Occurrence and load modeling of per- and polyfluoroalkyl substances in an urban watershed Vantaa River, southern Finland

Per- and polyfluoroalkyl substances (PFAS) are persistent environmental contaminants linked to multiple adverse impacts and they are ubiquitous in the Finnish aquatic environment. The Vantaa River watershed is densely populated and constitutes a reserve water source for water supply in the Helsinki Metropolitan area (1 million people).  The aim of this research was to quantify PFAS concentrations and test a load estimator software (LOADEST) to determine loads at the mouth of Vantaa River before it flows into the Baltic Sea. Weekly water sampling was conducted near a continuous gauging station (Oulunkylä station) between March 2023 and October 2023 resulting in 28 samples. Water samples were analyzed for 50 PFAS in a commercial laboratory in Finland. Instantaneous daily load (riverine export) of individual and total PFAS (g/day) were calculated from the measured PFAS concentration in the river and daily river discharge data. The USGS application LOADEST was used to calculate individual and total PFAS loads at a daily interval over the monitoring period.

Σ₅₀ PFAS concentration averaged 25 ng/L (range was 7-53 ng/L) and Σ₅₀ PFAS load averaged 27 g/day (range was 6-121 g/day). Six PFAS constituted 83,4% of total quantified PFAS, perfluorooctane sulphonic acid (PFOS) and perfluoropentane acid (PFPeA) accounted for 40%. The total Σ₅₀PFAS load at Oulunkylä was 4,7 kg over the entire monitoring period (197 days). In addition, the PFAS yield (kg/km² yr) calculated by dividing the annual PFAS load by the drainage area was 5.15 10^-3 kg/km² yr in Vantaa River. Statistical measures of model performance indicated that LOADEST models for Σ₅₀PFAS was within acceptable limits, with a Load Bias of –0.5% and a Nash-Sutcliffe Efficiency Index of 0.9. Further monitoring and modeling are warranted as this study shows that LOADEST can successfully be applied to the Vantaa River watershed, and it could be used to track the PFAS load reaching the Baltic Sea and follow the evolution of the system after restriction or ban of individual PFAS in Europe.