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Fractured Rock Hydrology Research

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CFC Transport to and Degradation Near a Shallow Water Table, Mirror Lake Area, Grafton County, New Hampshire

Abstract

The use of Chlorofluorocarbons (CFC's) as age-dating tracers for shallow ground water at the Mirror Lake fractured-rock research site may be limited because concentrations of CFC's in at least some areas are significantly reduced by anaerobic degradation. Oxygen transport through the vadose zone is limited by the low permeability and usually high moisture content of the glacial drift, which overlies crystalline bedrock. Samples from piezometers screened just below the water table indicate that CFC-11, CFC-12, and CFC-113 are degraded in areas where dissolved oxygen (DO) concentrations are low. Several samples with zero DO also have zero CFC-11 and CFC-113, and CFC-12 levels are about 1/3 of modern equilibrium concentrations. However, several samples with low DO, but high enough to be biologically aerobic, also have significantly reduced CFC's. Many samples fall on a linear mixing line for CFC's and DO, suggesting that they are a mixture of two end members: (1) waters saturated with oxygen and containing CFC's in equilibrium with the modern atmosphere, and (2) zero-DO waters containing no CFC-11 or CFC-113 and containing about 1/3 of modern CFC-12. Samples with high DO collected at the water table usually contain CFC's at atmospheric-equilibrium concentrations. A number of samples indicate contamination in that they contain CFC concentrations higher than those from equilibration with peak atmospheric levels. Evidence of active anaerobic biodegradation includes wells with high methane and ample iron (Fe-II) and sulfate and generally low DO. Hydrogen gas concentrations in anaerobic samples from water-table piezometers are consistent with methanogenesis or sulfate-reduction as the terminal electron-accepting process. Concentrations of dissolved nitrogen and argon gases in samples collected at the water table do not indicate entrapped air, in contrast to water samples collected from bedrock, suggesting in situ production, for example by denitrification in the case of nitrogen.

Citation:
Goode, D.J., 1997, CFC transport to and degradation near a shallow water table, Mirror Lake area, Grafton County, New Hampshire : (abs.), p. S176 in 1997 Spring Meeting, supplement to Eos, 29 April 1997, American Geophysical Union, Washington DC.

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