Combining field datasets and mathematical modeling to quantify PFAS leaching and mass discharge at an AFFF-impacted site

Vadose zones serve as significant reservoirs of per- and polyfluoroalkyl substances (PFAS) at contaminated sites, posing risks to the groundwater underneath. Partitioning of PFAS to the solid–water and air–water interfaces in soils complicates PFAS leaching in the vadose zone. We apply mathematical models representing PFAS-specific retention and transport processes to simulate vadose-zone leaching and mass discharge at a PFAS-contaminated field site. The mathematical models are constrained by detailed datasets collected at the site under both ambient rainfall and artificial flushing conditions. Predicted porewater concentrations generally agree with those sampled by suction lysimeters over a period of 2 months. Model-based analysis suggests: (1) minimal downward migration of PFOS and PFOA occurred over the 2-month period, (2) variations in observed porewater concentrations were caused by mass redistribution among the different phases in response to dynamic changes in soil moisture content and air–water interfacial area, (3) accounting for rate-limited solid-phase desorption reduces discrepancies between simulated and sampled porewater concentrations for PFOS and PFOA, particularly for the shallowest depth interval, and (4) porewater concentration and moisture data may be used to estimate air–water interfacial area. Additional 40-year long-term simulations indicate that the simulated leaching is consistent with field observations for PFOS, but it generally overestimates the leaching for PFOA, PFHxS, and PFBS, which appears to be caused by the simulations not accurately representing desorption kinetics, underestimating solid-phase adsorption, and/or not accounting for precursor transformation. Our results suggest that accurate quantification of source strength and mass discharge at a PFAS-contaminated site requires characterizing hydraulic and transport parameters especially kinetic solid-phase desorption behaviors, PFAS soil concentration profiles, precursor transformation, and site-specific infiltration rates.