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

Journal Article · Mon Jul 01 00:00:00 EDT 1991 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(91)90026-2· OSTI ID:5910670

Smith, R L ^[1]; Howes, B L ^[2]; Duff, J H ^[3]

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1. Geological Survey, Lakewood, CO (United States)
2. Woods Hole Oceanographic Inst., MA (United States)
3. Geological Survey, Menlo Park, CA (United States)

A relatively narrow vertical zone (5-6 m thick) of NO{sub 3}{sup {minus}} 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{sub 3}{sup {minus}} zone was anoxic and contained high concentrations of N{sub 2}O (16.5 {mu}M), 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){sup {minus}1} day{sup {minus}1}. Concentrations of dissolved inorganic carbon and N{sub 2} were close to atmospheric equilibrium in uncontaminated groundwater, but were more than 2 times higher within the contaminant plume. Excess CO{sub 2} and N{sub 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 {delta}{sup 15}N of NO{sub 3}{sup {minus}} (from +13.6 to +42.0{per thousand}) and the N{sub 2} produced, with an isotopic enrichment factor, {epsilon}, of {minus}13.9{per thousand}. Vertical profiles of NH{sub 4}{sup +} and {delta}{sup 15}N of NH{sub 4}{sup +} indicated that dissimilatory reduction of NO{sub 3}{sup {minus}} to NH{sub 4}{sup +} 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.

• OSTI ID: 5910670
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Vol. 55; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
58 GEOSCIENCES
AQUIFERS
WATER POLLUTION
GROUND WATER
DENITRIFICATION
MASSACHUSETTS
ANAEROBIC CONDITIONS
CARBON
ECOLOGICAL CONCENTRATION
ENVIRONMENTAL TRANSPORT
GEOCHEMISTRY
NITRATES
NITROGEN
REMOVAL
SEWAGE
CHEMICAL REACTIONS
CHEMISTRY
ELEMENTS
HYDROGEN COMPOUNDS
MASS TRANSFER
NITROGEN COMPOUNDS
NONMETALS
OXYGEN COMPOUNDS
POLLUTION
WASTES
WATER
540320* - Environment
Aquatic- Chemicals Monitoring & Transport- (1990-)
580000 - Geosciences