National Water-Quality Assessment (NAWQA) Program
By Larry J. Puckett and Timothy K. Cowdery[Journal of Environmental Quality, vol. 31, no. 3, p. 782-796 ]
A combination of ground-water modeling, chemical and dissolved gas analyses, and chlorofluorocarbon age dating of water was used to determine the relation between changes in agricultural practices, and NO3- concentrations in ground water of a glacial outwash aquifer in west-central Minnesota. The results revealed a redox zonation throughout the saturated zone with oxygen reduction near the water table, NO3- reduction immediately below it, and then a large zone of ferric iron reduction, with a small area of sulfate (SO42-) reduction and methanogenesis (CH4) near the end of the transect. Analytical and NETPATH modeling results supported the hypothesis that organic carbon served as the electron donor for the redox reactions. Denitrification rates were small, 0.005 to 0.047 mmol NO3- yr-1, and were limited by the small amounts of organic carbon, 0.01 to 1.45 percent. In spite of the organic carbon limitation, denitrification was virtually complete because residence time is sufficient to allow even slow processes to reach completion. Ground-water sample ages showed that maximum residence times were on the order of 50-70 years. Reconstructed NO3- concentrations, estimated from measured NO3- and dissolved nitrogen gas showed that NO3- concentrations have been increasing in the aquifer since the 1940s have been above the 714 mmol L-1 maximum contaminant level at most sites since the mid- to late-1960s. This increase in NO3- has been accompanied by a corresponding increase in agricultural use of fertilizer, identified as the major source of NO3- to the aquifer.
Table of Contents