In‐Stream Wetland Mitigation of Nitrogen Contamination in a USA Coastal Plain Stream

Abstract

Abstract 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 , NO 3 ‐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 NO 3 ‐N from 6.6 to 2.0 mg L −1 . Nitrate‐N mass removal was highly correlated to inflow NO 3 ‐N ( r = 0.93) in the warmer months when biological processes were more active. Ammonia‐N mass removal was opposite that of NO 3 ‐N. It was highly correlated to inflow NH 3 ‐N ( r = 0.81) during the cooler months. Removal of both NO 3 ‐N and total‐N (NO 3 ‐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.