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==Sediment Capping==
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==Lysimeters for Measuring PFAS Concentrations in the Vadose Zone==  
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Capping is an ''in situ'' remedial technology that involves placement of a clean substrate on the surface of [[Contaminated Sediments - Introduction | contaminated sediments]] to reduce contaminant uptake by benthic organisms and contaminant flux to surface water. Simple sand caps can be effective in reducing exposure of benthic organisms and in limiting oxygen transport into the contaminated sediments, resulting in precipitation of metal sulfides. Amendments are sometimes included in caps to reduce cap permeability and groundwater upwelling, to increase contaminant sorption or biodegradation, or to improve habitat.
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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.  
 
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
 
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
  
 
'''Related Article(s):'''
 
'''Related Article(s):'''
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*[[Contaminated Sediments - Introduction]]
 
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*[[In Situ Treatment of Contaminated Sediments with Activated Carbon]]
 
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*Sediment Risk Assessment
 
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*[[Passive Sampling of Sediments]]
 
  
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'''Contributor(s):'''
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*[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]
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*Danny Reible
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*[[PFAS Transport and Fate]]
  
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'''Key Resource(s):'''
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'''Contributors:''' Gorm Heron, Emily Crownover, Patrick Joyce, Ramona Iery
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*Processes, Assessment and Remediation of Contaminated Sediments<ref name="Reible2014">Reible, D. D., Editor, 2014. Processes, Assessment and Remediation of Contaminated Sediments. Springer, New York, NY. 462 pp. ISBN: 978-1-4614-6725-0</ref>
 
  
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* Guidance for In-Situ Subaqueous Capping of Contaminated Sediments<ref name="Palermo1998">Palermo, M., Maynord, S., Miller, J. and Reible, D., 1998. Guidance for In-Situ Subaqueous Capping of Contaminated Sediments. Assessment and Remediation of Contaminated Sediments (ARCS) Program, Great Lakes National Program Office, US EPA 905-B96-004. 147 pp.  [[Media: USEPA_905-B96-004.pdf | Report.pdf]]</ref>
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'''Key Resource:'''
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*Perfluoroalkyl and polyfluoroalkyl substances thermal desorption evaluation
  
 
==Introduction==
 
==Introduction==
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[[File:SedCapFig1.png|thumb|left|500px|Figure 1. Conceptual sketch of a cap configuration]]
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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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Capping is an ''in situ'' remedial technology for contaminated sediments that involves placement of a clean substrate on the sediment surface. Capping contaminated sediments following [[Wikipedia: Dredging | dredging operations]] and capping of dredged material to stabilize contaminants has been a common practice by the United States Army Corps of Engineers since the 1970s. Beginning in the 1980s, in Japan and subsequently elsewhere, capping has been used more widely as a remedial approach to improve the quality of the bottom substrate and reduce contaminant exposures to benthic organisms and fish. The USEPA published a capping guidance document in 1998 that summarizes past uses of sediment capping and outlines its basic design<ref name="Palermo1998"/>.   Although capping technology has developed substantially in the past 20 years, this early reference still provides useful information on the approach and its applications.  A more recent summary of capping is described in Reible 2014<ref name="Reible2014"/>.
 
