Modeling PFAS Subsurface Transport in the Presence of Groundwater Table Fluctuations: The Impact on Source‐Zone Leaching and Exploration of Model Simplifications.

Air–water interfacial adsorption represents a major source of retention for many per‐ and poly‐fluoroalkyl substances (PFAS). Therefore, transient hydrological fluxes that dynamically change the amount of air–water interfaces are expected to strongly influence PFAS retention in their source zones in the vadose zone. We employ mathematical modeling to study how seasonal groundwater table (GWT) fluctuations affect PFAS source‐zone leaching. The results suggest that, by periodically collapsing air–water interfaces, seasonal GWT fluctuations can lead to strong temporal variations in groundwater concentration and significantly enhance PFAS leaching in the vadose zone. The enhanced leaching is more pronounced for longer‐chain PFAS, coarser‐textured porous media, drier climates, and greater amplitudes of fluctuations. GWT fluctuations and lateral migration above the GWT introduce a downgradient persistent secondary source zone for longer‐chain PFAS. However, the enhanced leaching and the secondary source zone are greatly reduced when subsurface heterogeneity is present. In highly heterogeneous source zones, GWT fluctuations may even lead to overall slower leaching due to lateral flow (in the GWT fluctuation zone and above the GWT) moving PFAS into local regions with greater retention capacities. Model simplification analyses suggest that the enhanced source‐zone leaching due to GWT fluctuations may be approximated using a static but shallower GWT. Additionally, while vertical 1D models underestimate source‐zone leaching due to not representing lateral migration, they can be revised to account for lateral migration and provide lower‐ and upper‐bound estimates of PFAS source‐zone leaching under GWT fluctuations. Overall, our study suggests that representing GWT fluctuations is critical for quantifying source‐zone leaching of PFAS, especially the more interfacially active longer‐chain compounds. Plain Language Summary: Per‐ and poly‐fluoroalkyl substances (PFAS) are contaminants that are now widespread in the environment. Many PFAS are interfacially active and tend to accumulate at solid surfaces and air–water interfaces. A growing body of field data at PFAS‐contaminated sites has shown that significant amounts of PFAS have accumulated in soils, posing great risks to the groundwater underneath. Because the accumulation at air–water interfaces is a primary mechanism that retains PFAS in soils, any transient water fluxes that dynamically collapse air–water interfaces may accelerate the downward movement of PFAS. One important type of temporal change to air–water interfaces can be caused by the fluctuation of groundwater, such as seasonal groundwater table fluctuations (GWT). We use mathematical models to study the impact of GWT on enhancing the downward movement of PFAS from soils to groundwater. Our analyses suggest that seasonal GWT fluctuations can significantly enhance PFAS downward movement by periodically collapsing air–water interfaces in soils. They also lead to seasonal variations in groundwater PFAS concentrations that may have important implications for groundwater sampling and risk assessment. Overall, representing groundwater table fluctuations is critical for quantifying groundwater contamination risks of PFAS, especially the more interfacially active compounds with longer carbon chains. Key Points: Groundwater table (GWT) fluctuations enhance source‐zone poly‐fluoroalkyl substances (PFAS) leaching, but the enhanced leaching is reduced in heterogeneous source zonesGWT fluctuations and lateral migration above the GWT lead to a downgradient persistent secondary source zone for longer‐chain PFASDevelop simplified modeling strategies to estimate the lower‐ and upper‐bounds of PFAS source‐zone leaching under GWT fluctuations [ABSTRACT FROM AUTHOR]