Field-scale PFAS transport dynamics in a small urban catchment: Insights from Stormwater, Vadose Zone and Groundwater monitoring in a nature-based Infiltration facility.

The widespread use and extreme persistence of per- and polyfluoroalkyl substances (PFAS) present significant risks to vulnerable hydrologic systems, yet the fate of these compounds originating from diffuse urban runoff remains poorly understood. This study investigates the transport and fate of 25 PFAS compounds within a pilot-scale nature-based infiltration facility, utilizing a multi-parameter in-situ monitoring network to track concentrations across the runoff-soil-groundwater continuum. Through the analysis of three representative rain events across different seasons, results reveal a sharp contrast in PFAS dynamics between environmental compartments. While a pronounced "first flush" effect was observed in surface runoff with peak concentrations of 656 ng/L rapidly decreasing to 5 ng/L the soil matrix acted as a significant geochemical buffer, moderating vadose zone percolate to a narrow range of 26 to 55 ng/L. Interestingly, background PFAS levels in the broader aquifer remained consistently higher (169 - 226 ng/L) than those measured in the vadose zone, suggesting that pre-existing legacy contamination exerts a more dominant influence on groundwater quality than contemporary leaching from the infiltration site. Furthermore, a temporary dilution effect observed in downgradient monitoring wells during rainfall events indicates that urban infiltration practices may locally mitigate groundwater contamination levels. Multivariate analysis identifies low pH and high total organic carbon (TOC) as the primary physicochemical drivers associated with elevated PFAS mobility. Ultimately, this research demonstrates that while urban runoff introduces new PFAS loads, the primary risk at this site stems from background aquifer contamination, providing a strong scientific basis for the promotion of urban infiltration as a sustainable and potentially remedial stormwater management strategy.