Difference between revisions of "User:Jhurley/sandbox"

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(→‎Lysimeters for Measuring PFAS Concentrations in the Vadose Zone)
 
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A Conceptual Site Model (CSM) is a collection of information about a contaminated site that integrates the available evidence regarding its hydrogeologic setting, contaminant sources, exposure pathways, potential receptors, and site history.  A CSM for a [[Wikipedia: Light non-aqueous phase liquid | Light Non-Aqueous Phase Liquid (LNAPL)]] site focuses on several key concepts:  the stage in the LNAPL site life cycle, LNAPL distribution in the subsurface and the resulting mobility of the LNAPL, LNAPL as a source of dissolved and vapor plumes, and the attenuation of LNAPL sources over time.
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==Lysimeters for Measuring PFAS Concentrations in the Vadose Zone==
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[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]] are frequently introduced to the environment through soil surface applications which then transport through the vadose zone to reach underlying groundwater receptors. Due to their unique properties and resulting transport and retention behaviors, PFAS in the vadose zone can be a persistent contaminant source to underlying groundwater systems. Determining the fraction of PFAS present in the mobile porewater relative to the total concentrations in soils is critical to understanding the risk posed by PFAS in vadose zone source areas. Lysimeters are instruments that have been used by agronomists and vadose zone researchers for decades to determine water flux and solute concentrations in unsaturated porewater. Lysimeters have recently been developed as a critical tool for field investigations and characterizations of PFAS impacted source zones.  
 
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'''Related Article(s)'''
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'''Related Article(s):'''
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* [[LNAPL Remediation Technologies]]
 
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* [[NAPL Mobility]]
 
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* [[Natural Source Zone Depletion (NSZD)]]
 
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* [[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]
 
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* [[Monitored Natural Attenuation (MNA)]]
 
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* [[Biodegradation - Hydrocarbons]]
 
  
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'''CONTRIBUTOR(S):''' [[Dr. Charles Newell, P.E. | Charles Newell]]
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*[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]
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*[[PFAS Transport and Fate]]
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*[[PFAS Toxicology and Risk Assessment]]
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*[[Mass Flux and Mass Discharge]]
  
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'''Key Resource(s):'''
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'''Contributors:''' Dr. John F. Stults, Dr. Charles Schaefer
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* LNAPL Site Management: LCSM Evolution, Decision Process, and Remedial Technologies. LNAPL-3. ITRC.<ref name="LNAPL-3">Interstate Technology and Regulatory Council (ITRC), 2018. LNAPL Site Management: LCSM Evolution, Decision Process, and Remedial Technologies. LNAPL-3. ITRC, LNAPL Update Team, Washington, DC.  [https://lnapl-3.itrcweb.org LNAPL-3 Website]</ref>
 
  
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* Managing Risk at LNAPL Sites - Frequently Asked Questions, 2nd Edition. API.<ref name="Sale2018"> Sale, T., Hopkins, H., and Kirkman, A., 2018.  Managing Risk at LNAPL Sites - Frequently Asked Questions, 2nd Edition. American Petroleum Institute (API), Washington, DC. 72 pages. [https://www.api.org/oil-and-natural-gas/environment/clean-water/ground-water/lnapl/lnapl-faqs Free download from API.] [https://www.enviro.wiki/index.php?title=File:Sale-2018_LNAPL_FAQs_2nd_ed.pdf Report.pdf]</ref>
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'''Key Resources:'''
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*Assessment of PFAS in Collocated Soil and Porewater Samples at an AFFF-Impacted Source Zone: Field-Scale Validation of Suction Lysimeters<ref name="AndersonEtAl2022"/>
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*PFAS Concentrations in Soil versus Soil Porewater: Mass Distributions and the Impact of Adsorption at Air-Water Interfaces<ref name="BrusseauGuo2022"/>
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*Using Suction Lysimeters for Determining the Potential of Per- and Polyfluoroalkyl Substances to Leach from Soil to Groundwater: A Review<ref name="CostanzaEtAl2025"/>
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*Use of Lysimeters for Monitoring Soil Water Balance Parameters and Nutrient Leaching<ref name="MeissnerEtAl2020"/>
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*PFAS Porewater Concentrations in Unsaturated Soil: Field and Laboratory Comparisons Inform on PFAS Accumulation at Air-Water Interfaces<ref name="SchaeferEtAl2024"/>
  
