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| − | == | + | ==Lysimeters for Measuring PFAS Concentrations in the Vadose Zone== |
| − | + | [[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):''' | ||
| − | |||
| − | |||
| − | + | *[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]] | |
| + | *[[PFAS Transport and Fate]] | ||
| − | ''' | + | '''Contributors:''' Gorm Heron, Emily Crownover, Patrick Joyce, Ramona Iery |
| − | |||
| − | + | '''Key Resource:''' | |
| − | + | *Perfluoroalkyl and polyfluoroalkyl substances thermal desorption evaluation | |
| − | * | ||
==Introduction== | ==Introduction== | ||
| − | [ | + | 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] [[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] [[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] [[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] [[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] [[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] [[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] [[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] [[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] [[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. |
| − | + | ==Target Temperature and Duration== | |
| + | PFAS behave differently from most other organics subjected to TCH treatment. While the boiling points of individual PFAS fall in the range of 150-400°C, their chemical and physical behavior creates additional challenges. Some PFAS form ionic species in certain pH ranges and salts under other chemical conditions. This intricate behavior and our limited understanding of what this means for our ability to remove the PFAS from soils means that direct testing of thermal treatment options is warranted. Crownover and colleagues<ref name="CrownoverEtAl2019"/> subjected PFAS-laden soil to bench-scale heating to temperatures between 200 and 400°C which showed strong reductions of PFAS concentrations at 350°C and complete removal of many PFAS compounds at 400°C. The soil concentrations of targeted PFAS were reduced to nearly undetectable levels in this study. | ||
| − | == | + | ==Heating Method== |
| − | + | For semi-volatile compounds such as dioxins, furans, poly-chlorinated biphenyls (PCBs) and Poly-Aromatic Hydrocarbons (PAH), thermal conduction heating has evolved as the dominant thermal technology because it is capable of achieving soil temperatures higher than the boiling point of water, which are necessary for complete removal of these organic compounds. Temperatures between 200 and 500°C have been required to achieve the desired reduction in contaminant concentrations<ref name="StegemeierVinegar2001">Stegemeier, G.L., Vinegar, H.J., 2001. Thermal Conduction Heating for In-Situ Thermal Desorption of Soils. Ch. 4.6, pp. 1-37. In: Chang H. Oh (ed.), Hazardous and Radioactive Waste Treatment Technologies Handbook, CRC Press, Boca Raton, FL. ISBN 9780849395864 [[Media: StegemeierVinegar2001.pdf | Open Access Article]]</ref>. TCH has become a popular technology for PFAS treatment because temperatures in the 400°C range are needed. | |
| − | The | + | The energy source for TCH can be electricity (most commonly used), or fossil fuels (typically gas, diesel or fuel oil). Electrically powered TCH offers the largest flexibility for power input which also can be supplied by renewable and sustainable energy sources. |
| − | == | + | ==Energy Usage== |
| − | + | Treating PFAS-impacted soil with heat requires energy to first bring the soil and porewater to the boiling point of water, then to evaporate the porewater until the soil is dry, and finally to heat the dry soil up to the target treatment temperature. The energy demand for wet soils falls in the 300-400 kWh/cy range, dependent on porosity and water saturation. Additional energy is consumed as heat is lost to the surroundings and by vapor treatment equipment, yielding a typical usage of 400-600 kWh/cy total for larger soil treatment volumes. Wetter soils and small treatment volumes drive the energy usage towards the higher number, whereas larger soil volumes and dry soil can be treated with less energy. | |
| − | + | ==Vapor Treatment== | |
