Nitrogen-enriched groundwater has been proposed as an important anthropogenic source of atmospheric nitrous oxide (N{sub}2O), yet few measurements of N{sub}2O in large aquifer systems have been made. Concentrations of N{sub}2O in water samples collected from the 124 000 km{sup}2 central High Plains aquifer in 1999 ranged from < 1 to 940 nM, with a median concentration of 29 nM (n = 123). Eighty percent of the N{sub}2O concentrations exceeded the aqueous concentration expected from equilibration with atmospheric N{sub}2O. Measurements of N{sub}2O, NO{sub}3+, and {sup left}3H in unsaturated-zone sediments, recently recharged groundwater, and older groundwater indicate that concentrations of N{sub}2O in groundwater increased over time and will likely continue to increase in the future as N-enriched water recharges the aquifer. Large concentrations of O{sub}2 and NO{sub}(3-) and small concentrations of NH{sub}(4+) and dissolved organic carbon in the aquifer indicate that N{sub}2O in the central High Plains aquifer was produced primarily by nitrification. Calculations indicate that the flux of N{sub}2O from the central High Plains aquifer to the atmosphere from well pumping and groundwater discharge to streams was not a significant source of atmospheric N{sub}2O.
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