Secondary water quality impacts (SWQIs) from [[Bioremediation - Anaerobic | anaerobic bioremediation]] are changes in water chemistry that result from adding an organic substrate to the subsurface to enhance contaminant biodegradation. Common SWQIs that occur at most sites include increases in dissolved [[wikipedia: Manganese | manganese]], [[wikipedia: Iron | iron]], and [[wikipedia: Methane | methane]]. Fortunately, SWQIs are usually restricted to areas that are already contaminated and attenuate with distance downgradient in well-understood ways.

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

'''Related Article(s):'''
*[[Bioremediation - Anaerobic]]
*[[Injection Techniques for Liquid Amendments]]

'''CONTRIBUTOR(S):''' [[Dr. Robert Borden, P.E.]]

'''Key Resource(s):'''
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-2131/ER-2131/(language)/eng- Extent and Persistence of Secondary Water Quality Impacts after Enhanced Reductive Bioremediation] <ref name="Bordenetal2015">Borden, R. C., Tillotson, J. M., Ng, G.-H. C., Bekins, B. A., Kent, D. B., Curtis, G. P., 2015. Extent and persistence of secondary water quality impacts after enhanced reductive bioremediation. ER-2131. Strategic Environmental Research and Development Program, Arlington, VA. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-2131/ER-2131/(language)/eng- ER-2131]</ref>

==Introduction==
Electron donor addition can be very effective in stimulating enhanced [[Bioremediation - Anaerobic | anaerobic bioremediation]] (EAB) of a wide variety of groundwater contaminants including [[Chlorinated Solvents | chlorinated solvents]], high explosives, [[Perchlorate |perchlorate]], and certain [[Metal and Metalloid Contaminants | metals]] and radionuclides<ref>AFCEE (Air Force Center for Engineering and the Environment), NFESC (Naval Facilities Engineering Service Center), ESTCP (Environmental Security Technology Certification Program), 2004. Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents. Prepared by Parsons Infrastructure & Technology Group, Inc., Denver, CO, USA. [http://www.environmentalrestoration.wiki/images/d/d5/AFCEE_Principles_and_Practices.pdf Report pdf]</ref><ref>Stroo, H.F., M.R. West, B.H. Kueper, R.C. Borden, C.H. Ward, 2014. In Situ Bioremediation of Chlorinated Ethene Source Zones, Chlorinated Solvent Source Zone Remediation, Ed. B.H. Kueper, H.F. Stroo and C.H. Ward, Springer, New York, NY. [http://link.springer.com/chapter/10.1007/978-1-4614-6922-3_12 doi:10.1007/978-1-4614-6922-3_12]</ref><ref>Borden, R.C., 2007, Anaerobic Bioremediation of Perchlorate and 1,1,1-Trichloroethane in an Emulsified Oil Barrier, Journal of Contaminant Hydrology, 94, 13-33. [http://www.sciencedirect.com/science/article/pii/S016977220700071X doi:10.1016/j.jconhyd.2007.06.002]</ref><ref>Yurovsky, M., D. Cacciatore, L. Hudson, D.P. Leigh., C. Bhullar, and W. Shaheen, 2009. Operation of in situ biological system for treatment of hexavalent chromium at the Selma Pressure Treating Superfund Site. In: Proceedings of the Tenth International In Situ and On-Site Bioremediation Symposium, Baltimore, MD. Battelle Press. [https://clu-in.org/products/tins/tinsone.cfm?id=9474771&query=groundwater%20OR%20ground%20water&numresults=25&startrow=1801 ISBN:9780981973012]</ref>. However, this can result in secondary water quality impacts (SWQIs) including decreased levels of dissolved [[wikipedia: Oxygen | oxygen]] (O2), [[wikipedia: Nitrate | nitrate]] (NO3-), and [[wikipedia: Sulfate | sulfate]] (SO42-), and elevated levels of dissolved [[wikipedia: Manganese | manganese]] (Mn2+), dissolved [[wikipedia: Iron | iron]] (Fe2+), [[wikipedia: Methane | methane]] (CH4), [[wikipedia: Sulfide | sulfide]] (S2-), organic carbon, and naturally occurring hazardous compounds (e.g., [[wikipedia: Arsenic | arsenic]])<ref name="Bordenetal2015"/>. Fortunately, the impacted groundwater is usually confined within the original contaminant plume and is unlikely to adversely impact potable water supplies<ref name="TB2015">Tillotson, J. M. and R. C. Borden, 2015. Statistical Analysis of Secondary Water Quality Impacts from Enhanced Reductive Bioremediation: Groundwater Monitoring and Remediation [http://onlinelibrary.wiley.com/doi/10.1111/gwmr.12132/abstract doi:10.1111/gwmr.12132]</ref>.

==SWQI Production and Attenuation Conceptual Model==

During EAB, large amounts of easily fermented organic substrates are added to the target treatment area to degrade or immobilize the contaminants of concern (CoC). These substrates are fermented to hydrogen (H2), acetate, and other volatile fatty acids that are then used as electron donors by microbes to mediate oxidation-reduction (redox) reactions that reduce dissolved oxygen, nitrate, and sulfate as well as Fe3+ and Mn3/4+ containing minerals and the CoC. Figure 1 shows a typical pattern of SWQI parameters with time in the: 1) injection area; 2) near plume (25 m downgradient); 3) medium-distance plume (50 m downgradient); and 4) far plume (100 m downgradient). These curves were generated using a geochemical model developed to simulate SWQI production and attenuation<ref name="Ng2015">Crystal Ng, G.H., Bekins, B.A., Cozzarelli, I.M., Baedecker, M.J., Bennett, P.C., Amos, R.T. and Herkelrath, W.N., 2015. Reactive transport modeling of geochemical controls on secondary water quality impacts at a crude oil spill site near Bemidji, MN. Water Resources Research. [https://pubs.er.usgs.gov/publication/70159670 doi:10.1002/2015WR016964]</ref>, and calibrated to [[ Long-Term Monitoring (LTM) - Data Analysis | long-term monitoring data]] from field sites where organic-rich materials have entered the subsurface<ref name=Bordenetal2015 />. The time frame for production and attenuation of SWQIs can vary from 10 to 100+ years, depending on the amount and duration of substrate addition, [[ Advection and Groundwater Flow | groundwater flow]] velocity, and background electron acceptor concentrations.

===Total Organic Carbon (TOC)===
[[File:Fig1_SWQI.png|thumbnail|left|650px|Figure 1. Typical variation in SWQI Parameters over time with distance from injection. Graphs compare concentrations in injection area, 25 m, 50 m and 100 m downgradient for 40 years post-injection<ref name=Bordenetal2015 />.]]
Organic substrate addition causes a rapid increase in total organic carbon (TOC) within injection area monitoring wells with maximum concentrations typically ranging from 50 to 500 mg/L (Fig. 1). However, much higher TOC concentrations were observed at some sites<ref name="TB2015" />. TOC concentrations often remain high for several years in the injection area due to the use of slow release electron donors (e.g., emulsified vegetable oil or ((EVO)) and/or repeated substrate injections, and then decline once substrate addition ends. However, low levels of TOC may continue to be released from endogenous decay of accumulated biomass<ref>Sleep, B.E., A.J. Brown, and B.S. Lollar. 2005. Long-term tetrachlorethene degradation sustained by endogenous cell decay. Journal of Environmental Engineering Science, 4(1), 11–17. [http://www.nrcresearchpress.com/doi/abs/10.1139/s04-038#.VoqpE_krLcs doi:10.1139/s04-038]</ref><ref>Adamson, D.T., and C.J Newell. 2009. Support of source zone bioremediation through endogenous biomass decay and electron donor recycling. Bioremediation Journal, 13(1), 29-40. [http://www.tandfonline.com/doi/abs/10.1080/10889860802690539 doi:10.1080/10889860802690539]</ref>. Increases in carbon loading are expected to result in greater SWQI formation. However, these SWQIs will attenuate with time and distance downgradient. Reducing the carbon loading to decrease SWQIs production may reduce treatment efficiency, possibly resulting in greater exposure to chlorinated solvents and other contaminants.

