Sampling and analytical chemistry are costly, and therefore, measurements can only provide limited information in space and time on the occurrence of micropollutants in surface waters. What would the picture look like, if one measured in rivers downstream of all wastewater treatment plants? To make a prediction of pollutant loads from urban drainage a national mass flux model has been developed. For more than 100 wastewater treatment plants, the dilution factor of treated wastewater during dry weather conditions is smaller than 10. For specific substances, this can lead to the exceedance of critical concentrations in rivers. Furthermore, the model allows the calculation of different scenarios. Whole river catchments are taken inter consideration.