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
