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==Abiotic Reduction of Munitions Constituents==
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==Lysimeters for Measuring PFAS Concentrations in the Vadose Zone==  
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Munition compounds (MCs) often contain one or more nitro (-NO<sub>2</sub>) functional groups which makes them susceptible to abiotic reduction, i.e., transformation by accepting electrons from a chemical electron donor. In soil and groundwater, the most prevalent electron donors are natural organic carbon and iron minerals. Understanding the kinetics and mechanisms of abiotic reduction of MCs by carbon and iron constituents in soil is not only essential for evaluating the environmental fate of MCs but also key to developing cost-efficient remediation strategies. This article summarizes the recent advances in our understanding of MC reduction by carbon and iron based reductants.
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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):'''
 
'''Related Article(s):'''
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*[[Munitions Constituents]]
 
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*[[Munitions Constituents - Alkaline Degradation]]
 
  
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'''Contributor(s):'''
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*[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]
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*Dr. Jimmy Murillo-Gelvez
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*[[PFAS Transport and Fate]]
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*Paula Andrea Cárdenas-Hernández
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*[[PFAS Toxicology and Risk Assessment]]
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*Dr. Pei Chiu
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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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* Schwarzenbach, Gschwend, and Imboden, 2016. Environmental Organic Chemistry, 3rd ed.<ref name="Schwarzenbach2016">Schwarzenbach, R.P., Gschwend, P.M., and Imboden, D.M., 2016. Environmental Organic Chemistry, 3rd Edition. John Wiley and Sons, Ltd, 1024 pages.  ISBN: 978-1-118-76723-8</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"/>
  
