Effect of Riparian Buffers and Controlled Drainageon Shallow Groundwater Quality in the North Carolina Middle Coastal Plain
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2000-11-15
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Degradation of water quality in the streams and estuaries of North Carolina in recent years has resulted in regulations to reduce the introduction of numerous types of contaminants to this system. In the Neuse and Tar-Pamlico River Basins, excessive amounts of nitrogen have been identified as causing increased algal growth, low dissolved oxygen concentrations, and have been linked to increased growth of toxic microorganisms such as Pfiesteria piscicida. There are numerous sources of nitrogen to the basins; however, agricultural nonpoint sources have been identified as the largest contributor of nitrogen. Riparian buffers, controlled drainage, and nutrient management have been identified as effective BMPs for reducing nitrogen transport to streams under many landscape conditions. As a result, a combination of nutrient management, controlled drainage, and riparian buffer best management practices have been mandated in the Neuse River Basin to reduce the loss of agricultural nonpoint source pollution. A large portion of the agricultural nonpoint source nitrogen losses to surface waters in the Neuse River Basin originate in the Middle Coastal Plain. These lands are drained by irregularly spaced first and second order streams that have often been channelized (i.e. deepened) to enhance drainage. The riparian vegetation has often been removed from these channelized streams. The effectiveness of riparian buffers and controlled drainage are not well documented under these landscape conditions that are common in the Middle Coastal Plain region. Controlled drainage may not be economical in this region because multiple control structures would be required to maintain a suitable water table elevation in this gently sloping landscape. Implementation of riparian buffers has met strong resistance from the agricultural community due to the potential loss of land. A few studies have also found that nitrogen rich groundwater may enter deeply incised or channelized streams below the active treatment zone of the buffer, rendering the buffer ineffective. A study to evaluate the effect of riparian buffer vegetation type and width on shallow groundwater quality was implemented at the Center for Environmental Farming Systems near Goldsboro, North Carolina. The effect of controlled drainage, riparian buffers, and a combination of both was studied. The hydrologic portion of the riparian ecosystem management model (REMM) was evaluated and tested against field measurements.Five riparian buffer vegetation types were established as follows: cool season grass (fescue), deep-rooted grass (switch grass), forest (pine trees), native vegetation, and no buffer (no-till corn and rye rotation). These vegetation types were established at two buffer widths perpendicular to the channelized streams, 8 m (25 ft) and 15 m (50 ft). In addition, a continuous native vegetation buffer under free drainage and a continuous no buffer treatment under controlled drainage was established. For about 50% of the time monitored, the 15 m riparian buffer plots resulted in a statistically lower NO3-N concentration in the mid depth ditch wells (screen depth 1.5-2.1 m below ground surface) compared to the 8 m plots. Width was not a statistically significant variable at the deep well depth (2.1-3.5 m screen depth). Vegetation type had no statistically significant effect on NO3-N concentration. Nitrate concentration decreased 69 and 28% as groundwater flowed beneath the 8 m wide riparian buffer plots toward the channelized streams and 84 and 43% in the 15 m plots, at the deep and mid depth, respectively. The wider buffers were approximately 15% more effective at removing nitrate, but the improvement was not linearly correlated to the width increase. The primary reason vegetation differences were not observed was likely due to the limited time for vegetation establishment and development during the relatively short 2.5 year study period. Five years or longer may be required for some types of vegetation to mature to the point of impacting the nitrogen in the shallow groundwater. Furthermore, differences in localized groundwater flow paths and soil physical and chemical properties may indefinitely over shadow vegetation effects at this site. Controlled drainage did not raise the water table elevation near the ditch as compared to 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 0.96 and 1.45 m for the combination and controlled drainage treatments, respectively. Again, the lack of hydrologic treatment effect may be due to localized differences in soil properties and groundwater flow paths. Percent NO3-N concentration decrease for those treatments was 22 and 35%, 75 and 51%, and 77 and 69%, for the deep and mid depth wells, for each respective treatment. Although more nitrate was apparently removed from the groundwater on the controlled drainage treatments, this effect could not be correlated to water table depth.Daily predicted water table depth from the riparian ecosystem management model (REMM) was compared to observed depths over a simulation period of two years. The model performed well during some periods but poorly during large storm events. Average absolute errors ranged from 150 to 650 mm. Model instability during large storm events and anomalies in evapotranspiration calculations must be addressed before this model can be a reliable planning tool for regions such as the Middle Coastal Plain of North Carolina.Based on this research, several recommendations for further study are presented. Monitoring of riparian buffer vegetation plots should continue with the expectation that vegetation may have a significant impact over time as the vegetation types become established. Eventually the vegetation in the buffer will reach a steady state with respect to nitrogen in the buffer; however, this may take many years. Quantification of the relative proportion of dilution and denitrification for a given nitrate concentration decrease beneath the buffers should be investigated. One approach would be installation of redox probes at the deep well depth to give an indication if conditions are favorable (i.e. reducing) for denitrification. Also, the deep groundwater (i.e. below the impermeable layer) chemistry should be compared to the shallow groundwater chemistry to determine the relative proportions of constituents such as calcium and magnesium. This analysis would give an indication if dilution of the shallow groundwater were occurring as a result of deep groundwater upwelling. The REMM simulations may be improved by measuring groundwater velocity into the riparian buffer, improving the estimates of surface water runoff into the riparian buffers, and by modifying the model to simulate a single buffer zone rather than three.
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PhD
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Biological and Agricultural Engineering