  
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Capping serves to contain contaminated sediment solids, isolate contaminants from benthic organisms and reduce contaminant transport to the sediment surface and overlying water.  The clean substrate may be an inert material such as sand, a natural sorbing material such as other sediments or clays, or be amended with an active/reactive material to enhance the isolation of the contaminants. Amendments to enhance contaminant isolation include permeability reduction agents to divert groundwater flow, sorbents to retard contaminant migration through the capping layer or provide greater accumulation capacity, or reagents to encourage degradation or transformation of the contaminants.  
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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>
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Where ''C<sub>p</sub>'' is the porewater concentration, ''C<sub>t</sub>'' is the total PFAS concentration, ''ρ<sub>b</sub>'' is the bulk density of the soil, ''θ<sub>w</sub>'' is the volumetric water content, ''R<sub>d</sub>'' is the PFAS retardation factor, ''K<sub>d</sub>'' is the solid phase adsorption coefficient, ''K<sub>ia</sub>'' is the air-water interfacial adsorption coefficient, and ''A<sub>aw</sub>'' is the air-water interfacial area. The air-water interfacial area of the soil is primarily a function of both the soil properties and the degree of volumetric water saturation in the soil. There are several methods of estimating air-water interfacial areas including thermodynamic functions based on the soil moisture retention curve. However, the thermodynamic function has been shown to underestimate air-water interfacial area<ref name="Brusseau2023">Brusseau, M.L., 2023. Determining Air-Water Interfacial Areas for the Retention and Transport of PFAS and Other Interfacially Active Solutes in Unsaturated Porous Media. Science of The Total Environment, 884, Article 163730. [https://doi.org/10.1016/j.scitotenv.2023.163730 doi: 10.1016/j.scitotenv.2023.163730]&nbsp; [[Media: Brusseau2023.pdf  | Open Access Article]]</ref>, and must typically be scaled using empirical scaling factors. An empirical method recently developed to estimate air-water interfacial area is presented in Equation 2<ref name="Brusseau2023"/>.</br>
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:: <big>'''Equation 2:'''</big>&nbsp;&nbsp; [[File: StultsEq2.png | 400 px]]</br>
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Where ''S<sub>w</sub>'' is the water phase saturation as a ratio of the water content over the volumetric soil porosity, and ''d<sub>50</sub>'' is the median grain diameter.
  
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The basic concept of a cap is illustrated in Figure 1. The Figure also illustrates that a cap is often a thin layer or layers relative to water depth and generally causes little disturbance to the underlying sediments or body of water in which it is placed.   Depending upon the erosive forces to which the cap may be subjected, the surface layer may be composed of relatively coarse material to withstand those erosive forces.  
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==Lysimeters Background==
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[[File: StultsFig1.png | thumb | 600 px | Figure 1. (a) A field suction lysimeter with labeled parts typically used in field settings – Credit: Bibek Acharya and Dr. Vivek Sharma, UF/IFAS - https://edis.ifas.ufl.edu/publication/AE581. (b) Laboratory suction lysimeters used in Schaefer et al. 2024, which employed the use of micro-sampling suction lysimeters. (c) a field lysimeter used in Schaefer et al. 2023. (d) diagram of a drainage wicking lysimeter – Credit: Edaphic Scientific - https://edaphic.com.au/products/water/lysimeter-wick-for-drainage/]]
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Lysimeters, generally speaking, refer to instruments which collect water from unsaturated soils<ref name="MeissnerEtAl2020"/><ref name="RogersMcConnell1993"/>. However, there are multiple types of lysimeters which can be employed in field or laboratory settings. There are three primary types of lysimeters relevant to PFAS listed here and shown in Figure 1a-d.
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# <u>Suction Lysimeters (Figure 1a,b):</u> These lysimeters are the most relevant for PFAS sampling and are the majority of discussion in this article. These lysimeters operate by extracting liquid from the unsaturated vadose zone by applying negative suction pressure at the sampling head<ref name="CostanzaEtAl2025"/><ref name="SchaeferEtAl2024"/><ref name="QuinnanEtAl2021"/>. The sampling head is typically constructed of porous ceramic or stainless steel. A PVC case or stainless-steel case is attached to the sampling head and extends upward above the ground surface. Suction lysimeters are typically installed between 1 and 9 feet below ground surface, but can extend as deep as 40-60 feet in some cases<ref name="CostanzaEtAl2025"/>. Shallow lysimeters (< 10 feet) are typically installed using a hand auger. For ceramic lysimeters, a silica flour slurry should be placed at the base of the bore hole and allowed to cover the ceramic head before backfilling the hole partially with natural soil. Once the hole is partially backfilled with soil to cover the sampling head, the remainder of the casing should be sealed with hydrated bentonite chips. When sampling events occur, suction is applied at the ground surface using a rubber gasket seal and a hand pump or electric pump. After sufficient porewater is collected (the time for which can vary greatly based on the soil permeability and moisture content), the seal can be removed and a peristaltic pump used to extract liquid from the lysimeter.
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# <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.
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# <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.
  