Maximum TOC concentrations in downgradient wells are generally much lower than in the injection area, indicating TOC in the aqueous phase is rapidly consumed and does not migrate long distances downgradient. Rapid consumption of TOC in the injection area is due to reactions with background electron acceptors (e.g., O2, NO3-, Mn4+, Fe3+, SO42-), the target contaminants, and fermentation to CH4. Since TOC is largely restricted to the injection area, these redox reactions are also largely restricted to the same area. Thermodynamic calculations indicate that reduction reactions should proceed in the order of O2, NO3-, Mn4+, Fe3+, SO42-, and CO2. However, these processes often overlap (e.g., methane production occurring before complete sulfate reduction) due to spatial variability, energy limitations from low reactant concentrations, slow reaction kinetics, and addition of excess electron donor<ref>Cozzarelli, I.M., J.M. Sulfita, G.A. Ulrich, S.H. Harris, M.A. Scholl, J.L. Schlottmann, and S.C. Christenson, 2000. Geochemical and microbiological methods for evaluating anaerobic processes in an aquifer contaminated by landfill leachate. Environmental Science and Technology, 34(18), 4025-4033. [http://pubs.acs.org/doi/abs/10.1021/es991342b doi: 10.1021/es991342b]</ref><ref>Jakobsen, R., and Postma, D., 1999. Redox zoning, rates of sulfate reduction and interactions with Fe-reduction and methanogenesis in a shallow sandy aquifer, Rømø, Denmark. Geochimica et Cosmochimica Acta, 63(1), 137-151. [http://www.sciencedirect.com/science/article/pii/S0016703798002725 doi:10.1016/S0016-7037(98)00272-5]</ref>.

===Oxygen (O2) and Nitrate (NO3-)===
In the injection area, O2 and NO3- decline rapidly following substrate addition and often remain low for years after TOC declines due to reduction of O2 and NO3- by sediment organic carbon and/or reduced minerals. Concentrations of O2 and NO3- in downgradient wells decline with the arrival of anaerobic, oxygen- and nitrate-depleted water. In most cases, there is little or no increase in O2 or NO3- with distance downgradient due to the limited mixing between the anaerobic plume and background aerobic groundwater<ref name="BB1986">Borden, R.C. and P.B. Bedient, 1986. Transport of dissolved hydrocarbons influenced by reaeration and oxygen limited biodegradation: 1. Theoretical development. Water Resources Research, 22(13), 1973-1982. [http://onlinelibrary.wiley.com/doi/10.1029/WR022i013p01983/full doi:10.1029/WR022i013p01983]</ref><ref>Schirmer, M., G.C. Durrant, J.W. Molson, and E.O. Frind, 2001. Influence of transient flow on contaminant biodegradation. Groundwater, 39(2), 276-282. [http://onlinelibrary.wiley.com/doi/10.1111/j.1745-6584.2001.tb02309.x/abstract doi:10.1111/j.1745-6584.2001.tb02309.x]</ref>.

===Sulfate (SO42-)===
SO42- concentrations follow the same general pattern as O2 and NO3-, with an initial decline in the injection area following substrate addition. However, biodegradation coupled to sulfate reduction is less energetically favorable than biodegradation coupled to O2, NO3-, Mn4+, and Fe3+ reduction. As a result, SO42- is depleted more slowly than these other terminal electron acceptors (TEAs) and substantial amounts of SO42- may persist in the injection area if background SO42- levels are high. In downgradient wells, SO42- concentrations decline with the arrival of anaerobic, low SO42- groundwater. In most cases, there is little increase in SO42- with distance downgradient due to the very limited mixing between the anaerobic low SO42- plume and background higher SO42- groundwater. The amount of dissolved S2- produced from SO42- reduction depends on the SO42- concentration, extent of SO42- reduction, and the amount of Fe2+ and Mn2+ in groundwater. If S2- is in excess (Saturation Index of FeS>1), then S2- will persist and limit the extent of Fe2+ released to solution. In practice, at most sites sufficient sediment-bound Mn and Fe are present to react with S2- and precipitate as low solubility sulfide minerals<ref>Cozzarelli, I.M., J.S. Herman, M.J. Baedecker, and J.M. Fischer, 1999. Geochemical heterogeneity of a gasoline-contaminated aquifer. Journal of Contaminant Hydrology, 40(3), 261-284. [http://www.sciencedirect.com/science/article/pii/S0169772299000509 doi:10.1016/S0169-7722(99)00050-9]</ref>. As a result, aqueous sulfide concentrations are low in the injection area and downgradient aquifer<ref name="TB2015" />. In uncommon cases where SO42- concentrations are high and solid phase Fe is low, some dissolved sulfide may migrate a short distance downgradient before reacting or precipitating. Once injection area TOC declines, SO42- is expected to recover somewhat more rapidly than O2 or NO3-.

===Manganese, Iron, and Arsenic===
Excess TOC in the injection area will stimulate reduction of solid phase Mn4+ and Fe3+, causing a gradual increase in Mn2+ and Fe2+. Much of the reduced Mn2+ and Fe2+ produced in these reactions will be retained on the aquifer material within the injection area through a variety of processes including ion exchange and surface complexation reactions<ref>Davis, J.A., and D.B. Kent, 1990. Surface complexation modeling in aqueous geochemistry. In: M.F. Hochella and A.F. White (eds.), Mineral-Water Interface Geochemistry. Washington, DC. Mineralogical Society of America, 23, 177-260. [http://rimg.geoscienceworld.org/content/23/1/177.citation Article]</ref>. However, a portion of the reduced Mn2+ and Fe2+ will be in the aqueous phase and migrate downgradient with natural groundwater flow. Sorption reactions will diminish the rate of transport of Fe and Mn compared to the groundwater flow rate as well as the maximum concentrations observed in downgradient wells. However, significant increases in Mn2+ and Fe2+ have been observed in monitoring wells at significant distances downgradient<ref name="TB2015" />. In some aquifers, Mn2+ and Fe2+ concentrations have been observed to decline before TOC is depleted, presumably due to depletion of bioavailable Mn4+ and Fe3+ in the injection-area aquifer materials. Once TOC levels decline in the injection area, production of additional Mn2+ and Fe2+ will slow and dissolved Mn and Fe concentrations in monitoring wells should start to decline.

In some aquifers, arsenic (As) is naturally present as As4+ sorbed or coprecipitated with Fe3+ or other minerals<ref>Pierce, M.L., and C.B. Moore, 1982. Adsorption of arsenite and arsenate on amorphous iron hydroxide. Water Research, 16(7), 1247-1253. [http://www.sciencedirect.com/science/article/pii/0043135482901439 doi:10.1016/0043-1354(82)90143-9]</ref><ref>Smedley, P.L., and D.G. Kinniburgh, 2002. A review of the source, behaviour and distribution of arsenic in natural waters. Applied Geochemistry, 17(5), 517-568. [http://www.sciencedirect.com/science/article/pii/S0883292702000185 doi:10.1016/S0883-2927(02)00018-5]</ref><ref>Kent, D.B., and P.M. Fox, 2004. The influence of groundwater chemistry on arsenic concentrations and speciation in a quartz sand and gravel aquifer. Geochemical Transactions, 5(1), 1-12. [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1475781/ doi:10.1186/1467-4866-5-1]</ref>. The general term sorption is used to encompass all binding mechanisms of As to Fe3+ phases. If arsenic is present in these forms, the TOC addition to remediate contaminants could release dissolved arsenic to groundwater by both reduction of Fe3+ to Fe2+ and As5+ to As3+. As Fe-rich groundwater migrates downgradient, Fe2+ sorbs to the sediment or precipitates as Fe-bearing minerals (FeS, carbonates, magnetite)<ref>Fredrickson J.K., J.M. Zachara, D.W. Kennedy, H. Dong, T.C. Onstott, N.W. Hinman, and S. Li. 1998. Biogenic iron mineralization accompanying dissimilatory reduction of hydrous ferric oxide by a groundwater bacterium. Geochimica et Cosmochimica Acta, 62, 3239-3257. [http://www.sciencedirect.com/science/article/pii/S0016703798002439 doi:10.1016/S0016-7037(98)00243-9]</ref> and aqueous Fe concentrations decline<ref name="Ng2015"/>. Available monitoring data suggest that arsenic follows a similar pattern and aqueous arsenic concentrations decline as the anaerobic plume migrates downgradient and encounters Fe3+-rich sediments<ref>Cozzarelli, I.M., M.E. Schreiber, M.L. Erickson, and B.A. Ziegler, 2015. Arsenic cycling in hydrocarbon plumes: secondary effects of natural attenuation. Groundwater, 54, 35–45. [http://onlinelibrary.wiley.com/doi/10.1111/gwat.12316/full doi:10.1111/gwat.12316]</ref><ref name="TB2015" />. Once TOC concentrations in the injection area decline, Fe3+ and As5+ reduction is expected to decline with a concurrent decline in arsenic release. At sites where concentrations of sediment-bound As5+ are low, minimal arsenic will be released.