| + | During the TCH process a significant fraction of the PFAS compounds are volatilized by the heat and then removed from the soil by vacuum extraction. The vapors must be treated and eventually discharged while meeting local and/or federal standards. Two types of vapor treatment have been used in past TCH applications for organics: (1) thermal and catalytic oxidation and (2) condensation followed by granular activated charcoal (GAC) filtration. Due to uncertainties related to thermal destruction of fluorinated compounds and future requirements for treatment temperature and residence time, condensation and GAC filtration have been used in the first three PFAS treatment field demonstrations. It should be noted that PFAS compounds will stick to surfaces and that decontamination of the equipment is important. This could generate additional waste as GAC vessels, pipes and other wetted equipment need careful cleaning with solvents or rinsing agents such as PerfluorAd<sup><small>TM</small></sup>. | ||
| − | + | ==PFAS Reactivity and Fate== | |
| + | While evaluating initial soil treatment results, Crownover ''et al''<ref name="CrownoverEtAl2019"/> noted the lack of complete data sets when the soils were analyzed for non-targeted compounds or extractable precursors. Attempts to establish the fluorine balance suggest that the final fate of the fluorine in the PFAS is not yet fully understood. Transformations are likely occurring in the heated soil as demonstrated in laboratory experiments with and without calcium hydroxide (Ca(OH)<small><sub>2</sub></small>) amendment<ref>Koster van Groos, P.G., 2021. Small-Scale Thermal Treatment of Investigation-Derived Wastes Containing PFAS. [https://serdp-estcp.mil/ Strategic Environmental Research and Development Program (SERDP) - Environmental Security Technology Certification Program (ESTCP)], [https://serdp-estcp.mil/projects/details/2f1577ac-c8ea-4ae8-804e-c9f97a12edb3/small-scale-thermal-treatment-of-investigation-derived-wastes-idw-containing-pfas Project ER18-1556 Website], [[Media: ER18-1556_Final_Report.pdf | Final Report.pdf]]</ref>. Amendments such as Ca(OH)<sub><small>2</small></sub> may be useful in reducing the required treatment temperature by catalyzing PFAS degradation. With thousands of PFAS potentially present, the interactions are complex and may never be fully understood. Therefore, successful thermal treatment may require a higher target temperature than for other organics with similar boiling points – simply to provide a buffer against the uncertainty. | ||
| − | == | + | ==Case Studies== |
| − | + | ===Stockpile Treatment, Eielson AFB, Alaska ([https://serdp-estcp.mil/projects/details/62098505-de86-43b2-bead-ae8018854141 ESTCP project ER20-5198]<ref name="CrownoverEtAl2023">Crownover, E., Heron, G., Pennell, K., Ramsey, B., Rickabaugh, T., Stallings, P., Stauch, L., Woodcock, M., 2023. Ex Situ Thermal Treatment of PFAS-Impacted Soils, [[Media: ER20-5198 Final Report.pdf | Final Report.]] Eielson Air Force Base, Alaska. [https://serdp-estcp.mil/ Strategic Environmental Research and Development Program (SERDP) - Environmental Security Technology Certification Program (ESTCP)], [https://serdp-estcp.mil/projects/details/62098505-de86-43b2-bead-ae8018854141 Project ER20-5198 Website]</ref>)=== | |
| + | [[File: HeronFig1.png | thumb | 400 px | Figure 1. TCH treatment of a PFAS-laden stockpile at Eielson AFB, Alaska<ref name="CrownoverEtAl2023"/>]] | ||
| + | Since there has been no approved or widely accepted method for treating soils impacted by PFAS, a common practice has been to excavate PFAS-impacted soil and place it in lined stockpiles. Eielson AFB in Alaska is an example where approximately 50 stockpiles were constructed to temporarily store 150,000 cubic yards of soil. One of the stockpiles containing 134 cubic yards of PFAS-impacted soil was heated to 350-450°C over 90 days (Figure 1). Volatilized PFAS was extracted from the soil using vacuum extraction and treated via condensation and filtration by granular activated charcoal. Under field conditions, PFAS concentration reductions from 230 µg/kg to below 0.5 µg/kg were demonstrated for soils that reached 400°C or higher for 7 days. These soils achieved the Alaska soil standards of 3 µg/kg for PFOS and 1.7 µg/kg for PFOA. Cooler soils near the top of the stockpile had remaining PFOS in the range of 0.5-20 µg/kg with an overall average of 4.1 µg/kg. Sampling of all soils heated to 400°C or higher demonstrated that the soils achieved undetectable levels of targeted PFAS (typical reporting limit was 0.5 µg/kg). | ||
