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Nitrogen-enriched groundwater has been proposed as an important anthropogenic source of atmospheric nitrous oxide (N 2 O), yet few measurements of N 2 O in large aquifer systems have been made. Concentrations of N 2 O in water samples collected from the 124 000 km 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 2 O concentrations exceeded the aqueous concentration expected from equilibration with atmospheric N 2 O. Measurements of N 2 O, NO 3 -, and 3 H in unsaturatedzone sediments, recently recharged groundwater, and older groundwater indicate that concentrations of N 2 O 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 2 and NO 3 -and small concentrations of NH 4 + and dissolved organic carbon in the aquifer indicate that N 2 O in the central High Plains aquifer was produced primarily by nitrification. Calculations indicate that the flux of N 2 O 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 2 O.

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