|[[LNAPL Conceptual Site Models]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Long-Term Monitoring (LTM) - Data Analysis]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data analysis, analysis methods (in regards to LTM)
|-
|[[Long-Term Monitoring (LTM) - Data Variability]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data variability (in regards to LTM), LTM evaluation
|-
|-
|[[Munitions Constituents - Abiotic Reduction]]||[[Dr. Jimmy Murillo-Gelvez |Murillo-Gelvez, Jimmy, Ph.D.]]
|-
|[[Supercritical Water Oxidation (SCWO)]]
|Nagar, Kobe
|
|-
|[[Matrix Diffusion]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Groundwater Flow and Solute Transport]]||[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]||groundwater flow, advection, dispersion, diffusion, molecular diffusion, mechanical dispersion
|-
|[[Molecular Biological Tools - MBTs]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||MBT, Molecular Biological Tool(s)
|-
|[[Quantitative Polymerase Chain Reaction (qPCR)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||qPCR, Polymerase Chain Reaction
|-
|[[Sediment Capping]]
|[[Dr. Danny Reible|Reible, Danny]]
|
|-
|[[Stable Isotope Probing (SIP)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||SIP, Stable Isotope Probing
|-
|[[Metagenomics]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||metagenomics
|-
|[[Natural Source Zone Depletion (NSZD)]]||[[Tom Palaia |Palaia, Tom]]||natural source zone depletion, NSZD
|-
|[[Climate Change Effects on Wildlife]]
|[[Dr. Breanna F. Powers|Powers, Breanna, PhD.]]
|
|-
|[[Amendment Distribution in Low Conductivity Materials]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||
|-
|[[Polycyclic Aromatic Hydrocarbons (PAHs)]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||polycyclic aromatic hydrocarbons, PAH(s)
|-
|[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]
|[[Florent Risacher|Risacher, Florent, M.Sc.]]
|
|-
|[[1,2,3-Trichloropropane]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||TCP, trichloropropane
|-
|[[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR)]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||ZVI
|-
|[[Mercury in Sediments]]
|[[Dr. Grace Schwartz|Schwartz, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents – Sample Extraction and Analytical Techniques|Munitions Constituents - Sample Extraction and Analytical Techniques]]
|[[Dr. Austin Scircle|Scircle, Austin]]
|
|-
|[[Geophysical Methods]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[Geophysical Methods - Case Studies]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[PFAS Treatment by Anion Exchange]]
|[[Dr. Timothy J. Strathmann|Strathmann, Timothy, Ph.D.]]
|PFAS
|-
|[[Munitions Constituents - Dissolution]]||[[Dr. Susan Taylor |Taylor, Susan, Ph.D.]]||explosive(s), dissolution,
|-
|[[PFAS Treatment by Electrical Discharge Plasma]]
|[[Dr. Selma Mededovic Thagard|Thagard, Selma Mededovic, Ph.D.]]
|PFAS
|-
|[[Restoration of Ecological Function in Terrestrial Systems Impacted by Invasive Species]]
|Thierry, Hugo, Ph.D.
|climate change, invasive species, restoration ecology
|-
|[[Chemical Reduction (In Situ - ISCR)]]||[[Dr. Paul Tratnyek |Tratnyek, Paul, Ph.D.]]||In Situ Chemical Reduction, ISCR
|-
|[[Injection Techniques - Viscosity Modification]]||[[Michael Truex |Truex, Michael]]||viscosity, viscosity modifiers, viscosity modification
|-
|[[Soil Vapor Extraction (SVE)]]||[[Michael Truex |Truex, Michael]]||soil vapor extraction, SVE
|-
|[[Munitions Constituents - Deposition]]||[[Michael R. Walsh, P.E., M.E.|Walsh, Michael, P.E.]]||explosive deposition, energetics deposition
|-
|[[Vapor Intrusion - Separation Distances from Petroleum Sources]]
|[[Dr. James Weaver|Weaver, James, Ph.D.]]
|vapor intrusion
|-
|[[Zerovalent Iron Permeable Reactive Barriers]]
|[[Dr. Richard Wilkin|Wilkin, Rick, Ph.D.]]
|
|-
|[[Monitored Natural Attenuation (MNA)]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, In Situ MNA, natural attenuation, natural attenuation processes
|-
|[[Monitored Natural Attenuation (MNA) of Fuels]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to petroleum hydrocarbons and fuel components)
|-
|[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to chlorinated solvents)
|-
|[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
|[[Dr. John Wilson|Wilson, John, Ph.D.]]
