The Case for Direct Measures of Soil-to-Groundwater Contaminant Mass Discharge at AFFF-Impacted Sites.

Many entities around the world are initiating massive field campaigns to characterize the environmental distribution of per- and polyfluoroalkyl substances (PFAS), particularly at aqueous film-forming foam (AFFF) impacted sites where historic point-source discharges occurred at the ground surface. Concurrently, many regulatory agencies are publishing criteria used in practice to define the "nature and extent" of PFAS-impacted environmental media. Specific to the soil-to-groundwater transport pathway protective of the groundwater ingestion end point, these soil criteria (or screening values) are to date exclusively based on the traditional approach used for hydrophobic organics with a number of simplifying assumptions. Research has clearly contradicted these assumptions, yet alternative methodologies have yet to emerge from the literature. This Perspective provides a rationale for why alternative approaches are critically necessary and proposes the application of lysimetry as a practical solution to accurately assess PFAS transport within unsaturated vadose zone soils. Ultimately, there is an urgent need to justify soil remediation on the basis of groundwater protection and to prioritize remedial efforts in order to optimize limited fiscal resources.