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.

Stormwater runoff transports organic contaminants from urban areas to receiving water bodies, yet its contribution to these pollutants in the aquatic environment is still poorly understood. Additionally, contaminants behave differently in receiving waters, with some binding to particles and accumulating in sediments while others stay dissolved in the water. This study was carried out three Swedish urban streams receiving stormwater discharges through separate sewer systems, under dry and wet weather conditions. Stream water and bottom sediment samples were collected along an urbanization gradient, from rural upstream to urban downstream sections, and analyzed for 50 stormwater-related organic contaminants to assess the impact of stormwater on contaminant levels. Polycyclic aromatic hydrocarbons (PAHs) and phthalates were more prevalent in sediment samples, with concentrations increasing along the urbanization gradient, indicating contributions from urban areas and stormwater runoff. In contrast, organotin compounds and phenols showed no clear pattern indicating transport through stormwater runoff in the water phase. Per and polyfluoroalkyl substances (PFAS) behaved differently from other contaminant groups by exhibiting a clear contribution from stormwater runoff in both phases. Though carried out in streams passing through relatively small urban settings, the findings clearly demonstrate that stormwater discharges can impact receiving waters. Of the 50 analyzed contaminants, three exceeded toxicity-based limits in dry weather (DW), seven in wet weather (WW), and twenty in bottom sediments. In the water phase, under DW and WW conditions, the three contaminants with the highest exceedance of toxicity-based limits were Perfluorooctanesulfonic acid (PFOS), Tributyltin (TBT), and 4-nonylphenol (4-NP). In the sediment phase, 4‑tert-octylphenol (4-t-OP), Tributyltin (TBT), and di-2-ethylhexyl phthalate (DEHP) were the three compounds with the highest exceedance of toxicity-based limits. Compared to relatively hydrophilic contaminants (e.g., PFAS), hydrophobic organic contaminants, particularly those accumulating in sediments (e.g. phenols, phthalates), posed a greater risk to the aquatic environment with exceedance levels reaching up to 105 times the thresholds. These findings raise concerns about the long-term impact on aquatic environments and highlight the need for mitigation strategies, including regulatory or operational restrictions on the contaminant sources and implementation of stormwater treatment facilities.

Per- and polyfluoroalkyl substances (PFAS) are extensively used in urban environments and are, thus, found in urban stormwater. However, the relevance of stormwater as a pathway for PFAS to urban streams is largely unknown. This study evaluated the impact of urban stormwater runoff on PFAS concentrations and spatial distribution in three urban streams affected by stormwater discharges from separate sewer systems. River water was sampled during dry (DW) and wet weather (WW) upstream, immediately downstream, and further downstream of three urbanized areas with separate sewer systems and with and without point sources (i.e. waste water treatment plant, airports). Water samples were analyzed for 34 targeted PFAS compounds and sediment samples for 35 targeted PFAS and 30 PFAS compounds using a total oxidizable precursor assay. The sum of the quantified PFAS concentrations ranged from the reporting limit (RL) to 84.7 ng/L during DW and increased as the streams were affected by WW discharges (0.87 to 102.3 ng/L). The highest PFAS concentrations were found downstream of urban areas and/or point sources (i.e. airports) during WW, indicating a clear contribution from stormwater discharges. A consistent PFAS contribution from the WWTP was observed under both DW and WW conditions. During WW events, concentrations of perfluorooctanesulfonic acid (PFOS) and total PFAS (PFOA equivalents) exceeded the annual average environmental quality standards, which are an established limit of 0.65 ng/L for PFOS and a proposed limit of 4.4 ng/L for total PFAS. Notably, except for the legacy PFAS, PFOS and perfluorooctanoic acid (PFOA), the most frequently quantified PFAS during DW were short-chain. For WW, long-chain perfluorocarboxylic acids (PFCAs) and a precursor, 6:2 Fluorotelomer sulfonic acid (6:2 FTS), were more frequently quantified, suggesting stormwater is a source of these longer-chain and particle-associated PFAS. The detection of unregulated fluorotelomer sulfonates (FTSs) such as 6:2 and 8:2 FTS during WW suggests a need for regulatory action, as these compounds can degrade into more stable PFAS. In sediment, higher concentrations, and a greater variety of PFAS were found at sites with known point sources i.e. airports. Long-chain PFCAs (C7–C13), perfluoroalkyl sulfonates (PFSAs) (C6), and precursors (i.e. N-Ethyl perfluorooctane sulfonamidoacetic acid), were more prevalent in sediments than in the water. Notably, PFOS concentrations in sediment exceeded the lowest Predicted No-Effect Concentration (PNEC) across sites, posing a potential long-term environmental risk, though current PNECs for other PFAS may underestimate such risks. The findings of the study highlight urban stormwater as a source of PFAS to urban streams indicating the need to minimize PFAS sources in the urban environment and to effectively treat stormwater to protect receiving water bodies.

