Hydrology shapes PFAS profiles and precursor contributions in a wastewater-impacted river

• Diffuse sources contributed the most to riverine PFAS load even in dry season. • Snowmelt-driven high flow revealed a high amount of short-chain PFAA precursors. • Four previously unreported short-chain PFAS precursors were identified. Per- and polyfluoroalkyl substances (PFAS) are a large group of compounds with a wide range of sources, uses and physicochemical properties, resulting in highly variable environmental behaviour and analytical detectability. This study investigates the occurrence of a large spectrum of PFAS, including precursors of perfluoroalkyl acids, in the River Porvoonjoki system in Southern Finland during two contrasting hydrological periods: high flow (April 2024) and low flow (June 2024). Surface water from up to ten sampling sites, including a stormwater ditch, and three wastewater treatment plants (WWTPs) discharging into the river were analysed using target analysis, Total Oxidizable Precursors (TOP) assay and suspect screening supported by non-target analysis (NTA). Short-chain perfluorocarboxylic acids dominated the PFAS profile in the river. Higher concentrations were observed during the low flow period, suggesting an increased impact from wastewater discharges when dilution is limited. However, mass load calculations propose that a substantial fraction of total PFAS load originates from diffuse sources, such as stormwater, snowmelt-driven surface runoff and background catchment inputs. Oxidizable precursors contributed substantially to total PFAS loads in both wastewater effluents and the four surface water samples that were analysed for TOP in both hydrological periods. Their contribution in these samples was higher in April than in June, potentially reflecting enhanced mobilization during spring snowmelt and wet-weather conditions. Suspect screening showed the presence of four previously unreported PFAS, however, the suspects or their semi-quantitative concentrations could not explain the TOP assay results. The findings emphasize the importance of integrating precursor analysis in PFAS monitoring and to consider varying hydrological conditions, to improve load estimation and to establish management strategies for reducing PFAS inputs to surface waters.