 
==Introduction==
 
==Introduction==
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[[File:AbioMCredFig1.PNG | thumb |left|300px|Figure 1. Common munitions compounds. TNT and RDX are legacy explosives. DNAN, NTO, and NQ are insensitive MCs (IMCs) widely used as replacement for legacy explosives.]]
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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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Legacy and insensitive MCs (Figure 1.) are susceptible to reductive transformation in soil and groundwater. Many redox-active constituents in the subsurface, especially those containing organic carbon, Fe(II), and sulfur can mediate MC reduction. Specific examples include Fe(II)-organic complexes<ref name="Naka2006">Naka, D., Kim, D., and Strathmann, T.J., 2006. Abiotic Reduction of Nitroaromatic Compounds by Aqueous Iron(II)−Catechol Complexes. Environmental Science and Technology 40(9), pp. 3006–3012. [https://doi.org/10.1021/es060044t DOI: 10.1021/es060044t]</ref><ref name="Naka2008">Naka, D., Kim, D., Carbonaro, R.F., and Strathmann, T.J., 2008. Abiotic reduction of nitroaromatic contaminants by iron(II) complexes with organothiol ligands. Environmental Toxicology and Chemistry, 27(6), pp. 1257–1266. [https://doi.org/10.1897/07-505.1 DOI: 10.1897/07-505.1]</ref><ref name="Hartenbach2008">Hartenbach, A.E., Hofstetter, T.B., Aeschbacher, M., Sander, M., Kim, D., Strathmann, T.J., Arnold, W.A., Cramer, C.J., and Schwarzenbach, R.P., 2008. Variability of Nitrogen Isotope Fractionation during the Reduction of Nitroaromatic Compounds with Dissolved Reductants. Environmental Science and Technology 42(22), pp. 8352–8359. [https://doi.org/10.1021/es801063u DOI: 10.1021/es801063u]</ref><ref name="Kim2009">Kim, D., Duckworth, O.W., and Strathmann, T.J., 2009. Hydroxamate siderophore-promoted reactions between iron(II) and nitroaromatic groundwater contaminants. Geochimica et Cosmochimica Acta, 73(5), pp. 1297–1311. [https://doi.org/10.1016/j.gca.2008.11.039 DOI: 10.1016/j.gca.2008.11.039]</ref><ref name="Kim2007">Kim, D., and Strathmann, T.J., 2007. Role of Organically Complexed Iron(II) Species in the Reductive Transformation of RDX in Anoxic Environments. Environmental Science and Technology, 41(4), pp. 1257–1264.  [https://doi.org/10.1021/es062365a DOI: 10.1021/es062365a]</ref>, iron oxides in the presence of aqueous Fe(II)<ref name="Colón2006">Colón, D., Weber, E.J., and Anderson, J.L., 2006. QSAR Study of the Reduction of Nitroaromatics by Fe(II) Species. Environmental Science and Technology, 40(16), pp. 4976–4982.  [https://doi.org/10.1021/es052425x DOI: 10.1021/es052425x]</ref><ref name="Luan2013">Luan, F., Xie, L., Li, J., and Zhou, Q., 2013. Abiotic reduction of nitroaromatic compounds by Fe(II) associated with iron oxides and humic acid. Chemosphere, 91(7), pp. 1035–1041. [https://doi.org/10.1016/j.chemosphere.2013.01.070 DOI: 10.1016/j.chemosphere.2013.01.070]</ref><ref name="Gorski2016">Gorski, C.A., Edwards, R., Sander, M., Hofstetter, T.B., and Stewart, S.M., 2016. Thermodynamic Characterization of Iron Oxide–Aqueous Fe<sup>2+</sup> Redox Couples. Environmental Science and Technology, 50(16), pp. 8538–8547.  [https://doi.org/10.1021/acs.est.6b02661 DOI: 10.1021/acs.est.6b02661]</ref><ref name="Fan2016">Fan, D., Bradley, M.J., Hinkle, A.W., Johnson, R.L., and Tratnyek, P.G., 2016. Chemical Reactivity Probes for Assessing Abiotic Natural Attenuation by Reducing Iron Minerals. Environmental Science and Technology, 50(4), pp. 1868–1876. [https://doi.org/10.1021/acs.est.5b05800 DOI: 10.1021/acs.est.5b05800]</ref><ref name="Jones2016">Jones, A.M., Kinsela, A.S., Collins, R.N., and Waite, T.D., 2016. The reduction of 4-chloronitrobenzene by Fe(II)-Fe(III) oxide systems - correlations with reduction potential and inhibition by silicate. Journal of Hazardous Materials, 320, pp. 143–149. [https://doi.org/10.1016/j.jhazmat.2016.08.031 DOI: 10.1016/j.jhazmat.2016.08.031]</ref><ref name="Klausen1995">Klausen, J., Troeber, S.P., Haderlein, S.B., and Schwarzenbach, R.P., 1995. Reduction of Substituted Nitrobenzenes by Fe(II) in Aqueous Mineral Suspensions. Environmental Science and Technology, 29(9), pp. 2396–2404. [https://doi.org/10.1021/es00009a036 DOI: 10.1021/es00009a036]</ref><ref name="Strehlau2016">Strehlau, J.H., Stemig, M.S., Penn, R.L., and Arnold, W.A., 2016. Facet-Dependent Oxidative Goethite Growth As a Function of Aqueous Solution Conditions. Environmental Science and Technology, 50(19), pp. 10406–10412. [https://doi.org/10.1021/acs.est.6b02436 DOI: 10.1021/acs.est.6b02436]</ref><ref name="Elsner2004">Elsner, M., Schwarzenbach, R.P., and Haderlein, S.B., 2004. Reactivity of Fe(II)-Bearing Minerals toward Reductive Transformation of Organic Contaminants. Environmental Science and Technology, 38(3), pp. 799–807.  [https://doi.org/10.1021/es0345569 DOI: 10.1021/es0345569]</ref><ref name="Colón2008">Colón, D., Weber, E.J., and Anderson, J.L., 2008. Effect of Natural Organic Matter on the Reduction of Nitroaromatics by Fe(II) Species. Environmental Science and Technology, 42(17), pp. 6538–6543.  [https://doi.org/10.1021/es8004249 DOI: 10.1021/es8004249]</ref><ref name="Stewart2018">Stewart, S.M., Hofstetter, T.B., Joshi, P. and Gorski, C.A., 2018. Linking Thermodynamics to Pollutant Reduction Kinetics by Fe<sup>2+</sup> Bound to Iron Oxides. Environmental Science and Technology, 52(10), pp. 5600–5609. [https://doi.org/10.1021/acs.est.8b00481 DOI: 10.1021/acs.est.8b00481]&nbsp;&nbsp; [https://pubs.acs.org/doi/pdf/10.1021/acs.est.8b00481 Open access article.]</ref><ref name="Klupinski2004">Klupinski, T.P., Chin, Y.P., and Traina, S.J., 2004. Abiotic Degradation of Pentachloronitrobenzene by Fe(II):  Reactions on Goethite and Iron Oxide Nanoparticles. Environmental Science and Technology, 38(16), pp. 4353–4360. [https://doi.org/10.1021/es035434j DOI: 10.1021/es035434j]</ref>, magnetite<ref name="Klausen1995"/><ref name="Elsner2004"/><ref name="Heijman1993">Heijman, C.G., Holliger, C., Glaus, M.A., Schwarzenbach, R.P., and Zeyer, J., 1993. Abiotic Reduction of 4-Chloronitrobenzene to 4-Chloroaniline in a Dissimilatory Iron-Reducing Enrichment Culture. Applied and Environmental Microbiology, 59(12), pp. 4350–4353. [https://doi.org/10.1128/aem.59.12.4350-4353.1993 DOI: 10.1128/aem.59.12.4350-4353.1993]&nbsp;&nbsp; [https://journals.asm.org/doi/reader/10.1128/aem.59.12.4350-4353.1993 Open access article.]</ref><ref name="Gorski2009">Gorski, C.A., and Scherer, M.M., 2009. Influence of Magnetite Stoichiometry on Fe<sup>II</sup> Uptake and Nitrobenzene Reduction. Environmental Science and Technology, 43(10), pp. 3675–3680.  [https://doi.org/10.1021/es803613a DOI: 10.1021/es803613a]</ref><ref name="Gorski2010">Gorski, C.A., Nurmi, J.T., Tratnyek, P.G., Hofstetter, T.B. and Scherer, M.M., 2010. Redox Behavior of Magnetite: Implications for Contaminant Reduction. Environmental Science and Technology, 44(1), pp. 55–60. [https://doi.org/10.1021/es9016848 DOI: 10.1021/es9016848]</ref>, Fe(II)-bearing clays<ref name"Hofstetter2006">Hofstetter, T.B., Neumann, A., and Schwarzenbach, R.P., 2006. Reduction of Nitroaromatic Compounds by Fe(II) Species Associated with Iron-Rich Smectites. Environmental Science and Technology, 40(1), pp. 235–242. [https://doi.org/10.1021/es0515147 DOI: 10.1021/es0515147]</ref><ref name"Schultz2000">Schultz, C. A., and Grundl, T.J., 2000. pH Dependence on Reduction Rate of 4-Cl-Nitrobenzene by Fe(II)/Montmorillonite Systems. Environmental Science and Technology 34(17), pp. 3641–3648. [https://doi.org/10.1021/es990931e DOI: 10.1021/es990931e]</ref><ref name"Luan2015a">Luan, F., Gorski, C.A., and Burgos, W.D., 2015. Linear Free Energy Relationships for the Biotic and Abiotic Reduction of Nitroaromatic Compounds. Environmental Science and Technology, 49(6), pp. 3557–3565. [https://doi.org/10.1021/es5060918 DOI: 10.1021/es5060918]</ref><ref name"Luan2015b">Luan, F., Liu, Y., Griffin, A.M., Gorski, C.A. and Burgos, W.D., 2015. Iron(III)-Bearing Clay Minerals Enhance Bioreduction of Nitrobenzene by ''Shewanella putrefaciens'' CN32. Environmental Science and Technology, 49(3), pp. 1418–1426. [https://doi.org/10.1021/es504149y DOI: 10.1021/es504149y]</ref><ref name"Hofstetter2003">Hofstetter, T.B., Schwarzenbach, R.P. and Haderlein, S.B., 2003. Reactivity of Fe(II) Species Associated with Clay Minerals. Environmental Science and Technology, 37(3), pp. 519–528. [https://doi.org/10.1021/es025955r DOI: 10.1021/es025955r]</ref><ref name"Neumann2008">Neumann, A., Hofstetter, T.B., Lüssi, M., Cirpka, O.A., Petit, S., and Schwarzenbach, R.P., 2008. Assessing the Redox Reactivity of Structural Iron in Smectites Using Nitroaromatic Compounds As Kinetic Probes. Environmental Science and Technology, 42(22), pp. 8381–8387.  [https://doi.org/10.1021/es801840x DOI: 10.1021/es801840x]</ref><ref name"Hofstetter2008">Hofstetter, T.B., Neumann, A., Arnold, W.A., Hartenbach, A.E., Bolotin, J., Cramer, C.J., and Schwarzenbach, R.P., 2008. Substituent Effects on Nitrogen Isotope Fractionation During Abiotic Reduction of Nitroaromatic Compounds. Environmental Science and Technology, 42(6), pp. 1997–2003.  [https://doi.org/10.1021/es702471k DOI: 10.1021/es702471k]</ref>, hydroquinones (as surrogates of natural organic matter)<ref name="Hartenbach2008"/><ref name="Schwarzenbach1990">Schwarzenbach, R.P., Stierli, R., Lanz, K., and Zeyer, J., 1990. Quinone and Iron Porphyrin Mediated Reduction of Nitroaromatic Compounds in Homogeneous Aqueous Solution. Environmental Science and Technology, 24(10), pp. 1566–1574.  [https://doi.org/10.1021/es00080a017 DOI: 10.1021/es00080a017]</ref><ref name="Tratnyek1989">Tratnyek, P.G., and Macalady, D.L., 1989. Abiotic Reduction of Nitro Aromatic Pesticides in Anaerobic Laboratory Systems. Journal of Agricultural and Food Chemistry, 37(1), pp. 248–254.  [https://doi.org/10.1021/jf00085a058 DOI: 10.1021/jf00085a058]</ref><ref name="Hofstetter1999">Hofstetter, T.B., Heijman, C.G., Haderlein, S.B., Holliger, C. and Schwarzenbach, R.P., 1999. Complete Reduction of TNT and Other (Poly)nitroaromatic Compounds under Iron-Reducing Subsurface Conditions. Environmental Science and Technology, 33(9), pp. 1479–1487.  [https://doi.org/10.1021/es9809760 DOI: 10.1021/es9809760]</ref><ref name="Murillo-Gelvez2019">Murillo-Gelvez, J., Hickey, K.P., Di Toro, D.M., Allen, H.E., Carbonaro, R.F., and Chiu, P.C., 2019. Experimental Validation of Hydrogen Atom Transfer Gibbs Free Energy as a Predictor of Nitroaromatic Reduction Rate Constants. Environmental Science and Technology, 53(10), pp. 5816–5827.  [https://doi.org/10.1021/acs.est.9b00910 DOI: 10.1021/acs.est.9b00910]</ref><ref name="Niedźwiecka2017">Niedźwiecka, J.B., Drew, S.R., Schlautman, M.A., Millerick, K.A., Grubbs, E., Tharayil, N. and Finneran, K.T., 2017. Iron and Electron Shuttle Mediated (Bio)degradation of 2,4-Dinitroanisole (DNAN). Environmental Science and Technology, 51(18), pp. 10729–10735.  [https://doi.org/10.1021/acs.est.7b02433 DOI: 10.1021/acs.est.7b02433]</ref><ref name="Kwon2006">Kwon, M.J., and Finneran, K.T., 2006. Microbially Mediated Biodegradation of Hexahydro-1,3,5-Trinitro-1,3,5- Triazine by Extracellular Electron Shuttling Compounds. Applied and Environmental Microbiology, 72(9), pp. 5933–5941.  [https://doi.org/10.1128/AEM.00660-06 DOI: 10.1128/AEM.00660-06]&nbsp;&nbsp; [https://journals.asm.org/doi/reader/10.1128/AEM.00660-06 Open access article.]</ref>, dissolved organic matter<ref name="Dunnivant1992">Dunnivant, F.M., Schwarzenbach, R.P., and Macalady, D.L., 1992. Reduction of Substituted Nitrobenzenes in Aqueous Solutions Containing Natural Organic Matter. Environmental Science and Technology, 26(11), pp. 2133–2141.  [https://doi.org/10.1021/es00035a010 DOI: 10.1021/es00035a010]</ref><ref name="Luan2010">Luan, F., Burgos, W.D., Xie, L., and Zhou, Q., 2010. Bioreduction of Nitrobenzene, Natural Organic Matter, and Hematite by Shewanella putrefaciens CN32. Environmental Science and Technology, 44(1), pp. 184–190.  [https://doi.org/10.1021/es901585z DOI: 10.1021/es901585z]</ref><ref name="Murillo-Gelvez2021">Murillo-Gelvez, J., di Toro, D.M., Allen, H.E., Carbonaro, R.F., and Chiu, P.C., 2021. Reductive Transformation of 3-Nitro-1,2,4-triazol-5-one (NTO) by Leonardite Humic Acid and Anthraquinone-2,6-disulfonate (AQDS). Environmental Science and Technology, 55(19), pp. 12973–12983.  [https://doi.org/10.1021/acs.est.1c03333 DOI: 10.1021/acs.est.1c03333]</ref>, black carbon<ref name="Oh2013">Oh, S.-Y., Son, J.G., and Chiu, P.C., 2013. Biochar-Mediated Reductive Transformation of Nitro Herbicides and Explosives. Environmental Toxicology and Chemistry, 32(3), pp. 501–508.  [https://doi.org/10.1002/etc.2087 DOI: 10.1002/etc.2087]&nbsp;&nbsp; [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/etc.2087 Open access article.]</ref><ref name="Oh2009">Oh, S.-Y., and Chiu, P.C., 2009. Graphite- and Soot-Mediated Reduction of 2,4-Dinitrotoluene and Hexahydro-1,3,5-trinitro-1,3,5-triazine. Environmental Science & Technology, 43(18), pp. 6983–6988.  [https://doi.org/10.1021/es901433m DOI: 10.1021/es901433m]</ref><ref name="Xu2015">Xu, W., Pignatello, J.J., and Mitch, W.A., 2015. Reduction of Nitroaromatics Sorbed to Black Carbon by Direct Reaction with Sorbed Sulfides. Environmental Science and Technology, 49(6), pp. 3419–3426.  [https://doi.org/10.1021/es5045198 DOI: 10.1021/es5045198]</ref><ref name="Oh2002">Oh, S.-Y., Cha, D.K., and Chiu, P.C., 2002. Graphite-Mediated Reduction of 2,4-Dinitrotoluene with Elemental Iron. Environmental Science and Technology, 36(10), pp. 2178–2184.  [https://doi.org/10.1021/es011474g DOI: 10.1021/es011474g]</ref><ref name="Amezquita-Garcia2013">Amezquita-Garcia, H.J., Razo-Flores, E., Cervantes, F.J., and Rangel-Mendez, J.R., 2013.  Activated carbon fibers as redox mediators for the increased reduction of nitroaromatics. Carbon, 55, pp. 276–284. [https://doi.org/10.1016/j.carbon.2012.12.062 DOI: 10.1016/j.carbon.2012.12.062]</ref><ref name="Xin2022">Xin, D., Girón, J., Fuller, M.E., and Chiu, P.C., 2022. Abiotic Reduction of 3-Nitro-1,2,4-triazol-5-one (NTO) and Other Munitions Constituents by Wood-Derived Biochar through Its Rechargeable Electron Storage Capacity. Environmental Science: Processes and Impacts, 24(2), pp. 316-329.  [https://doi.org/10.1039/D1EM00447F DOI: 10.1039/D1EM00447F]</ref>, and sulfides<ref name="Hojo1960">Hojo, M., Takagi, Y. and Ogata, Y., 1960. Kinetics of the Reduction of Nitrobenzenes by Sodium Disulfide. Journal of the American Chemical Society, 82(10), pp. 2459–2462.  [https://doi.org/10.1021/ja01495a017 DOI: 10.1021/ja01495a017]</ref><ref name="Zeng2012">Zeng, T., Chin, Y.P., and Arnold, W.A., 2012. Potential for Abiotic Reduction of Pesticides in Prairie Pothole Porewaters. Environmental Science and Technology, 46(6), pp. 3177–3187.  [https://doi.org/10.1021/es203584d DOI: 10.1021/es203584d]</ref>. These geo-reductants may control the fate and half-lives of MCs in the environment and can be used to promote MC degradation in soil and groundwater through enhanced natural attenuation<ref name="USEPA2012">US EPA, 2012. A Citizen’s Guide to Monitored Natural Attenuation. EPA document 542-F-12-014.  [https://www.epa.gov/sites/default/files/2015-04/documents/a_citizens_guide_to_monitored_natural_attenuation.pdf Free download.]</ref>.
 
