CFC Degradation under Anaerobic Water Table Conditions in Glacial Drift at Mirror Lake New Hampshire
The use of chlorofluorocarbons (CFC's) as age-dating tracers for 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. CFC concentrations in water samples from high-conductivity fractures in schist and granite bedrock indicate ages ranging from greater than 45 years to about 7 years, but the spatial pattern of ages is complex and the youngest measured ages occur near the lake at the bottom of the forested watershed. Equilibrium between CFC in the atmosphere and at the water table is needed to date saturated-zone waters. However, samples from piezometers screened just below the water table in the glacial drift indicate that CFC-11, CFC-12, and CFC-113 are substantially degraded where dissolved oxygen (DO) concentrations are low. Anaerobic (DO < 0.1 mg/L) conditions existing in a sandy terrace deposit may be caused by biodegradation of organic carbon in infiltrating stream water. Samples from this and other shallow anaerobic zones lack CFC-11 and CFC-113 and have CFC-12 concentrations that are as low as 1/3 of modern equilibrium concentrations. Furthermore, several hypoxic (0.1 < DO < 3 mg/L) samples have significantly reduced CFC's. As expected, samples with high DO collected at the water table generally contain CFC's at atmospheric-equilibrium concentrations. Many samples plot on a mixing line for CFC's and DO, suggesting that the water sampled can be 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. Several samples contain CFC-11 and CFC-113 concentrations higher than those from equilibration with peak atmospheric levels, suggesting a non-atmospheric source. Evidence of active anaerobic biodegradation, which can degrade CFCs, includes 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.
Goode, D.J., Busenberg, Eurybiades, Plummer, L.N., Shapiro, A.M., and Vroblesky, D.A., 1997, CFC degradation under anaerobic water table conditions in glacial drift at Mirror Lake New Hampshire: (abs.), Geological Society of America 1997 Annual Meeting, 20-23 October 1997, Salt Lake City, Utah, GSA Abstracts with Programs, vol. 29, no. 6, p. 77.