Currently, there is little understanding of how per- and polyfluoroalkyl substances (PFAS) sources in the unsaturated zone decay over time, and how PFAS mass discharge and composition vary as the sources decay. Furthermore, it remains 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 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 overall goals 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 overall approach employed in this study 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.

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.

Overall, results from 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-chained 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. (Project Completion - 2023)