A mathematical model for the release, transport, and retention of per- and polyfluoroalkyl substances (PFAS) in the vadose zone

Abstract:

Per- and polyfluoroalkyl substances (PFAS) are emerging contaminants of critical concern. As surfactants, PFAS tend to accumulate at air-water interfaces and may stay in the vadose zone for long times before contaminating groundwater. Yet not well understood, the extent of retention in the vadose zone has critical implications for risk management and remediation strategies. We present the first mathematical model that accounts for surfactant-induced flow and nonlinear rate-limited solid-phase and air-water interfacial adsorption under transient variably saturated flow. We apply the model to simulate PFOS (a PFAS compound of primary concern) transport in the vadose zone at a model fire-training area site impacted by aqueous film-forming foam (AFFF). Air-water interfacial adsorption, amplified by the low water content due to gravity drainage, is shown to have a significant impact. The retardation factors range from 233 to 1355 for the sand and 146 to 792 for the soil used in the present study. The simulations illustrate that it can take several decades or longer for PFOS to reach groundwater. Counterintuitively, the lower water content in the sand—due to stronger drainage and weaker capillary retention—leads to retardation factors greater than that for the soil. Also, most PFOS is adsorbed at air-water interfaces with only 1-2% in the aqueous phase. Our findings imply that 1) fine-texture materials could have lower retardation factors than that for sand due to higher retained water content, 2) soil PFAS concentrations are likely to be orders of magnitude higher than those in the groundwater beneath source zones. Both implications are supported by recent field observations at hundreds of AFFF-impacted sites.