Impacts of Impervious Surfaces on Urban Stormwater Quality: Minneapolis Case Study Analysis

Urban stormwater runoff is a major concern for water quality. Impervious surfaces, especially in urban environments, can allow stormwater direct access to receiving waterbodies and make up nearly 90% of land cover in downtown Minneapolis, Minnesota. A study of stormwater runoff from impervious surfaces in downtown Minneapolis, Minnesota, USA, was conducted to understand the potential impacts of different types of impervious surfaces (i.e., streets, sidewalks, parking lots, and rooftops). A rainfall simulator delivered water to the street, sidewalk, and parking lot sites and the rooftop runoff characteristics were studied separately using automated samplers and rain gauges. ANOVA statistical analysis was used to determine whether stormwater runoff pollutant concentrations (chloride, total suspended solids, and total phosphorus) varied significantly within surface types, between surface types, and across seasons. Results showed that the first flush of runoff contained higher pollutant concentrations than the whole rain event, and water quality differences for all surfaces were relatively minor for the summer and fall seasons. In contrast, pollutant concentrations were significantly higher in the spring, particularly on streets. Among all surface types, streets exhibited the highest event-mean concentrations (EMCs) for all pollutants. The study highlights the importance of surface-specific stormwater management strategies and the need for tailored BMP design and policies to enhance the effectiveness of mitigating water quality impairments in urban environments.