Evaluation of Storm-Water Wetlands in Series in Piedmont North Carolina

Three storm-water wetlands in series were monitored in a heavily urbanized 12.5 ha watershed in Mooresville, North Carolina. Monitoring of this system allowed an examination of the diminishing returns provided by three successive best management practices (BMPs) of a similar type. At least 80% of the total concentration reduction for all pollutants occurred within the first wetland cell. Only the first wetland cell significantly (p<0.05) reduced all pollutants tested. No pollutant was significantly reduced from the outlet of Wetland Cell 2 to the outlet of Wetland Cell 3 (p<0.05). Median complete system (outlet of Wetland Cell 3) effluent concentrations for total suspended solids, total phosphorus, total nitrogen, and turbidity were 8, 0.09, 0.73 mg/L, and 10 NTU, respectively, which compared favorably to published results. Organic nitrogen generated from wetland vegetation seemed to result in a background source of nitrogen in the wetlands, supporting the idea of an irreducible concentration for nitrogen in these systems. The results indicate that the successive BMPs in a series do not perform as well as the first when each BMP uses similar removal mechanisms.