|MNA, natural attenuation
|-
|[[Chlorinated Solvents]]||[[Dr. Bilgen Yuncu, P.E. |Yuncu, Bilgen, Ph.D., P.E.]]||chlorinated solvents
|-
|[[Petroleum Hydrocarbons (PHCs)]]
|[[Dr. Bilgen Yuncu, P.E.|Yuncu, Bilgen, Ph.D., P.E.]]
|Petroleum Hydrocarbons (PHCs)
|-
|[[Photoactivated Reductive Defluorination - PFAS Destruction]]
|[[Dr. Suzanne Witt|Witt, Suzanne, Ph.D.]]
|PFAS destruction
|
|-
|[[Hydrogeophysical Methods for Characterization and Monitoring of Groundwater-Surface Water Exchanges]]
|[[Dr. Lee Slater|Slater, Lee, Ph.D.]]
|geophysics, hydrogeophysical methods,
|
|-
|[[Hydrothermal Alkaline Treatment (HALT)]]
|[[Dr. Brian Pinkard|Pinkard, Brian]]
|
|-
|[[1,4-Dioxane]]
|[[Matthew Zenker|Zenker, Matthew]]
|
|-
|[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
|[[Dr. John F. Stults|Stults, Dr. John]]
|PFAS, vadose zone, lysimeter, field investigation
|
|-
|[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]||[[Dr. Yida Fang |Fang, Yida, Ph.D.]]||PFAS destruction
|
|-
|}

Articles

2026-03-13T21:29:10Z

Admin:

{| class="wikitable sortable"
|-
!Title!!First Author!!Linking Phrases
|-
|[[Groundwater Sampling - No-Purge/Passive]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||passive sampling, no purge sampling, grab samplers, diffusion samplers, sorptive samplers
|-
|[[ Long-Term Monitoring (LTM)]]||[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]||long-term monitoring, LTM, LTM objectives, LTM programs, LTM challenges
|-
|[[PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)]]
|[[Dr. David Adamson, P.E. |Adamson, David, Ph.D., P.E.]]
|PFAS, MNA, natural attenuation
|-
|[[Sorption of Organic Contaminants]]||[[Richelle Allen-King|Allen-King, Richelle]]||
|-
|[[PFAS Transport and Fate]]
|[[Dr. Richard Anderson|Anderson, Richard, Ph.D.]]
|PFAS, fate and transport
|-
|[[Mass Flux and Mass Discharge]]||[[Dr. Michael Annable, P.E. |Annable, Michael, Ph.D., P.E.]]||source reduction
|-
|[[PFAS Toxicology and Risk Assessment]]
|[[Jennifer Arblaster|Arblaster, Jennifer]]
|PFAS, toxicology, risk assessment
|-
|[[Metal(loid)s - Small Arms Ranges]]|| Dr. Amanda Barker |[[Dr. Amanda Barker|Barker, Amanda, Ph.D.]]
|
|-
|[[Munitions Constituents – Photolysis|Munitions Constituents - Photolysis]]
|[[Dr. Warren Kadoya|Kadoya, Warren, Ph.D.]]
|
|-
|[[Munitions Constituents - Soil Sampling]]
|[[Dr. Samuel Beal|Beal, Samuel, Ph.D.]]
|
|-
|[[Vapor Intrusion – Sewers and Utility Tunnels as Preferential Pathways|Vapor Intrusion - Sewers and Utility Tunnels as Preferential Pathways]]
|[[Lila Beckley|Beckley, Lila]]
|vapor intrusion
|-
|[[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
|[[Dr. Barbara Bekins|Bekins, Barbara, Ph.D.]]||
|-
|[[Bioremediation - Anaerobic Secondary Water Quality Impacts]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||secondary impacts, water quality (in regards to anaerobic conditions)
|-
|[[Design Tool - Base Addition for ERD]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||aquifer acidity, base addition
|-
|[[Emulsified Vegetable Oil (EVO) for Anaerobic Bioremediation]]
|[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||
|-
|[[Low pH Inhibition of Reductive Dechlorination]]||[[Dr. Robert Borden, P.E. |Borden, Robert, Ph.D., P.E.]]||low pH inhibition
|-
|[[In Situ Toxicity Identification Evaluation (iTIE)]]||Burton, Allen, P.E.||toxicity evaluation
|-
|[[OPTically-based In-situ Characterization System (OPTICS)]]
|[[Dr. Grace Chang|Chang, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents - Electrochemical Treatment]]
|[[Dr. Brian P. Chaplin|Chaplin, Brian, Ph.D.]]