| − | In | + | ===''In situ'' Vadose Zone Treatment, Beale AFB, California ([https://serdp-estcp.mil/projects/details/94949542-f9f7-419d-8028-8ba318495641/er20-5250-project-overview ESTCP project ER20-5250]<ref name="Iery2024">Iery, R. 2024. In Situ Thermal Treatment of PFAS in the Vadose Zone. [https://serdp-estcp.mil/ Strategic Environmental Research and Development Program (SERDP) - Environmental Security Technology Certification Program (ESTCP)], [https://serdp-estcp.mil/projects/details/94949542-f9f7-419d-8028-8ba318495641 Project ER20-5250 Website]. [[Media: ER20-5250 Fact Sheet.pdf | Fact Sheet.pdf]]</ref>)=== |
| + | [[File: HeronFig2.png | thumb | 600 px | Figure 2. ''In situ'' TCH treatment of a PFAS-rich vadose zone hotspot at Beale AFB, California]] | ||
| + | A former fire-training area at Beale AFB had PFAS concentrations as high as 1,970 µg/kg in shallow soils. In situ treatment of a PFAS-rich soil was demonstrated using 16 TCH borings installed in the source area to a depth of 18 ft (Figure 2). Soils which reached the target temperatures were reduced to PFAS concentrations below 1 µg/kg. Perched water which entered in one side of the area delayed heating in that area, and soils which were affected had more modest PFAS concentration reductions. As a lesson learned, future in situ TCH treatments will include provisions for minimizing water entering the treated volume<ref name="Iery2024"/>. It was demonstrated that with proper water management, even highly impacted soils can be treated to near non-detect concentrations (greater than 99% reduction). | ||
| − | == | + | ===Constructed Pile Treatment, JBER, Alaska ([https://serdp-estcp.mil/projects/details/eb7311db-6233-4c7f-b23a-e003ac1926c5/pfas-treatment-in-soil-using-thermal-conduction-heating ESTCP Project ER23-8369]<ref name="CrownoverHeron2024">Crownover, E., Heron, G., 2024. PFAS Treatment in Soil Using Thermal Conduction Heating. Defense Innovation Unit (DIU) and [https://serdp-estcp.mil/ Strategic Environmental Research and Development Program (SERDP) - Environmental Security Technology Certification Program (ESTCP)], [https://serdp-estcp.mil/projects/details/eb7311db-6233-4c7f-b23a-e003ac1926c5/pfas-treatment-in-soil-using-thermal-conduction-heating Project ER23-8369 Website]</ref>)=== |
| − | + | [[File: HeronFig3.png | thumb | 600 px | Figure 3. Treatment of a 2,000 cubic yard soil pile at JBER, Alaska]] | |
| + | In 2024, a stockpile of 2,000 cubic yards of PFAS-impacted soil was thermally treated at Joint Base Elmendorf-Richardson (JBER) in Anchorage, Alaska<ref name="CrownoverHeron2024"/>. This ESTCP project was implemented in partnership with DOD’s Defense Innovation Unit (DIU). Three technology demonstrations were conducted at the site where approximately 6,000 cy of PFAS-impacted soil was treated (TCH, smoldering and kiln-style thermal desorption). Figure 3 shows the fully constructed pile used for the TCH demonstration. In August 2024 the soil temperature for the TCH treatment exceeded 400°C in all monitoring locations. At an energy density of 355 kWh/cy, Alaska Department of Environmental Conservation (ADEC) standards and EPA Residential Regional Screening Levels (RSLs) for PFAS in soil were achieved. At JBER, all 30 post-treatment soil samples were near or below detection limits for all targeted PFAS compounds using EPA Method 1633. The composite of all 30 soil samples was below all detection limits for EPA Method 1633. Detection limits ranged from 0.0052 µg/kg to 0.19 µg/kg. | ||
| − | + | ==Advantages and Disadvantages== | |
| + | Thermal treatment of PFAS in soils is energy intensive, and the cost of that energy may be prohibitive for some clients. Also, while it often is the least costly option for complete PFAS removal when compared to excavation followed by offsite disposal or destruction, heating soil to treatment temperatures on site or ''in situ'' typically takes longer than excavation. Major advantages include: | ||
| + | *On site or ''in situ'' treatment eliminates the need to transport and dispose of the contaminated soil | ||
| + | *Site liabilities are removed once and for all | ||
| + | *Treatment costs are competitive with excavation, transportation and off-site treatment or disposal. | ||
| − | == | + | ==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== | ==References== | ||
| − | |||
<references /> | <references /> | ||
==See Also== | ==See Also== | ||
Latest revision as of 15:53, 8 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.
