Modeling the long-term retention and leaching of PFAS in the vadose zone

Abstract:

Per- and polyfluoroalkyl substances (PFAS) are emerging contaminants that are widespread in the environment. Synthesized as surfactants, when released to the subsurface PFAS tend to accumulate at air-water interfaces and can be retained in the vadose zone for long times before migrating to groundwater. The primary factors that control the timescale of retention for PFAS in the vadose zones remain poorly understood, especially under dynamic changes of air-water interfaces driven by time-dependent infiltration. Here, we simulate the transport and retention of a group of six dominant PFAS using a state-of-the-art mathematical model that incorporates transient variably saturated flow, surfactant-induced flow, and nonlinear solid-phase and air-water interfacial (AWI) adsorption processes. Two typical source zones---fire training area and biosolids-amended land---are simulated under a wide range of conditions including climatic conditions, soil type, and solution ionic strength. The simulations show that---driven by stronger solid-phase and AWI adsorption---long-chain PFAS are much more strongly retained in the vadose zone than their short-chain counterparts: (1) retardation factors for the former range from a hundred to several thousand or greater, while for the latter are consistently smaller than ten; (2) long-chain PFAS can be retained in the vadose zone for several decades after PFAS-release contamination events have ceased. Additionally, retardation factors for PFOS increase by more than 10 times in synthetic groundwater than those in deionized water caused by stronger AWI adsorption, demonstrating the strong impact of solution ionic strength. The simulation results are supported by field observations at many PFAS-contaminated sites.