Blue-green infrastructure (BGI) options are considered to be more sustainable practices for water management and bring arange of benefits over and above water management. Davidshall in Malmö, Sweden, has been used as a case area toassess the multiple benefits of implementing BGI, considering seven alternative BGI schemes systematically developed alongtwo scales: naturalness (i.e. more/less engineered/complex) and spatial distribution (e.g. decentral vs. end-of-pipe). The baseline alternative was the existing situation. The Benefit Estimation Tool (B£ST) was used to carry out a socio-economicassessment. The overall benefits varied significantly (two orders of magnitude), depending on the BGI scheme implemented:the greatest values were associated with natural decentral, natural decentral/end-of-pipe, and engineered decentral/endof-pipe alternatives, those including sub-surface and open dry detention, stormwater tree pits, and rain gardens. The threeB£ST categories providing the greatest benefits were enhancing amenity, benefiting health, and reducing flooding. Cultural ecosystem services were provided by all alternatives, and two alternatives (natural decentral and natural decentral/end-of-pipe)also provided regulating ecosystem services. The study showed that amenity and health were the most significant benefitsof BGI implementation, contrasting with the main aim of BGI implementation, which was stormwater management (water quality and flood protection).

The Schuylkill River Watershed in southeastern PA provides essential ecosystem services, including drinking water, power generation, recreation, transportation, irrigation, and habitats for aquatic life. The impact of changing climate and land use on these resources could negatively affect the ability of the watershed to continually provide these services. This study applies a hydrologic model to assess the impact of climate and land use change on water resources in the Schuylkill River Basin. A hydrologic model was created within the Structural Thinking Experiential Learning Laboratory with Animation (STELLA) modeling environment. Downscaled future climate change scenarios were generated using Localized Constructed Analogs (LOCA) from 2020 to 2040 for Representative Concentration Pathways (RCP) 4.5 and RCP 8.5 emission scenarios. Three regional land use change scenarios were developed based on historical land use and land cover change trends. The calibrated model was then run under projected climate and land use scenarios to simulate daily streamflow, reservoir water levels, and investigate the availability of water resources in the basin. Historically, the streamflow objective for the Schuylkill was met 89.8% of the time. However, the model forecasts that this will drop to 67.2–76.9% of the time, depending on the climate models used. Streamflow forecasts varied little with changes in land use. The two greenhouse gas emission scenarios considered (high and medium emissions) also produced similar predictions for the frequency with which the streamflow target is met. Barring substantial changes in global greenhouse gas emissions, the region should prepare for substantially greater frequency of low flow conditions in the Schuylkill River.

This study investigated how urban stormwater runoff, known to contain various chemical substances, alters pollutant concentrations in the receiving water bodies. Samples were collected under dry and wet weather conditions at 4 sampling stations along the Fyrisån, a river that passes along the city of Uppsala, Sweden. Samples were analyzed for 80 organic substances, 19 metals (total and dissolved phase), and conventional physicochemical parameters. 19 of 80 organic substances were qualified above the limit of quantification (LOQ) in at least one sampling event. The most detected substance family was poly-fluoroalkyl substances (PFAS). The concentrations of detected organic substances and metals increased in wet weather conditions. Organic substance and metal concentrations showed similar spatial variation with higher concentrations measured at sampling locations close to urbanized areas.

Cette étude a examiné comment le ruissellement des eaux pluviales urbaines, connues pour contenir diverses substances chimiques, modifie les concentrations de polluants dans les masses d'eau réceptrices. Des échantillons ont été collectés dans des conditions de temps sec et humide dans 4 stations d'échantillonnage le long de la Fyrisån, une rivière qui passe le long de la ville d'Uppsala, en Suède. Les échantillons ont été analysés pour 80 substances organiques, 19 métaux (phase totale et dissoute), et des paramètres physico-chimiques conventionnels. 19 des 80 substances organiques ont été qualifiées au-dessus de la limite de quantification (LOQ) à au moins un des événements d'échantillonnage. La famille de substances la plus détectée était les substances polyfluoroalkyles (PFAS). Les concentrations des substances organiques et des métaux détectés ont augmenté par temps humide. Les concentrations de substances organiques et de métaux présentaient une variation spatiale similaire, les concentrations les plus élevées étant mesurées aux points d'échantillonnage proches des zones urbanisées.