Browsing by Author "J. Wendell Gilliam, 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.
Phosporus Loss in Surface Runoff from Piedmont Soils Receiving Animal Manure and Fertilizer Additions
(2001-09-26) Tarkalson, David Dale; Robert L. Mikkelsen, Chair; J. Wendell Gilliam, Member; John E. Parsons, Member; Eugene J. Kamprath, Member
The purpose of this research was to measure P losses in runoff from agricultural land in the Piedmont region of the southeastern U.S. with varying soil P levels and receiving broiler litter and inorganic P fertilizers. The experimental results will be helpful for the development of the P Loss Assessment Tool in North Carolina and other P Index approaches in states with similar soil characteristics and crop management practices. A net influx of P into many areas due to high animal populations has resulted in increased potential P losses to sensitive surface waters. A typical North Carolina broiler farm and dairy farm were found to have annual P surpluses of 65 kg P/ha and 20 kg P/ha respectively. The use of low phytic acid corn varieties and phytase enzyme has the potential to reduce the P surplus on broiler farms by 25 to 58%. Phosphorus losses in runoff from Piedmont conventional till (CT) and no-till (NT) soils with varying soil P concentrations and from soils currently receiving broiler litter and fertilizer P applications were assessed. In these studies, rainfall simulation at rates of 6 and 7.6 cm/hr were utilized to collect runoff samples from crop land with a range of initial P concentrations and from plots with varying fertilizer P and broiler litter application rates, both incorporated and broadcast. Runoff samples were collected at 5-min intervals for 30 min and analyzed for reactive P (RP), algal-available (AAP), and total P (TP). Concentration of RP in runoff from CT and NT plots was positively correlated with Mehlich-3 extractable P (r2 = 0.61 and 0.7 respectively) and oxalate extractable degree of P saturation (DPS) (r2 = 0.6 and 0.61 respectively). However, only TP mass loss (kg TP/ha) in runoff from CT was correlated with DPS (r2 = 0.57). A Mehlich 3 extractable P concentration of 350 mg P/kg and a DPS of 84% corresponded to 1 mg RP/L in runoff. Incorporation of broiler litter and inorganic P fertilizer into the soil at all P application rates virtually eliminated P runoff loses and had similar P losses in runoff as the unfertilized control. Surface application of broiler litter resulted in runoff containing between 2.9 and 24.5 mg RP/L for application rates of 8 to 82 kg P/ha respectively. Mass loss of TP in runoff from surface-applied broiler litter ranged from 1.3 to 8.5 kg P/ha over the same application rates. There was no significant relationship between surface applied inorganic P application rate and RP concentrations or TP mass losses in runoff. However, there was a trend for increased RP concentrations and TP mass losses in runoff with increasing application rate. Concentration of RP and mass loss of TP in runoff from surface applied inorganic P averaged 4.9 mg RP/L and 1.1 kg P/ha over all application rates. There was no significant difference between P losses in runoff from plots receiving surface applied conventional broiler litter and broiler litter derived from birds fed a low phytic acid corn (High Available P corn).
Quantification and Modeling of In-Stream Processes in Agricultural canals of the lower coastal plain
(2000-08-09) Birgand, Francois; R. Wayne Skaggs, Chair; J. Wendell Gilliam, Member; John E. Parsons, Member; JoAnn M. Burkholder, Member
Excess nutrient loads have been recognized to be the major cause of serious water quality problems recently encountered in the North Carolina estuaries and coastal waters. There has been a particular concern in coastal watersheds because agricultural and forested lands are located in close proximity to recreational and environmentally sensitive waters. The key to nutrient management at the watershed scale is the understanding and quantification of the fate of nutrients at the field scale and after they enter the aquatic environment. There is no accepted method to describe and predict fate of nutrients in canals and streams. The purpose of this research was to investigate the magnitude of the effects of in-stream processes in agricultural canals of the lower coastal plain and to propose a modeling approach for quantifying nitrogen transformations in such canals. This was accomplished in four steps.The first step was an extensive review of the literature on nitrogen retention in agricultural streams. Nitrogen removal rates in most agricultural canals and streams vary between 50 and 800 mg N/m²/d, with mass transfer coefficient varying between 0.01 and 0.10 m/d. The magnitude of nitrogen retention in streams and canals of agricultural watersheds has been reported to vary between less than 5% to more the 60% of the gross load. In the second step, the effects of biogeochemical processes on chemical and nutrient loads was evaluated in a 1125-m long agricultural canal reach of the lower coastal plain near the town of Plymouth, NC. Chemical and nutrient loads at both ends of the reach were measured by continuous measurement of flow and concentrations. Flow measurements were made using trapezoidal flumes in which flow velocity and depth was continuously measured and recorded with velocity meters. Nutrient concentrations were measured on water samples taken both manually and automatically at strategic times along the hydrographs so that linear interpolation between two consecutive samples could be made. Nutrient addition due to seepage along the reach was estimated. After corrections for lateral contribution, it was estimated that, over the 14-month measuring campaign, 3% of the total nitrogen load entering the upstream end was retained within the reach. This was mostly due to the combination of nitrate retention and release of organic nitrogen (ON) within the reach. Up to 10.2 % of the total phosphorus load measured at the upstream station was retained while 10% of the total suspended solids was also retained. There was a release of inorganic carbon equal to 18.7% more that the load measured at the upstream end.Measurements of algae and macrophyte biomass within the reach, and, measurements of nitrogen and carbon concentration profiles at the sediment-water interface revealed that most of nitrate retention was likely due to denitrification after diffusion from the water-column to the sediment. Release of organic nitrogen was attributed to flux of refractory organic nitrogen from the sediment into the water-column. Assimilation by algae and macrophytes may have accounted for as much as 20% of the total retention of inorganic nitrogen. Rates of nitrate removal and release of organic nitrogen were estimated using the model DUFLOW. Nitrate removal rates varied between 200 and 800 mg NO3-N/m²/d, while release rates of organic nitrogen varied between 100 and 400 mg ON/m²/d. A mass transfer coefficient of 0.3 m/d was obtained for nitrate at two distinct periods of the year.A simple approach was proposed for modeling nitrogen transformations in canals of the lower coastal plain. Transformations are simplified as the combination of downward diffusion of water-column nitrate into the sediment and an upward diffusion of organic nitrogen from the sediment.