===Methane===
TOC fermentation products (H2 and acetate) in the injection area that are not consumed in microbially-mediated reactions with contaminants or background electron acceptors will be fermented to CH4. This can result in high CH4 concentrations in the injection area and downgradient aquifer<ref>Jacob, C., E.F. Weber, J.N. Bet, and A.K. Macnair, 2005. Full-scale enhanced reductive dechlorination using sodium lactate and vegetable oil. In: Proceedings of the Eighth International In Situ and On-Site Bioremediation Symposium (Baltimore, MD; June 2005), Battelle Press, Columbus, OH.</ref>. If the sum of gas partial pressures (mainly N2 + CO2 + CH4) exceeds the hydrostatic pressure, these gases will come out of solution and form bubbles<ref>Amos, R.T. and K.U. Mayer, 2006. Investigating the roles of gas bubble formation and entrapment in contaminated aquifers: reactive transport modelling. Journal of Contaminant Hydrology, 87(1-2), 123-154. [http://www.sciencedirect.com/science/article/pii/S0169772206000891 doi:10.1016/j.jconhyd.2006.04.008]</ref>. This occurs primarily near the water table due to the lower hydrostatic pressure there compared to deeper levels. At greater depths, groundwater can appear to be supersaturated with CH4 due to the higher pressure. In relatively homogeneous, coarse grained sediments, gas bubbles can migrate upward into the vadose zone, removing CH4 from the aquifer. However, in finer grained sediments, upward migration of gas bubbles will be more limited. If the gas bubbles are not released to the vadose zone, they may eventually dissolve, releasing CH4 back into groundwater. Dissolved CH4 produced in the injection area will migrate downgradient with groundwater flow. Since mixing between the anaerobic plume and aerobic background groundwater is low, aerobic methane oxidation will be limited<ref name="BB1986" />. Recent research suggests that CH4 plume migration may be limited by anaerobic oxidation of methane (AOM) using NO32-<ref>Raghoebarsing, A.A., A. Pol, K.T. van de Pas-Schoonen, A.J.P. Smolders, K.F. Ettwig, W.I.C. Rijpstra, S. Schouten, J.S.S. Damste, H.J.M. Op den Camp, M.S.M. Jetten, and M. Strous. 2006. A microbial consortium couples anaerobic methane oxidation to denitrification. Nature 440, no. 7086, 918-921. [http://www.nature.com/nature/journal/v440/n7086/abs/nature04617.html doi:10.1038/nature04617]</ref>, Mn3/4+ and Fe3+<ref>Beal, E.J., C.H. House, and V.J. Orphan, 2009. Manganese- and iron-dependent marine methane oxidation. Science 325, no. 5937: 184-187, [http://www.sciencemag.org/content/325/5937/184.abstract doi:10.1126/science.1169984]</ref><ref>Amos, R.T., B.A Bekins, I.M Cozzarelli, M.A. Voytek, J.D. Kirshtein, E.J.P. Jones, and D.W. Blowes, 2012. Evidence for iron-mediated anaerobic methane oxidation in a crude oil-contaminated aquifer. Geobiology, 10(6), 506-517. [http://onlinelibrary.wiley.com/doi/10.1111/j.1472-4669.2012.00341.x/abstract doi:10.1111/j.1472-4669.2012.00341.x]</ref> or SO42-<ref>Caldwell, S.L., Laidler, J.R., Brewer, E.A., Eberly, J.O., Sandborgh, S.C. and Colwell, F.S., 2008. Anaerobic oxidation of methane: mechanisms, bioenergetics, and the ecology of associated microorganisms. Environmental sScience & Technology, 42(18), 6791-6799. [http://dx.doi.org/10.1016/j.jconhyd.2007.06.002 doi:10.1021/es800120b]</ref><ref>Grossman, E.L., L.A. Cifuentes, and I.M. Cozzarelli, 2002. Anaerobic methane oxidation in a landfill-leachate plume. Environmental Science & Technology, 36(11), 2436-2442. [http://pubs.acs.org/doi/abs/10.1021/es015695y doi:10.1021/es015695y]</ref> as terminal electron acceptors. However, this reaction may be slow or may not occur at some sites. As a result, dissolved CH4 can migrate long distances in some aquifers. Once TOC in the injection area declines, CH4 production will stop and dissolved CH4 should be transported downgradient by groundwater flow.

==Implications for Remediation==
When implementing enhanced anaerobic bioremediation systems, designers should be aware that some level of SWQIs occur at every site. However, concentrations of all parameters decline with distance downgradient following reasonably well understood and predictable processes. Importantly, elevated levels of SWQI parameters are usually confined within the original contaminant plume and are unlikely to adversely impact potable water supplies<ref name="TB2015" />.

==References==

<references/>

==See Also==
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-2131 Numerical Modeling of Post-Remediation Impacts of Anaerobic Bioremediation on Groundwater Quality]

Thermal Remediation - Smoldering

2018-05-03T17:48:00Z

Astenger:

Smoldering remediation is a [[Thermal Remediation | thermal technology]] for treating non-aqueous phase liquids (NAPLs) where air is forced through the material to be treated to propagate a low-temperature, flameless form of combustion. The self-sustaining reaction travels from an ignition location through NAPL-contaminated soil, destroying most of the NAPL, while a small fraction is recovered as vapors and treated. The technology can be used to treat a wide range of NAPLs, including [[wikipedia: Coal tar | coal tar]], crude oil sludge, and other difficult-to-treat materials. This approach can be used for ''ex situ'' treatment of excavated soils and sludges, and ''in situ'' above and below the [[wikipedia: Water table | water table]]. However, [[wikipedia: Smoldering | smoldering]] is not appropriate for remediating materials with low air permeability (e.g., clay or fractured rock) and requires a minimum amount of NAPL to propagate a self-sustaining reaction.