Target Temperature and Duration
PFAS behave differently from most other organics subjected to TCH treatment. While the boiling points of individual PFAS fall in the range of 150-400°C, their chemical and physical behavior creates additional challenges. Some PFAS form ionic species in certain pH ranges and salts under other chemical conditions. This intricate behavior and our limited understanding of what this means for our ability to remove the PFAS from soils means that direct testing of thermal treatment options is warranted. Crownover and colleagues[24] subjected PFAS-laden soil to bench-scale heating to temperatures between 200 and 400°C which showed strong reductions of PFAS concentrations at 350°C and complete removal of many PFAS compounds at 400°C. The soil concentrations of targeted PFAS were reduced to nearly undetectable levels in this study.
Heating Method
For semi-volatile compounds such as dioxins, furans, poly-chlorinated biphenyls (PCBs) and Poly-Aromatic Hydrocarbons (PAH), thermal conduction heating has evolved as the dominant thermal technology because it is capable of achieving soil temperatures higher than the boiling point of water, which are necessary for complete removal of these organic compounds. Temperatures between 200 and 500°C have been required to achieve the desired reduction in contaminant concentrations[25]. TCH has become a popular technology for PFAS treatment because temperatures in the 400°C range are needed.
The energy source for TCH can be electricity (most commonly used), or fossil fuels (typically gas, diesel or fuel oil). Electrically powered TCH offers the largest flexibility for power input which also can be supplied by renewable and sustainable energy sources.
Energy Usage
Treating PFAS-impacted soil with heat requires energy to first bring the soil and porewater to the boiling point of water, then to evaporate the porewater until the soil is dry, and finally to heat the dry soil up to the target treatment temperature. The energy demand for wet soils falls in the 300-400 kWh/cy range, dependent on porosity and water saturation. Additional energy is consumed as heat is lost to the surroundings and by vapor treatment equipment, yielding a typical usage of 400-600 kWh/cy total for larger soil treatment volumes. Wetter soils and small treatment volumes drive the energy usage towards the higher number, whereas larger soil volumes and dry soil can be treated with less energy.
Vapor Treatment
During the TCH process a significant fraction of the PFAS compounds are volatilized by the heat and then removed from the soil by vacuum extraction. The vapors must be treated and eventually discharged while meeting local and/or federal standards. Two types of vapor treatment have been used in past TCH applications for organics: (1) thermal and catalytic oxidation and (2) condensation followed by granular activated charcoal (GAC) filtration. Due to uncertainties related to thermal destruction of fluorinated compounds and future requirements for treatment temperature and residence time, condensation and GAC filtration have been used in the first three PFAS treatment field demonstrations. It should be noted that PFAS compounds will stick to surfaces and that decontamination of the equipment is important. This could generate additional waste as GAC vessels, pipes and other wetted equipment need careful cleaning with solvents or rinsing agents such as PerfluorAdTM.
PFAS Reactivity and Fate
While evaluating initial soil treatment results, Crownover et al[24] noted the lack of complete data sets when the soils were analyzed for non-targeted compounds or extractable precursors. Attempts to establish the fluorine balance suggest that the final fate of the fluorine in the PFAS is not yet fully understood. Transformations are likely occurring in the heated soil as demonstrated in laboratory experiments with and without calcium hydroxide (Ca(OH)2) amendment[26]. Amendments such as Ca(OH)2 may be useful in reducing the required treatment temperature by catalyzing PFAS degradation. With thousands of PFAS potentially present, the interactions are complex and may never be fully understood. Therefore, successful thermal treatment may require a higher target temperature than for other organics with similar boiling points – simply to provide a buffer against the uncertainty.
