At the initiation of this project, there was little understanding of how sources of per- and polyfluoroalkyl substances (PFAS) in the unsaturated zone decayed over time, and how PFAS mass discharge and composition varied as the sources decay. It remained unclear the extent to which equilibrium parameters that govern PFAS distribution (soil-water distribution and accumulation at the air-water interface) impact PFAS leaching in situ. This project was focused on obtaining improved insights into the long-term mass discharge of PFAS from the unsaturated zone of a source area historically impacted by aqueous film-forming foam (AFFF). 

The goals of Phase I of this project were 1) to attain insight into the transformation and mass discharge of PFAS from AFFF sources that reside in the unsaturated zone, and 2) to understand how these processes change over time and with AFFF composition and mass. The results of the Phase I effort are available in the Phase I Final Report. 

Additional efforts as part of Phase II of this project are focused on the extent to which partial excavation of the highly AFFF-impacted surface soil has on decreasing the PFAS mass flux to underlying groundwater. Specifically, this additional research will provide a comprehensive data set that could justify (in some cases) a remedial approach that minimizes the volume of soil requiring excavation, as well as provide a science-based rationale for leaving PFAS-impacted soils in the subsurface. Thus, the specific objectives of Phase II are to: 

• Directly measure the decrease in PFAS porewater concentrations, and the corresponding decrease in PFAS mass flux to groundwater, as a result of soil surface excavation. PFAS porewater concentrations in test plots with and without soil excavation will be compared to facilitate determination of the impacts of soil excavation; 
• Attain insight into the timeframe needed to observe the impact of the excavation on percolating porewater; and 
• Demonstrate the utility of porewater lysimeters for assessing treatment effectiveness.

The overall approach employed in Phase I consisted of installing a highly instrumented test cell (approximately 14 ft x 14 ft) within an AFFF source area to monitor PFAS porewater concentrations as a function of moisture content, time, and depth within the unsaturated zone. An irrigation system was used to assess PFAS porewater concentrations during and after enhanced flushing, and to provide insight into the relationship between PFAS mass flux and mass removal. To provide insight into the mechanisms controlling the PFAS porewater concentrations, a parallel set of bench-scale experiments were performed using site soils to determine PFAS soil desorption kinetics and isotherms and sorption coefficients at the air-water interface.

Phase II of this project focuses on excavation of the uppermost foot of soil, which contains the most PFAS-impacted soils, from one quadrant of the test cell. Using the currently installed network of lysimeters, as well as two newly installed lysimeters, PFAS porewater concentrations are subsequently being monitored for up to 1.5 years. If needed, the existing irrigation system can be used to accelerate flushing due to soil infiltration. This robust set of field data will be used to compare PFAS porewater concentrations and mass flux between the excavated and non-excavated portions of the field plot, thus providing a direct measure of the impacts of partial excavation on PFAS mass flux to underlying groundwater.

Phase I Results

Under Phase I, results from the bench-scale soil desorption testing showed that desorption isotherms were reasonably described by a linear model and that a fraction of the soil-bound PFAS mass was not readily desorbed (non-labile). The non-labile fraction of PFAS mass sorbed to the soil ranged from 0.17 g g-1 for perfluorohexane sulfonamide in the deep soil to 0.87 g g-1 for 8:2 fluorotelomer sulfonate in the shallow soil. PFAS porewater concentrations measured in the field lysimeters were well predicted using the soil isotherm data coupled with an estimate of the PFAS mass sorbed at the air-water interface, thereby demonstrating the role of air-water interfacial adsorption on PFAS mass flux during leaching. Enhanced flushing showed that PFAS mass flux decreased much more rapidly than PFAS within the test cell, particularly for long-chained perfluorinated sulfonates such as perfluorooctane sulfonate; this was observed at both the bench-scale and in the field.

Results from Phase I of this study demonstrated that PFAS leaching can be reasonably predicted based on equilibrium-based parameters that can be readily quantified in the laboratory, particularly for longer-chain compounds. Furthermore, both the bench- and field-scale results suggest that elevated PFAS concentrations may persist in soils even after leaching has been substantially reduced. This observation can facilitate a scientifically based justification for basing soil cleanup criteria based on site-specific leaching/desorption values rather than generic regulatory limits. Successful completion of the Phase II effort will ultimately improve the cost effectiveness of PFAS treatment technologies, directly benefiting the warfighter and installation communities. (Anticipated Phase II Project Completion – 2026)

Schaefer, C .E., G. Lavorgna, D.R. Lippincott, D. Nguyen, E. Christie, S. Shea, S. O’Hare, M. Lemes, C.P. Higgins, and J. Field. 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. doi.org/10.1016/j.jconhyd.2022.104001.

Schaefer, C .E., G.M. Lavorgna, D.R. Lippincott, D. Nguyen, A. Schaum, C.P. Higgins, and J. Field. 2023. Leaching of Perfluoroalkyl Acids During Unsaturated Zone Flushing at a Field Site Impacted with Aqueous Film Forming Foam. Environmental Science & Technology, 57(5):1940-1948. doi.org/10.1021/acs.est.2c06903.

Schaefer, C .E., D. Nguyen, E. Christie, S. Shea, C.P. Higgins, and J.A. Field. 2022. Desorption Isotherms for Poly-and Perfluoroalkyl Substances in Soil Collected from an Aqueous Film-Forming Foam Source Area. Journal of Environmental Engineering, 148(1):04021074.

Schaefer, C .E., D. Nguyen, E. Christie, S. Shea, C.P. Higgins, and J.A. Field. 2021. Desorption of Poly- and Perfluoroalkyl Substances from Soil Historically Impacted with Aqueous Film-Forming Foam. Journal of Environmental Engineering, 147(2):06020006. doi.org/10.1061/(asce)ee.1943-7870.0001846.

Shea, S .M., C .E. Schaefer, T. Illangasekare, and C .P. Higgins. 2025. Release of Poly-and Perfluoroalkyl Substances from AFFF-Impacted Soils: Effects of Water Saturation in Vadose Zone Soils. J. of Contaminant Hydrology., 269: 104506. doi.org/10.1016/j.jconhyd.2025.104506.