A whole series of hazardous substances may be emitted from urban surfaces during rainfall and washed out into the receiving waters. Among these are various biocides which are used in large amounts in renders and wall paints against algae and fungi growth. A rainfall-discharge model combined with an experimentally based leaching model has been developed and used in a real urban catchment to simulate concentration and load dynamics of the biocides Terbutryne and Cybutryne in facade runoff and in the receiving water. It also allowed predicting statistical information on ecotoxicological risks in the receiving water. The results reveal that for both biocides the concentrations in the receiving water exceed acute and chronic quality standards fairly often, especially in small rivers. The most severe case was identified to be an area with new or newly renovated buildings discharging the surface runoff through separate sewers directly into the receiving water.