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==Life Cycle of LNAPL Sites==
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==Introduction==
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[[File:Newell1w2Fig1.png |thumb|left|250px| Figure 1. Early, Middle, and Late Stage LNAPL releases<ref name= "Sale2018"/>. The key distinctions are the presence of continuous LNAPL that can be mobile and the amount of time that has elapsed for NSZD to remove LNAPL.]]
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Lysimeters are devices that are placed in the subsurface above the groundwater table to monitor the movement of water through the soil<ref name="GossEhlers2009">Goss, M.J., Ehlers, W., 2009. The Role of Lysimeters in the Development of Our Understanding of Soil Water and Nutrient Dynamics in Ecosystems. Soil Use and Management, 25(3), pp. 213–223. [https://doi.org/10.1111/j.1475-2743.2009.00230.x doi: 10.1111/j.1475-2743.2009.00230.x]</ref><ref>Pütz, T., Fank, J., Flury, M., 2018. Lysimeters in Vadose Zone Research. Vadose Zone Journal, 17 (1), pp. 1-4. [https://doi.org/10.2136/vzj2018.02.0035 doi: 10.2136/vzj2018.02.0035]&nbsp; [[Media: PutzEtAl2018.pdf | Open Access Article]]</ref><ref name="CostanzaEtAl2025">Costanza, J., Clabaugh, C.D., Leibli, C., Ferreira, J., Wilkin, R.T., 2025. Using Suction Lysimeters for Determining the Potential of Per- and Polyfluoroalkyl Substances to Leach from Soil to Groundwater: A Review. Environmental Science and Technology, 59(9), pp. 4215-4229. [https://doi.org/10.1021/acs.est.4c10246 doi: 10.1021/acs.est.4c10246]</ref>. Lysimeters have historically been used in agricultural sciences for monitoring nutrient or contaminant movement, soil moisture release curves, natural drainage patterns, and dynamics of plant-water interactions<ref name="GossEhlers2009"/><ref>Bergström, L., 1990. Use of Lysimeters to Estimate Leaching of Pesticides in Agricultural Soils. Environmental Pollution, 67 (4), 325–347. [https://doi.org/10.1016/0269-7491(90)90070-S doi: 10.1016/0269-7491(90)90070-S]</ref><ref>Dabrowska, D., Rykala, W., 2021. A Review of Lysimeter Experiments Carried Out on Municipal Landfill Waste. Toxics, 9(2), Article 26. [https://doi.org/10.3390/toxics9020026 doi: 10.3390/toxics9020026]&nbsp; [[Media: Dabrowska Rykala2021.pdf | Open Access Article]]</ref><ref>Fernando, S.U., Galagedara, L., Krishnapillai, M., Cuss, C.W., 2023. Lysimeter Sampling System for Optimal Determination of Trace Elements in Soil Solutions. Water, 15(18), Article 3277. [https://doi.org/10.3390/w15183277 doi: 10.3390/w15183277]&nbsp; [[Media: FernandoEtAl2023.pdf | Open Access Article]]</ref><ref name="MeissnerEtAl2020">Meissner, R., Rupp, H., Haselow, L., 2020. Use of Lysimeters for Monitoring Soil Water Balance Parameters and Nutrient Leaching. In: Climate Change and Soil Interactions. Elsevier, pp. 171-205. [https://doi.org/10.1016/B978-0-12-818032-7.00007-2 doi: 10.1016/B978-0-12-818032-7.00007-2]</ref><ref name="RogersMcConnell1993">Rogers, R.D., McConnell, J.W. Jr., 1993. Lysimeter Literature Review, Nuclear Regulatory Commission Report Numbers: NUREG/CR--6073, EGG--2706. [https://www.osti.gov/] ID: 10183270. [https://doi.org/10.2172/10183270 doi: 10.2172/10183270]&nbsp; [[Media: RogersMcConnell1993.pdf | Open  Access Article]]</ref><ref>Sołtysiak, M., Rakoczy, M., 2019. An Overview of the Experimental Research Use of Lysimeters. Environmental and Socio-Economic Studies, 7(2), pp. 49-56. [https://doi.org/10.2478/environ-2019-0012 doi: 10.2478/environ-2019-0012]&nbsp; [[Media: SołtysiakRakoczy2019.pdf | Open Access Article]]</ref><ref name="Stannard1992">Stannard, D.I., 1992. Tensiometers—Theory, Construction, and Use. Geotechnical Testing Journal, 15(1), pp. 48-58. [https://doi.org/10.1520/GTJ10224J doi: 10.1520/GTJ10224J]</ref><ref name="WintonWeber1996">Winton, K., Weber, J.B., 1996. A Review of Field Lysimeter Studies to Describe the Environmental Fate of Pesticides. Weed Technology, 10(1), pp. 202-209. [https://doi.org/10.1017/S0890037X00045929 doi: 10.1017/S0890037X00045929]</ref>. Recently, there has been strong interest in the use of lysimeters to measure and monitor movement of per- and polyfluoroalkyl substances (PFAS) through the vadose zone<ref name="Anderson2021">Anderson, R.H., 2021. The Case for Direct Measures of Soil-to-Groundwater Contaminant Mass Discharge at AFFF-Impacted Sites. Environmental Science and Technology, 55(10), pp. 6580-6583. [https://doi.org/10.1021/acs.est.1c01543 doi: 10.1021/acs.est.1c01543]</ref><ref name="AndersonEtAl2022">Anderson, R.H., Feild, J.B., Dieffenbach-Carle, H., Elsharnouby, O., Krebs, R.K., 2022. Assessment of PFAS in Collocated Soil and Porewater Samples at an AFFF-Impacted Source Zone: Field-Scale Validation of Suction Lysimeters. Chemosphere, 308(1), Article 136247. [https://doi.org/10.1016/j.chemosphere.2022.136247 doi: 10.1016/j.chemosphere.2022.136247]</ref><ref name="SchaeferEtAl2024">Schaefer, C.E., Nguyen, D., Fang, Y., Gonda, N., Zhang, C., Shea, S., Higgins, C.P., 2024. PFAS Porewater Concentrations in Unsaturated Soil: Field and Laboratory Comparisons Inform on PFAS Accumulation at Air-Water Interfaces. Journal of Contaminant Hydrology, 264, Article 104359. [https://doi.org/10.1016/j.jconhyd.2024.104359 doi: 10.1016/j.jconhyd.2024.104359]&nbsp; [[Media: SchaeferEtAl2024.pdf | Open Access Manuscript]]</ref><ref name="SchaeferEtAl2023">Schaefer, C.E., Lavorgna, G.M., Lippincott, D.R., Nguyen, D., Schaum, A., Higgins, C.P., Field, J., 2023. Leaching of Perfluoroalkyl Acids During Unsaturated Zone Flushing at a Field Site Impacted with Aqueous Film Forming Foam. Environmental Science and Technology, 57(5), pp. 1940-1948. [https://doi.org/10.1021/acs.est.2c06903 doi: 10.1021/acs.est.2c06903]</ref><ref name="SchaeferEtAl2022">Schaefer, C.E., Lavorgna, G.M., Lippincott, D.R., Nguyen, D., Christie, E., Shea, S., O’Hare, S., Lemes, M.C.S., Higgins, C.P., Field, J., 2022. A Field Study to Assess the Role of Air-Water Interfacial Sorption on PFAS Leaching in an AFFF Source Area. Journal of Contaminant Hydrology, 248, Article 104001. [https://doi.org/10.1016/j.jconhyd.2022.104001 doi: 10.1016/j.jconhyd.2022.104001]&nbsp; [[Media: SchaeferEtAl2022.pdf | Open Access Manuscript]]</ref><ref name="QuinnanEtAl2021">Quinnan, J., Rossi, M., Curry, P., Lupo, M., Miller, M., Korb, H., Orth, C., Hasbrouck, K., 2021. Application of PFAS-Mobile Lab to Support Adaptive Characterization and Flux-Based Conceptual Site Models at AFFF Releases. Remediation, 31(3), pp. 7-26. [https://doi.org/10.1002/rem.21680 doi: 10.1002/rem.21680]</ref>. PFAS are frequently introduced to the environment through land surface application and have been found to be strongly retained within the upper 5 feet of soil<ref name="BrusseauEtAl2020">Brusseau, M.L., Anderson, R.H., Guo, B., 2020. PFAS Concentrations in Soils: Background Levels versus Contaminated Sites. Science of The Total Environment, 740, Article 140017. [https://doi.org/10.1016/j.scitotenv.2020.140017 doi: 10.1016/j.scitotenv.2020.140017]</ref><ref name="BiglerEtAl2024">Bigler, M.C., Brusseau, M.L., Guo, B., Jones, S.L., Pritchard, J.C., Higgins, C.P., Hatton, J., 2024. High-Resolution Depth-Discrete Analysis of PFAS Distribution and Leaching for a Vadose-Zone Source at an AFFF-Impacted Site. Environmental Science and Technology, 58(22), pp. 9863-9874. [https://doi.org/10.1021/acs.est.4c01615 doi: 10.1021/acs.est.4c01615]</ref>. PFAS recalcitrance in the vadose zone means that environmental program managers and consultants need a cost-effective way of monitoring concentration conditions within the vadose zone. Repeated soil sampling and extraction processes are time consuming and only give a representative concentration of total PFAS in the matrix<ref name="NickersonEtAl2020">Nickerson, A., Maizel, A.C., Kulkarni, P.R., Adamson, D.T., Kornuc, J. J., Higgins, C.P., 2020. Enhanced Extraction of AFFF-Associated PFASs from Source Zone Soils. Environmental Science and Technology, 54(8), pp. 4952-4962. [https://doi.org/10.1021/acs.est.0c00792 doi: 10.1021/acs.est.0c00792]</ref>, not what is readily transportable in mobile porewater<ref name="SchaeferEtAl2023"/><ref name="StultsEtAl2024">Stults, J.F., Schaefer, C.E., Fang, Y., Devon, J., Nguyen, D., Real, I., Hao, S., Guelfo, J.L., 2024. Air-Water Interfacial Collapse and Rate-Limited Solid Desorption Control Perfluoroalkyl Acid Leaching from the Vadose Zone. Journal of Contaminant Hydrology, 265, Article 104382. [https://doi.org/10.1016/j.jconhyd.2024.104382 doi: 10.1016/j.jconhyd.2024.104382]&nbsp; [[Media: StultsEtAl2024.pdf | Open Access Manuscript]]</ref><ref name="StultsEtAl2023">Stults, J.F., Choi, Y.J., Rockwell, C., Schaefer, C.E., Nguyen, D.D., Knappe, D.R.U., Illangasekare, T.H., Higgins, C.P., 2023. Predicting Concentration- and Ionic-Strength-Dependent Air–Water Interfacial Partitioning Parameters of PFASs Using Quantitative Structure–Property Relationships (QSPRs). Environmental Science and Technology, 57(13), pp. 5203-5215. [https://doi.org/10.1021/acs.est.2c07316 doi: 10.1021/acs.est.2c07316]</ref><ref name="BrusseauGuo2022">Brusseau, M.L., Guo, B., 2022. PFAS Concentrations in Soil versus Soil Porewater: Mass Distributions and the Impact of Adsorption at Air-Water Interfaces. Chemosphere, 302, Article 134938. [https://doi.org/10.1016/j.chemosphere.2022.134938 doi: 10.1016/j.chemosphere.2022.134938]&nbsp; [[Media: BrusseauGuo2022.pdf | Open Access Manuscript]]</ref>. Fortunately, lysimeters have been found to be a viable option for monitoring the concentration of PFAS in the mobile porewater phase in the vadose zone<ref name="Anderson2021"/><ref name="AndersonEtAl2022"/>. Note that while some lysimeters, known as weighing lysimeters, can directly measure water flux, the most commonly utilized lysimeters in PFAS investigations only provide measurements of porewater concentrations.
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A Conceptual Site Model (CSM) is a collection of information about a contaminated site that integrates the available evidence regarding its hydrogeologic setting, contaminant sources, exposure pathways, potential receptors, and site history (see ASTM E1689-95(2014)<ref name="ASTM2014a"> ASTM, 2014. Standard Guide for Developing Conceptual Site Models for Contaminated Sites. ASTM E1689-95(2014), ASTM International, West Conshohocken, PA. [https://doi.org/10.1520/E1689-95R14 DOI: 10.1520/E1689-95R14]  http://www.astm.org/cgi-bin/resolver.cgi?E1689</ref> and ASTM E2531-06(2014)<ref name="ASTM2014b"> ASTM, 2014. Standard Guide for Development of Conceptual Site Models and Remediation Strategies for Light Nonaqueous-Phase Liquids Released to the Subsurface. ASTM E2531-06(2014), ASTM International, West Conshohocken, PA. [https://doi.org/10.1520/E2531-06R14  DOI: 10.1520/E2531-06R14] http://www.astm.org/cgi-bin/resolver.cgi?E2531</ref>). When developing a CSM for an LNAPL site, it is important to understand that LNAPL releases evolve and change from what are referred to as Early Stage sites to Middle Stage and then to Late Stage sites<ref name="Sale2018"/> (Figure 1).
 