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[[File:AbioMCredFig2.png | thumb |450px|Figure 2. General mechanism for the reduction of NACs/MCs.]]
 
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[[File:AbioMCredFig3.png | thumb |450px|Figure 3. Schematic of natural attenuation of MCs-impacted soils through chemical reduction.]]
 
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Although the chemical structures of MCs can vary significantly (Figure 1), most of them contain at least one nitro functional group (-NO<sub>2</sub>), which is susceptible to reductive transformation<ref name="Spain2000">Spain, J.C., Hughes, J.B., and Knackmuss, H.J., 2000. Biodegradation of Nitroaromatic Compounds and Explosives. CRC Press, 456 pages. ISBN: 9780367398491</ref>. Of the MCs shown in Figure 1, 2,4,6-trinitrotoluene (TNT), 2,4-dinitroanisole (DNAN), and 3-nitro-1,2,4-triazol-5-one (NTO)<ref name="Harris1996">Harris, N.J., and Lammertsma, K., 1996. Tautomerism, Ionization, and Bond Dissociations of 5-Nitro-2,4-dihydro-3H-1,2,4-triazolone. Journal of the American Chemical Society, 118(34), pp. 8048–8055.  [https://doi.org/10.1021/ja960834a DOI: 10.1021/ja960834a]</ref> are nitroaromatic compounds (NACs) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and nitroguanidine (NQ) are nitramines. The structural differences may result in different reactivities and reaction pathways. Reduction of NACs results in the formation of aromatic amines (i.e., anilines) with nitroso and hydroxylamine compounds as intermediates (Figure 2)<ref name="Schwarzenbach2016"/>.
 
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Although the final reduction products are different for non-aromatic MCs, the reduction process often starts with the transformation of the -NO<sub>2</sub> moiety, either through de-nitration (e.g., RDX<ref name="Kwon2008">Kwon, M.J., and Finneran, K.T., 2008. Biotransformation products and mineralization potential for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in abiotic versus biological degradation pathways with anthraquinone-2,6-disulfonate (AQDS) and ''Geobacter metallireducens''. Biodegradation, 19(5), pp. 705–715.  [https://doi.org/10.1007/s10532-008-9175-5 DOI: 10.1007/s10532-008-9175-5]</ref><ref name="Halasz2011">Halasz, A., and Hawari, J., 2011. Degradation Routes of RDX in Various Redox Systems. Aquatic Redox Chemistry, American Chemical Society, 1071(20), pp. 441-462.  [https://doi.org/10.1021/bk-2011-1071.ch020 DOI: 10.1021/bk-2011-1071.ch020]</ref>) or reduction to nitroso<ref name="Kwon2006"/><ref name="Tong2021">Tong, Y., Berens, M.J., Ulrich, B.A., Bolotin, J., Strehlau, J.H., Hofstetter, T.B., and Arnold, W.A., 2021. Exploring the Utility of Compound-Specific Isotope Analysis for Assessing Ferrous Iron-Mediated Reduction of RDX in the Subsurface. Environmental Science and Technology, 55(10), pp. 6752–6763.  [https://doi.org/10.1021/acs.est.0c08420 DOI: 10.1021/acs.est.0c08420]</ref> followed by ring cleavage<ref name="Kim2007"/><ref name="Halasz2011"/><ref name="Tong2021"/><ref name="Larese-Casanova2008">Larese-Casanova, P., and Scherer, M.M., 2008. Abiotic Transformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by Green Rusts. Environmental Science and Technology, 42(11), pp. 3975–3981.  [https://doi.org/10.1021/es702390b DOI: 10.1021/es702390b]</ref>.
 
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Figure 3 illustrates a typical MC reduction reaction. A redox-active soil constituent, such as organic matter or iron mineral, donates electrons to an MC and transforms the nitro group into an amino group (R-NH<sub>2</sub>). The rate at which an MC is reduced can vary by many orders of magnitude depending on the soil constituent, the MC, the reduction potential (''E<sub>H</sub>'') and other media conditions<ref name="Borch2010">Borch, T., Kretzschmar, R., Kappler, A., Cappellen, P.V., Ginder-Vogel, M., Voegelin, A., and Campbell, K., 2010. Biogeochemical Redox Processes and their Impact on Contaminant Dynamics. Environmental Science and Technology, 44(1), pp. 15–23.  [https://doi.org/10.1021/es9026248 DOI: 10.1021/es9026248]&nbsp;&nbsp; [https://pubs.acs.org/doi/pdf/10.1021/es9026248 Open access article.]</ref>.
 
