Stormwater Pollution Management at a Foundry Site – Monitoring, Modeling, and Design

Stormwater pollution from heavy manufacturing sites is generally regulated under federal and state regulations. Stormwater management requirements for these sites are often prescriptive, and not based on site-specific stormwater quality conditions. The actual pollutant reductions achieved for these sites are often based on literature values or predictive modeling. A heavy manufacturing site in Wisconsin was found to be in violation of the state guidelines (based on USEPA benchmarks) for total suspended solids, aluminum, nitrate/nitrite, COD, copper and zinc concentrations. These violations were based on simple "grab" sampling conducted once per year at the site's three outfalls. Through discussions with the Wisconsin Department of Natural Resources (WDNR), the manufacturer agreed on a course of action that included conducting event based monitoring at an outfall (recording rain and flow gauge; and automatic event sampling). The purpose of this monitoring was to: 1) better characterize the quality of the stormwater, 2) verify that permit violations were in fact occurring based on more sophisticated sampling, and 3) use the results of the monitoring to help design appropriate BMPs. Key to the monitoring program was the inclusion of particle size distribution analyses for the events. The particle size distribution found in the runoff has a significant influence on the type of BMP necessary to achieve the desired concentration levels at the storm sewer outfalls. A total of nine events were monitored in the fall of 2004 and spring of 2005. The monitoring data was also used to calibrate the urban pollution model: WinSLAMM. The model was used to evaluate various pollution control approaches. The regulatory agency agreed to use model results as evidence of BMP effectiveness. Based on the results of the monitoring and modeling, proprietary and non-proprietary (engineered/designed) BMPs were evaluated for the three storm sewer outfalls. Unlike most conventional BMP installation, the recommended structures for these outfalls were based on comprehensive monitoring data unique to the site and the BMPs were designed to capture the types of pollutants distinctive to the heavy manufacturing site. A series of underground settling vaults, lift stations, and a pond were selected and designed. Construction of the BMPs is scheduled for 2007.