Case Studies
Stockpile Treatment, Eielson AFB, Alaska (ESTCP project ER20-5198[27])
Figure 1. TCH treatment of a PFAS-laden stockpile at Eielson AFB, Alaska[27]
Since there has been no approved or widely accepted method for treating soils impacted by PFAS, a common practice has been to excavate PFAS-impacted soil and place it in lined stockpiles. Eielson AFB in Alaska is an example where approximately 50 stockpiles were constructed to temporarily store 150,000 cubic yards of soil. One of the stockpiles containing 134 cubic yards of PFAS-impacted soil was heated to 350-450°C over 90 days (Figure 1). Volatilized PFAS was extracted from the soil using vacuum extraction and treated via condensation and filtration by granular activated charcoal. Under field conditions, PFAS concentration reductions from 230 µg/kg to below 0.5 µg/kg were demonstrated for soils that reached 400°C or higher for 7 days. These soils achieved the Alaska soil standards of 3 µg/kg for PFOS and 1.7 µg/kg for PFOA. Cooler soils near the top of the stockpile had remaining PFOS in the range of 0.5-20 µg/kg with an overall average of 4.1 µg/kg. Sampling of all soils heated to 400°C or higher demonstrated that the soils achieved undetectable levels of targeted PFAS (typical reporting limit was 0.5 µg/kg).
In situ Vadose Zone Treatment, Beale AFB, California (ESTCP project ER20-5250[28])
A former fire-training area at Beale AFB had PFAS concentrations as high as 1,970 µg/kg in shallow soils. In situ treatment of a PFAS-rich soil was demonstrated using 16 TCH borings installed in the source area to a depth of 18 ft (Figure 2). Soils which reached the target temperatures were reduced to PFAS concentrations below 1 µg/kg. Perched water which entered in one side of the area delayed heating in that area, and soils which were affected had more modest PFAS concentration reductions. As a lesson learned, future in situ TCH treatments will include provisions for minimizing water entering the treated volume[28]. It was demonstrated that with proper water management, even highly impacted soils can be treated to near non-detect concentrations (greater than 99% reduction).
Constructed Pile Treatment, JBER, Alaska (ESTCP Project ER23-8369[29])
In 2024, a stockpile of 2,000 cubic yards of PFAS-impacted soil was thermally treated at Joint Base Elmendorf-Richardson (JBER) in Anchorage, Alaska[29]. This ESTCP project was implemented in partnership with DOD’s Defense Innovation Unit (DIU). Three technology demonstrations were conducted at the site where approximately 6,000 cy of PFAS-impacted soil was treated (TCH, smoldering and kiln-style thermal desorption). Figure 3 shows the fully constructed pile used for the TCH demonstration. In August 2024 the soil temperature for the TCH treatment exceeded 400°C in all monitoring locations. At an energy density of 355 kWh/cy, Alaska Department of Environmental Conservation (ADEC) standards and EPA Residential Regional Screening Levels (RSLs) for PFAS in soil were achieved. At JBER, all 30 post-treatment soil samples were near or below detection limits for all targeted PFAS compounds using EPA Method 1633. The composite of all 30 soil samples was below all detection limits for EPA Method 1633. Detection limits ranged from 0.0052 µg/kg to 0.19 µg/kg.
Advantages and Disadvantages
Thermal treatment of PFAS in soils is energy intensive, and the cost of that energy may be prohibitive for some clients. Also, while it often is the least costly option for complete PFAS removal when compared to excavation followed by offsite disposal or destruction, heating soil to treatment temperatures on site or in situ typically takes longer than excavation. Major advantages include:
- On site or in situ treatment eliminates the need to transport and dispose of the contaminated soil
- Site liabilities are removed once and for all
- Treatment costs are competitive with excavation, transportation and off-site treatment or disposal.
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.
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