Occurrence and turnover of nitric oxide in a nitrogen-impacted sand and gravel aquifer

Abstract

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 (NO 3 − ) gradient and characterized by low dissolved oxygen (<10 μM). NO concentrations correlated best with nitrite (NO 2 − ) (up to 35 μM), but nitrous oxide (N 2 O) (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 15 N[NO] demonstrated that oxidation to 15 N[NO 2 − ] occurred only during the initial stages, but after 4 h reduction to 15 N[N 2 O] was the primary reaction product. Added NO 2 − (31 μM) or NO 3 − (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.