Effect of Riparian Buffer Width and Vegetation Type on Shallow Groundwater Quality

Abstract

Agricultural nonpoint sources are a large contributor of nitrogen to many rivers in the Eastern United States, including the Neuse River in North Carolina. Best Management Practices (BMPs) such as nutrient management, riparian buffers, and controlled drainage have been shown effective at reducing nutrient transport from agricultural fields under certain landscape conditions. As a result, recent regulations in North Carolina have mandated that agricultural operations must use a combination of BMPs to reduce the loss of nitrogen. The Middle Coastal Plain is characterized by intensive agriculture on sandy soils with deeply incised or channelized streams. The effectiveness of riparian buffers has not been well evaluated for this landscape. This study was conducted to compare the effect of riparian buffer vegetation type and width on shallow groundwater quality in the Middle Coastal Plain of North Carolina. Five riparian buffer vegetation types were established as follows: cool season grass (fescue), deep-rooted grass (switch grass), forest (pine and mixed hardwood), native vegetation, and no buffer (no-till corn and rye rotation or pasture). These vegetation types were established at two buffer widths, 8 m and 15 m, perpendicular to channelized streams for a total of 10 plots on each of six stream replicates (ranging from intermittent to perennial). Each plot was 24 m long parallel to the stream. A groundwater monitoring well nest was installed at the field/buffer edge and the stream edge in the middle of each riparian buffer plot. Wells were installed at three well depths per well nest. Most deep, middle, and shallow wells were 3.0 m, 1.8 m, and 0.6 m deep from the ground surface to the top of the perforated section, respectively. The perforated section was 0.6 m long. Land use adjacent to the riparian buffer plots was agricultural and included beef cattle pasture along one replicate stream, dairy cattle pasture along two replicates, and row crop agriculture along three replicates. Wells were sampled for 23 months beginning July 1997. Nitrate nitrogen concentration was significantly lower (alpha = 0.1) on approximately half the sampling dates at the middle well depth on the 15 m wide riparian buffer plots compared to the 8 m wide plots. Buffer width was not a significant variable at the deep well depth. Effect of vegetation was not significant at any time. Nitrate removal from the groundwater was greater and less temporally variable in the deep wells than the mid depth wells. For plots where flow was toward the stream and dilutional effects taken into account, nitrate concentration decreased 69 and 28% as groundwater flowed beneath the 8 m wide riparian buffer plots toward the ditch; 84 and 43% beneath the 15 m plots, in the deep and mid depth wells, respectively. Overall, the 15 m wide buffers were approximately 15% more effective than the 8 m buffers at groundwater nitrate removal, while vegetation did not seem to play a key role. Reasons may be attributed to soil and hydrologic variability on the site, immaturity of vegetation, and differences in localized groundwater flow paths. Results of this study indicate that riparian buffer effectiveness was closely linked to the site hydrology. Establishment of buffers along streams where groundwater flowed away from the stream did not result in lower groundwater nitrate levels. Implementation of riparian buffers without knowledge of the site hydrology may lead to minimal water quality benefits.