Analysis of stormwater runoff from impervious surfaces in downtown Minneapolis, MN

Abstract

Urban stormwater runoff is a major concern for water quality. Impervious surfaces, especially in urban environments, can allow contaminated stormwater direct access to receiving waterbodies. Impervious surfaces make up nearly 90% of land cover in downtown Minneapolis, Minnesota. When rain falls or snow melts, pollutants quickly transfer from those surfaces into nearby waterways. A study of stormwater runoff from impervious surfaces in downtown Minneapolis, Minnesota USA was conducted to understand potential impacts of different types of impervious surfaces (i.e., streets, sidewalks, parking lots and rooftops). The results of this study could be used to inform urban stormwater management strategies, particularly when the makeup of the area is mostly impervious surfaces. Between summer 2017 and spring 2018, a rainfall simulator was used to deliver water upon street, sidewalk, and parking lot sites, which removed differences in rainfall characteristics, and tested the role of varying surface types and seasonal differences. Characteristics of rooftop runoff were studied using natural rainfall and snowmelt event data collected year-round with automated samplers and rain gauges. Results showed that the first flush of runoff contained higher pollutant concentrations compared to the whole rain event, and water quality differences for all of the surfaces were relatively minor for the summer and fall seasons. The greatest difference was observed with higher pollutant concentrations occurring in the spring for all sites, particularly on streets. Higher than expected concentrations of chloride in the winter occurred from roofs, though concentrations were overall much smaller than the ground sites in the spring. Street event mean concentrations (EMCs) were the highest across different stormwater constituents, including chloride, total phosphorus, and total suspended solids. For each stormwater constituent, when the average EMC value was used for calculating pollutant loading instead of individual surface type EMC values, street contributions were underestimated, and the other surface types were overestimated due to the higher pollutant concentrations from streets than other impervious surfaces.