'''Related Article(s):'''
*[[Thermal Remediation]]
*[[Thermal Remediation - Steam]]
*[[Thermal Remediation - Electrical Resistance Heating]]
*[[Thermal Remediation - Desorption]]
*[[Thermal Remediation - Combined Remedies]]

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

'''CONTRIBUTOR(S):''' [[Dr. Jason Gerhard]]

'''Key Resource(s):'''
*[http://www.eng.uwo.ca/research/restore/star.html Research for Subsurface Transport and Remediation (RESTORE)]
*[http://dx.doi.org/10.1021/acs.est.5b03177 Smoldering remediation of coal-tar-contaminated soil: STAR pilot field tests]<ref name="Scholes2015">Scholes, G.C., Gerhard, J.I., Grant, G.P., Major, D.W., Vidumsky, J.E., Switzer, C. and Torero, J.L., 2015. Smoldering remediation of coal-tar-contaminated soil: Pilot field tests of STAR. Environmental Science & Technology, 49(24), 1-9. [http://dx.doi.org/10.1021/acs.est.5b03177 doi: 10.1021/acs.est.5b03177]</ref>

==Introduction==
Remediation by [[wikipedia: Smoldering | smoldering]] combustion has rapidly evolved from a new idea in 2009<ref name="Switzer2009">Switzer, C., Pironi, P., Gerhard, J.I., Rein, G. and Torero, J.L., 2009. Self-sustaining smoldering combustion: a novel remediation process for non-aqueous-phase liquids in porous media. Environmental Science & Technology, 43(15), 5871-5877. [http://dx.doi.org/10.1021/es803483s doi: 10.1021/es803483s]</ref> to a technology being commercially applied to contaminated soils both ''in situ'' and ''ex situ''<ref name="Scholes2015" /><ref name="Grant2016">Grant, G.P., Major, D., Scholes, G.C., Horst, J., Hill, S., Klemmer, M.R. and Couch, J.N., 2016. Smoldering Combustion (STAR) for the Treatment of Contaminated Soils: Examining Limitations and Defining Success. Remediation Journal, 26(3), 27-51. [http://dx.doi.org/10.1002/rem.21468 doi: 10.1002/rem.21468]</ref>. It is appropriate for soils contaminated with both light non-aqueous phase liquids (LNAPLs) and dense non-aqueous phase liquids (DNAPLs). Since the smoldering reaction is self-sustaining, very little external energy is required to achieve clean up. Contaminant concentrations are often reduced to below detection because smoldering continues until no NAPL remains. Over 98% of the NAPL removed is typically destroyed ''in situ'', with < 2% released as emissions that require capture<ref name="Scholes2015"/>. There is no upper limit on the NAPL concentration, meaning the process is attractive as a mass reduction technology for highly contaminated sites. When concentrations are below a certain level (typically ~ 3,000 mg/kg, but very soil and contaminant dependent), the process is not self-sustaining.

Smoldering is relatively quick, with ''in situ'' propagation rates for the reaction on the order of 0.5 m/day<ref name="Scholes2015"/>. ''In situ'' smoldering works equally well above and below the water table. Field applications of the approach use standard air injection and vapour capture/treatment equipment, combined with special systems to initiate the reaction<ref name="Grant2016"/>. Treatability studies and field pilot tests<ref name="Grant2016"/> have shown smoldering remediation to be successful in a wide range of soil types from gravel-based fill to silt. However, since it depends on air injection to support the reaction, very low permeability materials such as clay are not appropriate. ''Ex situ'', smoldering can be used for both contaminated soils and for organic wastes.

==Scientific Principals==
Combustion is an [[wikipedia: Exothermic reaction | exothermic]] oxidation reaction:
::[[File:Gerhard Equation 1.JPG|350px]] 
[[File:Gerhard Fig1.jpg|thumbnail|right|Figure 1. Smoldering charcoal example of a flameless, self-sustaining combustion reaction.]]

It is activated by raising the fuel (i.e., the carbon compounds, in this case the contaminant) to its ignition temperature and ensuring the presence of oxygen. [[wikipedia: Smoldering | Smoldering]] is a flameless type of combustion. The reaction occurs within the surface of the fuel. Glowing red charcoal in a barbeque is a typical example (Fig. 1).

Smoldering requires a porous fuel, such as charcoal, or an inert porous matrix embedded with fuel, such as sand containing NAPL. This is because the reaction requires air to flow to, and then diffuse into, the fuel surface. After a small, local addition of energy to achieve ignition, the reaction will continue indefinitely as long as air and carbon compounds are available, since the reaction itself produces the energy required to initiate further reactions. For this reason, smoldering is ‘self-sustaining’. This means that the energy generated is more than the energy used to heat the fuel and lost externally.

The smoldering remediation concept for NAPL-contaminated soil is straightforward (Fig. 2). Heat and air are applied to a small portion of the contaminated soil until ignition temperatures (specific to the NAPL type, but typically ~200°C) are reached, after which the smoldering reaction initiates. At this time, the heater is no longer needed and is turned off. Air injection continues and the burning NAPL is converted primarily to CO2, H2O, and also heat; the latter is absorbed by the adjacent soil and NAPL, thereby allowing the process to continue. The reaction, propagating in the direction of the air flow, will continue as long as the pathway contains NAPL or until the air flow rate (i.e., O2 mass flux) falls below a critical amount required to support the reaction.

The smoldering front will propagate through the NAPL-contaminated soil as a relatively thin (few centimeters) reaction zone, with a heated zone preceding it and a cooling, clean zone behind it (e.g., Fig. 2). This can be clearly observed in a webcam video of a self-sustaining smoldering reaction in coal tar-contaminated soil (Fig. 3). In the video, the reaction propagates at ~ 0.5 cm/min such that it takes ~ 15 min to travel from the bottom to the top of the 30 cm quartz column. This propagation rate is linearly dependent on the rate that air is injected<ref name="Pironi2011">Pironi, P., Switzer, C., Gerhard, J.I., Rein, G. and Torero, J.L., 2011. Self-sustaining smoldering combustion for NAPL remediation: laboratory evaluation of process sensitivity to key parameters. Environmental Science & Technology, 45(7), 2980-2986. [http://dx.doi.org/10.1021/es102969z doi: 10.1021/es102969z]</ref>. This means that the velocity of the smoldering front (and thus the mass destruction rate) is easily controlled by the operator by manipulating the air injection rate. The reaction zone temperature depends primarily on NAPL type, typically ranging from 500°C (e.g., for diesel) to 1000°C (e.g., for coal tar). Water-filled porosity tends to have little effect, since there is a boiling front that travels ahead of the reaction and air flows from the injection pipe through dry sand to the heated NAPL participating in the reaction. In fact, smoldering remediation below the water table is common<ref name="Scholes2015"/><ref name="Grant2016"/>.

[[File:Gerhard Figure2.gif|thumbnail|left|400px|Figure 2. Animation showing the concept of NAPL destruction in soil by self-sustaining smoldering.]][[File:Figure3.gif|thumbnail|right|510px|Figure 3. Webcam video of a self-sustaining smoldering reaction traveling upwards in a 30 cm tall transparent quartz column containing coal tar contaminated soil (accelerated 50 times).]]

[[File:Gerhard Fig4.jpg|thumbnail|right|500px|Figure 4. Comparison of coal-tar contaminated soil before and after smoldering treatment in the laboratory.]]
 A self-sustaining smoldering reaction terminates on its own when no NAPL remains. After the process is complete, it is typical to observe dry, organic-free soil (Fig. 4). Typical reductions in soil of total petroleum hydrocarbons are > 99.9%<ref name="Switzer2009"/><ref name="Scholes2015"/><ref name="Grant2016"/>. The reaction can be terminated sooner, if desired, by turning off the airflow. NAPL smoldering cannot continue in the absence of forced air injection, therefore a runaway reaction is not possible (unlike [[wikipedia: Coal seam fire | underground burning coal]]). In some cases, the amount of energy generated is not enough to overcome heat losses and the reaction is not self-sustaining. This can occur if not enough NAPL is present; typically ~ 3,000 mg/kg, with the exact value being soil and NAPL dependent. It also can occur when the NAPL is present in too narrow a finger, so that heat losses to the surrounding (clean) soil are severe. Laboratory results suggest that a NAPL finger thicker than ~ 2 cm can be self-sustaining (publication in preparation). Details about the fundamentals of smoldering NAPLs in soil are available<ref name="Switzer2009"/><ref name="Pironi2011"/>.

