Partitioning of Poly- and Perfluoroalkyl Substances from Soil to Groundwater within Aqueous Film Forming Foam Source Zones

Metadata from ongoing site investigations at U.S. Air Force sites impacted by Aqueous Film-Forming Foam (AFFF) were used to evaluate primary factors that affect transport of poly- and perfluoroalkyl substances (PFAS) within source zones. Soil-to-groundwater ratios (γSoil-GW) were calculated for each site and PFAS as an empirical transport metric and regressed against select soil chemical and physical properties, as well as relevant interactions thereof, using analysis of covariance (ANCOVA). Total organic carbon and percent total clay content were significant covariates and best fit log-transformed γSoil-GW with a parallel lines model. Regression parameters were used to derive apparent KOC estimates for 15 perfluoroalkyl acids, perfluorooctane sulfonamide, and the 6:2 and 8:2 fluorotelomer sulfonates, which were bimodal where significant differences were primarily observed between PFAS with fewer than 8 perfluorinated carbons (including perfluorooctoanoic acid; PFOA) and those with 8 or more (including perfluorooctane sulfonate; PFOS). Surprisingly, clay content was observed with a negative slope (i.e., γSoil-GW decreased with increasing clay content), which may reflect greater retardation at more permeable sites due to air-water interfacial partitioning. Further analysis demonstrated that γSoil-GW is also affected by the degree of flushing in a way that suggests mass transfer (as opposed to hysteretic desorption) primarily controls mass discharge at poorly flushed sites. Overall, this meta-analysis demonstrates that PFAS retention in soil is significant, but depends on a combination of fundamental partitioning mechanisms that may not be predictable in all cases based on transport models that assume an equilibrium condition.