Realistic exposure scenarios in combined sewer overflows: how temporal resolution and selection of micropollutants impact risk assessment.

Organic micropollutants in combined sewer overflows (CSOs) pose a potential risk to aquatic ecosystems. Previous studies mainly reported event mean concentrations (EMCs) and often focused on a small number of substances. This study presents realistic exposure scenarios using high-temporal resolution (10-minute) data from 24 events at two CSO sites. We analyzed 49 dissolved organic micropollutants for all events and 198 for four events, including pharmaceuticals, pesticides, and road-related compounds, of which we detected 83 substances at least once. From these, we assessed the mixed chemical risk by applying acute quality criteria and evaluated how the risk assessment outcome changes for two aspects: temporal resolution and selection of substances. Our results reveal that total risk quotients (RQ) can vary greatly within CSO events, with 10-minute data capturing peak concentrations that are missed with EMCs. Using EMCs underestimates the maximum RQ of an event by a median factor of 4.9, up to a maximum factor of 6.9. When comparing a selection of 20 substances from the Swiss Waters Protection Ordinance to a broader list of 49 substances commonly detected at CSOs and a comprehensive list of 198 substances, the estimated RQ increases between 1.1 to 2.3-fold. RQ values exceed the threshold of 1 in 75 % of the events, requiring further dilution in the receiving water body. All three pollutant classes (pharma, pesticide, road) drive the total risk, and no specific phase during overflow events consistently poses higher risk than other phases, which challenges the design of effective mitigation measures. Furthermore, the exposure scenarios presented here offer essential input for future ecotoxicological research as they reveal high short-term fluctuations in RQ whose ecological significance is still largely unknown.