Assessment of PFAS in urban stormwater, vadose zone and groundwater: Insights from three different rainfall events

While per- and polyfluoroalkyl substances (PFAS) pose a persistent threat to aquifers, the interplay between urban runoff and the remobilization of legacy PFAS stocks in soils within infiltration facilities remains poorly understood. This study investigates these dynamics by monitoring a pilot-scale infiltration system over three seasons, tracking PFAS concentrations across the runoff-soil-groundwater continuum. The specific objectives are to: (1) Quantify 25 PFAS compounds in urban runoff originating from a small urban catchment; (2) Assess the retention and buffering capacity of the underlying contaminated urban soil by monitoring PFAS breakthrough in vadose zone percolate; (3) Evaluate the downgradient impact of the infiltration process on groundwater quality; (4) Identify the key biogeochemical drivers governing PFAS mobility within both the runoff and the vadose zone. Results revealed a 'first flush' in runoff (peak 656 ng/L, dropping to 5 ng/L), while the soil matrix acted as a geochemical buffer, stabilizing vadose zone concentrations (26 -55 ng/L). Paradoxically, background aquifer levels (169 -274 ng/L) significantly exceeded these vadose zone inputs, identifying legacy contamination as the primary driver of groundwater quality at this site. Infiltration consequently led to a transient dilution effect observed downgradient. Multivariate analysis revealed that runoff concentrations were strongly correlated with seasonal temperature and Total Organic Carbon (TOC), whereas transport within the vadose zone was primarily governed by temperature and ionic strength. The persistent lack of correlation with pH suggests that theoretical chemical equilibria are overshadowed by transport inertia inherent to the urban subsurface. These findings indicate that PFAS mobilization is most critical during rainfall events following prolonged dry and hot periods. From a management perspective, these dynamics suggest that mitigating PFAS entry into infiltration facilities requires pre-treatment strategies focused on fine fraction decantation to achieve TOC reduction. Furthermore, the sensitivity of PFAS mobility in the vadose zone to ionic strength necessitates careful monitoring of runoff salinity, particularly in contexts where road salts or high mineralization are prevalent. Ultimately, while runoff introduces new PFAS loads, the buffering capacity of the vadose zone allows urban infiltration to effectively mitigate groundwater risks through dilution. This supports the use of decentralized infiltration as a sustainable PFAS management strategy, provided that monitoring protocols account for transient seasonal breakthrough windows and include long-term oversight to evaluate the evolution of the vadose zone's behavior as its retention capacity reaches saturation.