|munitions constituents remediation
|-
|[[PFAS Sources]]
|[[Dr. Dora Chiang|Chiang, Dora, Ph.D.]]
|
|-
|[[Biodegradation - Cometabolic]]||[[Dr. Kung-Hui (Bella) Chu |Chu, Kung-Hui (Bella), Ph.D]]||cometabolic biodegradation
|-
|[[Munitions Constituents - Composting]]
|[[Harry Craig|Craig, Harry]]
|
|-
|[[Chemical Oxidation (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||ISCO, chemical oxidation
|-
|[[Chemical Oxidation Oxidant Selection (In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||chemical oxidant, oxidant (in regards to ISCO)
|-
|[[Chemical Oxidation Design Considerations(In Situ - ISCO)]]||[[Dr. Michelle Crimi |Crimi, Michelle, Ph.D]]||screening, design, implementation, oxidant delivery (in regards to ISCO)
|-
|[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]||[[Dr. Rula Deeb |Deeb, Rula, Ph.D.]]||PFAS, perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS)
|-
|[[Metal and Metalloid Contaminants]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal contaminant(s), metalloid contaminant(s), metal(s), metalloid(s),
|-
|[[Metals and Metalloids - Mobility in Groundwater]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||metal mobility, aqueous speciation, adsorption, precipitation, colloidal transport (in regards to metals)
|-
|[[Monitored Natural Attenuation (MNA) of Metal and Metalloids]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||MNA, attenuation of metal(s), natural attenuation processes, attenuation (in regards to metals)
|-
|[[Metal and Metalloids - Remediation]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||remediation, in situ technologies, contaminant removal (in regards to metals)
|-
|[[pH Buffering in Aquifers]]||[[Dr. Miles Denham |Denham, Miles, Ph.D.]]||ph buffer, natural pH buffer, engineered pH buffer
|-
|[[Munitions Constituents - Sorption]]||[[Dr. Katerina Dontsova |Dontsova, Katerina, Ph.D.]]||energetics, sorption
|-
|[[Biodegradation - Hydrocarbons]]||[[Dr. Elizabeth Edwards |Edwards, Elizabeth, Ph.D.]]||hydrocarbon, biodegradation
|-
|[[Source Zone Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||source zone modeling
|-
|[[Plume Response Modeling]]||[[Dr. Ron Falta |Falta, Ron, Ph.D.]]||plume response modeling
|-
|[[REMChlor - MD]]
|[[Dr. Ron Falta|Falta, Ron, Ph.D.]]
|
|-
|[[In Situ Groundwater Treatment with Activated Carbon]]
|[[Dr. Dimin Fan|Fan, Dimin, Ph.D.]]
|
|-
|[[LNAPL Remediation Technologies]]||[[Dr. Shahla Farhat |Farhat, Shahla, Ph.D.]]||
|-
|[[Sustainable Remediation]]||[[Paul Favara |Favara, Paul]]||sustainable remediation, social, economic and environmental impacts
|-
|[[Munitions Constituents]]||[[Dr. Kevin Finneran |Finneran, Kevin, Ph.D.]]||Explosives, energetics, insensitive munitions
|-
|[[Biodegradation - Reductive Processes]]||[[Dr. David Freedman |Freedman, David, Ph.D.]]||biotic reduction, biotic reductive processes, hydrogenolysis, dihaloelimination, coupling, organohalide respiration
|-
|[[Remediation of Stormwater Runoff Contaminated by Munition Constituents|Munitions Constituents - Remediation of Stormwater Runoff]]||Fuller, Mark, Ph.D.||energetics, insensitive munitions, stormwater runoff
|-
|[[Subgrade Biogeochemical Reactor (SBGR)]]||[[Jeff Gamlin |Gamlin, Jeff]]||SBGR, subgrade biogeochemical reactor, bioreactor
|-
|[[Thermal Remediation - Smoldering]]||[[Dr. Jason Gerhard |Gerhard, Jason, Ph.D.]]||smouldering remediation, self-sustaining treatment for active remediation, STAR
|-
|[[Contaminated Sediments - Introduction]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[Contaminated Sediment Risk Assessment]]
|[[Richard Wenning|Wenning, Richard]]
|
|-
|[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
|[[Dr. Upal Ghosh|Ghosh, Upal, Ph.D.]]
