Model validation and analyses of parameter sensitivity and uncertainty for modeling long-term retention and leaching of PFAS in the vadose zone

Abstract

PFAS are emerging contaminants widespread in the environment. As surfactants, PFAS tend to accumulate at solid–water and air–water interfaces in the vadose zone, which may pose long-term threats to groundwater. The primary factors that control the long-term retention of PFAS in the vadose zone remain poorly understood. To address this knowledge gap, we first use multiple datasets from transport experiments to validate a state-of-the-art mathematical model that incorporates transient variably saturated flow, surfactant-induced flow, and rate-limited and nonlinear solid-phase and air–water interfacial adsorption. We then employ the validated model to simulate and analyze the primary processes and parameters controlling the retention and leaching of PFAS in the vadose zone at a model fire-training-area site. Our simulations show that adsorption at solid–water and air–water interfaces leads to strong retention of PFAS in the vadose zone. The strength of retention increases with PFAS chain length and porewater ionic strength, while it decreases at greater PFAS concentrations due to nonlinear adsorption. Comprehensive parameter sensitivity analyses reveal that model predictions are most sensitive to parameters related to the air–water interfacial area and PFAS interfacial properties when air–water interfacial adsorption (AWIA) is more important than solid-phase adsorption (SPA). Predicted PFAS leaching rates vary by a wide range resulting from uncertainties in the input parameters, but the uncertainty range is much greater for longer-chain PFAS than that of their shorter-chain counterparts. The simulated arrival times to groundwater were found to follow log-normal distributions. Finally, model complexity analysis reveals that nonlinearity in AWIA and kinetic SPA and kinetic AWIA have a minimal impact on the long-term retention of PFAS under the wide range of field conditions examined in the present study.