Assessment of Urban Stormwater Chloride and its Impact on Surface Water Trends

Abstract

This project examined chloride in stormwater and in urban streams and lakes in Minnesota to better understand the factors that influence chloride concentrations, long term trends, and seasonal dynamics. The project built on a large Twin Cities metro stormwater monitoring dataset assembled by the team in a previous project by incorporating stream and lake monitoring datasets and spatial data across the state. Analyses of this substantial dataset (over 100,000 observations of chloride in surface waters and runoff) provided insights into spatial patterns of stormwater chloride, and seasonality and long-term trends in Minnesota lakes and streams. We found strong, increasing long-term trends in lakes and streams, with higher chloride in surface waters associated with greater urban land cover and road density. Hydrologic flow paths impacted warm season chloride concentrations, with generally low chloride in stormwater runoff but much higher in baseflow and groundwater-dominated streams, while lakes showed contrasting patterns of accumulation versus seasonal flushing that depended on lake hydrology and morphology. We used long-term data to provide an assessment of chloride pollution risk in lakes and streams, identifying 80 lakes and streams at high or moderate risk that are not currently included on the MPCA’s lists of impaired and at-risk waters. Project results can help watershed managers better understand the where and when of lake and stream vulnerability to chloride pollution and provide a foundation for future studies of methods to assess un-monitored surface waters and of understanding the impact of stormwater management practices on chloride transport in urban watersheds.