A framework for determining site-specific soil screening levels for PFAS

This study illustrates the application of a framework for determining site-specific soil screening levels (SSLs) for PFAS. The framework was used at four sites at which multiple source zones have been impacted by aqueous film forming foam (AFFF). Soil and porewater samples were collected for determination of PFAS concentrations and for measurements of multiple physical and geochemical properties. PFAS-LEACH, a comprehensive modeling platform developed specifically for PFAS was used to conduct detailed mathematical modeling and to determine site-specific SSLs for the three most prevalent PFAS, perfluorooctane sulfonic acid (PFOS), perfluorooctanoic acid (PFOA), and perfluorohexanesulfonic acid (PFHxS). Model simulations successfully predicted measured water contents and PFAS porewater concentrations, delineating the relevant transport processes and determining the most representative input parameters. This information was used to apply a simplified analytical-solution-based screening model and a revised dilution-attenuation factor (DAF) equation implemented in PFAS-LEACH to determine site-specific SSLs. The SSLs determined with the site-specific approach were a factor of three to more than two orders-of-magnitude larger than generic EPA screening values. The disparities were shown to result from the consideration of PFAS retention and attenuation in the vadose zone and in particular the impact of air-water interfacial adsorption. The SSLs determined for PFOS were larger than those for PFOA and PFHxS due to the greater retention properties of PFOS. The influence of input-parameter uncertainty was assessed, producing a factor of four range in SSLs. SPLP leaching-test measurements and soil/porewater concentrations from paired field samples were tested as alternative methods to parameterize the DAF equation.