|
|-
|[[PFAS Ex Situ Water Treatment]]
|[[Dr. Scott Grieco |Grieco, Scott, Ph.D.]]
|
|-
|[[Stream Restoration]]
|[[Dr. Natalie Griffiths|Griffiths, Natalie, Ph.D.]]
|
|-
|[[Passive Sampling of Sediments]]
|[[Dr. Philip M. Gschwend|Gschwend, Philip]]
|
|-
|[[Phytoplankton (Algae) Blooms]]
|[[Dr. Nathan Hall|Hall, Nathan]]
|
|-
|[[PFAS Soil Remediation Technologies]]||[[James_Hatton |Hatton, Jim]]||PFAS, Soil source zones
|-
|[[Proteomics and Proteogenomics]]
|[[Dr. Kate Kucharzyk|Kucharzyk, Kate, Ph.D.]]
|
|-
|[[N-nitrosodimethylamine (NDMA)]]
|[[Paul Hatzinger|Hatzinger, Paul, Ph.D.]]
|
|-
|[[Alternative Endpoints]]||[[Elisabeth Hawley |Hawley, Elisabeth]]||management of complex sites||
|-
|[[Thermal Remediation]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, in situ thermal
|-
|[[Thermal Remediation - Steam]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Steam Enhanced Extraction
|-
|[[Thermal Remediation - Electrical Resistance Heating]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||Electrical Resistance Heating
|-
|[[Thermal Conduction Heating (TCH)]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal desorption
|-
|[[Thermal Remediation - Combined Remedies]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation
|-
|[[Thermal Conduction Heating for Treatment of PFAS-Impacted Soil]]||[[Dr. Gorm Heron |Heron, Gorm, Ph.D.]]||thermal remediation, PFAS
|-
|[[Predicting Species Responses to Climate Change with Population Models]]
|[[Dr. Brian Hudgens|Hudgens, Brian, Ph.D.]]
|climate change
|-
|[[Infrastructure Resilience]]
|[[Dr. John Hummel|Hummel, John, Ph.D.]]
|
|-
|[[Munitions Constituents- TREECS™ Fate and Risk Modeling|Munitions Constituents - TREECS™ Fate and Risk Modeling]]||[[Dr. Billy E. Johnson |Johnson, Billy, Ph.D.]]||munitions constituents fate and transport modeling, TREECS
|-
|[[Munitions Constituents - Alkaline Degradation]]
|[[Jared Johnson|Johnson, Jared]]
|
|-
|[[Assessing Vapor Intrusion (VI) Impacts in Neighborhoods with Groundwater Contaminated by Chlorinated Volatile Organic Chemicals (CVOCs)|Vapor Intrusion - Assessing VI Impacts in Neighborhoods with Groundwater Contaminated CVOCs]]
|[[Dr. Paul C. Johnson|Johnson, Paul, Ph.D.]]
|vapor intrusion
|-
|[[Munitions Constituents - IM Toxicology]]||-----||insensitive explosives, insensitive munitions, IMX-101, IMX
|-
|[[Landfarming]]
|[[Dr. Roopa Kamath|Kamath, Roopa, Ph.D.]]
|
|-
|[[NAPL Mobility]]
|[[Andrew Kirkman|Kirkman, Andrew]]
|
|-
|[[Climate Change Primer]]
|[[Dr. Rao Kotamarthi|Kotamarthi, Rao, Ph.D.]]
|
|-
|[[Perchlorate]]||[[Thomas Krug |Krug, Thomas]]||perchlorate
|-
|[[Injection Techniques for Liquid Amendments]]||[[Thomas Krug |Krug, Thomas]]||amendment injection
|-
|[[Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)]]
|[[Dr. Johnsie Ray Lang|Lang, Johnsie Ray, Ph.D.]]||PFAS
|
|-
|[[Characterization Methods – Hydraulic Conductivity]]
|[[Dr. Gaisheng Liu|Liu, Gaisheng, Ph.D.]]
