EXPERIMENTAL AND MODELING INVESTIGATIONS OF PFAS TRANSPORT AND RETENTION IN THE UNSATURATED ZONE (Invited Presentation)

This presentation provides an overview of coupled experimental and modeling research designed to explore the influence of interfacial accumulation and soil sorption processes on transport of selected per- and polyfluoroalkyl substances (PFAS) in Aqueous Film-Forming Foam (AFFF)-impacted soils. The surface tension of aqueous solutions containing perfluorooctanoic acid (PFOA), perfluorosulfonic acid (PFOS), and perfluorooctane sulfonamide (FOSA) at concentrations ranging from 0.1 mg/L to greater than 1,000 mg/L were measured using the Wilhelmy plate method for a range of dissolved solids concentrations representative of natural groundwaters. A series of batch reactor experiments was also conducted to measure the sorption of select PFAS on three representative porous media, 80-100 mesh Ottawa sand, Appling soil, and Mardin soil. Experimental results reveal that PFAS sorption is non-linear, conforming to a Freundlich isotherm, and that accumulation at the air-water interface is well-described by a Langmuir-Szyszkowski expression. Based upon these laboratory data, mass distribution calculations are presented to illustrate the relative importance of PFAS interfacial partitioning and sorption for varying soil type, aqueous phase composition, and moisture content. An industry-standard numerical simulator, HYDRUS, was modified to incorporate experimentally derived equilibrium partitioning relations and used to explore the influence of interfacial adsorption processes on PFAS transport and fate. Simulations reveal the impact of surface tension reduction on aqueous phase redistribution and PFAS migration in the soil profile, as well as the significance of nonlinear adsorption effects.