National Pollutant Removal Performance Database: Version 3

Center for Watershed Protection

The National Pollutant Removal Performance Database v. 2 was recently updated to include an additional 27 studies published through 2006. The updated database was statistically analyzed to derive the median and quartile removal values for each major group of stormwater BMPs. The data are presented as box and whisker plots for the various pollutants found in stormwater runoff. The National Pollutant Removal Performance Database, version 2 (Winer, 2000) consisted of 139 individual best management practice (BMP) performance studies published through 2000. An update of the database has since been conducted to include an additional 27 studies published through 2006. The source information for these additional studies is listed in the References section of this document. The updated database was statistically analyzed to derive the median and quartile removal values for each major group of stormwater BMPs (Figures 1-7). All BMP studies considered for inclusion into the database were reviewed with respect to three target criteria: 1. Five or more storm samples were collected 2. Automated equipment that enabled flow or time-based composite samples were used 3. The method used to compute removal efficiency was documented Pollutant removal efficiency, usually represented by a percentage, specifically refers to the pollutant reduction from the inflow to the outflow of a system. The two most common computation methods are event mean concentration (EMC) efficiency and mass or load efficiency. When more than one method was used to calculate pollutant removal in a specific BMP study, mass or load-based measurements of removal efficiency were entered into the database rather than concentration-based measurements. While EMC efficiency averages the inflow and outflow concentrations for all storm events, it does not account for water volume. Mass efficiency, on the other hand, is influenced by the volume of water entering the BMP and water losses within the BMP (e.g., evapotranspiration and infiltration) (Winer, 2000). This method is based on the sum of incoming and outgoing loads and is considered a more accurate calculation than EMC efficiency, which gives equal weight to both small and large storm events. As a general rule, the concentration-based technique often results in slightly lower performance efficiencies than the mass-based technique.