Browsing by Author "R. Wayne Skaggs, Member"

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Effect of Riparian Buffers and Controlled Drainageon Shallow Groundwater Quality in the North Carolina Middle Coastal Plain
(2000-11-15) Dukes, Michael Dale; Robert O. Evans, Chair; J. Wendell Gilliam, Member; R. Wayne Skaggs, Member; John E. Parsons, Member
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.
The Use of Constructed Wetlands to Remove Nitrogen and Phosphorus from Pumped Shallow Groundwater
(2002-01-31) Cook, Michael J.; Robert Evans, Chair; R. Wayne Skaggs, Member; Garry Grabow, Member; Stephen Broome, Minor Representative, Member
Non-point pollution has received significant attention nationwide with seepage from lagoons one potential source. The research presented here discusses the hydrology and shallow groundwater quality associated with leakage from an old, unlined lagoon located in the Middle Coastal Plain of North Carolina. Ammonium-N concentrations from wells installed between the lagoon and a nearby stream averaged 121 mg/L, with the highest concentrations exceeding 170 mg/L. Mean annual NH4-N concentrations in the stream ranged from 10 to 25 mg/L indicating that the seepage plume was reaching the stream. A water control structure was installed down stream of the lagoon to reduce the hydraulic gradient towards the stream. In addition, the lagoon was closed out in April 2001. The hydraulic gradient has decreased from 0.023 m/m when the lagoon was in production to 0.0026 m/m since closure. Over a 33-month pre- and post- closure period, NH4-N concentrations in wells 15 m down gradient of the lagoon have decreased from 121 mg/L to 96 mg/L. A series of pumping wells were installed in the seepage plume to remove and route the contaminated groundwater to a 0.35 ha constructed wetland for treatment. Inflow and outflow of the wetland were continuously monitored to determine nutrient loading and reduction rates. Fourteen monthly mass balances were computed to compare the inflow and outflow of the wetland and to assess monthly nutrient reduction for TKN, NH4-N, NO3-N, TP and OP. Overall, greater than 79 % of the nitrogen and 26 % of the phosphorus were assimilated on a mass basis while concentrations decreased by more than 87 % across all nutrient species. Oxidation-reduction, air and water temperature, pH, and dissolved oxygen were measured weekly at several locations within the constructed wetland. Regression analyses were conducted to examine the relationship between these parameters and monthly nutrient reductions. Nutrient export from the wetland was positively correlated to water temperature (i.e., nutrient export increased as water temperature increased). In general, lower redox and DO were correlated to higher nutrient levels within the wetland and subsequently to higher export from the wetland. Caution should be taken on interpretation of these regression analyses as the conditions in the wetland changed over the course of the study. The loading rate was doubled at the beginning of the first full growing season (i.e., the loading rate was two times higher during the growing season than the previous dormant season). Another factor that likely impacted N and P assimilation is many plants in the upper portion of the wetland died in July and August of 2001. The hydrology of the site was evaluated using MODFLOW-GMS. MODFLOW was calibrated using water level data from the site and was used to evaluate the influence of water levels in the adjacent channelized stream on the movement of contaminants in the groundwater plume. Model results indicated a decrease in the hydraulic gradient from the former lagoon from 0.0045 m/m in the free drainage case to 0.0027 m/m in the controlled drainage case. From the gradient calculations, travel time of the seepage plume to the stream increased from 380 days free drainage scenario to 640 days in the controlled drainage case. MODFLOW analysis of the pumping wells indicated that 6.3 gpm was required in the free drainage mode to reverse the gradient from the stream and capture the seepage plume. In the controlled drainage mode, 4.7 gpm was required to reverse the direction of groundwater flow. A hydrologic analysis was also conducted to evaluate pumping requirements to mitigate an actively leaking lagoon. Simulations were performed using an interceptor drain (French drain) adjacent to the lagoon for collection of seepage discharge from the lagoon. Under controlled drainage, the interceptor drain collected 51.5 m3/day (9.4 gpm) while under free drainage 67.4 m3/day (12.4 gpm) would be collected. This analysis assumed a worst-case scenario where all wastewater deposited in the lagoon was lost to seepage. The research presented here provides strategies for clean-up of leaking lagoons or those lagoons targeted for closure. Overall, the wetland assimilated 383 kg of total nitrogen and 60 kg of total phosphorus during the 14 month study period. The wetland surface area was originally based on a pumping rate of 1.5 gpm which was the estimated seepage rate of the lagoon. In March 2001, the pumping rate was increased to 3.5 gpm to match growing season ET rates. Initial results indicated that the wetland assimilated this increased flux of nutrients; but as loading continued at this rate, nutrient concentrations in outflow began to increase, suggesting that this higher rate exceeded the assimilative capacity of the wetland. The MODFLOW analysis indicated that a pumping rate of 4.7 to 6.3 gpm was required to reverse the groundwater gradient from the stream. These pumping rates would require a wetland surface area three to four times larger than the wetland area used. The mass balance indicated that phosphorus mass exports were higher than imports for the final two months. From this, further study is needed to determine processes that are affecting removal of phosphorus. Plant uptake was responsible for only 9 % (35 kg) of the N and 18 % (11 kg) of the P assimilated. Oxidation-reduction reactions were judged to be the dominant nitrogen reduction mechanism. Considering the importance of plants in providing conditions conducive for nutrient assimilation, further investigations are needed to determine the cause of the plant die-off. The influence of temperature, dissolved oxygen and redox on nutrient assimilation should be further documented under conditions of constant loading.