  
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An Early Stage site is characterized by the presence of a continuous LNAPL zone where a thick layer of LNAPL accumulation (also known as free product) is observed in monitoring wells. The continuous LNAPL zone (or LNAPL body) may be mobile at Early Stage sites, migrating into previously non-impacted areas. Removal of significant LNAPL mass by active pumping may be feasible at these sites. Early Stage sites are now relatively rare in the United States due to stringent environmental regulations enacted in the 1980s which emphasized preventing releases.
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==PFAS Background==
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[[File:Newell1w2Fig2a.png |thumb|500px| Figure 2a. Time lapse conceptualization of the formation of an LNAPL body<ref name="ITRC2019"> Interstate Technology and Regulatory Council (ITRC), 2019. LNAPL Training: Connecting the Science to Managing Sites. Part 1: Understanding LNAPL Behavior in the Subsurface. ITRC, Washington, DC. [[Media: ITRC2019_LNAPLtrainingPart1.pdf | Slides.pdf]]</ref>.]]
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PFAS are a broad class of chemicals with highly variable chemical structures<ref>Moody, C.A., Field, J.A., 1999. Determination of Perfluorocarboxylates in Groundwater Impacted by Fire-Fighting Activity. Environmental Science and Technology, 33(16), pp. 2800-2806. [https://doi.org/10.1021/es981355+ doi: 10.1021/es981355+]</ref><ref name="MoodyField2000">Moody, C.A., Field, J.A., 2000. Perfluorinated Surfactants and the Environmental Implications of Their Use in Fire-Fighting Foams. Environmental Science and Technology, 34(18), pp. 3864-3870. [https://doi.org/10.1021/es991359u doi: 10.1021/es991359u]</ref><ref name="GlügeEtAl2020">Glüge, J., Scheringer, M., Cousins, I.T., DeWitt, J.C., Goldenman, G., Herzke, D., Lohmann, R., Ng, C.A., Trier, X., Wang, Z., 2020. An Overview of the Uses of Per- and Polyfluoroalkyl Substances (PFAS). Environmental Science: Processes and Impacts, 22(12), pp. 2345-2373. [https://doi.org/10.1039/D0EM00291G doi: 10.1039/D0EM00291G]&nbsp; [[Media: GlügeEtAl2020.pdf | Open Access Article]]</ref>. One characteristic feature of PFAS is that they are fluorosurfactants, distinct from more traditional hydrocarbon surfactants<ref name="MoodyField2000"/><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]&nbsp; [[Media: Brusseau2018.pdf | Open Access Manuscript]]</ref><ref>Dave, N., Joshi, T., 2017. A Concise Review on Surfactants and Its Significance. International Journal of Applied Chemistry, 13(3), pp. 663-672. [https://doi.org/10.37622/IJAC/13.3.2017.663-672 doi: 10.37622/IJAC/13.3.2017.663-672]&nbsp; [[Media: DaveJoshi2017.pdf  | Open Access Article]]</ref><ref>García, R.A., Chiaia-Hernández, A.C., Lara-Martin, P.A., Loos, M., Hollender, J., Oetjen, K., Higgins, C.P., Field, J.A., 2019. Suspect Screening of Hydrocarbon Surfactants in Afffs and Afff-Contaminated Groundwater by High-Resolution Mass Spectrometry. Environmental Science and Technology, 53(14), pp. 8068-8077. [https://doi.org/10.1021/acs.est.9b01895 doi: 10.1021/acs.est.9b01895]</ref>. Fluorosurfactants typically have a fully or partially fluorinated, hydrophobic tail with ionic (cationic, zwitterionic, or anionic) head group that is hydrophilic<ref name="MoodyField2000"/><ref name="GlügeEtAl2020"/>. The hydrophobic tail and ionic head group mean PFAS are very stable at hydrophobic adsorption interfaces when present in the aqueous phase<ref>Krafft, M.P., Riess, J.G., 2015. Per- and Polyfluorinated Substances (PFASs): Environmental Challenges. Current Opinion in Colloid and Interface Science, 20(3), pp. 192-212. [https://doi.org/10.1016/j.cocis.2015.07.004 doi: 10.1016/j.cocis.2015.07.004]</ref>. Examples of these interfaces include naturally occurring organic matter in soils and the air-water interface in the vadose zone<ref>Schaefer, C.E., Culina, V., Nguyen, D., Field, J., 2019. Uptake of Poly- and Perfluoroalkyl Substances at the Air–Water Interface. Environmental Science and Technology, 53(21), pp. 12442-12448. [https://doi.org/10.1021/acs.est.9b04008 doi: 10.1021/acs.est.9b04008]</ref><ref>Lyu, Y., Brusseau, M.L., Chen, W., Yan, N., Fu, X., 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]</ref><ref>Costanza, J., Arshadi, M., Abriola, L.M., Pennell, K.D., 2019. Accumulation of PFOA and PFOS at the Air-Water Interface. Environmental Science and Technology Letters, 6(8), pp. 487-491. [https://doi.org/10.1021/acs.estlett.9b00355 doi: 10.1021/acs.estlett.9b00355]</ref><ref>Li, F., Fang, X., Zhou, Z., Liao, X., Zou, J., Yuan, B., Sun, W., 2019. Adsorption of Perfluorinated Acids onto Soils: Kinetics, Isotherms, and Influences of Soil Properties. Science of The Total Environment, 649, pp. 504-514. [https://doi.org/10.1016/j.scitotenv.2018.08.209 doi: 10.1016/j.scitotenv.2018.08.209]</ref><ref>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]&nbsp; [[Media: NguyenEtAl2020.pdf | Open Access Article]]</ref>. Their strong adsorption to both soil organic matter and the air-water interface is a major contributor to elevated concentrations of PFAS observed in the upper 5 feet of the soil column<ref name="BrusseauEtAl2020"/><ref name="BiglerEtAl2024"/>. While several other PFAS partitioning processes exist<ref name="Brusseau2018"/>, adsorption to solid phase soils and air-water interfaces are the two primary processes present at nearly all PFAS sites<ref>Brusseau, M.L., Yan, N., Van Glubt, S., Wang, Y., Chen, W., Lyu, Y., Dungan, B., Carroll, K.C., 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]</ref>. The total PFAS mass obtained from a vadose zone soil sample contains the solid phase, air-water interfacial, and aqueous phase PFAS mass, which can be converted to porewater concentrations using Equation 1<ref name="BrusseauGuo2022"/>.</br>
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[[File:Newell1w2Fig2b.png |thumb|500px| Figure 2b. Sand tank experiment of an LNAPL release<ref name="ITRC2019"/>.]]
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:: <big>'''Equation 1:'''</big>&nbsp;&nbsp; [[File: StultsEq1.png | 400 px]]</br>
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Where ''C<sub>p</sub>'' is the porewater concentration, ''C<sub>t</sub>'' is the total PFAS concentration, ''ρ<sub>b</sub>'' is the bulk density of the soil, ''θ<sub>w</sub>'' is the volumetric water content, ''R<sub>d</sub>'' is the PFAS retardation factor, ''K<sub>d</sub>'' is the solid phase adsorption coefficient, ''K<sub>ia</sub>'' is the air-water interfacial adsorption coefficient, and ''A<sub>aw</sub>'' is the air-water interfacial area. The air-water interfacial area of the soil is primarily a function of both the soil properties and the degree of volumetric water saturation in the soil. There are several methods of estimating air-water interfacial areas including thermodynamic functions based on the soil moisture retention curve. However, the thermodynamic function has been shown to underestimate air-water interfacial area<ref name="Brusseau2023">Brusseau, M.L., 2023. Determining Air-Water Interfacial Areas for the Retention and Transport of PFAS and Other Interfacially Active Solutes in Unsaturated Porous Media. Science of The Total Environment, 884, Article 163730. [https://doi.org/10.1016/j.scitotenv.2023.163730 doi: 10.1016/j.scitotenv.2023.163730]&nbsp; [[Media: Brusseau2023.pdf  | Open Access Article]]</ref>, and must typically be scaled using empirical scaling factors. An empirical method recently developed to estimate air-water interfacial area is presented in Equation 2<ref name="Brusseau2023"/>.</br>
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:: <big>'''Equation 2:'''</big>&nbsp;&nbsp; [[File: StultsEq2.png | 400 px]]</br>
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Where ''S<sub>w</sub>'' is the water phase saturation as a ratio of the water content over the volumetric soil porosity, and ''d<sub>50</sub>'' is the median grain diameter.
  
