In-stream wetland mitigation of nitrogen contamination in a USA coastal plain stream

Nonpoint source N from riverine origin is a major water quality problem throughout the world. Nitrogen removal from a contaminated (6.6 mg L-1, NO3-N) stream was evaluated in this study using an instream wetland (ISW). The ISW was established at the exit of a 425-ha USDA Water Quality Demonstration watershed in the Coastal Plain of North Carolina,It ranged in depth from about 0.2 to 2 m, and it was <1% (3.3 ha) the size of the watershed. The ISW dramatically lowered mean stream NO3-N from 6.6 to 2.0 mg L-(1.) Nitrate-N mass removal was highly correlated to inflow NO3-N (r = 0.93) in the warmer months when biological processes were more active. Ammonia-N mass removal was opposite that of NO3-N. It was highly correlated to inflow NH3-N (r = 0.81) during the cooler months. Removal of both NO3-N and total-N (NO3-N + TKN) were positively correlated to temperature with r values of 0.77 and 0.62, respectively. Total annual N removal for the ISW was approximately 3 kg ha(-1) d(-1), which was about 37% of the inflow N. The ISWs appear to be very good landscape features for mitigating excess nonpoint source N in the southeastern Coastal Plain of the USA. As such, they are a good complement to other best management practices for improved water quality.