On per- and polyfluoroalkyl substances (PFAS) redistribution in soil driven by evaporation

Evaporation-induced concentration of per- and polyfluoroalkyl substances (PFAS) in the vadose zone can enhance contaminant accumulation near the soil surface, thereby increasing the potential for transport into the food chain. While most existing transport models primarily describe downward advective-dispersive movement of PFAS in soils, upward fluxes driven by evaporation are often neglected. In this study, an analytical model is developed to describe evaporation-driven PFAS transport and redistribution in variably saturated soils. The governing transport equation, which incorporates advection, dispersion, and phase exchange between the aqueous and sorbed phases, is solved analytically using the Laplace transform technique. The model accounts for spatial variability in soil moisture content with depth, which influences both transport and storage processes. Arbitrary initial conditions are considered for both aqueous and sorbed PFAS phases. The influence of evaporation-induced upward flow on the evolution of initially heterogeneous concentration profiles is examined through representative cases. Furthermore, a sensitivity analysis is performed to quantify the effects of key hydrodynamic parameters, including moisture-dependent transport properties, on PFAS concentration dynamics. The proposed analytical solution provides a computationally efficient screening tool for evaluating the potential risk of PFAS accumulation at the soil surface under evaporative conditions, particularly in arid and semi-arid environments.