Understanding pollutant pathways to receiving water bodies is essential for implementing effective mitigation measures (including adequate treatment and/or source control) and meeting regulatory guidelines. The research for this Licentiate thesis aimed to improve knowledge of urban stormwater contributions to receiving water bodies, with a focus on common contaminants in stormwater runoff, and assess the potential risks these substances pose. The study is based on an extensive dataset collected from field sampling campaigns carried out along three urban streams in Sweden, all of which primarily receive untreated stormwater discharges through separate sewer systems when passing through urban areas.

Field sampling results indicate that during wet weather events, the concentrations of total suspended solids (TSS) and metals associated with anthropogenic activities originating from traffic-related activities and building and infrastructure materials – such as chromium (Cr), nickel (Ni), and zinc (Zn) – increase in the streams, confirming the impact of urban stormwater discharges. Further, the concentrations of per- and polyfluoroalkyl substances (PFAS) were also increased in the streams under wet weather conditions, with total quantified concentrations (sum of 34 PFAS) ranging from below the reporting limit up to 84.7 ng/L during dry weather and increasing under wet weather conditions (0.87 to 102.3 ng/L). The stormwater discharges introduced a great variety of PFAS to the streams, particularly longer chain PFAS. In sediments, both higher concentrations and a greater variety of PFAS were found at sampling sites located downstream of urban areas as well as sites close by known point sources, such as airports. Of all contaminant groups analyzed, PFAS stood out as the only compound family consistently detected in both the water and sediment phases. Most other organic contaminants, including phthalates and polycyclic aromatic hydrocarbons (PAHs), were not detected above reporting limits in stream water but were quantified in bottom sediments. The concentration and occurrence of these hydrophobic organic contaminants in bottom sediment followed the urbanization gradient, with higher concentrations observed at downstream sampling sites near urban areas or known point sources, such as industrial areas and airports, compared to upstream sites. This suggests that urban runoff mobilizes and transports these contaminants from urban areas to receiving water bodies, leading to their accumulation in sediments. In contrast, organotin compounds likely originate from sources other than stormwater runoff (remains unidentified), while phenolic substances did not exhibit a clear pattern indicative of transport through urban runoff.

Risk assessment of the streams revealed contamination levels in bottom sediments, where 20 out of 82 assessed compounds – including PAHs, phenols, phthalates, and PFAS – exceeded toxicity-based limits (i.e. Predicted No-Effect Concentration (PNEC) and Annual Average Environmental Quality Standards (AA-EQS)). In the sediment phase, exceedance rates ranged from 2 to 105 times. Notably, 4-tert-octylphenol (4-t-OP) exceeded PNEC by up to 105 times, while perfluorooctane sulfonate (PFOS) consistently surpassed the critical threshold, with concentrations reaching 140 times the toxicity-based threshold (PNEC). In the water phase, in general exceedance rates were lower than in bottom sediment, nevertheless exceedance rates for PFOS and Total PFOA Eqv remained critically high, particularly under wet weather conditions. Findings also highlighted the limitations of current risk assessment methods, which may underestimate risks primarily due to the lack of experimentally derived PNEC values.