Mechanisms of PFAS Transport in Groundwater Driven by Seasonal Rainfall

Current research on the transport of per- and polyfluoroalkyl substances (PFAS) in groundwater is often confined to steady-state assumptions or laboratory scales, which inadequately capture the nonlinear transport mechanisms driven by seasonal precipitation. To address this gap, this study developed a numerical model based on groundwater flow theory, incorporating adsorption at the air-water interface (AWI). Using perfluorooctanoic acid (PFOA) as a representative PFAS, we conducted a 30-year (1990—2020) dynamic simulation to quantitatively analyze the transport mechanisms under the influence of seasonal precipitation. The results indicate that the AWI in the vadose zone exerts a significant nonlinear retardation effect on PFOA. During dry periods, PFOA accumulates at the AWI; however, abrupt increases in water content during intense rainfall events compress the AWI area, triggering rapid desorption (time lag < 7 d) and subsequent leaching of PFOA into the groundwater. In highly permeable aquifers, this process facilitates the formation of a PFOA plume characterized by low concentrations (< 1 μg/L) but high discharge flux (>0.5 g/(m²·d)). Under high hydraulic gradients (dh/dx> 0.003), advection becomes the dominant transport mechanism, resulting in an elongated plume morphology (length-to-width ratio> 3∶1). This study elucidates the nonlinear regulatory mechanisms of transient hydrological processes on the multi-interfacial behavior of PFAS.