|
|-
|[[Compound Specific Isotope Analysis (CSIA)]]||[[Dr. Barbara Sherwood Lollar, F.R.S.C. |Lollar, Barbara S., FRSC]]||Compound Specific Isotope Analysis (CSIA)
|-
|[[Passive Sampling of Munitions Constituents|Munitions Constituents - Passive Sampling]]
|[[Dr. Guilherme Lotufo|Lotufo, Guilerme, Ph.D.]]
|
|-
|[[Vapor Intrusion (VI)]]||[[Chris Lutes |Lutes, Chris]]||vapor intrusion
|-
|[[Bioremediation - Anaerobic]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||anaerobic bioremediation
|-
|[[Bioremediation - Anaerobic Design Considerations]]||[[Leah MacKinnon, M.A.Sc., P. Eng.|MacKinnon, Leah]]||
|-
|[[Biodegradation - 1,4-Dioxane]]
|[[Dr. Shaily Mahendra|Mahendra, Shaily, Ph.D.]]
|
|-
|[[Direct Push (DP) Technology]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push, DP, DP machines, DP technology
|-
|[[Direct Push Sampling]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push sampling, soil sampling, groundwater sampling, well installation, soil vapor sampling (in regards to DP)
|-
|[[Direct Push Logging]]||[[Wesley McCall, M.S., P.G. |McCall, Wesley, M.S., P.G.]]||direct push logging, Cone Penetration Testing, CPT, Electrical Conductivity, EC, Hydraulic Profiling Tool, HPT, Membrane Interface Probe, MIP, Optical Imaging Profiler, OIP
|-
|[[Downscaled High Resolution Datasets for Climate Change Projections]]
|[[Dr. Rao Kotamarthi|Kotamarthi, Rao, Ph.D.]]
|
|-
|[[Remediation Performance Assessment at Chlorinated Solvent Sites]]||[[Travis McGuire|McGuire, Travis]]||multi-site studies
|-
|[[LNAPL Conceptual Site Models]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Long-Term Monitoring (LTM) - Data Analysis]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data analysis, analysis methods (in regards to LTM)
|-
|[[Long-Term Monitoring (LTM) - Data Variability]]||[[Dr. Thomas McHugh |McHugh, Thomas, Ph.D.]]||data variability (in regards to LTM), LTM evaluation
|-
|-
|[[Munitions Constituents - Abiotic Reduction]]||[[Dr. Jimmy Murillo-Gelvez |Murillo-Gelvez, Jimmy, Ph.D.]]
|-
|[[Supercritical Water Oxidation (SCWO)]]
|Nagar, Kobe
|
|-
|[[Matrix Diffusion]]
|[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]
|
|-
|[[Groundwater Flow and Solute Transport]]||[[Dr. Charles Newell, P.E. |Newell, Charles, Ph.D., P.E.]]||groundwater flow, advection, dispersion, diffusion, molecular diffusion, mechanical dispersion
|-
|[[Molecular Biological Tools - MBTs]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||MBT, Molecular Biological Tool(s)
|-
|[[Quantitative Polymerase Chain Reaction (qPCR)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||qPCR, Polymerase Chain Reaction
|-
|[[Sediment Capping]]
|[[Dr. Danny Reible|Reible, Danny]]
|
|-
|[[Stable Isotope Probing (SIP)]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||SIP, Stable Isotope Probing
|-
|[[Metagenomics]]||[[Dora Ogles-Taggart |Ogles-Taggart, Dora]]||metagenomics
|-
|[[Natural Source Zone Depletion (NSZD)]]||[[Tom Palaia |Palaia, Tom]]||natural source zone depletion, NSZD
|-
|[[Climate Change Effects on Wildlife]]
|[[Dr. Breanna F. Powers|Powers, Breanna, PhD.]]
|
|-
|[[Amendment Distribution in Low Conductivity Materials]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||
|-
|[[Polycyclic Aromatic Hydrocarbons (PAHs)]]||[[Dr. Stephen Richardson |Richardson, Stephen, Ph.D.]]||polycyclic aromatic hydrocarbons, PAH(s)
|-
|[[Sediment Porewater Dialysis Passive Samplers for Inorganics (Peepers)]]
|[[Florent Risacher|Risacher, Florent, M.Sc.]]
|
|-
|[[1,2,3-Trichloropropane]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||TCP, trichloropropane
|-
|[[Zerovalent Iron (ZVI) (Chemical Reduction - ISCR)]]||[[Dr. Alexandra Salter-Blanc |Salter-Blanc, Alexandra, Ph.D.]]||ZVI
|-
|[[Mercury in Sediments]]
|[[Dr. Grace Schwartz|Schwartz, Grace, Ph.D.]]
