Hillslope groundwater discharges provide localized stream ecosystem buffers from regional per‐ and polyfluoroalkyl substances contamination

Emerging groundwater contaminants such as per‐ and polyfluoroalkyl substances (PFAS) may impact surface‐water quality and groundwater‐dependent ecosystems of gaining streams. Although complex near‐surface hydrogeology of stream corridors challenges sampling efforts, recent advances in heat tracing of discharge zones enable efficient and informed data collection. For this study, we used a combination of streambed temperature push‐probe and thermal infrared methods to guide a discharge‐zone‐oriented sample collection along approximately 6 km of a coastal trout stream on Cape Cod, MA. Eight surface‐water locations and discharging groundwater from 24 streambed and bank seepages were analysed for dissolved oxygen (DO), specific conductance, stable water isotopes, and a range of PFAS compounds, which are contaminants of emerging concern in aquatic environments. The results indicate a complex system of groundwater discharge source flowpaths, where the sum of concentrations of six PFAS compounds (corresponding to the U.S. Environmental Protection Agency third Unregulated Contaminant Monitoring Rule “UCMR 3”) showed a median concentration of 52 ± 331 (SD) ng/L with two higher outliers and three discharges with PFAS concentrations below the quantification limit. Higher PFAS concentration was related (−0.66 Spearman rank, p