Occurrence of nitrous oxide in the central High Plains aquifer, 1999

Nitrogen-enriched groundwater has been proposed as an important anthropogen ic source of atmospheric nitrous oxide (N2O), yet few measurements of N2O i n large aquifer systems have been made. Concentrations of N2O in water samp les collected from the 124 000 km(2) central High Plains aquifer in 1999 ra nged from < 1 to 940 nM, with a median concentration of 29 nM (n = 123). Ei ghty percent of the N2O concentrations exceeded the aqueous concentration e xpected from equilibration with atmospheric N2O. Measurements of N2O, NO3-, and H-3 in unsaturated-zone sediments, recently recharged groundwater, and older groundwater indicate that concentrations of N2O in groundwater incre ased over time and will likely continue to increase in the future as N-enri ched water recharges the aquifer. Large concentrations of O-2 and NO3- and small concentrations of NH4+ and dissolved organic carbon in the aquifer in dicate that N2O in the central High Plains aquifer was produced primarily b y nitrification. Calculations indicate that the flux of N2O from the centra l High Plains aquifer to the atmosphere from well pumping and groundwater d ischarge to streams was not a significant source of atmospheric N2O.