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Although a cap is typically thin compared to the water depth, it generally must be thicker than the biologically active zone (BAZ) of the sediments.  The biologically active zone is that zone in which benthic organisms live and interact with the sediment.  Their activities tend to mix the BAZ (known as [[Wikipedia: Bioturbation | bioturbation]]) over the course of a few years and thus a cap that is thinner than the BAZ will tend to become intermixed with the underlying contaminated sediments.   Processes other than bioturbation including diffusion, advection or groundwater upwelling, hyporheic exchange near the interface, biogenic gas production and migration and underlying sediment consolidation can all lead to contaminant migration into and through a cap.  These occur at different rates and intensities and their assessment and evaluation ultimately governs the effectiveness of a cap and the feasibility of its use as a sediment remediation technology for a particular site.
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==Analysis of PFAS Concentrations in Soil and Porewater==
 +
[[File: StultsFig2.png | thumb | 500 px | Figure 2. Field Measured PFAS concentration Data (Orange) and Lab Core Measured Concentration Data (Blue) for four PFAS impacted sites<ref name="AndersonEtAl2022"/>]]  
 +
Schaefer ''et al.''<ref name="SchaeferEtAl2024"/> 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.
  
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In general, capping is an effective remedial technology for contaminants that are strongly associated with the sediment solids including hydrophobic organic compounds such as high molecular weight [[Polycyclic Aromatic Hydrocarbons (PAHs) | polycyclic aromatic hydrocarbons (PAHs)]], [[Wikipedia: Polychlorinated biphenyl | polychlorinated biphenyls (PCBs)]], [[Wikipedia: Dioxins and dioxin-like compounds | dioxins]] and [[Wikipedia: DDT | DDTx]], but also [[Metal and Metalloid Contaminants | heavy metals]].  Hydrophobic organic compounds tend to strongly associate with the organic fraction of sediments so organic rich sediments or the addition of organic phases to the capping material can be very effective at containing these contaminants. Many of the common heavy metals of concern, including cadmium, copper, nickel, zinc, lead and mercury, tend to be associated with insoluble sulfides under strongly reducing conditions.  Since oxygen penetration into a capping layer is typically limited to a few cm or less at the surface, a cap serves to drive the underlying contaminated sediment toward strongly reducing conditions and, particularly in marine and estuarine sediments, encourage sulfate reduction leading to the formation of these insoluble sulfides.  The low solubility of these sulfides encourages retention by a capping layer and makes the cap extremely effective as a remedial approach for sediments with elevated concentrations of heavy metals.
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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.  
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A variety of tools have been developed to evaluate the processes leading to sorption and retardation of contaminants as well as processes leading to contaminant migration and release. The original references quantifying contaminant behavior in a sediment cap were explored in a series of papers in the early 1990s<ref name="Wang1991">Wang, X.Q., Thibodeaux, L.J., Valsaraj, K.T. and Reible, D.D., 1991. Efficiency of Capping Contaminated Bed Sediments in Situ. 1. Laboratory-Scale Experiments on Diffusion-Adsorption in the Capping Layer. Environmental Science and Technology, 25(9), pp.1578-1584.  [https://doi.org/10.1021/es00021a008 DOI: 10.1021/es00021a008]</ref><ref name="Thoma1993">Thoma, G.J., Reible, D.D., Valsaraj, K.T. and Thibodeaux, L.J., 1993. Efficiency of Capping Contaminated Bed Sediments in Situ 2. Mathematics of Diffusion-Adsorption in the Capping Layer. Environmental Science and Technology, 27(12), pp.2412-2419.  [https://doi.org/10.1021/es00048a015 DOI: 10.1021/es00048a015]</ref>.  Since that time, design tools have been continuously improved. [https://www.depts.ttu.edu/ceweb/research/reiblesgroup/downloads.php CapSim] is a commonly used and current tool developed by Dr. Reible and collaborators. This tool can evaluate contaminant release from uncapped, capped, and treated sediments for purposes of design and evaluation.  The model formulation and structure is described in Shen et al. 2018<ref name="Shin2018">Shen, X., Lampert, D., Ogle, S. and Reible, D., 2018. A software tool for simulating contaminant transport and remedial effectiveness in sediment environments. Environmental Modelling and Software, 109, pp. 104-113.  [https://doi.org/10.1016/j.envsoft.2018.08.014 DOI: 10.1016/j.envsoft.2018.08.014]</ref>. One common use of such a tool is to evaluate the effect of various cap materials and thicknesses on the performance of a cap.
 