==Contaminant Treatability==
Smoldering remediation is most readily applied to complex, long-chain hydrocarbons such as coal tar, fuel oils, and unrefined or heavy petroleum hydrocarbons. These NAPLs contain significant embedded energy. For example, coal tar has a heat of combustion of ~35,000 kJ/kg, which is more than double that of wood. However, the suitability of a NAPL for smoldering cannot be evaluated by its heat of combustion because other characteristics of the NAPL compounds and the soil play an important role. For instance, compound volatility and the composition of mixed NAPLs are important. Coal tar and heavy hydrocarbons exhibit low volatility in the majority of their hundreds of compounds, making their smoldering reaction highly self-sustaining. This means that their removal during smoldering is dominated by ''in situ'' oxidation rather than volatilization. For example, < 2% of coal tar mass treated by smoldering was volatilized with > 98% destroyed ''in situ''<ref name="Scholes2015"/>. The high suitability of heavy hydrocarbons to smoldering is fortuitous since these NAPLs tend to be the most resistant to standard physical, chemical, thermal, and biological treatments.

More volatile NAPLs, or the more volatile components of mixed NAPLs, can exhibit higher mass fractions volatilized due to the heating front that moves ahead of the smoldering reaction. If a NAPL is too volatile, such as gasoline, it may be difficult to ignite. Diesel has been demonstrated to be treated with self-sustained smoldering in some circumstances on both laboratory and field pilot tests (publication in preparation); this is likely among the most volatile NAPL compounds that can be directly smoldered. Lighter compounds, such as gasoline and chlorinated solvents, tend to be too volatile to be smoldered directly. In these cases, neat or emulsified vegetable oil can be injected into the soil as a non-toxic fuel to support smoldering. This has been demonstrated for trichloroethylene (TCE) in laboratory studies, producing soil that had no detectable chlorinated solvents (or vegetable oil) remaining<ref name="Salman2015">Salman, M., Gerhard, J.I., Major, D.W., Pironi, P. and Hadden, R., 2015. Remediation of trichloroethylene-contaminated soils by STAR technology using vegetable oil smoldering. Journal of Hazardous Materials, 285, 346-355. [http://dx.doi.org/10.1016/j.jhazmat.2014.11.042 doi: 10.1016/j.jhazmat.2014.11.042]</ref>. That work demonstrated that, of the chlorinated solvent mass removed from the soil during smoldering, ~ 25% was destroyed ''in situ'' and ~ 75% was volatilized and captured in the emissions. A similar ratio of destroyed to volatilized mass was observed in a recent field pilot test for emulsified vegetable oil (EVO) smoldering to remediate gasoline (publication in preparation). In such cases, EVO smoldering may be an inexpensive means to accomplish thermal treatment, the goal of which is to volatilize most of the contaminant for recovery and treatment at the surface.

Other gaseous compounds can be produced in minor quantities that require careful attention to capture and treatment. Carbon monoxide is typically produced as a byproduct of smoldering. Although it is typically a fraction of the amount of carbon dioxide (i.e., most combustion is complete, not incomplete), it is a regulated compound that must be carefully monitored, captured, and handled. Incineration of chlorinated compounds and vegetable oil may, under certain conditions, produce gaseous byproducts including volatile organic compounds, phosgene, and dioxins/furans. Laboratory studies of smoldering TCE NAPL found perchlorethylene (PCE) generated at concentrations two orders of magnitude less than TCE emissions, and phosgene concentrations an order of magnitude less than regulatory limits<ref name="Salman2015"/>. The same study found that aliphatic hydrocarbons in trace amounts were observed due to smoldering the injected vegetable oil. Dioxin/furan formation is complex and highly dependent on the type and mass fraction of chlorinated compounds in the NAPL, the smoldering temperature, the smoldering rate, and numerous other factors. The potential for its formation should be evaluated on a site-specific basis using laboratory treatability studies and confirmed with an on-site pilot test. In all cases where minor gaseous compounds of concern are expected, standard surface vapors treatment techniques are implemented (e.g., adsorption on activated carbon, regenerative thermal oxidation).

Bench scale treatability studies are the best way to evaluate whether a particular contaminated soil or liquid waste is a good candidate for smoldering remediation. Standard laboratory tests can determine whether the contaminated soil sample is self-sustaining alone or self-sustaining with the addition of EVO. In the case of liquid waste, a homogeneous mixture with sand is typically created first. The treatability study can provide the peak smoldering temperature, the expected smoldering front velocity (linked to estimates of mass destruction rates), and before/after contaminant concentrations. Specialized treatability studies can also examine site-specific questions, including the fraction destroyed versus volatilized, the amount of CO that might be generated due to incomplete combustion, the fate of metals, or the potential for dioxins and furans to be formed (e.g., with chlorinated compounds). A treatability study can reveal if the soil permeability is too low to permit sufficient air flow to sustain the reaction. Smoldering reactions at the bench scale are less robust (due to higher heat losses) than at the field scale<ref name="Switzer2014">Switzer, C., Pironi, P., Gerhard, J.I., Rein, G. and Torero, J.L., 2014. Volumetric scale-up of smouldering remediation of contaminated materials. Journal of Hazardous Materials, 268, 51-60. [http://dx.doi.org/10.1016/j.jhazmat.2013.11.053 doi: 10.1016/j.jhazmat.2013.11.053]</ref> and bench treatability studies are usually conservative (i.e., if it smolders in the lab, it is very likely to smolder in the field, and may do so at lower contaminant concentrations and/or with lower air flow rates).

Current research is pushing the boundaries of the material types that can be remediated with smoldering. Wastes that are typically considered too wet to be combusted in a conventional manner have been shown to be amenable to smoldering due to its high-energy efficiency. A recent study demonstrated that smoldering is a promising alternative for converting wastewater treatment plant biosolids into heat and inorganic ash<ref>Rashwan, T., J.I. Gerhard, G. Grant, 2016. Application of self-sustaining smouldering combustion for the destruction of wastewater biosolids, Waste Management, pp. 201-212. [http://www.sciencedirect.com/science/article/pii/S0956053X1630037X doi:10.1016/j.wasman.2016.01.037]</ref>. Faeces mixed with sand can be smoldered, potentially offering a low-energy sanitation solution for the developing world<ref>Yermán, L., Hadden, R.M., Carrascal, J., Fabris, I., Cormier, D., Torero, J.L., Gerhard, J.I., Krajcovic, M., Pironi, P. and Cheng, Y.L., 2015. Smouldering combustion as a treatment technology for faeces: exploring the parameter space. Fuel, 147, 108-116. [http://dx.doi.org/10.1016/j.fuel.2015.01.055 doi: 10.1016/j.fuel.2015.01.055]</ref>.

==Field Applications==

===''In Situ'' Smoldering===
Smoldering has been demonstrated as a successful ''in situ'' technology in several pilot tests<ref name="Scholes2015"/><ref name="Grant2016"/>. The equipment and typical pilot test stages are animated (Fig. 5). As an example, ''in situ'' smoldering was applied to a coal tar layer present at 8 m depth in an alluvial aquifer, 7.5 m below the watertable. Ignition was achieved after ~ 3 hours of heating a region immediately around the well-screen. Thereafter, the self-sustaining reaction travelled outwards over the following 11 days in an approximately uniform manner in all four compass directions, reaching a radius of influence (ROI) of ~ 3.5 m. During that time, coal tar destruction rates were measured at ~ 80 kg/day, as determined from combustion gases collected at the surface. An estimated total of 860 kg was destroyed, with < 2% recovered as organic compounds by the vapor collection system<ref name="Scholes2015"/>. Total petroleum hydrocarbon (TPH) concentrations were reduced from a pre-treatment average of 18,500 mg/kg to a post-treatment average of 450 mg/kg, based on soil core sampling (Fig. 6). The fact that the zone being treated was below the watertable was no barrier to ''in situ'' smoldering. The test also demonstrated that the smoldering reaction rapidly terminates when air injection is stopped. The pilot test results were used to design the full scale site remediation (Fig. 7). Note that heterogeneity of ''in situ'' permeability, which dictates the spatial distribution of air flow, can affect the distribution of smoldering propagation rates. Further details about smoldering pilot tests, full scale applications, and challenges associated with ''in situ'' application are described elsewhere<ref name="Scholes2015"/><ref name="Grant2016"/>.

