Nitrogen-15/14 isotope ratios (δ¹⁵N) were measured in groundwater, infiltrating wastewater, and natural uncontaminated soil to determine the source of high NO₃^- concentrations in two desert basins at Fort Irwin National Training Center in the Mojave Desert. The abundance and isotopic composition of dissolved nitrogen gas were measured in some of the groundwater samples to determine the effects of denitrification on the abundance and isotopic composition of the NO₃^-. Delta ¹⁵N-NO₃ values in groundwaters most likely to include sewage effluent ranged from about 11 to 23‰. Delta ¹⁵N values in groundwater not near sources of sewage effluent ranged from 6 to 11‰, a range similar to that found in the overlying soils, which contain large concentrations of leachable NO₃^- presumed to be of natural origin. Some of the samples with relatively low NO₃^- concentrations and high δ¹⁵N values contained excess non-atmospheric nitrogen gas attributed to denitrification. These data indicate that high NO₃^- concentrations in groundwater unaffected by sewage effluent may result from leaching of naturally occurring nitrogen in desert soils, and that some of the NO₃^- may be removed by denitrification at the water table.

Nitrogen-15/14 isotope ratios (δ¹⁵N) were measured in groundwater, infiltrating wastewater, and natural uncontaminated soil to determine the source of high NO₃^- concentrations in two desert basins at Fort Irwin National Training Center in the Mojave Desert. The abundance and isotopic composition of dissolved nitrogen gas were measured in some of the groundwater samples to determine the effects of denitrification on the abundance and isotopic composition of the NO₃^-. Delta ¹⁵N-NO₃ values in groundwaters most likely to include sewage effluent ranged from about 11 to 23 per mil. Delta ¹⁵N values in groundwater not near sources of sewage effluent ranged from 6 to 11 per mil, a range similar to that found in the overlying soils, which contain large concentrations of leachable NO₃^-presumed to be of natural origin. Some of the samples with relatively low NO₃^- concentrations and high δ¹⁵N values contained excess non-atmospheric nitrogen gas attributed to denitrification. These data indicate that high NO₃^- concentrations in groundwater unaffected by sewage effluent may result from leaching of naturally occurring nitrogen in desert soils, and that some of the NO₃^- may be removed by denitrification at the water table.