Evaluating the Effectiveness of Two Stormwater Wetlands in North Carolina

Abstract

Two constructed stormwater wetlands in North Carolina were monitored to document their efficiencies at reducing nitrogen, phosphorus, and sediment loads in runoff from urban areas. Inflow to and outflow from the two wetlands were monitored during at least ten storm events, and flow-proportional samples of each were collected and analyzed. The first wetland (CMS), located in the Piedmont region, had a relatively low wetland surface to drainage area ratio of 2.2%, while the second (UNC), located in the mountains region, had less storage and a wetland surface to drainage area ratio of 4.7%. The UNC wetland had a much more diverse and extensive plant community. For the CMS wetland, the median load reduction efficiencies of nitrogen forms ranged from 47% to 54%, while the corresponding reductions for the UNC wetland ranged from 57% to 71%. The median reduction efficiencies for phosphorus forms for the CMS wetland ranged from 59% to 76%, while those for the UNC wetland ranged from -95% to 70%. The median reduction efficiencies for TSS were 72% and 88% for the CMS and UNC wetlands, respectively. The median load reduction efficiencies of nitrogen forms, total phosphorus, and sediment for the UNC wetland were greater than for the CMS wetland and were considerably greater than corresponding median efficiencies of other completed studies. These results indicate that both of these wetlands were, in general, more efficient at reducing pollutant loading than many others reported in the literature. The greater efficiencies may be the result of a relatively high surface area to drainage area ratio and a diverse plant community in the UNC wetland and a considerable storage capacity of the CMS wetland.