Nitrate (NO2+NO3–N) in ground water of the Upper Snake River basin, Idaho and western Wyoming, 1991–95

Factors related to contamination of ground water by dissolved nitrite plus nitrate as nitrogen (NO₂+NO₃-N) in parts of the upper Snake River Basin were evaluated at regional and local scales. Regional-scale relations between NO₂+NO₃-N concentrations and depth to first-encountered ground water, land use, precipitation, and soils were evaluated using a geographic information system. Local-scale relations between NO ₂+NO₃-N concentrations and other nutrients, major ions, nitrogen isotopes, stable isotopes, and tritium in five areas with different hydrogeologic settings, land use, and sources of irrigation water were evaluated to determine the factors causing differences in NO₂+NO₃-N. Data were collected and analyzed as part of the U.S. Geological Survey's National Water-Quality Assessment Program, which began in 1991.