Browsing by Author "William Showers, Committee Chair"

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    Nitrate Transport in Shallow Flow Systems at the Neuse River Waste Water Treatment Plant
    (2006-04-23) Fountain, Matthew; William Showers, Committee Chair
    In recent years a number of studies have indicated that riparian buffers have a high efficiency of nitrogen removal from shallow groundwater flow systems. However, little work has focused on establishing what relative effect field drainage streams may have on allowing contaminated water to bypass these riparian buffer systems. Even less attention has been focused on how storm event hydrology may affect these systems in terms of nitrate flux. Three groundwater monitoring well transects were installed in a riparian buffer and a weir flow control structure was installed on a surface drainage bordering a waste application field at the Neuse River Waste Water Treatment Plant. This application field has been in use for over 20 years. Well water and surface water were sampled for a year beginning in February 2005. Samples were analyzed for nitrate, chloride, silicate, ammonium, and phosphate concentrations as well as natural abundance nitrate-nitrogen, nitrate-oxygen, water-oxygen, and water-hydrogen stable isotope ratios. Water quality measurements were made through the transition from dormant to growing season and from high to low water table gradients and elevations. The summer and fall of 2005 were a time of extreme drought in the region and allowed examination of low flow conditions and system recovery to normal flow patterns. The buffer groundwater ion concentrations and nitrogen isotope compositions remained extremely consistent throughout the different conditions. Nitrate concentration 10 m inside the buffer averaged 33.7 mgN/L and 20 m farther into the buffer at wells 6 m from the river edge averaged 0.30 mgN/L, for a nitrate reduction of 99.1% after factoring in rainwater dilution (approximately 34.8%) calculated from chloride data. This study indicates that even shallow groundwater systems at 3-5m of depth can experience high efficient nitrogen removal by denitrification in riparian buffer zones. The surface drainage system transitioned from low flow rates and nitrate concentrations in the dry summer and fall, to high flow rates and high nitrate concentrations in the higher water table elevations of the winter months. During rain events significant nitrate flux increases were noted in the drainage system. The drainage was found to be exporting an amount of nitrogen much larger than that of a proportionally loaded and sized buffer system, indicating that significant nitrogen is bypassing the riparian buffer strategy.
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    River nitrate gains from groundwater discharge at waste application fields. Neuse River, North Carolina.
    (2009-12-03) Reyes, Manuel Miguel; William Showers, Committee Chair; DelWayne Bohnenstiehl, Committee Member; David Genereux, Committee Member
    Nitrate is a common pollutant in rivers worldwide. Samples of groundwater discharging into the Neuse River adjacent the Neuse River Wastewater Treatment Plant (NRWWTP) waste application fields (WAFs) have been measured as high as 86 mg NO3/L. The NC surface water standard is 10 mg NO3/L (N.C. Division of Water Quality, 2007). Geophysical surveys of the NRWWTP WAFs revealed diabase dikes intruded into the granitic bedrock. Groundwater flow is occluded across these dikes; however, fractures around the contact metamorphic zone along these dikes are likely candidates for high groundwater flow. At the NRWWTP, four seasonal sampling periods were conducted at a dike/river crossing in the NW, and one sampling period at another crossing in the E. Samples were analyzed for NO3, NH4, Si, and Cl concentrations. At each sample point, parameters of hydraulic conductivity (K), hydraulic gradient (j) were measured yielding specific discharge (v) and nitrate flux. A ‘hot spot’ of nitrate flux discharging into the river was found downstream of the dike at the NW reach. Using sampling data from each period, a multiquadric radial basis function with anisotropy interpolation was computed and prediction maps for NO3, Si, Cl conc., j, K, v, riverbed elevation, and nitrate flux were made. Assuming an accurate interpolation, the nitrate flux at this ‘hot spot’ was measured in some hydrologic conditions to discharge amounts of nitrate as high as 27 kg NO3/d over an area of 1181.25 m3. This nitrate has an isotopic signature of bio-solid nitrate with denitrification in some areas. In areas of high nitrate, dissolved organic carbon (DOC) concentration was found to be a controlling factor for denitrification.