Occurrence and Turnover of Nitric Oxide in a Nitrogen-Impacted Sand and Gravel Aquifer

Little is known about nitric oxide (NO) production or consumption in the subsurface, an environment which may be conducive to NO accumulation. A study conducted in a nitrogen-contaminated aquifer on Cape Cod, Massachusetts assessed the occurrence and turnover of NO within a contaminant plume in which nitrification and denitrification were known to occur. NO (up to 8.6 nM) was detected in restricted vertical zones located within a nitrate (NO3-) gradient and characterized by low dissolved oxygen (< 10 microM). NO concentrations correlated best with nitrite (NO2-) (up to 35 microM), but nitrous oxide (N2O) (up to 1 microM) also was present. Single-well injection tests were used to determine NO production and consumption in situ within these zones. First-order rate constants for NO consumption were similar (0.05-0.08 h(-1)) at high and low (260 and 10 nM) NO concentrations, suggesting a turnover time at in situ concentrations of 10-20 h. Tracer tests with 15N[NO] demonstrated that oxidation to 15N[NO2-] occurred only during the initial stages, but after 4 h reduction to 15N[N2O] was the primary reaction product. Added NO2- (31 microM) or NO3- (53 microM) resulted in a linear NO accumulation at 2.4 and 1.0 nM h(-1) for the first 6 h of in situ tests. These results suggest that NO was primarily produced by denitrification within this aquifer.