Modelling the risk of rainfall events leading to momentary pollution levels exceeding maximum allowed concentrations - A Swedish case study of urban runoff in the Fyris river

Abstract: The purpose of this study was (1) to study the proportion (X) of the flow in a watercourse that consists of urban runoff during a rain event and (2) to evaluate the risk that a few chosen pollutants, transported by urban runoff, exceed the maximum allowed concentration in the watercourse according to the environmental quality standards (MAC-EQS). The Fyris river in Uppsala, Sweden, was selected as a case study. Urban runoff quickflow was estimated with a water balance model using precipitation data and flow data from three stations. Precipitation data was used to identify 31 rain events with a minimum rain volume of 10 mm and at least a maximum rain intensity of three mm/h during the study period 2017-2020. Pollutants in urban runoff were sampled during the winter of 2020-2021. The highest concentrations obtained during sampling were used to estimate momentary pollution concentration and to evaluate the risk of exceeding MAC-EQS. The highest X found during a rain event was 71%. Low flow conditions in the river prior to a rain event in summertime are circumstances when X can be expected to be high. It is therefore advised to include rain events under such circumstances when monitoring MAC-EQS or sampling momentary pollution concentrations in the Fyris river. The pollutant category polycyclic aromatic hydrocarbons (PAH), and especially the pollutant fluoranthene, showed risk of momentary pollution concentration exceeding MAC-EQS. Therefore, the author recommends that future studies of urban runoff should include PAHs.