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==Cap Design and Materials for Chemical Containment==
 
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An inert material such as sand can be effective as a capping material where contaminants are strongly associated with solids and where the operative site specific transport mechanisms do not lead to rapid contaminant migration through such a material. Additional contaminant containment can often be achieved through the placement of clean sediment, e.g. dredged material from a nearby location.  Other materials as cap layers or amendments may be useful to address particularly mobile contaminants or when particular degradative mechanisms can be exploited. The Anacostia River was the site of a demonstration that first tested “active” or “amended” capping in the field<ref name="Reible2003">Reible, D., Constant, D.W., Roberts, K. and Zhu, Y., 2003. Active capping demonstration project in anacostia DC. In Second International Conference on the Remediation of Contaminated Sediments: October.  Free download available from: [https://www.researchgate.net/profile/Danny-Reible/publication/237747790_ACTIVE_CAPPING_DEMONSTRATION_PROJECT_IN_ANACOSTIA_DC/links/0c96053861030b7699000000/ACTIVE-CAPPING-DEMONSTRATION-PROJECT-IN-ANACOSTIA-DC.pdf ResearchGate]</ref><ref name="Reible2006">Reible, D., Lampert, D., Constant, D., Mutch Jr, R.D. and Zhu, Y., 2006. Active Capping Demonstration in the Anacostia River, Washington, DC. Remediation Journal: The Journal of Environmental Cleanup Costs, Technologies and Techniques, 17(1), pp. 39-53.  [https://doi.org/10.1002/rem.20111 DOI: 10.1002/rem.20111]  Free download available from: [https://www.academia.edu/download/44146457/Remediation_Journal_Paper_2006.pdf Academia.edu]</ref>. Amended caps are often the best option when groundwater upwelling or other advective processes promote significant mobility of contaminants and the addition of sorbents can slow that contaminant migration<ref name="Ghosh2011">Ghosh, U., Luthy, R.G., Cornelissen, G., Werner, D. and Menzie, C.A., 2011. In-situ Sorbent Amendments: A New Direction in Contaminated Sediment Management. Environmental Science and Technology, 45(4), pp. 1163-1168. [https://doi.org/10.1021/es102694h DOI: 10.1021/es102694h]  Open access article from: [https://pubs.acs.org/doi/pdf/10.1021/es102694h American Chemical Society]&nbsp;&nbsp; [[Media: Ghosh2011.pdf | Report.pdf]]</ref>.  Although a variety of materials have been proposed for sediment caps, a far smaller number of options have been successfully employed in the field.  
 
 
   
 
   
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Metals migration is very site dependent due to the potential for many metals to complex with other species in the interstitial water and the specific metal speciation present at a site.  Often, the strongly reducing environment beneath a cap renders many common metals unavailable through the formation of metal sulfides.  In such cases, a simple sand cap can be very effective.  Amended caps to manage metal contaminated sediments may be advantageous when site specific conditions lead to elevated metals mobility, but should be supported with site specific testing<ref name="Viana2008">Viana, P.Z., Yin, K. and Rockne, K.J., 2008. Modeling Active Capping Efficacy. 1. Metal and Organometal Contaminated Sediment Remediation. Environmental Science and Technology, 42(23), pp. 8922-8929. [https://doi.org/10.1021/es800942t DOI: 10.1021/es800942t]</ref>.
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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<ref name="AndersonEtAl2022"/>).
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For hydrophobic organic contaminants, cap amendments that directly control groundwater upwelling and also sorbents that can remove migrating contaminants from that groundwater have been successfully employed.  Examples include clay materials such as AquaBlok for permeability control, sorbents such as [[Wikipedia: Activated carbon | activated carbon]] for truly dissolved contaminants, and [[Wikipedia: Organoclay | organophilic clays]] for separate phase contaminants. 
 
