Results of Analyses of the Expanded EPA/ASCE National BMP Database

The US EPA/ASCE National BMP Database has grown significantly since the first evaluation of BMP performance data in the database was completed in 2000. The project team is currently performing a 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. Although this analysis has not been fully completed, several initial results are presented in this paper. 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, and vaults. 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, and 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. 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 (with wet pools) vs. the average size storm event volume monitored has been established, showing that those with a ratio of 1 or have been observed to achieve better effluent quality.