Impact of suburbanization on ground water quality and denitrification in coastal aquifer sediments

The South Carolina coastal plain is currently facing rapid population growth and suburbanization. Suburbanization brings the potential for surface- and ground water contamination from the use of nitrogen-based fertilizers, which can render water toxic to humans and fish, and lead to eutrophication. Additionally, nitrate is highly mobile in sediments and poses the potential for contamination of receiving waters, downstream areas, and ground water. The objectives of this study were to evaluate the differences in ground water quality and sediment denitrification rates at two sites, an undeveloped forested area (Oyster Creek, North Inlet, SC) and an area which has been developed for residential and commercial use (Dog Creek, Murrells Inlet, SC). Ground water monitoring wells were installed at the two sites at several sampling depths ranging from 0.6 m to 5 m. Ground water samples were collected every 4-8 weeks for 16 months, and analyzed in the field for pH, conductivity, temperature, and dissolved oxygen (DO), and in the laboratory for nitrate, nitrite, ammonia, phosphate and total organic carbon (TOC). Additionally, sediment samples were collected from two locations in both creek bottoms from approximately 1.0 m depth, and microbial denitrification was estimated using the acetylene block technique by measuring the accumulation of nitrous oxide (N2O). Ground water at both sites was microaerophilic, ranging from 0.4 to 1 mg O-2/l. Ammonia and TOC concentrations were significantly higher at the forested site due to higher inputs of organic matter in the form of leafy vegetation, whereas nitrate concentrations were significantly higher at the suburban site. Sediments from both sites were able to rapidly convert NO3 to N2O with progressive depletion of NO3 in extracted sediments. Both the rate of N2O production and the conversion efficiencies were found to increase with increasing nitrate concentrations from 0.1 to 0.5 mg/g. The smallest nitrate concentration had the lowest N2O production and NO3 conversion efficiency. However, for the intermediate treatment (0.25 mg/g) conversion efficiencies were variable. In addition to potential increased NO3 inputs, increased drainage for development present at suburban sites may cause aeration of near channel soils and favor the oxidized, more mobile form of nitrogen. Because the suburban site has steeper hydraulic gradients, and nitrate is highly mobile, there is potential for both nitrate transport to the estuary and accumulation in the shallow water-table aquifer at the suburban site. However, it appears that the microbial communities from both sites were well adapted to denitrifying inputs of nitrate in the concentration ranges tested. (C) 1997 Elsevier Science B.V.