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.

In the area where the river enters the sea, due to the weak hydrodynamic strength of the river and the water upstream, the fine suspended matter carried by the river forms a low permeability covering layer on the river bed surface, thus limiting the water-salt interaction between the river and groundwater. The in-depth study on the exchange of water and salt between tidal river flow and groundwater under the influence of low permeability silt layer has important scientific significance and application value for the utilization and protection of coastal groundwater resources. Taking the Dagu River estuary area of Shandong Peninsula as the study area, a site-scale groundwater numerical model is established on the basis of on-site hydrogeological investigation and monitoring. The groundwater salt dynamics characteristics and the fluxes of tidal river-groundwater water and salt exchange under the influence of low permeability silt layer on the river bed surface are studied for the first time. The effect of permeability and thickness change of low permeability layer on tidal river-groundwater exchange process is quantitatively analyzed. The research shows that under different conditions of low permeability layer, the length of the saline wedge increases and the regression rate of the saline wedge decreases at the initial stage, and gradually reaches the minimum value after about 100 days, then the length of the saline wedge suddenly decreases and the regression rate of the saline wedge increases rapidly, and the regression rate gradually becomes stable after about 600 days. In addition, the fluxes of river-groundwater water and salt exchange show periodic changes, and the fluxes of river-groundwater water and salt exchange decrease with the decrease of permeability or the increase of thickness of the low permeability layer.

Widespread use of subsurface dams as an effective control mechanism may lead to residual saltwater in landward directions and limit the exploitation of groundwater. However, most of the current studies neglect the influence of heterogeneity, or are limited to laboratory scale. In this paper, the seawater intrusion process before and after the construction of subsurface dams under different layered heterogeneous conditions was simulated by a site-scale numerical model. The effects of different layered heterogeneous conditions on the desalination process of residual saltwater and groundwater discharge process are discussed by taking the wedge regression rate of residual saltwater, the removal rate of residual total salt and the percentage of groundwater discharge as indexes. The results show that the change of hydraulic conductivity of the lower aquifer has a significant influence on the regression effect of residual saltwater. The larger the value of low permeability layer, the faster the regression rate. However, the thickness of low permeability layer has little influence on the desalination process of residual saltwater.