Per- and poly-fluoroalkyl substances (PFAS) contamination of surface waters by historic landfills <i>via</i> groundwater plumes: ecosystem exposure and downstream mass loading

Many historic landfill sites have groundwater plumes that discharge to nearby surface waters. Recent research indicates that leachate of historic landfills can contain elevated concentrations of per- and polyfluoroalkylated substances (PFAS), but there is limited data on resulting PFAS inputs to aquatic ecosystems as might inform on this potential environmental threat. The objective of this study was to evaluate PFAS exposure in three ecological zones and PFAS mass loading downstream, over 1 year, at two historic landfill sites where landfill plumes discharge to nearby surface waters (1 pond with outlet stream, called HB site; 1 urban stream, called DC site). The three zones experienced different magnitudes and patterns of PFAS concentration exposure (<i>i.e.</i>, contaminant presence in the zone). The endobenthic zone of the sediments receiving the landfill plumes experienced the highest concentrations (∑PFAS >4000 ng L^-1 (HB) and >20 000 ng L^-1 (DC)), often year-round and over a substantial area at each site. Dilution of landfill PFAS in surface waters was observed though concentrations were still elevated (∑PFAS: >120 ng L^-1 (HB) and >60 ng L^-1 (DC)), with evidence of year-round pelagic zone exposure. PFAS concentrations in the epibenthic zones could vary between that of the endobenthic and pelagic zones, sometimes with daily, event-based, and longer-term patterns. Together these findings suggest historic landfill plumes can lead to substantial PFAS exposure to a variety of aquatic life. Downstream PFAS mass loadings during base flows were relatively small individually (15 (HB) and 36 (DC) g per year (∑PFAS)); however, collective loadings from the numerous historic landfills in a watershed could contribute to increasing PFAS concentrations of connected water bodies, with implications for ecological health, drinking water sources, and fisheries.