An Investigation of the Performance of a Constructed Wetland in Treating Urban Stormwater

Abstract

This study investigated the pollutant removal performance of a constructed wetland treating stormwater runoff from a residential townhome complex in Manassas, Virginia. The facility was constructed by retrofitting a dry detention basin to retain a permanent shallow pool and included additional temporary storage to detain roughly the first half inch of rainfall for approximately 24 hours. Vegetation was allowed to establish itself through volunteer colonization, rather than active planting of selected wetland species. Flow measurements showed substantially greater volume passing through the outlet than entering through the single gaged inlet. The extra volume (about 41% of the total) was attributed to ungaged overland flow which drained a wooded/grassy area adjacent to the site. Mass balance calculations employing the rational method with a runoff coefficient of 0.2 to estimate the flow from this area showed good agreement between long-term total estimated inflow and measured outflow. However, this method was not effective in accounting for the discrepancies between inlet and outlet volumes of individual storms. Thirty-three runoff events were monitored between April, 1996 and May, 1997. Because of greater flow volumes passing through the outlet, constituent mass calculations which ignored the overland contribution generally exhibited higher loads exiting than entering the facility. With the results from a limited number of grab samples representing concentrations in overland input, estimated efficiencies improved substantially, showing overall net removal for most constituents. Less than one year after being retrofitted, the basin showed signs of beginning to develop a diverse wetland flora.