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The placement of clean sediment as an ''in situ'' cap can be difficult when the material is fine grained or has a low density.  Capping with a layer of coarse grained material such as clean sand mitigates this issue although clean sands have minimal sorption capacity.  Because of this limitation, sand caps may not be sufficient for achieving remedial goals in sites where contamination levels are high or transport rates are fast due to pore water upwelling or tidal pumping effects. Conditions such as these may require the use of “active” amendments to reduce transport rates.
 
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Capping with clean sand provides a physical barrier between the underlying contaminated material and the overlying water, stabilizes the underlying sediment to prevent re-suspension of contaminated particles, and can reduce chemical exposure under certain conditions. Sand primarily provides a passive barrier to the downward penetration of bioturbating organisms and the upward movement of sediment or contaminants.  Although conventional sandy caps can often be an effective means of managing contaminated sediments, there are conditions when sand caps may not be capable of achieving design objectives.  Some factors that reduce the effectiveness of sand caps include:
 
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*erosion and loss of cap integrity
 
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*high groundwater upwelling rates
 
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*mobile (low sorption) contaminants of concern (COCs)
 
−
*high COC concentrations
 
−
*unusually toxic COCs
 
−
*the presence of tidal influences
 
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*the presence of non-aqueous phase liquids (NAPLs)
 
−
*high rates of gas ebullition
 
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Of these, the first three are common limitations to capping and often control the ability to effectively design and implement a cap as a sediment remedial strategy. In these cases, it may be possible to offset these issues by increasing the thickness of the cap.  However, the required thickness can reach infeasible levels in shallow streams or navigable water bodies.  In addition, increased construction costs associated with thick caps may become prohibitive.  As a result of these issues, caps that use alternative materials (also known as active caps) to reduce the thickness or increase the protectiveness of a cap may be necessary.  The materials in active caps are designed to interact with the COCs to enhance the containment properties of the cap. 
 
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[[Wikipedia: Apatite | Apatites]] are a class of naturally occurring minerals that have been investigated as a sorbent for metals in soils and sediments<ref name="Melton2003">Melton, J.S., Crannell, B.S., Eighmy, T.T., Wilson, C. and Reible, D.D., 2003. Field Trial of the UNH Phosphate-Based Reactive Barrier Capping System for the Anacostia River. EPA Grant R819165-01-0</ref><ref name="Reible2003"/><ref name="Knox2012">Knox, A.S., Paller, M.H. and Roberts, J., 2012. Active Capping Technology—New Approaches for In Situ Remediation of Contaminated Sediments. Remediation Journal, 22(2), pp.93-117.  [https://doi.org/10.1002/rem.21313 DOI: 10.1002/rem.21313]  Free download available from: [https://www.researchgate.net/profile/Anna-Knox-2/publication/233374607_Active_Capping_Technology-New_Approaches_for_In_Situ_Remediation_of_Contaminated_Sediments/links/5a7de4c5aca272a73765c344/Active-Capping-Technology-New-Approaches-for-In-Situ-Remediation-of-Contaminated-Sediments.pdf ResearchGate]</ref>.  Apatites consist of a matrix of calcium phosphate and various other common anions, including fluoride, chloride, hydroxide, and occasionally carbonate. Metals are sequestered either through direct ion exchange with the calcium atom or dissolution of hydroxyapatite followed by precipitation of lead apatite.  [[Wikipedia: Zeolite | Zeolites]], which are microporous aluminosilicate minerals with a high cationic exchange capacity (CEC), have also been proposed to manage metal species<ref name="Zhan2019">Zhan, Y., Yu, Y., Lin, J., Wu, X., Wang, Y. and Zhao, Y., 2019. Simultaneous control of nitrogen and phosphorus release from sediments using iron-modified zeolite as capping and amendment materials. Journal of Environmental Management, 249, p.109369.  [https://doi.org/10.1016/j.jenvman.2019.109369 DOI: 10.1016/j.jenvman.2019.109369]</ref>.
 