{{#ev:youtube|https://www.youtube.com/watch?v=656B57-yxpo&feature=youtu.be|500|center|Figure 5. Animation of the stages of an ''in situ'' smoldering remediation treatment below the water table including typical equipment employed.|frame}}
 
[[File:Gerhard Fig6.jpg|thumbnail|500px|center|Figure 6. Soil cores from before and after an ''in situ'' smoldering remediation pilot test at a coal tar contaminated site.]]
 
[[File:Gerhard Fig7.jpg|thumbnail|500px|center|Figure 7. Photograph of smoldering remediation full scale site treatment at a former chemical manufacturing facility in the USA.]]

===''Ex Situ'' Smoldering===
"Ex situ" treatment with smoldering is relatively straightforward because issues associated with heterogeneity, air flow distribution, and vapor management are more easily controlled and optimized. The smoldered material can either be (i) excavated contaminated soils, or (ii) organic liquids from lagoons or industrial processes that are mixed intentionally with sand. In the latter case, the sand matrix can be reused in subsequent batches, since treatment renders it clean and relatively unaltered<ref>Yermán, L., Wall, H., Torero, J., Gerhard, J.I. and Cheng, Y.L., 2016. Smoldering Combustion as a Treatment Technology for Feces: Sensitivity to Key Parameters. Combustion Science and Technology, 188(6), 968-981. [http://dx.doi.org/10.1080/00102202.2015.1136299 doi: 10.1080/00102202.2015.1136299]</ref>. When ''ex situ'' treatment was scaled-up from a bench column to an oil drum to a pilot field system, the technology was shown to be equally or more effective at larger scales<ref name="Switzer2014"/>. Pre-test concentrations averaging 31,000 mg/kg TPH were reduced to 10 mg/kg (Fig. 8). An example design of an ''ex situ'' smoldering treatment system is rather simple to illustrate (Fig. 9). Large scale treatment systems have been designed to treat approximately 100,000 kg of sludge or 500 m3 of contaminated soil per day in a semi-continuous manner.

[[File:Gerhard Fig8.jpg|thumbnail|500px|right|Figure 8. Pilot test of ''ex situ'' STAR treatment for coal tar liquid waste intentionally mixed with coarse sand<ref name="Switzer2009" />.]]

[[File:Gerhard Fig9.jpg|thumbnail|500px|right|Figure 9. Illustration of one design for an ''ex situ'' smoldering system.]]

===Practical Considerations===
Smoldering remediation is typically used as a source treatment technology where the goal is NAPL destruction to the extent practicable. Mass destruction rates on the order of hundreds of kilograms per day per ignition point are typical, with propagation rates on the order of feet per day.

Typical sites amenable to ''in situ'' smoldering remediation are those impacted by low-volatility recalcitrant compounds such as coal tar, creosote, or petroleum hydrocarbons at soil concentrations equal to or greater than approximately 3,000 mg/kg TPH within silty sand or coarser geologic units. This concentration is required to maintain self-sustaining smoldering (i.e., combustion without the input of external energy). The ability for smoldering to propagate outwards from a local, small energy input (e.g., few hours application at an ignition point) in a self-sustaining manner for many days in all directions where contamination exists, represents a major advantage in terms of cost savings, carbon footprint, and life cycle assessment. The suitability of a soil/NAPL site for smoldering treatment is easily determined with a laboratory treatability study. The radius of influence for a single ignition well is a key design parameter that is best determined by a field pilot test.

The smoldering remediation process can tolerate some site heterogeneity. For example, pockets exhibiting soil concentrations below the required minimum will be treated. In addition, the self-sustaining process can tolerate “clean gaps” in the contaminant distribution, transferring accumulated heat and starting combustion on the far side of the gap. Clay lenses can be tolerated, and will even be treated (pyrolized) as the treatment front passes by, but the overall unit to be treated requires adequate permeability for air injection to support the smoldering reaction. The rate of air injection provides a control on the mass destruction rate, and is accomplished with standard air injection equipment (e.g., compressor).

Depth below the watertable is an important design consideration, but does not impact the suitability of the smoldering remediation process (i.e., smoldering can be applied both above and below the watertable). Intermediate volatility contaminants, such as diesel, can often be smoldered directly. However, highly volatile NAPLs including gasoline and chlorinated solvents require a supplemental, non-toxic fuel such as EVO to be injected to support smoldering. In these cases, standard EVO injection equipment can be used and smoldering should be considered as a potentially inexpensive means to achieve treatment by heat-induced volatilization and surface capture/treatment. In any case where emissions of concern may be generated, standard vapor extraction and treatment systems are implemented.

===Summary===
Smoldering remediation results in the near complete destruction of organic compounds wherever combustion occurs. Concentration reductions > 99% are common in treated areas and post-treatment concentrations above non-detect or near non-detect levels are typically only associated with small areas where combustion did not occur within the target treatment zone. It is expected that, after smoldering remediation applications, groundwater contaminant mass flux may be reduced to a point that it can be addressed through [[Monitored Natural Attenuation (MNA) | monitored natural attenuation (MNA)]].

Smoldering remediation can also be applied ''ex situ''. The same types of impacted soils and contaminants apply as for ''in situ'' treatment, with the added benefit that materials can easily be manipulated to optimize the application. For example, the blending of highly contaminated soils with lightly contaminated soils can be carried out to create a mixture ideal for smoldering treatment. In addition, sand can be added to increase the permeability of material, thereby increasing the range of soil types suitable for the process. In addition to soil treatment, ''ex situ'' smoldering remediation systems can be used as a waste disposal technology for liquid organic wastes. In such cases, sand is mixed with the waste and the treated sand is then recycled for subsequent batches.

==References==

<references/>

==See Also==
*[http://www.eng.uwo.ca/research/restore/star.html Environmentally-Beneficial Smoldering Research]
*[http://www.savronsolutions.com/ Commercial Applications]

Soil Vapor Extraction (SVE)

2018-05-03T17:48:00Z

Astenger: Created page with "Soil vapor extraction (SVE) is a common and typically effective physical treatment process for remediation of volatile contaminants in vadose zone (unsaturated) soils. During..."

Soil vapor extraction (SVE) is a common and typically effective physical treatment process for remediation of volatile contaminants in vadose zone (unsaturated) soils. During SVE, contaminated soil gas is extracted from the [[wikipedia: Vadose zone | vadose zone]], treated, and then discharged to the atmosphere. SVE is most effective at sites where airflow can be induced through contaminated zones. SVE is a mature technology and has been applied in multiple configurations. Like other extraction-based technologies, SVE effectiveness typically diminishes over time as readily extracted contaminant mass is removed and mass transfer limitations begin to control the recovery of remaining contaminant mass.
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
'''Related Article(s):'''
*[[Remediation Technologies]]

'''CONTRIBUTOR(S):''' [[Michael Truex]] 

'''Key Resource(s):'''
*[http://dx.doi.org/10.2136/vzj2012.0137 Characterization and Remediation of Chlorinated Volatile Organic Contaminants in the Vadose Zone]<ref name= "Brusseau2013">Brusseau, M.L., Carroll, K.C., Truex, M.J., Becker, D.J., 2013. Characterization and remediation of chlorinated volatile organic contaminants in the vadose zone. Vadose Zone Journal, 12(4). [http://dx.doi.org/10.2136/vzj2012.0137 doi:10.2136/vzj2012.0137]</ref>
*[http://www.environmentalrestoration.wiki/images/0/00/Truex-2013-SVE_System_optimization%2C_Transition_and_Closure_Guidance-PNNL-21843.pdf Soil Vapor Extraction System Optimization, Transition, and Closure Guidance] <ref name= "Truex2013">Truex, M.J., Becker, D., Simon, M.A., Oostrom, M., Rice, A.K., Johnson, C.D., 2013. Soil vapor extraction system optimization, transition, and closure guidance (No. PNNL-21843). Pacific Northwest National Laboratory (PNNL), Richland, WA. [http://www.environmentalrestoration.wiki/images/0/00/Truex-2013-SVE_System_optimization%2C_Transition_and_Closure_Guidance-PNNL-21843.pdf Report pdf]</ref>

