River Bio-Solid Nitrate Gains from Groundwater Discharge at WAFs

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. Geophysical surveys of the WAFs revealed diabase dikes intruded into the bedrock. Groundwater flow is occluded across the dikes; however, fractures along the dikes are likely areas of high groundwater flow. At the NRWWTP, four sampling periods were conducted at a dike/river crossing. Samples were analyzed for NO3, NH4, Si, and Cl concentrations. At each sample point, parameters of hydraulic conductivity and hydraulic gradient were measured yielding specific discharge and nitrate flux. A hot spot of nitrate discharging into the river was found downstream of the dike at the NW reach. Using sampling data from each period, an interpolation was computed and prediction maps were made. 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 some denitrification.