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It is possible to create a hydrophobic, sorbing layer for non-polar organics by exchanging a cationic surfactant onto the surface of clays such as zeolites and bentonites,. Organoclay is a modified bentonite containing such substitutions that has been evaluated for control of non-aqueous phase NAPLs and other organic contaminants<ref name="Reible2007">Reible, D.D., Lu, X., Moretti, L., Galjour, J. and Ma, X., 2007. Organoclays for the capping of contaminated sediments. AIChE Annual Meeting.  ISBN: 978-081691022-9</ref>.  An organoclay cap has been implemented for sediment remediation at the McCormick and Baxter site in Portland, OR<ref name="Parrett2005">Parrett, K. and Blishke, H., 2005. 23-Acre Multilayer Sediment Cap in Dynamic Riverine Environment Using Organoclay an Adsorptive Capping Material. Presentation to Society of Environmental Toxicology and Chemistry (SETAC), 26th Annual Meeting.</ref>.  A similar organic sorbing phase can be formed by treating zeolites with surfactants but this approach has not been reported for contaminated sediments.
 
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Activated carbon is a strong sorbent of hydrophobic organic compounds and has been used as a [[In Situ Treatment of Contaminated Sediments with Activated Carbon | treatment for sediments]] or as an active sorbent within a capping layer<ref name="Zimmerman2004">Zimmerman, J.R., Ghosh, U., Millward, R.N., Bridges, T.S. and Luthy, R.G., 2004. Addition of Carbon Sorbents to Reduce PCB and PAH Bioavailability in Marine Sediments: Physicochemical Tests. Environmental Science and Technology, 38(20), pp. 5458-5464.  [https://doi.org/10.1021/es034992v DOI: 10.1021/es034992v]</ref><ref name="Werner2005">Werner, D., Higgins, C.P. and Luthy, R.G., 2005. The sequestration of PCBs in Lake Hartwell sediment with activated carbon. Water Research, 39(10), pp. 2105-2113.  [https://doi.org/10.1016/j.watres.2005.03.019 DOI: 10.1016/j.watres.2005.03.019]</ref><ref name="Abel2018">Abel, S. and Akkanen, J., 2018. A Combined Field and Laboratory Study on Activated Carbon-Based Thin Layer Capping in a PCB-Contaminated Boreal Lake. Environmental Science and Technology, 52(8), pp. 4702-4710. [https://doi.org/10.1021/acs.est.7b05114 DOI: 10.1021/acs.est.7b05114] Open access article available from: [https://pubs.acs.org/doi/pdf/10.1021/acs.est.7b05114 American Chemical Society]&nbsp;&nbsp; [[Media: Abel2018.pdf | Report.pdf]]</ref><ref name="Payne 2018">Payne, R.B., Ghosh, U., May, H.D., Marshall, C.W. and Sowers, K.R., 2019. A Pilot-Scale Field Study: In Situ Treatment of PCB-Impacted Sediments with Bioamended Activated Carbon. Environmental Science and Technology, 53(5), pp. 2626-2634. [https://doi.org/10.1021/acs.est.8b05019 DOI: 10.1021/acs.est.8b05019]</ref><ref name="Yan2020">Yan, S., Rakowska, M., Shen, X., Himmer, T., Irvine, C., Zajac-Fay, R., Eby, J., Janda, D., Ohannessian, S. and Reible, D.D., 2020. Bioavailability Assessment in Activated Carbon Treated Coastal Sediment with In situ and Ex situ Porewater Measurements. Water Research, 185, p. 116259.  [https://doi.org/10.1016/j.watres.2020.116259 DOI: 10.1016/j.watres.2020.116259]</ref>.  Placement of activated carbon for sediment capping is difficult due to the near neutral buoyancy of the material but it has been applied in this manner in relatively low energy environments such as Onondaga Lake, Syracuse, NY<ref name="Vlassopoulos2017">Vlassopoulos, D., Russell, K., Larosa, P., Brown, R., Mohan, R., Glaza, E., Drachenberg, T., Reible, D., Hague, W., McAuliffe, J. and Miller, S., 2017. Evaluation, Design, and Construction of Amended Reactive Caps to Restore Onondaga Lake, Syracuse, New York, USA. Journal of Marine Environmental Engineering, 10(1), pp. 13-27.  Free download available from: [https://www.researchgate.net/publication/317762995_Evaluation_design_and_construction_of_amended_reactive_caps_to_restore_Onondaga_lake_Syracuse_New_York_USA ResearchGate]</ref>.  Alternatives in higher energy environments include placement of activated carbon in a mat such as the CETCO Reactive Core Mat (RCM)® or Huesker Tektoseal®, or as a composite material such as SediMite® or AquaGate®.  In the case of the mats, powdered or granular activated carbon can be placed in a controlled layer while the density of the composite materials is such that they can be broadcast from the surface and allowed to settle to the bottom.  In a sediment treatment application, the composite material would either be worked into the surface or allowed to intermix gradually by bioturbation and other processes.  In a capping application, the mat or composite material would typically be combined or overlain with a sand or other capping layer to keep it in place and to provide a chemical isolation layer away from the sediment surface.
 
