National Water-Quality Assessment (NAWQA) Program
Go to:
By Larry J. Puckett and Timothy K. Cowdery
[Journal of Environmental Quality, vol. 31, no. 3, p. 782-796 ]Abstract
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