NITRATE-N DISTRIBUTION AND TRENDS IN SHALLOW GROUNDWATER ON AN EASTERN COASTAL PLAINS WATERSHED

Nonpoint source pollution from agriculture has been a major concern, p articularly where intensive agricultural operations exist near environ mentally sensitive waters. To address these nonpoint source pollution concerns, a Water Quality Demonstration Project (WQDP) was initiated o n the Herrings Marsh Run (HMR) watershed in Duplin County North Caroli na. The WQDP was implemented to determine water quality benefits from voluntary adoption of improved management practices. In the WQDP, 84 g roundwater monitoring well sites were established on 21 farms selected to represent the major farming practices on the watershed. On the HMR watershed nitrate-N contamination of groundwater was not a wide sprea d problem. Seventy-four percent of the groundwater monitoring sites ha d nitrate-N less than the drinking water standard of 10 mg/L. Mean nit rate-N concentrations were below 10 mg/L on 16 of the 21 farms. Of the four farms with nitrate-N exceeding 10 mg/L, one farm had mean nitrat e-N that exceeded 20 mg/L. This farm had an undersized and overloaded swine wastewater spray field. After the spray field was expanded and a pplication rates were reduced, groundwater nitrate-N concentrations de clined; but they continued to exceed 20 mg/L. Other farms with swine w aste spray fields had mean groundwater nitrate-hr concentrations <20 m g/L throughout the study period. Groundwater nitrate-N concentrations under row crops were <10 mg/L on all but two farms. Three of the four farms with nitrate-IV concentrations exceeding 10 mg/L were in a subwa tershed of the HMR that had the highest concentration of animal waste application and excess nitrogen applied. Of the 21 farms, three farms had a significant increasing trend in groundwater nitrate-N while four farms had a significant decreasing trend. The overloaded swine wastew ater spray field had a significant decreasing nitrate-N trend. Most fa rms with concentrations less than 10 mg/L had no detectable trend in n itrate-N concentration during the study. These findings indicate that nitrate-N contamination of groundwater is not a widespread problem on the HMR watershed even though it is intensively farmed.