==Introduction==
SVE (also referred to as in situ soil venting or vacuum extraction) is a vadose zone remediation method for volatile contaminants<ref>U.S. Environmental Protection Agency, 2012. A citizen’s guide to soil vapor extraction and air sparging. Office of Solid Waste and Emergency Response, Washington, D.C. EPA/542/F-2/018. [http://www.environmentalrestoration.wiki/images/c/c4/EPA-2012-A_Citizens_Guide_to_soil_vapor_extraction_and_air_sparging.pdf Report pdf]</ref>. SVE is based on mass transfer of contaminant from the solid (sorbed) and liquid (aqueous or non-aqueous) phases into the gas phase, followed by collection and extraction of the contaminated soil gas. Extracted contaminant mass in the gas phase (and any condensed liquid phase) is then treated in aboveground systems. SVE is most effective for contaminants with higher [[wikipedia: Henry's law | Henry's Law] constants, including a range of chlorinated solvents and hydrocarbons. SVE is a well-demonstrated, mature remediation technology that has been identified by the U.S. Environmental Protection Agency (EPA) as a presumptive remedy<ref>U.S. Environmental Protection Agency, 1993. Presumptive Remedies: Site characterization and technology selection for CERCLA sites with volatile organic compounds in soils. Office of Solid Waste and Emergency Response, Washington, D.C. EPA/540/F-93/048. [http://www.environmentalrestoration.wiki/images/3/3d/EPA-1993-Site_Charact_and_Tech_Selection_for_CERCLA_Sites.pdf Report pdf]</ref><ref>U.S. Environmental Protection Agency (USEPA), 1996. User's guide to the VOCs in soils presumptive remedy. U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington, D.C. EPA/540/F-96/008. [http://www.environmentalrestoration.wiki/images/1/1b/EPA-1996-User%E2%80%99s_Guide_to_the_VOCs_in_Soils_Presumptive_Remedy.pdf Report pdf]</ref><ref>U.S. Environmental Protection Agency, 2011. Presumptive remedies: policy and procedures. U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington, D.C. [http://www.epa.gov/superfund/policy/remedy/presump/pol.htm US EPA Superfund site]</ref>.

==SVE Configuration==
SVE remediation technology uses vacuum blowers and vapor extraction wells to induce gas flow through the subsurface, collecting contaminated soil vapor, which is subsequently treated aboveground (Fig. 1)<ref>Hutzler, N.J., Murphy, B.E., Gierke, J.S., 1990. State of technology review: soil vapor extraction systems. U.S. Environmental Protection Agency, Risk Reduction Engineering Laboratory, Cincinnati, Ohio. EPA/600/S2-89/024. [http://www.environmentalrestoration.wiki/images/5/5a/EPA-1990-State_of_Tech_Rev-Soil_Vapor_Extraction_System.pdf Report pdf]</ref><ref>Pedersen, T.A., Curtis, J.T., 1991. Soil vapor extraction technology. Noyes Data Corporation, Park Ridge, New Jersey.</ref><ref>Noyes, R. 1994. Unit operations in environmental engineering. Noyes Publications, Park Ridge, New Jersey. [http://store.elsevier.com/Unit-Operations-in-Environmental-Engineering/Robert-Noyes/isbn-9780815513438/ ISBN: 978-0-8155-1343-8]</ref><ref>Stamnes, R., Blanchard, J., 1997. Engineering forum issue paper: soil vapor extraction implementation experiences. U.S. Environmental Protection Agency, Washington, D.C. EPA 540/F-95/030. [http://www.environmentalrestoration.wiki/images/4/4e/Stamnes-1997-Soil_Vapor_Extraction_Implementation_Experiences.pdf Report pdf]</ref><ref>Suthersan, S.S., 1999. Soil vapor extraction. In: Remediation Engineering Design Concepts. CRC Press, Suthersan, S.S., ed., Boca Raton, Florida. [https://www.crcpress.com/Remediation-Engineering-Design-Concepts/Suthersan/p/book/9781566701372 ISBN: 978-1-5667-0137-2]</ref><ref>Khan, F.I., Husain, T., Hejazi, R., 2004. An overview and analysis of site remediation technologies. Journal of Environmental Management, 71(2), 95-122. [http://dx.doi.org/10.1016/j.jenvman.2004.02.003 doi: 10.1016/j.jenvman.2004.02.003]</ref><ref>Damera, R., Bhandari, A., 2007. Physical treatment technologies. In: Remediation Technologies for Soils and Groundwater. Edited by A. Bhandari, R.Y. Surampalli, P. Champagne, S.K. Ong, R.D. Tyagi, I.M.C. Lo. American Society of Civil Engineers, Reston, Virginia. [http://dx.doi.org/10.1061/9780784408940.ch03 doi: 10.1061/9780784408940.ch03]</ref><ref name= "Brusseau2013"/>. The system can be implemented with standard wells and off-the-shelf equipment (blowers, instrumentation, vapor treatment, etc.). The vacuum extraction of soil gas induces gas flow through the vadose zone, increasing the rate of mass transfer from aqueous (soil moisture), non-aqueous (pure phase), and solid (soil) phases into the gas phase. Some systems only use soil gas extraction wells. Other systems also include air injection wells to help guide airflow through contaminated zones. Enhancements for improving SVE effectiveness can include directional drilling (to access under surface features that prevent vertical drilling or to align better with the contaminant distribution and/or subsurface features), pneumatic and hydraulic fracturing (to enhance subsurface airflow), and [[Thermal Remediation | thermal enhancement]] (e.g., hot air or steam injection to increase contaminant volatility)<ref>Frank, U., Barkley, N., 1995. Remediation of low permeability subsurface formations by fracturing enhancement of soil vapor extraction. Journal of Hazardous Materials, 40(2), 191-201. [http://dx.doi.org/10.1016/0304-3894(94)00069-s doi:10.1016/0304-3894(94)00069-S]</ref><ref>U.S. Environmental Protection Agency, 1997. Analysis of selected enhancements for soil vapor extraction. Office of Solid Waste and Emergency Response, Washington, D.C. EPA/542/R-97/007. [http://www.environmentalrestoration.wiki/images/4/48/EPA-1997-Analysis_of_Selected_Enhancements_for_SVE.pdf Report pdf]</ref><ref>Peng, S., Wang, N., Chen, J., 2013. Steam and air co-injection in removing residual TCE in unsaturated layered sandy porous media. Journal of Contaminant Hydrology, 153, 24-36. [http://dx.doi.org/10.1016/j.jconhyd.2013.07.002 doi: 10.1016/j.jconhyd.2013.07.002]</ref>. Passive SVE systems that rely on barometric pumping for soil gas extraction may also be employed<ref>Early, T., Borden, B., Heitkamp, M., Looney, B.B., Major, D., Waugh, W.J., Wein, G., Wiedemeier, T. Vangelas, K.M., Adams, K.M., Sink, C.H., 2006. Enhanced attenuation: a reference guide on approaches to increase the natural treatment capacity of a system. Washington Savannah River Company, Aiken, South Carolina. WSRC-STI-2006-00083, Rev.1. [http://www.environmentalrestoration.wiki/images/4/48/Early-2006-Enhanced_Attenuation-A_Ref_Guide_on_Approaches...natural_treatment_cap.pdf Report pdf]</ref><ref>Kamath, R., Adamson, D.T. , Newell, C.J., Vangelas, K.M., Looney, B.B., 2010. Passive soil vapor extraction. Savannah River National Laboratory, Aiken, South Carolina. SRNL-STI-2009-00571, Rev. 1. [http://www.environmentalrestoration.wiki/images/0/06/Kamath-2010-Passive_Soil_Vapor_Extraction-SRNL-STI-2009-00571.pdf Report pdf]</ref>.
[[File:Truex 3 Fig1.png|thumbnail|right|400 px|Figure 1. Conceptual diagram of basic SVE system for vadose zone remediation.]]