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Many sites in the U.S. are now considered to be in the Middle Stage, where the LNAPL thickness in wells has been largely depleted by natural spreading of the LNAPL body, [[Natural Source Zone Depletion (NSZD)]], smearing of the water table, and/or active remediation, and where the LNAPL bodies are stable or shrinking<ref name="LNAPL-3"/><ref name="Sale2018"/> (Figure 1). Active pumping characteristically only recovers LNAPL at relatively low rates of under 100 gallons per acre per year at Middle Stage sites, but NSZD rates may be much higher, on the order of 100s to 1,000s of gallons per acre per year.  Middle Stage dissolved phase plumes, typically comprised of monoaromatics such as benzene, toluene, ethyl benzene, and xylenes, are stable or shrinking over time.
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==Lysimeters Background==
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[[File: StultsFig1.png |thumb|600 px|Figure 1. (a) A field suction lysimeter with labeled parts typically used in field settings – Credit: Bibek Acharya and Dr. Vivek Sharma, UF/IFAS, https://edis.ifas.ufl.edu/publication/AE581. (b) Laboratory suction lysimeters used in Schaefer ''et al.'' 2024<ref name="SchaeferEtAl2024"/>, which employed the use of micro-sampling suction lysimeters. (c) A field lysimeter used in Schaefer ''et al.'' 2023<ref name="SchaeferEtAl2023"/>. (d) Diagram of a drainage wicking lysimeter – Credit: Edaphic Scientific, https://edaphic.com.au/products/water/lysimeter-wick-for-drainage/]]
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Lysimeters,&nbsp;generally&nbsp;speaking, refer to instruments which collect water from unsaturated soils<ref name="MeissnerEtAl2020"/><ref name="RogersMcConnell1993"/>. However, there are multiple types of lysimeters which can be employed in field or laboratory settings. There are three primary types of lysimeters relevant to PFAS listed here and shown in Figure 1a-d.
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# <u>Suction Lysimeters (Figure 1a,b):</u> These lysimeters are the most relevant for PFAS sampling and are the majority of discussion in this article. These lysimeters operate by extracting liquid from the unsaturated vadose zone by applying negative suction pressure at the sampling head<ref name="CostanzaEtAl2025"/><ref name="SchaeferEtAl2024"/><ref name="QuinnanEtAl2021"/>. The sampling head is typically constructed of porous ceramic or stainless steel. A PVC case or stainless-steel case is attached to the sampling head and extends upward above the ground surface. Suction lysimeters are typically installed between 1 and 9 feet below ground surface, but can extend as deep as 40-60 feet in some cases<ref name="CostanzaEtAl2025"/>. Shallow lysimeters (< 10 feet) are typically installed using a hand auger. For ceramic lysimeters, a silica flour slurry should be placed at the base of the bore hole and allowed to cover the ceramic head before backfilling the hole partially with natural soil. Once the hole is partially backfilled with soil to cover the sampling head, the remainder of the casing should be sealed with hydrated bentonite chips. When sampling events occur, suction is applied at the ground surface using a rubber gasket seal and a hand pump or electric pump. After sufficient porewater is collected (the time for which can vary greatly based on the soil permeability and moisture content), the seal can be removed and a peristaltic pump used to extract liquid from the lysimeter.
 +
# <u>Field Lysimeters (Figure 1c):</u> These large lysimeters can be constructed from plastic or metal sidings. They can range from approximately 2 feet in diameter to as large as several meters in diameter<ref name="MeissnerEtAl2020"/>. Instrumentation such as soil moisture probes and tensiometers, or even multiple suction lysimeters, are typically placed throughout the lysimeter to measure the movement of water and determine characteristic soil moisture release curves<ref name="Stannard1992"/><ref name="WintonWeber1996"/><ref name="SchaeferEtAl2023"/><ref name="SchaeferEtAl2022"/><ref>van Genuchten, M.Th. , 1980. A Closed‐form Equation for Predicting the Hydraulic Conductivity of Unsaturated Soils. Soil Science Society of America Journal, 44(5), pp. 892-898. [https://doi.org/10.2136/sssaj1980.03615995004400050002x doi: 10.2136/sssaj1980.03615995004400050002x]</ref>. Water is typically collected at the base of the field lysimeter to determine net recharge through the system. These field lysimeters are intended to represent more realistic, intermediate scale conditions of field systems.
 +
# <u>Drainage Lysimeters (Figure 1d):</u> Also known as a “wick” lysimeter, these lysimeters typically consist of a hollow cup attached to a spout which protrudes above ground to relieve air pressure from the system and act as a sampling port. The hollow cup typically has filters and wicking devices at the base to collect water from the soil. The cup is filled with natural soil and collects water as it percolates through the vadose zone. These lysimeters are used to directly monitor net recharge from the vadose zone to the groundwater table and could be useful in determining PFAS mass flux.
  
−
Late Stage sites only have a sparse distribution of residual (trapped) LNAPL due to long-term NSZD and any active remediation that has been performed at the site. The potential risks to receptors are typically low at Late Stage sites due to relatively low concentrations of LNAPL constituents in the dissolved phase and/or vapor plumes.
+
==Analysis of PFAS Concentrations in Soil and Porewater==
 +
{| class="wikitable mw-collapsible" style="float:left; margin-right:20px; text-align:center;"
 +
|+Table 1. Measured and Predicted PFAS Concentrations in Porewater for Select PFAS in Three Different Soils
 +
|-
 +
!Site
 +
!PFAS
 +
!Field</br>Porewater</br>Concentration</br>(&mu;g/L)
 +
!Lab Core</br>Porewater</br>Concentration</br>(&mu;g/L)
 +
!Predicted</br>Porewater</br>Concentration</br>(&mu;g/L)
 +
|-
 +
|Site A||PFOS||6.2 ± 3.4||3.0 ± 0.37||6.6 ± 3.3
 +
|-
 +
|Site B||PFOS||2.2 ± 2.0||0.78 ± 0.38||2.8
 +
|-
 +
|rowspan="3"|Site C||PFOS||13 ± 4.1||680 ± 460||164 ± 75
 +
|-
 +
|8:2 FTS||1.2 ± 0.46||52 ± 13||16 ± 6.0
 +
|-
 +
|PFHpS||0.36 ± 0.051||2.9 ± 2.0||5.9 ± 3.4
 +
|}
 +
[[File: StultsFig2.png | thumb | 600 px | Figure 2. Field Measured PFAS concentration Data (Orange) and Lab Core Measured Concentration Data (Blue) for four PFAS impacted sites<ref name="AndersonEtAl2022"/>]]
 +
[[File: StultsFig3.png | thumb | 400 px | Figure 3. Measured and predicted data for PFAS concentrations from a single site field lysimeter study. Model predictions both with and without PFAS sorption to the air-water interface were considered<ref name="SchaeferEtAl2023"/>.]]
 +
Schaefer&nbsp;''et&nbsp;al.''<ref name="SchaeferEtAl2024"/>&nbsp;measured&nbsp;PFAS porewater concentrations with field and laboratory suction lysimeters across several sites. Intact cores from the site were collected for soil water extraction using laboratory lysimeters. The lysimeters were used to directly compare field derived measurements of PFAS concentration in the mobile porewater phase. Results from measurements are for four sites presented in Figure 2.
  
−
==LNAPL Body Formation==
+
Data from sites A and B showed reasonably good agreement (within ½ order of magnitude) for most PFAS measured in the systems. At site C, more hydrophobic constituents (> C6 PFAS) tended to have higher concentrations in the lab core than the field site while less hydrophobic constituents (< C6) had higher concentrations in the field than lab cores. Site D showed substantially greater (1 order of magnitude or more) PFAS concentrations measured in the laboratory-collected porewater sample compared to what was measured in the field lysimeters. This discrepancy for the Site D soil can likely be attributed to soil heterogeneity (as indicated by ground penetrating radar) and the fact that the soil consisted of back-filled materials rather than undisturbed native soils.
−
LNAPLs released from tanks, pits, pipelines, or other sources will percolate downwards under the influence of gravity through permeable pathways in the unsaturated zone (e.g., soil pore space, fractures, and macropores) depending on the volume and pressure head of the LNAPL release, until encountering an impermeable layer or the water table, causing the LNAPL body to spread laterally. The Interstate Technology and Regulatory Council (ITRC)<ref name="LNAPL-3"/> describes this downward movement toward the water table this way:
+
 +
Site&nbsp;C&nbsp;showed&nbsp;elevated PFAS concentrations in the laboratory collected porewater for the more surface-active compounds. This increase was attributed to the soil wetting that occurred at the bench scale, which was reasonably described by the model shown in Equations 1 and 2 (see Table 1<ref name="AndersonEtAl2022"/>). Equations 1 and 2 were also used to predict PFAS porewater concentrations (using porous cup lysimeters) in a highly instrumented test cell<ref name="SchaeferEtAl2023"/>(Figure 3). The ability to predict soil concentrations from recurring porewater samples is critical to the practical application of lysimeters in field settings<ref name="AndersonEtAl2022"/>.
  
