CFC’s in the Unsaturated Zone and in Shallow Ground Water at Mirror Lake, New Hampshire

At the U.S. Geological Survey's fractured-rock hydrology research site at Mirror Lake, N.H., concentrations of chlorofluorocarbons (CFC's) and other chemical constituents in ground water were measured to help identify large-scale flow and transport properties of the crystalline bedrock. The use of CFC’s as environmental tracers in ground water is based on the assumption that CFC concentrations in recharge are in equilibrium with atmospheric levels. Results of a 3-year field study to identify the input source function for CFC’s in ground-water recharge indicate that use of CFC’s as an environmental tracer at this site, and presumably at other forested, humid sites underlain by glacial deposits, may be significantly complicated by anaerobic degradation reactions in shallow ground water. CFC concentrations near the water table are depleted where dissolved oxygen levels are low. CFC-11 and CFC-113 are absent under anaerobic conditions; CFC-12 is as low as one-third of modern concentrations. Methanogenic and sulfate-reduction conditions have been identified by use of hydrogen-gas measurements, and methane has been detected in several anaerobic samples. One area of active degradation appears to be associated with streamflow loss to ground water. Soil-gas concentrations are generally close to atmospheric levels, although limited spatial correlation i