Riparian buffer width and nitrate removal in a lagoon-effluent irrigated agricultural area

ABSTRACT:

The ideal buffer width required to maximize water quality benefits while minimizing unnecessary land utilization is difficult to determine. This study examined the effect of increasing buffer width on nitrate (NO₃-N) reduction in shallow groundwater in the middle Coastal Plain of North Carolina. The study site was a streamside buffer zone where previous research determined the buffer did not sufficiently reduce NO₃-N discharge to stream water. Buffer width was increased from 9 to 30 m (30 to 98 ft) by fencing out cattle and allowing volunteer vegetation to emerge, thus forming a grass and shrub buffer along the newly widened area. Buffer functions were assessed by comparing groundwater NO₃-N concentrations from this new system with the same area prior to widening. A significant increase in percent NO₃-N reduction was observed in shallow (0.8 to 1.2 m; 2.6 to 4.0 ft) groundwater on the east (34.7 percent to 95.0 percent; p = 0.02) side of stream, most likely due to an increase (p = 0.01) in dissolved organic carbon (C) at this depth. Reduction at the 30-m (98-ft) width in shallow groundwater at the west side of stream was measured at 93.3 percent, although this was not a statistically significant increase from the 9-m (30-ft) width (53.1 percent; p = 0.2). Percent NO₃-N reduction was less efficient in deep (2.4 to 4.5 m; 7.9 to 14.5 ft) groundwater, although a significant increase (-37.8 percent to 39.2 percent; p = 0.003) at the east side of stream was observed. Reduction on the west side at this depth significantly decreased (19.5 percent to -5.1 percent; p = 0.05) suggesting that the deeper confining layer along this side of stream allowed groundwater to bypass the zone of high dissolved organic C enhanced by riparian vegetation, thus minimizing the effects of buffer widening in this area. Stream sampling indicated no significant difference in upstream and downstream NO₃-N between the two buffer widths (p = 0.61). Although increasing the buffer width did improve reduction in some areas, the severity of the groundwater NO₃-N situation at this site was not completely resolved by increasing the buffer width to 30 m (98 ft).