Stream nitrogen changes in an eastern Coastal Plain watershed

Agricultural nonpoint source pollution (NPS) is a major water quality concern throughout the United States and the world. Concerns over agricultural nonpoint source pollution are heightened where intensive agricultural operations exist near environmentally sensitive waters. To address these environmental concerns, a water quality demonstration project involving federal, state, and local agencies, private industry, and local landowners was initiated in 1990 on the Herrings Marsh Run watershed in the Cape Fear River Basin in Duplin County, North Carolina. Best management practices (BMPs) to reduce nutrient losses to the environment included nutrient and animal waste management plans, soil conservation practices, and an in-stream wetland (ISW). Stream nitrate-N and ammonia-N were measured at the watershed outlet and at three subwatershed outlets from 1990-1998 to evaluate the effectiveness of the best management practices. The project was divided into pre-in-stream wetland (September 1990-May 1993) and post-in-stream wetland (June 1993-December 1998) time periods because the majority of the best management practices were implemented at the time of the in-stream wetland establishment. Post-in-steam wetland stream nitrate-N concentrations were significantly reduced on the watershed (56%) and on each of the three subwatersheds (4% to 56%). The watershed nitrate-N concentrations were reduced from 2.01 to 0.88 mg/L (ppm). One subwatershed had stream nitrate-N concentrations reduced from 5.63 to 2.74 mg/L (ppm). Nitrate-N mass export from the watershed was significantly reduced on an annual basis from 7.14 to 3.88 kg/ha (6.37 to 3.46 lb/ac). Ammonia-N concentrations and mass export from the watershed were unchanged from the pre- to post-in-stream wetland periods. The results of this study indicate that the implemented best management practices were effective in reducing nitrogen loss from the Herrings Marsh Run watershed.