River nitrate gains from groundwater discharge at waste application fields. Neuse River, North Carolina.

Abstract

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.