−
<blockquote>''During the downward movement of LNAPL through the soil, the presence of confining layers, subsurface heterogeneities, or other preferential pathways may result in irregular and complex lateral spreading and/or perching of LNAPL before the water table is encountered. Once at the water table, the LNAPL will spread laterally in a radial fashion as well as penetrate vertically downward into the saturated zone, displacing water to some depth proportional to the driving force of the vertical LNAPL column (or LNAPL head). The vertical penetration of LNAPL into the saturated zone will continue to occur as long as the downward force produced by the LNAPL head or pressure from the LNAPL release exceeds the counteracting forces produced by the resistance of the soil matrix and the buoyancy resulting from the density difference between LNAPL and groundwater.''<ref name="LNAPL-3"/></blockquote>
+
Results from suction lysimeters studies and field lysimeter studies show that PFAS concentrations in porewater predicted from soil concentrations using Equations 1 and 2 generally have reasonable agreement with measured ''in situ'' porewater data when air-water interfacial partitioning is considered. Results show that for less hydrophobic components like PFOA, the impact of air-water interfacial adsorption is less significant than for highly hydrophobic components like PFOS. The soil for the field lysimeter in Figure 3 was a sandy soil with a relatively low air-water interfacial area. The effect of air-water interfacial partitioning is expected to be much more significant for a greater range of PFAS in soils with high capillary pressure (i.e. silts/clays) with higher associated air-water interfacial areas<ref name="Brusseau2023"/><ref>Peng, S., Brusseau, M.L., 2012. Air-Water Interfacial Area and Capillary Pressure: Porous-Medium Texture Effects and an Empirical Function. Journal of Hydrologic Engineering, 17(7), pp. 829-832. [https://doi.org/10.1061/(asce)he.1943-5584.0000515 doi: 10.1061/(asce)he.1943-5584.0000515]</ref><ref>Brusseau, M.L., Peng, S., Schnaar, G., Costanza-Robinson, M.S., 2006. Relationships among Air-Water Interfacial Area, Capillary Pressure, and Water Saturation for a Sandy Porous Medium. Water Resources Research, 42(3), Article W03501, 5 pages. [https://doi.org/10.1029/2005WR004058 doi: 10.1029/2005WR004058]&nbsp; [[Media: BrusseauEtAl2006.pdf | Free Access Article]]</ref>.
  
−
While the release at the surface is still active, the LNAPL body can expand until the LNAPL addition rate is equal to the NSZD depletion rate. However, once the release at the surface is stopped, the expansion will stop relatively quickly, and the LNAPL body will stabilize. Figure 2a shows a conceptual depiction of this release scenario and Figure 2b shows a sand tank experiment of an LNAPL release.  Because of the buoyancy effects, LNAPL releases that reach the water table will form LNAPL bodies that “like icebergs, are partially above and below the water table”.<ref name="Sale2018"/>
+
==Summary and Recommendations==
 +
The majority of research with lysimeters for PFAS site investigations has been done using porous cup suction lysimeters<ref name="CostanzaEtAl2025"/><ref name="AndersonEtAl2022"/><ref name="SchaeferEtAl2024"/><ref name="QuinnanEtAl2021"/>. Porous cup suction lysimeters are advantageous because they can be routinely sampled or sampled after specific wetting or drying events much like groundwater wells. This sampling is easier and more efficient than routinely collecting soil samples from the same locations. Co-locating lysimeters with soil samples is important for establishing the baseline soil concentration levels at the lysimeter location and developing correlations between the soil concentrations and the mobile porewater concentration<ref name="CostanzaEtAl2025"/>. Appropriate standard operation procedures for lysimeter installation and operation have been established and have been reviewed in recent literature<ref name="CostanzaEtAl2025"/><ref name="SchaeferEtAl2024"/>. Lysimeters should typically be installed near the source area and just above the maximum groundwater level elevation to obtain accurate results of porewater concentrations year round. Depending upon the geology and vertical PFAS distribution in the soil, multilevel lysimeter installations should also be considered.
  
−
==Key Implications of the LNAPL Conceptual Site Model==
+
Results from several lysimeters studies across multiple field sites and modelling analysis has shown that lysimeters can produce reasonable results between field and laboratory studies<ref name="SchaeferEtAl2024"/><ref name="SchaeferEtAl2023"/><ref name="SchaeferEtAl2022"/>. Transient effects of wetting and drying as well as media heterogeneity affects appear to be responsible for some variability and uncertainty in lysimeter based PFAS measurements in the vadose zone. These mobile porewater concentrations can be coupled with effective recharge estimates and simplified modelling approaches to determine mass flux from the vadose zone to the underlying groundwater<ref name="Anderson2021"/><ref name="StultsEtAl2024"/><ref name="BrusseauGuo2022"/><ref>Stults, J.F., Schaefer, C.E., MacBeth, T., Fang, Y., Devon, J., Real, I., Liu, F., Kosson, D., Guelfo, J.L., 2025. Laboratory Validation of a Simplified Model for Estimating Equilibrium PFAS Mass Leaching from Unsaturated Soils. Science of The Total Environment, 970, Article 179036. [https://doi.org/10.1016/j.scitotenv.2025.179036 doi: 10.1016/j.scitotenv.2025.179036]</ref><ref>Smith, J. Brusseau, M.L., Guo, B., 2024. An Integrated Analytical Modeling Framework for Determining Site-Specific Soil Screening Levels for PFAS. Water Research, 252, Article121236. [https://doi.org/10.1016/j.watres.2024.121236 doi: 10.1016/j.watres.2024.121236]</ref>.
−
The nature of multi-phase flow processes in porous media (e.g., the interaction of LNAPL, water, and air in the pore spaces of an unconsolidated aquifer) has several important implications for environmental professionals in areas including interpretation of LNAPL thickness in monitoring wells and assessment of the long-term risk associated with LNAPL source zones. A few of the key implications are described below.
 
  
−
===Three States of LNAPL===
+
Future research opportunities should address the current key uncertainties related to the use of lysimeters for PFAS investigations, including:
−
LNAPL can be found in the subsurface in three different states:
+
#<u>Collect larger datasets of PFAS concentrations</u> to determine how transient wetting or drying periods and media type affect PFAS concentrations in the mobile porewater. Some research has shown that non-equilibrium processes can occur in the vadose zone, which can affect grab sample concentration in the porewater at specific time periods.  
−
 
+
#<u>More work should be done with flux averaging lysimeters</u> like the drainage cup or wicking lysimeter. These lysimeters can directly measure net recharge and provide time averaged concentrations of PFAS in water over the sampling period. However, there is little work detailing their potential applications in PFAS research, or operational considerations for their use in remedial investigations for PFAS.
−
# '''Residual LNAPL''' is trapped and immobile but can undergo composition and phase changes and generate dissolved hydrocarbon plumes in saturated zones and/or vapors in unsaturated zones. The fraction of the total pore space occupied by this discontinuous LNAPL is referred to as the residual saturation, with other phases such as water and air in the remainder of the pore space.
+
#<u>Lysimeters should be coupled with monitoring of wetting and drying</u> in the vadose zone using ''in situ'' soil moisture sensors or tensiometers and groundwater levels. Direct measurements of soil saturation at field sites are vital to directly correlate porewater concentrations with soil concentrations. Similarly, groundwater level fluctuations can inform net recharge estimates. By collecting these data we can continue to improve partitioning and leaching models which can relate porewater concentrations to total PFAS mass in soils and PFAS leaching at field sites.
−
# '''Mobile LNAPL''' is LNAPL at greater than the residual saturation. Mobile LNAPL can accumulate in a well and is potentially recoverable, but is not migrating (i.e., the LNAPL body is not expanding).
+
#<u>Comparisons of various bench-scale leaching or desorption tests to field-based lysimeter data</u> are recommended. The ability to correlate field measurements of PFAS concentrations with estimates of leaching from laboratory studies would provide a powerful method to empirically estimate PFAS leaching from field sites.
−
# '''Migrating LNAPL''' is LNAPL at greater than the residual concentration which is observed to expand into previously non-impacted locations over time (e.g., LNAPL appears in a monitoring well that had previously been clean). 
 
−
 
 
−
These three LNAPL states can cause different concerns and in some cases require different remediation goals.
 
−
 
 
−
===LNAPL “Apparent Thickness” is a Poor Metric for Risk Management===
 
−
[[File:Newell1w2Fig3.png |thumb|left|600px| Figure 3.  Five LNAPL Thickness Scenarios for five different physical settings<ref name="Sale2018"/>.]]
 
−
[[File:Newell1w2Fig4.png |thumb|350px| Figure 4.  Apparent LNAPL thickness versus LNAPL transmissivity, showing no correlation<ref name="Hawthorne2015">Hawthorne, J.M., 2015.  Nationwide (USA) Statistical Analysis of LNAPL Transmissivity, in: R. Darlington and A.C. Barton (Chairs), Bioremediation and Sustainable Environmental Technologies—2015. Third International Symposium on Bioremediation and Sustainable Environmental Technologies (Miami, FL), page C-017, Battelle Memorial Institute, Columbus, OH.  www.battelle.org/biosymp  [[Media:Hawthorne2015.pdf | Abstract.pdf]]</ref>.]]
 
−
LNAPL thickness in monitoring wells is often referred to as the “apparent LNAPL thickness” because at first glance this LNAPL thickness might be expected to be the thickness of LNAPL that is in the formation, but in reality it is not well correlated with the thickness of the LNAPL zone in the subsurface for several reasons.
 
