Effect of Riparian Buffers and Controlled Drainage on Shallow Groundwater Quality

Abstract

As a result of recent surface water quality problems in North Carolina, riparian buffersand controlled drainage are being used to reduce the loss of nonpoint source nitrogen. Theeffect of controlled drainage and riparian buffers as best management practices (BMPs) toreduce the loss of agricultural nonpoint source nitrogen from the Middle Coastal Plain has notbeen well documented. A two-year study was conducted to determine the effectiveness ofcontrolled drainage, riparian buffers, and a combination of both in the Middle Coastal Plain ofNorth Carolina. It was thought that raising the water table near the ditch might enhance NO3-Nremoval. Controlled drainage did not raise the water table near the ditch to a greater degree than observed on the free drainage treatment. Over seventeen storm events, the riparian buffer(free drainage) treatment had an average groundwater table depth of 0.92 m, compared to 0.96and 1.45 m for the combination (riparian buffer and controlled drainage) and controlled drainagetreatments, respectively. Treatment locations were assigned randomly. Strictly by chance, thefree drainage treatment was installed along a ditch with a shallower impermeable layercompared to the impermeable layer on the controlled drainage treatments (i.e. 2 m versus 3-4 mdeep). Percent NO3-N concentration decrease between the field/buffer wells and ditch edgewells for those treatments averaged 22 and 35%, 75 and 51%, and 77 and 69%, for the deepand mid depth wells, in each respective treatment. The distance from the ground surface to thetop of the well screen ranged 2.1-3.5 m for the deep wells and 1.5-2.1 m for the mid depth wells,with most at 3.0 m and 1.8 m, respectively. Although apparently more nitrate was removed fromthe groundwater on the controlled drainage treatments, the controlled drainage treatment watertable was not raised closer to the ground surface compared to the free drainage treatment.Nitrate removal effectiveness was attributed to local soil and landscape properties. Differencesin local soil properties between the combination riparian buffer and controlled drainagetreatment and controlled drainage only were not obvious; however, the impermeable layer onthe free drainage treatment was closer to the surface compared to the other treatments.