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 μM). NO concentrations correlated best with nitrite (NO2−) (up to 35 μM), but nitrous oxide (N2O) (up to 1 μM) 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 μM) or NO3− (53 μM) 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.