−
 
 
−
First, different physical settings can produce different LNAPL thicknesses in monitoring wells.  Sale et al. (2018) show five different scenarios that produce very different responses with regard to apparent LNAPL thickness (Figure 3).  Scenario A shows an LNAPL apparent thickness in the monitoring well that is at static equilibrium with LNAPL in an unconfined aquifer. Scenario B, while also an unconfined aquifer, is comprised of very fine-grained soils that cause the LNAPL thickness in the well to be much higher than in Scenario A.  In Scenario C, the LNAPL has accumulated under a confined unit (likely due to an underground release of LNAPL below the confining unit), and the LNAPL has risen above the groundwater potentiometric surface, leading to a large (and misleading) LNAPL thickness in the monitoring well.  Scenario D, LNAPL in a perched unit, also shows a very different response from the other scenarios.  Scenario E, LNAPL in fractured system, shows that the LNAPL can penetrate below the water table, and that LNAPL thickness in a well is dependent on the pressure from accumulation of LNAPL in the fractures<ref name="Sale2018"/>.
 
−
 
 
−
Second, apparent LNAPL thickness is affected by changes in the groundwater surface elevation (or water table). Generally, when groundwater elevations are higher than typical, the LNAPL thickness in monitoring wells will decrease or go to zero because the groundwater will redistribute any mobile LNAPL into what previously was the unsaturated zone.  During lower groundwater elevation periods, much more of the LNAPL will occur as a continuous phase near the water table, leading to higher LNAPL thicknesses in wells.
 
−
 
 
−
Overall, LNAPL thickness measurements are useful for delineating the extent of mobile LNAPL in the saturated zone and can provide useful data for understanding the vertical distribution of LNAPL in the formation<ref name="Hawthorne2011">Hawthorne, J.M., 2011. Diagnostic Gauge Plots—Simple Yet Powerful LCSM Tools. Applied NAPL Science Review (ANSR), 1(2). [http://naplansr.com/diagnostic-gauge-plots-volume-1-issue-2-february-2011/ Website] [[Media:Hawthorne2011.pdf | Report.pdf]]</ref><ref name="Kirkman2013">Kirkman, A.J., Adamski, M., and Hawthorne, M., 2013. Identification and Assessment of Confined and Perched LNAPL Conditions. Groundwater Monitoring and Remediation, 33 (1), pp. 75–86. [https://doi.org/10.1111/j.1745-6592.2012.01412.x  DOI:10.1111/j.1745-6592.2012.01412.x]</ref>. But LNAPL thickness by itself is a very poor indicator of the feasibility of LNAPL recovery<ref name="LNAPL-2">Interstate Technology and Regulatory Council (ITRC), 2009. Evaluating LNAPL Remedial Technologies for Achieving Project Goals. LNAPL-2. ITRC, LNAPLs Team, Washington, DC. www.itrcweb.org  [[Media:ITRC-LNAPL-2.pdf | Report.pdf]]</ref><ref name="Hawthorne2015"/> (see [[NAPL Mobility]]) (Figure 4).  Because there is little correlation between apparent LNAPL thickness and LNAPL mobility, there is also little correlation between apparent thickness and the risk to receptors from the LNAPL.
 
−
 
 
−
===Complete LNAPL Remediation Is Very Challenging===
 
−
Sale et al. (2018) described the problems with attaining complete LNAPL remediation this way:
 
−
 
 
−
<blockquote>''Experience of the last few decades has taught us: 1) our best efforts often leave some LNAPL in place, and 2) the remaining LNAPL often sustains exceedances of drinking water standards in release areas for extended periods. Entrapment of LNAPLs at residual saturations is a primary factor constraining our success. Other challenges include the low solubility of LNAPL, the complexity of the subsurface geologic environment, access limitations associated with surface structures, and concentration goals that are often three to five orders of magnitude less than typical initial concentrations within LNAPL zones.''<ref name="Sale2018"/></blockquote>
 
−
 
 
−
In particular, the discontinuous residual LNAPL cannot be removed (or recovered) by pumping, and ''in situ'' remediation is expensive and not completely effective (see [[LNAPL Remediation Technologies]]).  However, many regulatory programs require “LNAPL recovery to the extent practicable.”  The lack of quantitative metrics and the lack of correlation between apparent LNAPL thicknesses and subsurface LNAPL makes this a problematic requirement in many cases and the ITRC (2018) cautions “Thickness or concentration data alone may not provide a sound basis for defining the point at which a cleanup objective is achieved.”<ref name="LNAPL-3"/>  However, Sale et al. (2018) describe metrics such as LNAPL transmissivity, limited/infrequent well thicknesses, decline curve analysis, asymptotic analysis, and comparison to NSZD rates that can be used to determine when LNAPL has been removed the extent practicable<ref name="Sale2018"/>.
 
−
 
 
−
===Attenuation Processes are Active and Important===
 
−
Both LNAPL source zones and their dissolved phase hydrocarbon plumes are attenuated by biodegradation and other attenuation process.  In the source zone, this attenuation is called [[Natural Source Zone Depletion (NSZD)]] (see also [[Natural Attenuation in Source Zone and Groundwater Plume - Bemidji Crude Oil Spill]]).  In the dissolved plume it is called [[Monitored Natural Attenuation (MNA)]] (see also  [[Biodegradation - Hydrocarbons]]).  These processes generally limit the length of dissolved phase hydrocarbon plumes to a few hundred feet<ref name="Newell1998">Newell, C.J., and Connor, J.A., 1998. Characteristics of Dissolved Hydrocarbon Plumes: Results from Four Studies, Version 1.1. American Petroleum Institute, Soil/Groundwater Technical Task Force, Washington, DC. [https://www.enviro.wiki/index.php?title=File:Newell-1998-chararacterization_of_dissolved_Pet._Hydro_Plumes.pdf  Report.pdf]</ref> via processes that have been well known and understood since the mid-1990s.
 
−
 
 
−
However, NSZD is “by far, the biggest new idea for LNAPLs in the last decade.”<ref name="Sale2018"/>  Originally, LNAPL bodies were thought to attenuate very slowly via dissolution and volatilization.  In 2006, it was discovered that NSZD rates are orders of magnitude higher than originally thought, largely due to direct biodegradation of LNAPL constituents to methane and carbon dioxide by methanogenic consortiums of naturally occurring bacteria<ref name="Lundegard2006">Lundegard, P.D., and Johnson, P.C., 2006. Source Zone Natural Attenuation at Petroleum Spill Sites—II: Application to a Former Oil Field. Groundwater Monitoring and Remediation. 26(4), pp. 93-106.  [https://doi.org/10.1111/j.1745-6592.2006.00115.x  DOI: 10.1111/j.1745-6592.2006.00115.x]</ref><ref name="Garg2017">Garg, S., Newell, C., Kulkarni, P., King, D., Adamson, D.T., Irianni Renno, M., and Sale, T., 2017. Overview of Natural Source Zone Depletion: Processes, Controlling Factors, and Composition Change. Groundwater Monitoring and Remediation, 37(3), pp. 62-81.  [https://doi.org/10.1111/gwmr.12219 DOI:  10.1111/gwmr.12219] [[Media:Garg2017gwmr.12219.pdf | Report.pdf]]</ref>.  NSZD processes play an important role in risk mitigation and the long-term stability of LNAPL bodies<ref name="Mahler2012">
 
−
Mahler, N., Sale, T., and Lyverse, M., 2012. A Mass Balance Approach to Resolving LNAPL Stability. Groundwater, 50(6), pp 861-871.  [https://doi.org/10.1111/j.1745-6584.2012.00949.x DOI: 10.1111/j.1745-6584.2012.00949.x]</ref><ref name="LNAPL-3"/>.
 
−
 
 
−
===Risk from LNAPL Source Zones Diminishes Over Time===
 
−
At Early Stage LNAPL sites, the expansion of the LNAPL body is a risk that needs to be addressed.  Fortunately, this type of site is relatively rare.  For Middle and Late Stage sites, the primary risks are associated with phase changes (dissolution of the LNAPL forming a dissolved plume and volatilization from the LNAPL or dissolved plume forming hydrocarbon vapors). As described above, MNA can often control the dissolved phase (see [[Monitored Natural Attenuation (MNA) of Fuels]]), while aerobic biodegradation in the unsaturated zone greatly reduces the vapor intrusion risk from hydrocarbon vapors (see [[Vapor Intrusion - Separation Distances from Petroleum Sources]]).
 
−
 
 
−
Understanding LNAPL body mobility and stability is important to understand the potential risks posed by LNAPL.  The relative magnitude of LNAPL mobility can be determined by measuring the LNAPL transmissivity (see [[NAPL Mobility]]).  If the transmissivity is below a threshold level (in the range of 0.1 to 0.8 ft<sup>2</sup>/day) then the LNAPL likely cannot be recovered efficiently by pumping, but above this transmissivity level recovery is feasible<ref name="LNAPL-3"/>.  Michigan’s LNAPL guidance states “if the NAPL has a transmissivity greater than 0.5 ft<sup>2</sup>/day, it is likely that the NAPL can be recovered in a cost-effective and efficient manner unless a demonstration is made to show otherwise.”  Kansas LNAPL guidance requires “recovery of all LNAPL with a transmissivity greater than 0.8 ft<sup>2</sup>/day that can be recovered in an efficient, cost-effective manner.”<ref name="LNAPL-3"/>.  The stability of the entire LNAPL body can be evaluated using statistical tools to determine if migration of LNAPL is occurring<ref name="Hawthorne2013">Hawthorne, J.M., Stone, C.D., Helsel, D., 2013. LNAPL Body Stability Part 2: Daughter Plume Stability via Spatial Moments Analysis. Applied NAPL Science Review (ANSR), 3(5).  [http://naplansr.com/lnapl-body-stability-part-2-daughter-plume-stability-via-spatial-moments-analysis-volume-3-issue-5-september-2013/ Website] [[Media:Hawthorne2013.pdf | Report.pdf]]</ref>.
 