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The most prevalent reductants in soils are iron minerals and organic carbon such as that found in natural organic matter. It has been suggested that Fe(II)<sub>aq</sub> and dissolved organic matter concentrations could serve as indicators of NAC reducibility in anaerobic sediments<ref name="Zhang2009">Zhang, H., and Weber, E.J., 2009. Elucidating the Role of Electron Shuttles in Reductive Transformations in Anaerobic Sediments. Environmental Science and Technology, 43(4), pp. 1042–1048.  [https://doi.org/10.1021/es8017072 DOI: 10.1021/es8017072]</ref>. The following sections summarize these two classes of reductants separately and present advances in our understanding of the kinetics of NAC/MC reduction by these geo-reductants.
 
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==Carbonaceous Reductants==
 
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The two most predominant forms of organic carbon in natural systems are natural organic matter (NOM) and black carbon (BC)<ref name="Schumacher2002">Schumacher, B.A., 2002. Methods for the Determination of Total Organic Carbon (TOC) in Soils and Sediments. U.S. EPA, Ecological Risk Assessment Support Center. [http://bcodata.whoi.edu/LaurentianGreatLakes_Chemistry/bs116.pdf Free download.]</ref>. Black carbon includes charcoal, soot, graphite, and coal. Until the early 2000s black carbon was considered to be a class of (bio)chemically inert geosorbents<ref name="Schmidt2000">Schmidt, M.W.I., and Noack, A.G., 2000. Black carbon in soils and sediments: Analysis, distribution, implications, and current challenges. Global Biogeochemical Cycles, 14(3), pp. 777–793.  [https://doi.org/10.1029/1999GB001208 DOI: 10.1029/1999GB001208]&nbsp;&nbsp; [https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/1999GB001208 Open access article.]</ref>. However, it has been shown that BC can contain abundant quinone functional groups and thus can store and exchange electrons<ref name="Klüpfel2014">Klüpfel, L., Keiluweit, M., Kleber, M., and Sander, M., 2014. Redox Properties of Plant Biomass-Derived Black Carbon (Biochar). Environmental Science and Technology, 48(10), pp. 5601–5611.  [https://doi.org/10.1021/es500906d DOI: 10.1021/es500906d]</ref> with chemical<ref name="Xin2019">Xin, D., Xian, M., and Chiu, P.C., 2019. New methods for assessing electron storage capacity and redox reversibility of biochar. Chemosphere, 215, 827–834.  [https://doi.org/10.1016/j.chemosphere.2018.10.080 DOI: 10.1016/j.chemosphere.2018.10.080]</ref> and biological<ref name="Saquing2016">Saquing, J.M., Yu, Y.-H., and Chiu, P.C., 2016. Wood-Derived Black Carbon (Biochar) as a Microbial Electron Donor and Acceptor. Environmental Science and Technology Letters, 3(2), pp. 62–66.  [https://doi.org/10.1021/acs.estlett.5b00354 DOI: 10.1021/acs.estlett.5b00354]</ref> agents in the surroundings. Specifically, BC such as biochar has been shown to reductively transform MCs including NTO, DNAN, and RDX<ref name="Xin2022"/>.
 
