Evaluating Bioretention Areas from Two Field Sites in North Carolina

Abstract

Due to recent fish kills along the Mid-Atlantic coast, new regulations require all future land disturbing activities — including the urbanization or development of land — to be environmentally friendly. Consequently, stormwater management practices are being designed to remove nutrients, particularly nitrogen and phosphorus, before runoff reaches streams. One stormwater practice, bioretention, is gaining popularity in commercial development because it can easily be sited in the required natural areas of parking lot medians; however, the current design criteria used for bioretention have not been tested extensively. Pollutant removal, especially for species of nitrogen, has not been tested under various conditions. Currently used bioretention monitoring protocol assumes bioretention devices retain little water. Current designs do not reduce nitrate-nitrogen levels sufficiently, as bioretention is constructed without any zone designed to be saturated. For nitrate-nitrogen to be converted to nitrogen gas, thus enhancing total nitrogen (TN) removal, an anaerobic zone may be necessary. This research determined the effect of an anaerobic layer within bioretention devices on the concentrations and loadings of TN, nitrate-nitrogen (NO 3 -N), and other nutrient and pollutant species in stormwater runoff including ammonia-nitrogen (NH 3 -N), total kjeldahl nitrogen (TKN), total phosphorus (TP), ortho-phosphate (Ortho-P), zinc (Zn), iron (Fe), copper (Cu), lead (Pb), and total suspended solids (TSS). Four field bioretention areas were installed and monitored in two North Carolina cities. The field sites were used to test the pollutant removal abilities and hydraulic retention of bioretention devices that ranged in depth from 0.6 m (2 ft) to 1.2 m (4 ft) and were designed with or without an anaerobic layer. The field studies in North Carolina confirmed high annual TN mass removal rates at two conventionally drained bioretention cells (40% reduction). NO 3 -N mass removal rates varied between 75% and 13%. The presence of an anaerobic zone was tested at the Greensboro field site and significantly (p<0.10) reduced TP and Fe outflow concentrations. Annual metal mass removal rates at the Greensboro site were over 98% for both Zn and Cu and 81% for Pb. A very large mass of Fe (nearly 17 Kg from a 0.20-ha (0.5-acre) watershed was added to the storm sewer network by the bioretention cell. TP and TSS removal rates were highly variable from cell to cell.