−
 
 
−
==Overview of Modern LNAPL Conceptual Site Model==
 
−
[[File:Newell1w2Fig5.png |thumb|500px| Figure 5.  A higher tier of LNAPL CSM is useful as LNAPL site complexity increases<ref name="LNAPL-3"/>.]]
 
−
The ITRC (2018) describes the typical evolution of an LCSM over the course of the remediation process which can be broken into three separate stages:
 
−
* An ''Initial LCSM'' focuses on identifying the LNAPL concerns, such as a risk to health or safety, any LNAPL migration, LNAPL-specific regulations, and physical or aesthetic impacts.
 
−
* A ''Remedy Selection LCSM'' supports remedial technology evaluation by characterizing aspects of the LNAPL and site subsurface that may impact remedial technology performance.
 
−
* A ''Design and Performance LCSM'' focuses on presenting the technical information needed to establish remediation objectives, design and implement remedies or control measures, and track progress toward defined remediation endpoints.
 
−
 
 
−
One key question when developing an LCSM is “how much data is enough.”  In general, the answer is that the existing data is sufficient for the current stage of the remediation project when it allows the stakeholders to agree on a path forward<ref name="LNAPL-3"/>. Figure 5 shows that as the level of complexity of a site increases, a higher tier of LCSM is useful to provide enough information for making decisions<ref name="LNAPL-3"/><ref name="ASTM2014a"/>.  The higher tier of information could be higher data density, additional tools for a given line of evidence, or other evaluations.
 
−
 
 
−
==LNAPL Concerns, Remediation Goals and Objectives==
 
−
Finally, the ITRC (2018) provides a methodology for identifying LNAPL concerns, verifying those concerns, selecting LNAPL remediation goals, and determining LNAPL remediation objectives.  Examples of each of these concepts are provided below:
 
−
 
 
−
* '''Potential Concerns:'''  Human or ecological risk concerns, fire or explosivity issues, LNAPL migration, LNAPL-specific regulatory concerns, other concerns such as odors or geotechnical issues.
 
−
* '''Verifying Concerns:'''  Measure LNAPL transmissivity to determine if it is recoverable; measure vertical and horizontal separation distances between buildings and LNAPL bodies to screen for vapor intrusion concerns.
 
−
* '''Remediation Goals:'''  Reduce mobile LNAPL saturation, abate unacceptable soil concentrations, terminate LNAPL body migration, abate unacceptable constituent concentrations in dissolved and vapor phases.
 
−
* '''Remediation Objectives:'''  Recover LNAPL to the extent practicable based on transmissivity, reduce soil concentrations to below regulatory limits, stop LNAPL migration with a barrier, contain migrating groundwater plume (if present), reduce groundwater and vapor concentration to acceptable levels.
 
−
* '''Remediation Technologies:'''  LNAPL Mass Recovery technologies, LNAPL phase change technologies, LNAPL Mass Control technologies, combinations of technologies.
 
−
 
 
−
Overall, a LNAPL Conceptual Site Model that integrates key site specific information and current technical knowledge about LNAPL sites in general is instrumental to successful site management, where LNAPL concerns drive remediation goals, goals drive remediation objectives, and the objectives form the basis for the selection of remediation technologies.  
 
  
 
==References==
 
==References==
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+
<references />
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<references/>
 
  
 
==See Also==
 
==See Also==
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Latest revision as of 15:50, 15 January 2026

Lysimeters for Measuring PFAS Concentrations in the Vadose Zone

PFAS are frequently introduced to the environment through soil surface applications which then transport through the vadose zone to reach underlying groundwater receptors. Due to their unique properties and resulting transport and retention behaviors, PFAS in the vadose zone can be a persistent contaminant source to underlying groundwater systems. Determining the fraction of PFAS present in the mobile porewater relative to the total concentrations in soils is critical to understanding the risk posed by PFAS in vadose zone source areas. Lysimeters are instruments that have been used by agronomists and vadose zone researchers for decades to determine water flux and solute concentrations in unsaturated porewater. Lysimeters have recently been developed as a critical tool for field investigations and characterizations of PFAS impacted source zones.

Related Article(s):

Contributors: Dr. John F. Stults, Dr. Charles Schaefer

Key Resources:

  • Assessment of PFAS in Collocated Soil and Porewater Samples at an AFFF-Impacted Source Zone: Field-Scale Validation of Suction Lysimeters[1]
  • PFAS Concentrations in Soil versus Soil Porewater: Mass Distributions and the Impact of Adsorption at Air-Water Interfaces[2]
  • Using Suction Lysimeters for Determining the Potential of Per- and Polyfluoroalkyl Substances to Leach from Soil to Groundwater: A Review[3]
  • Use of Lysimeters for Monitoring Soil Water Balance Parameters and Nutrient Leaching[4]
  • PFAS Porewater Concentrations in Unsaturated Soil: Field and Laboratory Comparisons Inform on PFAS Accumulation at Air-Water Interfaces[5]

Introduction

Lysimeters are devices that are placed in the subsurface above the groundwater table to monitor the movement of water through the soil[6][7][3]. Lysimeters have historically been used in agricultural sciences for monitoring nutrient or contaminant movement, soil moisture release curves, natural drainage patterns, and dynamics of plant-water interactions[6][8][9][10][4][11][12][13][14]. Recently, there has been strong interest in the use of lysimeters to measure and monitor movement of per- and polyfluoroalkyl substances (PFAS) through the vadose zone[15][1][5][16][17][18]. PFAS are frequently introduced to the environment through land surface application and have been found to be strongly retained within the upper 5 feet of soil[19][20]. PFAS recalcitrance in the vadose zone means that environmental program managers and consultants need a cost-effective way of monitoring concentration conditions within the vadose zone. Repeated soil sampling and extraction processes are time consuming and only give a representative concentration of total PFAS in the matrix[21], not what is readily transportable in mobile porewater[16][22][23][2]. Fortunately, lysimeters have been found to be a viable option for monitoring the concentration of PFAS in the mobile porewater phase in the vadose zone[15][1]. Note that while some lysimeters, known as weighing lysimeters, can directly measure water flux, the most commonly utilized lysimeters in PFAS investigations only provide measurements of porewater concentrations.

PFAS Background

PFAS are a broad class of chemicals with highly variable chemical structures[24][25][26]. One characteristic feature of PFAS is that they are fluorosurfactants, distinct from more traditional hydrocarbon surfactants[25][27][28][29]. Fluorosurfactants typically have a fully or partially fluorinated, hydrophobic tail with ionic (cationic, zwitterionic, or anionic) head group that is hydrophilic[25][26]. The hydrophobic tail and ionic head group mean PFAS are very stable at hydrophobic adsorption interfaces when present in the aqueous phase[30]. Examples of these interfaces include naturally occurring organic matter in soils and the air-water interface in the vadose zone[31][32][33][34][35]. Their strong adsorption to both soil organic matter and the air-water interface is a major contributor to elevated concentrations of PFAS observed in the upper 5 feet of the soil column[19][20]. While several other PFAS partitioning processes exist[27], adsorption to solid phase soils and air-water interfaces are the two primary processes present at nearly all PFAS sites[36]. The total PFAS mass obtained from a vadose zone soil sample contains the solid phase, air-water interfacial, and aqueous phase PFAS mass, which can be converted to porewater concentrations using Equation 1[2].

Equation 1:   StultsEq1.png

Where Cp is the porewater concentration, Ct is the total PFAS concentration, ρb is the bulk density of the soil, θw is the volumetric water content, Rd is the PFAS retardation factor, Kd is the solid phase adsorption coefficient, Kia is the air-water interfacial adsorption coefficient, and Aaw is the air-water interfacial area. The air-water interfacial area of the soil is primarily a function of both the soil properties and the degree of volumetric water saturation in the soil. There are several methods of estimating air-water interfacial areas including thermodynamic functions based on the soil moisture retention curve. However, the thermodynamic function has been shown to underestimate air-water interfacial area[37], and must typically be scaled using empirical scaling factors. An empirical method recently developed to estimate air-water interfacial area is presented in Equation 2[37].

Equation 2:   StultsEq2.png

Where Sw is the water phase saturation as a ratio of the water content over the volumetric soil porosity, and d50 is the median grain diameter.

