Managing Stormwater Pollutants from a Heavy Manufacturing Site: Monitoring, Modeling, and Design

When applying a pollutant concentration limit to a stormwater discharge many issues must be clarified to demonstrate compliance. This paper summarizes how a modeling approach was used to meet pollutant concentration requirements in stormwater. Neenah Foundry, Neenah, Wisconsin was in violation of the state guidelines for total suspended solids, aluminum, nitrate/nitrite, copper, and zinc stormwater concentrations. These violations were based on "grab" sampling conducted once per year at the site's three outfalls. Through discussions with the Wisconsin Department of Natural Resources (WDNR), a course of action was developed that included conducting event based monitoring at an outfall and incorporation of the monitoring information into a model. The monitoring: 1) characterize the site's stormwater pollutant concentrations, 2) verified that permit benchmarks were in fact being exceeded, and 3) provided data (including particle size distribution) to calibrate a pollutant loading model (WinSLAMM) for use in the management practice design. Through application of this approach the regulatory agency accepted a modeled pollutant concentration target expressed as an average annual concentration for permit compliance purposes.