|
|-
|[[Munitions Constituents – Sample Extraction and Analytical Techniques|Munitions Constituents - Sample Extraction and Analytical Techniques]]
|[[Dr. Austin Scircle|Scircle, Austin]]
|
|-
|[[Geophysical Methods]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[Geophysical Methods - Case Studies]]||[[Dr. Lee Slater |Slater, Lee, Ph.D.]]||geophysics
|-
|[[PFAS Treatment by Anion Exchange]]
|[[Dr. Timothy J. Strathmann|Strathmann, Timothy, Ph.D.]]
|PFAS
|-
|[[Munitions Constituents - Dissolution]]||[[Dr. Susan Taylor |Taylor, Susan, Ph.D.]]||explosive(s), dissolution,
|-
|[[PFAS Treatment by Electrical Discharge Plasma]]
|[[Dr. Selma Mededovic Thagard|Thagard, Selma Mededovic, Ph.D.]]
|PFAS
|-
|[[Restoration of Ecological Function in Terrestrial Systems Impacted by Invasive Species]]
|Thierry, Hugo, Ph.D.
|climate change, invasive species, restoration ecology
|-
|[[Chemical Reduction (In Situ - ISCR)]]||[[Dr. Paul Tratnyek |Tratnyek, Paul, Ph.D.]]||In Situ Chemical Reduction, ISCR
|-
|[[Injection Techniques - Viscosity Modification]]||[[Michael Truex |Truex, Michael]]||viscosity, viscosity modifiers, viscosity modification
|-
|[[Soil Vapor Extraction (SVE)]]||[[Michael Truex |Truex, Michael]]||soil vapor extraction, SVE
|-
|[[Munitions Constituents - Deposition]]||[[Michael R. Walsh, P.E., M.E.|Walsh, Michael, P.E.]]||explosive deposition, energetics deposition
|-
|[[Vapor Intrusion - Separation Distances from Petroleum Sources]]
|[[Dr. James Weaver|Weaver, James, Ph.D.]]
|vapor intrusion
|-
|[[Zerovalent Iron Permeable Reactive Barriers]]
|[[Dr. Richard Wilkin|Wilkin, Rick, Ph.D.]]
|
|-
|[[Monitored Natural Attenuation (MNA)]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, In Situ MNA, natural attenuation, natural attenuation processes
|-
|[[Monitored Natural Attenuation (MNA) of Fuels]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to petroleum hydrocarbons and fuel components)
|-
|[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]||[[Dr. John Wilson |Wilson, John, Ph.D.]]||MNA, natural attenuation, attenuate (when used in context related to chlorinated solvents)
|-
|[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
|[[Dr. John Wilson|Wilson, John, Ph.D.]]
|MNA, natural attenuation
|-
|[[Chlorinated Solvents]]||[[Dr. Bilgen Yuncu, P.E. |Yuncu, Bilgen, Ph.D., P.E.]]||chlorinated solvents
|-
|[[Petroleum Hydrocarbons (PHCs)]]
|[[Dr. Bilgen Yuncu, P.E.|Yuncu, Bilgen, Ph.D., P.E.]]
|Petroleum Hydrocarbons (PHCs)
|-
|[[Photoactivated Reductive Defluorination - PFAS Destruction]]
|[[Dr. Suzanne Witt|Witt, Suzanne, Ph.D.]]
|PFAS
|
|-
|[[Hydrogeophysical Methods for Characterization and Monitoring of Groundwater-Surface Water Exchanges]]
|[[Dr. Lee Slater|Slater, Lee, Ph.D.]]
|geophysics, hydrogeophysical methods,
|
|-
|[[Hydrothermal Alkaline Treatment (HALT)]]
|[[Dr. Brian Pinkard|Pinkard, Brian]]
|
|-
|[[1,4-Dioxane]]
|[[Matthew Zenker|Zenker, Matthew]]
|
|-
|[[Lysimeters for Measuring PFAS Concentrations in the Vadose Zone]]
|[[Dr. John F. Stults|Stults, Dr. John]]
|PFAS, vadose zone, lysimeter, field investigation
|
|-
|[[PFAS Destruction by Ultraviolet/Sulfite Treatment]]||[[Dr. Yida Fang |Fang, Yida, Ph.D.]]||
|
|-
|}