Water Quality Improvement by a Small In-Stream Constructed Wetland in North Carolina's Coastal Plain

Wetlands have shown promise as a stormwater best management practice (BMP). Current recommendations suggest wetland surface areas be at least 1% of the contributing watershed area to ensure significant water quality improvement. Most urban areas are severely limited with regard to available land areas for wetland creation, thus, to be practical, smaller wetland:watershed area ratios will be necessary in many locations. A one ha (2.4 acre), in-stream constructed wetland was built to intercept drainage waters from approximately 240 hectares (600 acres) of agricultural and urban watershed, which resulted in a wetland:watershed area ratio of 0.004:1. The wetland was instrumented to monitor hydrology and chemistry. Monitoring began in early 1996 and continued through December of 1999. Water quality samples were analyzed for total kjeldahl nitrogen (TKN), ammonium-nitrogen (NH₄-N), nitrate-nitrogen (NO₃-N), total phosphorus (TP), and ortho-phosphorous (OP). Over the monitoring period, NO₃-N concentrations were reduced by +60% NH₄-N by +30%, and TKN by 9.5%. Total and ortho-phosphorous levels in increased 55%. Yearly and seasonal means indicate that significant improvements in NO₃-N and NH₄-N concentrations can be achieved with relatively small wetlands.