Lysimeters Background

Figure 1. (a) A field suction lysimeter with labeled parts typically used in field settings – Credit: Bibek Acharya and Dr. Vivek Sharma, UF/IFAS, https://edis.ifas.ufl.edu/publication/AE581. (b) Laboratory suction lysimeters used in Schaefer et al. 2024[5], which employed the use of micro-sampling suction lysimeters. (c) A field lysimeter used in Schaefer et al. 2023[16]. (d) Diagram of a drainage wicking lysimeter – Credit: Edaphic Scientific, https://edaphic.com.au/products/water/lysimeter-wick-for-drainage/

Lysimeters, generally speaking, refer to instruments which collect water from unsaturated soils[4][11]. However, there are multiple types of lysimeters which can be employed in field or laboratory settings. There are three primary types of lysimeters relevant to PFAS listed here and shown in Figure 1a-d.

  1. Suction Lysimeters (Figure 1a,b): These lysimeters are the most relevant for PFAS sampling and are the majority of discussion in this article. These lysimeters operate by extracting liquid from the unsaturated vadose zone by applying negative suction pressure at the sampling head[3][5][18]. The sampling head is typically constructed of porous ceramic or stainless steel. A PVC case or stainless-steel case is attached to the sampling head and extends upward above the ground surface. Suction lysimeters are typically installed between 1 and 9 feet below ground surface, but can extend as deep as 40-60 feet in some cases[3]. Shallow lysimeters (< 10 feet) are typically installed using a hand auger. For ceramic lysimeters, a silica flour slurry should be placed at the base of the bore hole and allowed to cover the ceramic head before backfilling the hole partially with natural soil. Once the hole is partially backfilled with soil to cover the sampling head, the remainder of the casing should be sealed with hydrated bentonite chips. When sampling events occur, suction is applied at the ground surface using a rubber gasket seal and a hand pump or electric pump. After sufficient porewater is collected (the time for which can vary greatly based on the soil permeability and moisture content), the seal can be removed and a peristaltic pump used to extract liquid from the lysimeter.
  2. Field Lysimeters (Figure 1c): These large lysimeters can be constructed from plastic or metal sidings. They can range from approximately 2 feet in diameter to as large as several meters in diameter[4]. Instrumentation such as soil moisture probes and tensiometers, or even multiple suction lysimeters, are typically placed throughout the lysimeter to measure the movement of water and determine characteristic soil moisture release curves[13][14][16][17][38]. Water is typically collected at the base of the field lysimeter to determine net recharge through the system. These field lysimeters are intended to represent more realistic, intermediate scale conditions of field systems.
  3. Drainage Lysimeters (Figure 1d): Also known as a “wick” lysimeter, these lysimeters typically consist of a hollow cup attached to a spout which protrudes above ground to relieve air pressure from the system and act as a sampling port. The hollow cup typically has filters and wicking devices at the base to collect water from the soil. The cup is filled with natural soil and collects water as it percolates through the vadose zone. These lysimeters are used to directly monitor net recharge from the vadose zone to the groundwater table and could be useful in determining PFAS mass flux.

Analysis of PFAS Concentrations in Soil and Porewater

Table 1. Measured and Predicted PFAS Concentrations in Porewater for Select PFAS in Three Different Soils
Site PFAS Field
Porewater
Concentration
(μg/L) 		Lab Core
Porewater
Concentration
(μg/L) 		Predicted
Porewater
Concentration
(μg/L)
Site A PFOS 6.2 ± 3.4 3.0 ± 0.37 6.6 ± 3.3
Site B PFOS 2.2 ± 2.0 0.78 ± 0.38 2.8
Site C PFOS 13 ± 4.1 680 ± 460 164 ± 75
8:2 FTS 1.2 ± 0.46 52 ± 13 16 ± 6.0
PFHpS 0.36 ± 0.051 2.9 ± 2.0 5.9 ± 3.4
Figure 2. Field Measured PFAS concentration Data (Orange) and Lab Core Measured Concentration Data (Blue) for four PFAS impacted sites[1]
Figure 3. Measured and predicted data for PFAS concentrations from a single site field lysimeter study. Model predictions both with and without PFAS sorption to the air-water interface were considered[16].

Schaefer et al.[5] measured PFAS porewater concentrations with field and laboratory suction lysimeters across several sites. Intact cores from the site were collected for soil water extraction using laboratory lysimeters. The lysimeters were used to directly compare field derived measurements of PFAS concentration in the mobile porewater phase. Results from measurements are for four sites presented in Figure 2.

Data from sites A and B showed reasonably good agreement (within ½ order of magnitude) for most PFAS measured in the systems. At site C, more hydrophobic constituents (> C6 PFAS) tended to have higher concentrations in the lab core than the field site while less hydrophobic constituents (< C6) had higher concentrations in the field than lab cores. Site D showed substantially greater (1 order of magnitude or more) PFAS concentrations measured in the laboratory-collected porewater sample compared to what was measured in the field lysimeters. This discrepancy for the Site D soil can likely be attributed to soil heterogeneity (as indicated by ground penetrating radar) and the fact that the soil consisted of back-filled materials rather than undisturbed native soils.

Site C showed elevated PFAS concentrations in the laboratory collected porewater for the more surface-active compounds. This increase was attributed to the soil wetting that occurred at the bench scale, which was reasonably described by the model shown in Equations 1 and 2 (see Table 1[1]). Equations 1 and 2 were also used to predict PFAS porewater concentrations (using porous cup lysimeters) in a highly instrumented test cell[16](Figure 3). The ability to predict soil concentrations from recurring porewater samples is critical to the practical application of lysimeters in field settings[1].

Results from suction lysimeters studies and field lysimeter studies show that PFAS concentrations in porewater predicted from soil concentrations using Equations 1 and 2 generally have reasonable agreement with measured in situ porewater data when air-water interfacial partitioning is considered. Results show that for less hydrophobic components like PFOA, the impact of air-water interfacial adsorption is less significant than for highly hydrophobic components like PFOS. The soil for the field lysimeter in Figure 3 was a sandy soil with a relatively low air-water interfacial area. The effect of air-water interfacial partitioning is expected to be much more significant for a greater range of PFAS in soils with high capillary pressure (i.e. silts/clays) with higher associated air-water interfacial areas[37][39][40].

Summary and Recommendations

The majority of research with lysimeters for PFAS site investigations has been done using porous cup suction lysimeters[3][1][5][18]. Porous cup suction lysimeters are advantageous because they can be routinely sampled or sampled after specific wetting or drying events much like groundwater wells. This sampling is easier and more efficient than routinely collecting soil samples from the same locations. Co-locating lysimeters with soil samples is important for establishing the baseline soil concentration levels at the lysimeter location and developing correlations between the soil concentrations and the mobile porewater concentration[3]. Appropriate standard operation procedures for lysimeter installation and operation have been established and have been reviewed in recent literature[3][5]. Lysimeters should typically be installed near the source area and just above the maximum groundwater level elevation to obtain accurate results of porewater concentrations year round. Depending upon the geology and vertical PFAS distribution in the soil, multilevel lysimeter installations should also be considered.

Results from several lysimeters studies across multiple field sites and modelling analysis has shown that lysimeters can produce reasonable results between field and laboratory studies[5][16][17]. Transient effects of wetting and drying as well as media heterogeneity affects appear to be responsible for some variability and uncertainty in lysimeter based PFAS measurements in the vadose zone. These mobile porewater concentrations can be coupled with effective recharge estimates and simplified modelling approaches to determine mass flux from the vadose zone to the underlying groundwater[15][22][2][41][42].

Future research opportunities should address the current key uncertainties related to the use of lysimeters for PFAS investigations, including:

  1. Collect larger datasets of PFAS concentrations to determine how transient wetting or drying periods and media type affect PFAS concentrations in the mobile porewater. Some research has shown that non-equilibrium processes can occur in the vadose zone, which can affect grab sample concentration in the porewater at specific time periods.
  2. More work should be done with flux averaging lysimeters like the drainage cup or wicking lysimeter. These lysimeters can directly measure net recharge and provide time averaged concentrations of PFAS in water over the sampling period. However, there is little work detailing their potential applications in PFAS research, or operational considerations for their use in remedial investigations for PFAS.
  3. Lysimeters should be coupled with monitoring of wetting and drying in the vadose zone using in situ soil moisture sensors or tensiometers and groundwater levels. Direct measurements of soil saturation at field sites are vital to directly correlate porewater concentrations with soil concentrations. Similarly, groundwater level fluctuations can inform net recharge estimates. By collecting these data we can continue to improve partitioning and leaching models which can relate porewater concentrations to total PFAS mass in soils and PFAS leaching at field sites.
  4. Comparisons of various bench-scale leaching or desorption tests to field-based lysimeter data are recommended. The ability to correlate field measurements of PFAS concentrations with estimates of leaching from laboratory studies would provide a powerful method to empirically estimate PFAS leaching from field sites.

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See Also

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