Denitrification in nitrate-contaminated groundwater: Occurrence in steep vertical geochemical gradients

A relatively narrow vertical zone (5–6 m thick) of NO 3 − containing groundwater was identified using multilevel sampling devices in a sand and gravel aquifer on Cape Cod, MA, USA. The aquifer has been chronically contaminated by surface disposal of treated sewage 0.3 km upgradient from the study area. The NO 3 − zone was anoxic and contained high concentrations of N 2O (16.5 μM), suggesting that it was a zone of active denitrification. Denitrifying activity was confirmed with direct measurement using acetylene block incubations with aquifer core material; the peak rate was 2.4 nmol N reduced (g sed) −1 day −1. Concentrations of dissolved inorganic carbon and N 2 were close to atmospheric equilibrium in uncontaminated groundwater, but were more than 2 times higher within the contaminant plume. Excess CO 2 and N 2 suggested in situ formation with a stoichiometry of C and N mineralized via denitrification of 0.8 (C/N). Denitrification within the aquifer resulted in an increase in the natural δ 15 N of NO 3 − (from +13.6 to +42.0%.) and the N 2 produced, with an isotopic enrichment factor, ϵ, of −13.9%.. Vertical profiles of NH 4 + and δ 15 N of NH 4 + indicated that dissimilatory reduction of NO 3 − to NH 4 + was also occurring but mass balance calculations indicated that denitrification was the predominant process. These results demonstrate that a combination approach using field mass balance, stable isotope analysis, and laboratory incubations yields useful insight as to the significance of denitrification in aquifer sediments and that closely spaced vertical sampling is necessary to adequately quantify the processes controlling C and N transport and transformation within these environments.