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As an alternative to a sorptive capping amendment, low-permeability cap amendments have been proposed to enhance cap design life by decreasing pore water advection.  Low permeability clays are an effective means to divert upwelling groundwater away from a contaminated sediment area but are difficult to place in the aqueous environment.  Bentonite clays can be placed in mats similar to what is done to provide a low permeability liner in landfills. There are also commercial products that can place clays directly such as the composite material AquaBlok®, a bentonite clay and polymer based mineral around an aggregate core<ref name="Barth2008">Barth, E.F., Reible, D. and Bullard, A., 2008. Evaluation of the physical stability, groundwater seepage control, and faunal changes associated with an AquaBlok® sediment cap. Remediation: The Journal of Environmental Cleanup Costs, Technologies and Techniques, 18(4), pp.63-70.  [https://doi.org/10.1002/rem.20183 DOI: 10.1002/rem.20183]</ref>.
 
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Sediment caps become colonized by microorganisms from the sediments and surface water and potentially become a zone of pollutant biotransformation over time. Aerobic degradation occurs only near the solids-water interface in which benthic organisms are active and thus there might still be significant benthic organism exposure to contaminants. Biotransformation in the anaerobic zone of a cap, which typically extends well beyond the zone of benthic activity, could significantly reduce the risk of pollutant exposure but successful caps encouraging deep degradation processes have not been demonstrated beyond the laboratory.  The addition of materials such as nutrients and oxygen releasing compounds for enhancing the attenuation of contaminants through biodegradation has also been assessed but not applied in the field.  Short term improvements in biodegradation rates can be achieved through tailoring of conditions or addition of nutrients but long term efficacy has not been demonstrated<ref name="Pagnozzi2020">Pagnozzi, G., Carroll, S., Reible, D.D. and Millerick, K., 2020. Biological Natural Attenuation and Contaminant Oxidation in Sediment Caps: Recent Advances and Future Opportunities. Current Pollution Reports, pp.1-14.  [https://doi.org/10.1007/s40726-020-00153-5 DOI: 10.1007/s40726-020-00153-5]</ref>. 
 
