Assessment of Underground Stormwater Management Devices on the Water Quality of Runoff

Project summary:

Many technologies have been developed to treat runoff and to remove suspended and dissolved contaminants. Various levels of government spend millions of dollars on implementation of storm water treatment practices, many of them commercial due to their small footprints, and so guidance is needed for evaluating the performance of storm water treatment practices and technologies. A storm water treatment evaluation and monitoring protocol was developed under funding from the Minnesota Pollution Control Agency (MPCA), and so the project objective was to apply and test the evaluation protocol on three storm water treatment sites assessing the pollutant removal effectiveness at each site. The project improved the effectiveness of storm water treatment evaluation and implementation through detailed assessment of performance, and ensured that the types of sites owned by Minnesota cities and counties were fully integrated into the evaluation protocol. The field evaluation methodology consisted of a series of tests on three underground structures selected from a list of possible sites in the Minneapolis-St. Paul metropolitan area. In these tests, several sediment gradations (sizes) were fed into the system at predefined concentrations and flow rates over a specified period of time, and the sediment collected in the systems were filtered, dried, and weighed to determine the true removal efficiency of each structure. A literature search helped determine correlations between specific pollutants (such as heavy metals and phosphorus) and sediment particle size. The results helped determine the efficiency of the tested structures in removing phosphorus and heavy metals and showed that controlled field tests are a practical, robust, and accurate means of determining an underground device's performance based on solid size distribution, density of the influent, and water discharge and temperature. This premise was successfully verified in field tests on four devices and in laboratory tests on two devices. The resulting protocol and test results are useful tools for consultants, manufacturers, local governments, and state agencies for selecting, sizing, and evaluating storm water treatment technologies to protect water resources.