Riverine PFAS Dynamics in an Agricultural-Urban Watershed

Forty targeted per- and polyfluoroalkyl substances (PFAS) were analyzed in samples collected weekly from mid-April to the end of November 2023 at four sites along the South Skunk River–Ioway Creek system in central Iowa. We evaluated spatial, seasonal, and hydrologic controls on PFAS concentrations, loads, and yields in a mixed-use agricultural-urban system. Fourteen PFAS were detected. Perfluorobutanesulfonic acid (PFBS) and 6:2 fluorotelomer sulfonate (6:2 FTS) were present in >50% of samples at all sites, and PFBS, perfluorohexanoic acid (PFHxA), and perfluorooctanoic acid (PFOA) were detected in >75% of samples from all South Skunk River (SSR) sites. Median total target PFAS concentrations increased from 5 ng/L upstream of Ames on the SSR to 28 ng/L below the Ames wastewater treatment plant (WWTP). Median area-normalized yields ranged from 667 μg/day/km2 at the upstream agriculturally impacted location to 3547 μg/day/km2 downstream of the Ames WWTP. Concentrations were inversely related to discharge. PFAS loads increased with discharge but scaled sublinearly, indicating dilution-limited loading. Summer and fall storm events produced high-flow PFAS concentration spikes downstream of the Ames urban area. Results indicate that the Ames WWTP is likely the dominant PFAS source in the watershed, while agriculture contributed relatively little target PFAS during the study period.