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NOM encompasses all the organic compounds present in terrestrial and aquatic environments and can be classified into two groups, non-humic and humic substances. Humic substances (HS) contain a wide array of functional groups including carboxyl, enol, ether, ketone, ester, amide, (hydro)quinone, and phenol<ref name="Sparks2003">Sparks, D.L., 2003. Environmental Soil Chemistry, 2nd Edition. Elsevier Science and Technology Books.  [https://doi.org/10.1016/B978-0-12-656446-4.X5000-2 DOI: 10.1016/B978-0-12-656446-4.X5000-2]</ref>. Quinone and hydroquinone groups are believed to be the predominant redox moieties responsible for the capacity of HS and BC to store and reversibly accept and donate electrons (i.e., through reduction and oxidation of HS/BC, respectively)<ref name="Schwarzenbach1990"/><ref name="Dunnivant1992"/><ref name="Klüpfel2014"/><ref name="Scott1998">Scott, D.T., McKnight, D.M., Blunt-Harris, E.L., Kolesar, S.E., and Lovley, D.R., 1998. Quinone Moieties Act as Electron Acceptors in the Reduction of Humic Substances by Humics-Reducing Microorganisms. Environmental Science and Technology, 32(19), pp. 2984–2989.  [https://doi.org/10.1021/es980272q DOI: 10.1021/es980272q]</ref><ref name="Cory2005">Cory, R.M., and McKnight, D.M., 2005. Fluorescence Spectroscopy Reveals Ubiquitous Presence of Oxidized and Reduced Quinones in Dissolved Organic Matter. Environmental Science & Technology, 39(21), pp 8142–8149.  [https://doi.org/10.1021/es0506962 DOI: 10.1021/es0506962]</ref><ref name="Fimmen2007">Fimmen, R.L., Cory, R.M., Chin, Y.P., Trouts, T.D., and McKnight, D.M., 2007. Probing the oxidation–reduction properties of terrestrially and microbially derived dissolved organic matter. Geochimica et Cosmochimica Acta, 71(12), pp. 3003–3015.  [https://doi.org/10.1016/j.gca.2007.04.009 DOI: 10.1016/j.gca.2007.04.009]</ref><ref name="Struyk2001">Struyk, Z., and Sposito, G., 2001. Redox properties of standard humic acids. Geoderma, 102(3-4), pp. 329–346.  [https://doi.org/10.1016/S0016-7061(01)00040-4 DOI: 10.1016/S0016-7061(01)00040-4]</ref><ref name="Ratasuk2007">Ratasuk, N., and Nanny, M.A., 2007. Characterization and Quantification of Reversible Redox Sites in Humic Substances. Environmental Science and Technology, 41(22), pp. 7844–7850.  [https://doi.org/10.1021/es071389u DOI: 10.1021/es071389u]</ref><ref name="Aeschbacher2010">Aeschbacher, M., Sander, M., and Schwarzenbach, R.P., 2010. Novel Electrochemical Approach to Assess the Redox Properties of Humic Substances. Environmental Science and Technology, 44(1), pp. 87–93.  [https://doi.org/10.1021/es902627p DOI: 10.1021/es902627p]</ref><ref name="Aeschbacher2011">Aeschbacher, M., Vergari, D., Schwarzenbach, R.P., and Sander, M., 2011. Electrochemical Analysis of Proton and Electron Transfer Equilibria of the Reducible Moieties in Humic Acids. Environmental Science and Technology, 45(19), pp. 8385–8394.  [https://doi.org/10.1021/es201981g DOI: 10.1021/es201981g]</ref><ref name="Bauer2009">Bauer, I., and Kappler, A., 2009. Rates and Extent of Reduction of Fe(III) Compounds and O<sub>2</sub> by Humic Substances. Environmental Science and Technology, 43(13), pp. 4902–4908.  [https://doi.org/10.1021/es900179s DOI: 10.1021/es900179s]</ref><ref name="Maurer2010">Maurer, F., Christl, I. and Kretzschmar, R., 2010. Reduction and Reoxidation of Humic Acid: Influence on Spectroscopic Properties and Proton Binding. Environmental Science and Technology, 44(15), pp. 5787–5792.  [https://doi.org/10.1021/es100594t DOI: 10.1021/es100594t]</ref><ref name="Walpen2016">Walpen, N., Schroth, M.H., and Sander, M., 2016. Quantification of Phenolic Antioxidant Moieties in Dissolved Organic Matter by Flow-Injection Analysis with Electrochemical Detection. Environmental Science and Technology, 50(12), pp. 6423–6432.  [https://doi.org/10.1021/acs.est.6b01120 DOI: 10.1021/acs.est.6b01120]&nbsp;&nbsp; [https://pubs.acs.org/doi/pdf/10.1021/acs.est.6b01120 Open access article.]</ref><ref name="Aeschbacher2012">Aeschbacher, M., Graf, C., Schwarzenbach, R.P., and Sander, M., 2012.  Antioxidant Properties of Humic Substances. Environmental Science and Technology, 46(9), pp. 4916–4925.  [https://doi.org/10.1021/es300039h DOI: 10.1021/es300039h]</ref><ref name="Nurmi2002">Nurmi, J.T., and Tratnyek, P.G., 2002. Electrochemical Properties of Natural Organic Matter (NOM), Fractions of NOM, and Model Biogeochemical Electron Shuttles. Environmental Science and Technology, 36(4), pp. 617–624.  [https://doi.org/10.1021/es0110731 DOI: 10.1021/es0110731]</ref>.  
 
  
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Hydroquinones have been widely used as surrogates to understand the reductive transformation of NACs and MCs by NOM. Figure 4 shows the chemical structures of the singly deprotonated forms of four hydroquinone species previously used to study NAC/MC reduction. The second-order rate constants (''k<sub>R</sub>'') for the reduction of NACs/MCs by these hydroquinone species are listed in Table 1, along with the aqueous-phase one electron reduction potentials of the NACs/MCs (''E<sub>H</sub><sup>1’</sup>'') where available. ''E<sub>H</sub><sup>1’</sup>'' is an experimentally measurable thermodynamic property that reflects the propensity of a given NAC/MC to accept an electron in water (''E<sub>H</sub><sup>1</sup>''(R-NO<sub>2</sub>)):
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==PFAS Background==
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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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:: <big>'''Equation 1:'''</big>&nbsp;&nbsp; [[File: StultsEq1.png | 400 px]]</br>
 +
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>
 +
:: <big>'''Equation 2:'''</big>&nbsp;&nbsp; [[File: StultsEq2.png | 400 px]]</br>
 +
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.
  
