Calculation of stormwater contaminant loads in Knivsta and suggested measures

Urban stormwater is an important pathway for the transport of nutrients, metals, and other pollutants to receiving waters, especially in rapidly developing municipalities. The aim of this thesis was to assess the potential contribution of stormwater to the pollution load in Knivsta municipality, Sweden, and to identify and prioritize the technical catchments with the greatest potential impact on local water bodies. The assessment is based on stormwater quality modelling using the StormTac model. The study shows that stormwater in Knivsta potentially constitutes a significant source of both nutrients and metals to several already impacted recipients. The results indicate that pollutant levels are generally high and that a limited number of technical catchments account for a large share of the total potential load. In particular, five areas were identified as critical due to consistently elevated modelled concentrations of multiple pollutants and a high proportion of impervious surfaces. Differences in runoff pathways and the presence or absence of treatment measures strongly influence the actual pressure on the receiving waters. Based on the modelling results, the thesis concludes that targeted stormwater management measures are necessary in the areas with highest levels of contaminants. A combination of new treatment facilities, improved maintenance of existing systems, and supplementary field sampling is considered essential to reduce the impact of stormwater and to support the long- term goal of achieving good ecological and chemical status in accordance with the EU Water Framework Directive.