Seasonal trends of PFAS concentrations in surface waters and fate during managed aquifer recharge (MAR)

Per- and polyfluoroalkyl substances (PFAS) are ubiquitous in surface waters, including those used for managed aquifer recharge (MAR). Yet, temporal PFAS variability and associated groundwater infiltration risks at MAR sites remain poorly understood. To address this research gap, we (a) monitored seasonal variations in surface water PFAS concentrations with a monthly to bi-weekly resolution, (b) assessed their correlation with other water quality parameters and (c) investigated PFAS fate after infiltration by examining potential point sources, precursor transformation, accumulation within the infiltration layer, and subsequent remobilization of PFAS. ∑ 29 PFAS concentrations in surface waters used for MAR varied substantially over time, ranging from 0.25 ng L⁻¹ to 63.0 ng L⁻¹ between months, and occasionally exceeded drinking water guideline values. This observed temporal variability enabled estimation of the annual ∑ 29 PFAS load infiltrating through MAR, ranging from 77 g to 141 g per year, while PFCAs and PFOS were selectively retained within the clogging layer of the infiltration ponds. Additionally, PFAS composition profiles changed during MAR, resulting in occasionally higher legacy PFAS concentrations in the artificially recharged groundwater, which could be attributed to point sources, accumulation in the infiltration layer, and subsequent release to underlying groundwater systems. Overall, the high temporal variability of PFAS concentrations in surface waters used for MAR highlights the need for regular and standardised PFAS monitoring. This aids to capture temporal variations, provide a robust basis for estimating PFAS loads infiltrating via MAR, and support improved treatment technologies and more refined MAR systems to meet newer PFAS drinking water guidelines.