MATHEMATICAL MODELING OF THE LONG-TERM RETENTION AND LEACHING OF PFAS IN THE VADOSE ZONE (Invited Presentation)

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. The primary factors that control the timescale of retention for PFAS in the vadose zone remain poorly understood, especially under dynamic changes of air-water interfaces driven by time-dependent infiltration. 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 the retention and leaching behaviors of a group of six dominant PFAS (including both short- and long-chain compounds) in the vadose zone at a model fire-training area site impacted by aqueous film-forming foam (AFFF). Real rainfall infiltration datasets obtained from two areas under humid and semi-arid meteorological conditions are employed. Our simulation results show that the adsorption at the air-water interfaces, amplified by the low water content due to gravity drainage, has much greater impact on the retention of the long-chain PFAS than that of their short-chain counterparts. The long-chain PFAS (such as PFOS) are strongly retained in the shallow vadose zone near the land surface even several decades after PFAS-release contamination events were stopped. Additionally, most of the long-chain PFAS in the vadose zone are adsorbed at air-water interfaces with only a few percent staying in the aqueous phase. Our findings clearly demonstrate that 1) vadose zones are going to be long-term sources zones of long-chain PFAS to groundwater, and 2) the soil concentrations of long-chain PFAS in source zones are likely to be orders of magnitude higher than those in the groundwater underneath. These findings are directly supported by recent field observations at hundreds of AFFF-impacted sites.