Reliability of NPDES Derived Loads in Urban Runoff

The US EPA mandates municipalities with populations greater than 100,000 to collect representative water quality data for their urban storm drainage basins and to subscribe to National Pollution Discharge Elimination System (NPDES) permitting procedures. Event Mean Concentrations (EMC) are determined by monitoring selected subcatchments that represent the dominant land uses. EMCs are multiplied by runoff volumes (e.g. from the Rational Method) to get loads for the entire drainage basin. A case study was conducted in watersheds in the Jefferson and Orleans Parishes of Louisiana to help answer the question: "How reliable are load estimates that are extrapolated from land use based monitoring of small subcatchments?" Both basins have separated sewer systems, pumped drainage, high subsidence (possibly causing cross-flow between sanitary and storm systems) and similar land use and topography. However, Orleans is an older area than Jefferson. NPDES monitoring studies were conducted at residential, commercial and industrial subcatchments in both Parishes. The EPA SWMM4.3 model was calibrated for each of the land use based subcatchments in Jefferson Parish and then these land use based parameters were applied to a large drainage basin in the adjacent Orleans Parish to check the transferability of these calibration parameters. Yu measured the load entering Pumping Station No. 4 (PS#4). It was found that hydrologic factors could be transferred using land use characteristics; however, this was not the case for nutrient loads. An explanation for this failure could be the difference in the age of the development of the two study areas. To account for the possible age effect, the SWMM model was recalibrated using the NPDES monitoring data for Orleans Parish and gave better agreement with measured nutrient loads. A rain chemistry study in the study area showed that a large fraction of the nutrient input is attributable to atmospheric deposition.