−
:::::'''Equation 1:'''&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;''R-NO<sub>2</sub> + e<sup>-</sup> ⇔ R-NO<sub>2</sub><sup>•-</sup>''
+
==Lysimeters Background==
−
 
+
[[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/]]
−
Knowing the identity of and reaction order in the reductant is required to derive the second-order rate constants listed in Table 1. This same reason limits the utility of reduction rate constants measured with complex carbonaceous reductants such as NOM<ref name="Dunnivant1992"/>, BC<ref name="Oh2013"/><ref name="Oh2009"/><ref name="Xu2015"/><ref name="Xin2021">Xin, D., 2021. Understanding the Electron Storage Capacity of Pyrogenic Black Carbon: Origin, Redox Reversibility, Spatial Distribution, and Environmental Applications. Doctoral Thesis, University of Delaware. [https://udspace.udel.edu/bitstream/handle/19716/30105/Xin_udel_0060D_14728.pdf?sequence=1 Free download.]</ref>, and HS<ref name="Luan2010"/><ref name="Murillo-Gelvez2021"/>, whose chemical structures and redox moieties responsible for the reduction, as well as their abundance, are not clearly defined or known. In other words, the observed rate constants in those studies are specific to the experimental conditions (e.g., pH and NOM source and concentration), and may not be easily comparable to other studies.
+
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.
 +
# <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.
  
 +
==Analysis of PFAS Concentrations in Soil and Porewater==
 
{| class="wikitable mw-collapsible" style="float:left; margin-right:20px; text-align:center;"
 
{| class="wikitable mw-collapsible" style="float:left; margin-right:20px; text-align:center;"
−
|+ Table 1. Aqueous phase one electron reduction potentials and logarithm of second-order rate constants for the reduction of NACs and MCs by the singly deprotonated form of the hydroquinones lawsone, juglone, AHQDS and AHQS, with the second-order rate constants for the deprotonated NAC/MC species (i.e., nitrophenolates and NTO–) in parentheses.
+
|+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
 
|-
 
|-
−
! Compound 
+
|Site B||PFOS||2.2 ± 2.0||0.78 ± 0.38||2.8
−
!
 
−
! colspan="4"| Hydroquinone (log ''k<sub>R</sub>''&nbsp;(M<sup>-1</sup>s<sup>-1</sup>))
 
 
|-
 
|-
−
! (NAC/MC)
+
|rowspan="3"|Site C||PFOS||13 ± 4.1||680 ± 460||164 ± 75
−
! ''E<sub>H</sub><sup>1'</sup>'' (V)
 
−
! LAW<sup>-</sup>
 
−
! JUG<sup>-</sup>
 
−
! AHQDS<sup>-</sup>
 
−
! AHQS<sup>-</sup>
 
 
|-
 
|-
−
| Nitrobenzene (NB) || -0.485 || 0.380 || -1.102 || 2.050 || 3.060
+
|8:2 FTS||1.2 ± 0.46||52 ± 13||16 ± 6.0
 
|-
 
|-
−
| 2-nitrotoluene (2-NT) || -0.590 || -1.432 || -2.523 || 0.775 ||
+
|PFHpS||0.36 ± 0.051||2.9 ± 2.0||5.9 ± 3.4
−
|-
 
−
| 3-nitrotoluene (3-NT) || -0.475 || 0.462 || -0.921 ||  ||
 
 
|}
 
|}
 +
[[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.
 +
 +
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&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"/>.
 +
 +
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>.
  
−
==Advantages and Limitations of the Technology for PFAS Treatment==
+
==Summary and Recommendations==
−
===Advantages:===
+
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.
−
* High removal rates of long-chain PFAS (C5-C8) due to the production of versatile reactive species
 
−
* Requires no chemical additions and produces no residual waste
 
−
* Total organic carbon (TOC) concentration and other non-surfactant co-contaminants do not influence the process efficiency
 
−
* The process is mobile and scalable
 
−
* Versatile: can be used in batch and continuous systems
 
  
−
===Limitations:===
+
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>.
−
* Limited removal of short-chain PFAS due to their inability to concentrate at plasma-liquid interfaces. Addition of surfactants such as CTAB improves their removal and degradation rates.
 
−
* Excessive foaming caused by bubbling argon gas through a solution containing high (>10 mg/L) concentrations of long-chain (surfactant) PFAS may interfere with the formation of plasma.
 
  
−
==Summary==
+
Future research opportunities should address the current key uncertainties related to the use of lysimeters for PFAS investigations, including:
−
PFAS are susceptible to plasma treatment because the hydrophobic PFAS accumulates at the gas-liquid interface, exposing more of the PFAS to the plasma. Plasma-based treatment of PFAS contaminated water successfully degrades PFOA and PFOS to below the EPA health advisory level of 70 ppt and accomplishes the near complete destruction of other PFAS within a short treatment time. PFAS concentration reductions of ≥90% and post-treatment concentrations below laboratory detection levels are common for long chain PFAS and precursors.
+
#<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.
−
The lack of sensitivity of plasma to co-contaminants, coupled with high PFAS removal and defluorination efficiencies, makes plasma-based water treatment a promising technology for the remediation of PFAS-contaminated water. The plasma treatment process is currently developed for ''ex situ'' application and can also be integrated into a treatment train<ref name="Richardson2021">Richardson, S., 2021. Nanofiltration Followed by Electrical Discharge Plasma for Destruction of PFAS and Co-occurring Chemicals in Groundwater: A Treatment Train Approach. Environmental Security Technology Certification Program (ESTCP), Project Number ER21-5136.  [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/ER21-5136  Project Overview]</ref>.
+
#<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.
 +
#<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.
 +
#<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.
  
 
==References==
 
==References==

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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