Nitrate transport in shallow flow systems at the Neuse River Waste Water Treatment Plant
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.
School:North Carolina State University
School Location:USA - North Carolina
Source Type:Master's Thesis
Keywords:marine earth and atmospheric sciences
Date of Publication:04/23/2006