Analyses of the Expanded EPA/ASCE International BMP Database and Potential Implications for BMP Design

Abstract

The US EPA/ASCE National Stormwater BMP Database has grown significantly since the first evaluation of BMP performance data in the database was completed in 2000. The project team has conducted a limited re-evaluation of the data contained in the database to assess the overall performance of BMPs as well as compare BMP design attributes to performance. The evaluations have included the assessment of various BMP types as categorized in the database with regards to their ability to reduce runoff volumes as well as the effluent quality they can achieve. It is apparent that certain BMP types can reduce the volumes of runoff through soil soaking and resulting evapotranspiration and/or infiltration as opposed to BMPs that are more “sealed,” such as wet ponds, wetlands, vaults and hydrodynamic devices. Runoff reductions contribute to pollutant loadings reductions as does improved effluent quality. On average, dry detention basins were found to reduce runoff volumes by an average of 30 percent (comparison of inflow to outflow), while biofilters reduced volumes by almost 40 percent. As expected, wet ponds, wetlands, hydrodynamic devices, and retention ponds show little or no runoff volume reductions. BMP types vary with regards to effluent quality that is achieved. BMPs such as wet ponds and wetlands appear to achieve lower concentrations in effluent quality than other BMPs such as detention ponds (dry) and hydrodynamic devices for some parameters. These differences vary with pollutant type. Finally, analyses of BMP design vs. performance are beginning to result in statistically valid results. For example, a relationship (ratio) between the treatment volume of retention ponds (wet ponds) vs. the average size measured storm event volume monitored has been established, showing that those with a ratio of 1 or more have been observed to achieve better effluent quality. Based upon these findings, this paper discusses potential BMP selection, design, and BMP design standards implications. A set of potential recommendations for how communities develop and specify BMP requirements such that more local goals for improved water quality is discussed.