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[[File: SedCapFig2.png | thumb |600px|Figure 2. A conceptualization of a cap with accompanying habitat layer]]
 
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==Cap Design and Materials for Habitat Restoration==
 
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In addition to providing chemical isolation and containment, a cap can also be used to provide improvements for organisms by enhancing the habitat characteristics of the bottom substrate<ref name="Yozzo2004">Yozzo, D.J., Wilber, P. and Will, R.J., 2004. Beneficial use of dredged material for habitat creation, enhancement, and restoration in New York–New Jersey Harbor. Journal of Environmental Management, 73(1), pp. 39-52.  [https://doi.org/10.1016/j.jenvman.2004.05.008 DOI: 10.1016/j.jenvman.2004.05.008]</ref><ref name="Zhang2016">Zhang, C., Zhu, M.Y., Zeng, G.M., Yu, Z.G., Cui, F., Yang, Z.Z. and Shen, L.Q., 2016. Active capping technology: a new environmental remediation of contaminated sediment. Environmental Science and Pollution Research, 23(5), pp.4370-4386.  [https://doi.org/10.1007/s11356-016-6076-8 DOI: 10.1007/s11356-016-6076-8]</ref><ref name="Vlassopoulos2017"/>.  Often, contaminated sediment environments are degraded for a variety of reasons in addition to the toxic constituents.  One way to overcome this is to provide both a habitat layer and chemical isolation or contaminant capping layer. Figure 2 illustrates just such a design providing a more appropriate habitat enhancing substrate, in this case by incorporation additional organic material, vegetation and debris, which is often used by fish species for protection, into the surface layer. In a high energy environment, it should be recognized that it may not be possible to keep a suitable habitat layer in place during high flow events.  This would be true of suitable habitat that had developed naturally as well as a constructed habitat layer and it is presumed that if such a habitat is the normal condition of the waterbody that it will recover over time between such high flow events.
 
  
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==Summary==
+
==Recommendations==
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Clean substrate can be placed at the sediment-water interface for the purposes of reducing exposure to and risk from contaminants in the sediments.  The cap can consist of simple materials such as sand designed to physically stabilize contaminated sediments and separate the benthic community from those contaminants or may include other materials designed to sequester contaminants even under adverse conditions including strong groundwater upwelling or highly mobile contaminants.  The surface of a cap may be designed of coarse material such as gravel or cobble to be stable under high flow events or designed to be more appropriate habitat for benthic and aquatic organisms.  As a result of its flexibility, simplicity and low cost relative to its effectiveness, capping is one of the most prevalent remedial technologies for sediments.  
+
Recent research suggests:
 +
*Successful thermal treatment of PFAS may require a higher target temperature than for other organics with similar boiling points
 +
*Prevention of influx of water into treatment zone may be necessary.
 +
Future studies should examine the potential for enhanced degradation during the thermal process by using soil amendments and/or manipulation of the local geochemistry to reduce the required treatment temperatures and therefore also reduce energy demand.
  
 
==References==
 
==References==

Latest revision as of 17:39, 9 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: Gorm Heron, Emily Crownover, Patrick Joyce, Ramona Iery

Key Resource:

  • Perfluoroalkyl and polyfluoroalkyl substances thermal desorption evaluation

Introduction

Lysimeters are devices that are placed in the subsurface above the groundwater table to monitor the movement of water through the soil[1][2][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[1][4][5][6][7][8][9][10][11]. 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[12][13][14][15][16][17]. 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[18][19]. 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[20], not what is readily transportable in mobile porewater[15][21][22][23]. 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[12][13]. 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[18][19]. 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[23].

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, which employed the use of micro-sampling suction lysimeters. (c) a field lysimeter used in Schaefer et al. 2023. (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[7][8]. 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][14][17]. 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[7]. 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[10][11][15][16][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

Figure 2. Field Measured PFAS concentration Data (Orange) and Lab Core Measured Concentration Data (Blue) for four PFAS impacted sites[13]

Schaefer et al.[14] 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[13]).

Recommendations

Recent research suggests:

  • Successful thermal treatment of PFAS may require a higher target temperature than for other organics with similar boiling points
  • Prevention of influx of water into treatment zone may be necessary.

Future studies should examine the potential for enhanced degradation during the thermal process by using soil amendments and/or manipulation of the local geochemistry to reduce the required treatment temperatures and therefore also reduce energy demand.

References

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See Also

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