Groundwater nitrate reduction processes in a riparian buffer enrolled in the NC conservation reserve enhancement program

In an effort to maximize water quality benefits of buffers enrolled in the NC Conservation Reserve Enhancement Program, several buffers were monitored for nitrate in groundwater and in the protected stream. One buffer monitored was located in the upper coastal plain of North Carolina. This buffer averaged 50 meters wide and consisted of 3 zones; a hardwood buffer near the stream, a wide loblolly pine zone, and a grassed zone at the field/buffer interface. Poultry litter was applied by the landowner to a pasture upslope from the grass buffer. Three blocks of groundwater wells, each containing three transects, were installed at shallow depths (1.5-2 m) and deep depths (2.5-3.0 m) and have been monitored for nutrients since 2005. Stream surface water samples were also collected at three points inside the buffer during this time. In June 2008, groundwater wells with depths ranging from 4.2-9.6 m were also installed in the upland and within each block. The mean nitrate concentration at the pasture edge before entering the buffer was 8.4 mg/L for shallow wells and 7.7 mg/L for deep wells. Mean nitrate concentrations at the stream edge were 1.3 mg/L and 1.4 mg/L for shallow and deep wells respectively (an 85% and 82% reduction). Based on several criteria, this decrease in nitrate concentration is thought to be due to denitrification. Pending analysis of data from the wells installed in 2008 should indicate if some of the reduction is due to mixing of groundwater from deeper depths.