The soil gas (vapor) that is extracted by the SVE system generally requires treatment prior to discharge back into the environment. The aboveground treatment is primarily for a gas stream, although condensation of liquid must be managed (and in some cases may specifically be desired). A variety of treatment techniques are available for aboveground treatment<ref>U.S. Environmental Protection Agency, 2006. Off-gas treatment technologies for soil vapor extraction systems: state of the practice. Office of Superfund Remediation and Technology Innovation, Washington, D.C. EPA/542/R-05/028. [http://www.environmentalrestoration.wiki/images/e/e0/EPA-2006-Off-Gas_Treament_Tech_for_SVE_Ssytems-542R05028.pdf Report pdf]</ref> including:

*Thermal destruction (e.g., direct flame thermal oxidation, catalytic oxidizers),
*Adsorption (e.g., granular activated carbon, zeolites, polymers),
*Biofiltration,
*Non-thermal plasma destruction,
*Photolytic/photocatalytic destruction,
*Membrane separation,
*Gas absorption, and
*Vapor condensation.

The most commonly applied aboveground treatment technologies are thermal oxidation and granular activated carbon adsorption. Selection of a particular aboveground treatment technology depends on the contaminant, concentrations in the extracted gas, throughput, and economic considerations.

==SVE Effectiveness==
The rate and extent of mass removal by SVE depends on a number of factors that influence contaminant mass transfer into the gas phase. SVE effectiveness is a function of the contaminant properties (e.g., [[wikipedia: Henry's law | Henry's Law]] constant, [[wikipedia: Vapor pressure | vapor pressure]], [[wikipedia: Boiling point | boiling point]], adsorption coefficient), temperature in the subsurface, vadose zone soil properties (e.g., soil grain size, soil moisture content, permeability, carbon content), subsurface heterogeneity, and the airflow driving force (applied pressure gradient). As an example, a highly volatile contaminant (such as [[wikipedia: Trichloroethylene | trichloroethene]]) in a homogeneous sand with high permeability and low carbon content (i.e., low/negligible adsorption) will be readily treated with SVE. In contrast, a low vapor pressure contaminant like [[wikipedia: Naphthalene | naphthalene]] would require a longer treatment time and/or SVE enhancements, especially if it were present in a heterogeneous vadose zone with one or more low-permeability layers. SVE effectiveness is also related to the ability to induce airflow through contaminated portions of the vadose zone. Airflow in porous media is affected by the amount of connected gas-filled porosity, which is a function of the total porosity and the moisture content. Because low permeability zones typically have higher water saturation, gas-filled porosity can be low and airflow is inhibited in those zones. The SVE mass removal rate can decline over time (e.g., observed as tailing on a mass removal curve) when contaminants are present in zones with lower airflow (i.e., low-permeability zones or zones of high moisture content) and/or lower volatility (or higher adsorption). Recent work<ref>Switzer, C., Kosson, D.S., 2007. Soil vapor extraction performance in layered vadose zone materials. Vadose Zone Journal, 6(2), 397-405. [http://dx.doi.org/10.2136/vzj2005.0131 doi: 10.2136/vzj2005.0131]</ref><ref>Oostrom, M., Rockhold, M.L., Thorne, P.D., Truex, M.J., Last, G.V., Rohay, V.J., 2007. Carbon tetrachloride flow and transport in the subsurface of the 216-Z-9 trench at the Hanford Site. Vadose Zone Journal, 6(4), 971-984. [http://dx.doi.org/10.2136/vzj2006.0166 doi: 10.2136/vzj2006.0166]</ref> has investigated layering and low permeability zones in the subsurface to understand how they affect SVE operations.

==Design, Optimization, Performance, Assessment, and Closure==
Implementation of SVE involves the following elements: system design, operation, optimization, performance assessment, and closure. Several guidance documents provide information on these implementation aspects. EPA and U.S. Army Corps of Engineers (USACE) guidance documents<ref>DiGiulio, D.C., Varadhan, R., 2001. Development of recommendations and methods to support assessment of soil venting performance and closure. U.S. Environmental Protection Agency, EPA/600/R-01/070. [http://www.environmentalrestoration.wiki/images/e/e1/Digiulio-2001-Development_of_Recommendations_and_Methods...Soil_Venting.pdf Report pdf]</ref><ref>U.S. Environmental Protection Agency, 2004. How to evaluate alternative cleanup technologies for underground storage tank sites. Office of Solid Waste and Emergency Response, Washington, D.C. EPA/510/R-04/002. [http://www.environmentalrestoration.wiki/images/9/90/EPA-2004-How_to_Evaluate_Alternative_Cleanup_technologies_for_USTs.pdf Report pdf]</ref><ref>U. S. Army Corps of Engineers, 2002. Soil vapor extraction and bioventing. EM 1110-1-4001. Washington, D.C. [http://www.environmentalrestoration.wiki/images/2/26/USACE-2002-Soil_Vapor_Extraction_and_Bioventing-EM_1110-1-4001.pdf Report pdf]</ref> establish an overall framework for design, operation, optimization, and closure of a SVE system. The Air Force Center for Engineering and the Environment (AFCEE) guidance<ref>United States Air Force Environmental Restoration Program (AFCEE), 2001. Guidance on soil vapor extraction optimization. Brooks Air Force Base, Texas. [http://www.environmentalrestoration.wiki/images/a/a4/AFCEE-2001-SVE-optimization.pdf Report pdf]</ref> presents actions and considerations for SVE system optimization, but has limited information related to approaches for SVE closure and meeting remediation goals. As time goes on, it is typical for a SVE system to exhibit a diminishing rate of contaminant extraction due to mass transfer limitations for removal of contaminant mass. Performance assessment is a key aspect to provide input for decisions about whether the system should be optimized, terminated, or transitioned to another technology to replace or augment SVE. Guidance from the Pacific Northwest National Laboratory (PNNL)<ref name= "Truex2013"/> supplements the aforementioned documents by discussing specific actions and decisions related to SVE optimization, transition, and/or closure. Assessment of rebound and mass flux<ref name = "Truex2013"/><ref>Switzer, C., Slagle, T., Hunter, D., Kosson, D.S., 2004. Use of rebound testing for evaluation of soil vapor extraction performance at the savannah river site. Ground Water Monitoring Remediation, 24(4), 106-117. [http://dx.doi.org/10.1111/j.1745-6592.2004.tb01308.x doi: 10.1111/j.1745-6592.2004.tb01308.x]</ref><ref>Brusseau, M.L., Rohay, V., Truex, M.J., 2010. Analysis of soil vapor extraction data to evaluate mass-transfer constraints and estimate source-zone mass flux. Ground Water Monitoring Remediation, 30(3), 57-64. [http://dx.doi.org/10.1111/j.1745-6592.2010.01286.x doi: 10.1111/j.1745-6592.2010.01286.x]</ref> provide approaches to evaluate system performance and obtain information on which to base decisions.

==References==

<references/>

==See Also==
Additional internet resources for SVE are available at:

*[https://clu-in.org/techfocus/default.focus/sec/Soil_Vapor_Extraction/cat/Overview/ Clu-in.org]
*[http://www.cpeo.org/techtree/ttdescript/soilve.htm CPEO.org]
*[https://frtr.gov/matrix2/section4/4-7.html FRTR.gov]