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Nearly all rural inhabitants and livestock in the Upper Republican Natural Resources District (URNRD) in southwestern Nebraska use ground water that can be affected by elevated nitrate concentrations. The development of ground-water irrigation in this area has increased the vulnerability of ground water to the introduction of fertilizers and other agricultural chemicals. In 1998, the U.S. Geological Survey, in cooperation with the Upper Republican Natural Resources District, began a study to characterize the quality of ground water in the Upper Republican Natural Resources District area with respect to physical properties and concentrations of major ions, coliform bacteria, nitrate, and pesticides, and to assess the presence of nitrogen concentrations in the unsaturated zone. At selected well sites, the ground-water characterization also included tritium and nitrogen-isotope analyses to provide information about the approximate age of the ground water and potential sources of nitrogen detected in ground-water samples, respectively.
In 1998, ground-water samples were collected from 101 randomly selected domestic-well sites. Of the 101 samples collected, 26 tested positive for total coliform bacteria, exceed¬ing the U.S. Environmental Protection Agency’s Maximum Contaminant Level (MCL) of zero colonies. In 1999, ground-water samples were collected from 31 of the 101 well sites, and 16 tested positive for coliform bacteria.