A chemical fate model for screening impacts from wet-weather discharges on Danish streams

Assessing the impacts of micropollutants released via urban wet weather discharges (combined sewer overflows and separate storm sewers) often requires resource-intensive monitoring and modelling activities. We present a screening tool to facilitate impact assessment and prioritize pollution reduction investments, utilizing public available data sources. The model is based on simple mass balances (i.e. dilution) and it includes six chemical fate processes for the pollutant transport in the streams. The model estimates the fate of 39 micropollutants in 1791 Danish streams. These streams were screened for a potential exceedance of Environmental Quality Standards. Sensitivity analysis and uncertainty analysis were performed to investigate the impact of the different sources of uncertainty on the results and importance of the different fate processes. The model outcomes indicates that > 40% streams potentially exceed the Maximum Allowed Concentrations for some heavy metals (Cu, Zn) and PAHs as consequence of wet-weather discharges. Considering the simple structure of the proposed approach, and the use of open data, the proposed approach can be transferred to other countries and regions. This will eventually help to prioritize the often limited resources for reducing the environmental impacts of urban wet weather discharges on the natural water environment.