USE OF 3-HELIUM/TRITIUM AGES TO DETERMINE CHLOROFLUOROCARBON DEGRADATION ASSOCIATED WITH NATURALLY OCCURRING ACID DRAINAGE, QUESTA, NEW MEXICO

As part of an investigation into pre-mining water quality, ground-water age dating techniques (3-helium/tritium (³He/³H) and chlorofluorocarbons (CFCs)) were utilized to define residence times of ground waters representing pre-mining water-quality conditions within a watershed near Questa, NM.  The ground waters flow within hydrothermally altered volcanic deposits that are comprised of competent and weathered bedrock associated with debris fans that extend from elevated drainages into the alluvial deposits of the Red River basin.  Ground-water chemistry in the debris fans is dominated by low pH and high dissolved metal content caused by pyrite oxidation; however, ground waters associated with the competent bedrock aquifer have a neutral pH. Comparison of the CFC and the ³He/³H ages illustrate large discrepancies between the two techniques, with a characteristic age distribution of apparent ³He/³H ages being younger than the CFC ages.  The CFC ages also are discordant with each other (CFC-11≠ CFC-113 ≠ CFC-12).  Minor variation between the ages may be explained by fundamental differences between the two techniques and by mixing of ground waters with different ages.  Eliminating such samples from the data set still leaves a subset of wells that show a clear degradation of CFCs with age.  CFC-11 and CFC-113 appear to degrade at similar rates, whereas CFC-12 degrades at a slower rate relative to the conservative ³He/³H age.  All of the wells associated with this unique degradation profile have low pH, low dissolved oxygen (< 1.0 to 6.0 mg/l) and high concentrations of reduced iron (Fe(II)).  Historically, CFC degradation has been documented to be microbially driven in anoxic ground waters associated with sulfate-reduction or  methanogenesis; however the ground-water geochemistry in the Red River basin links the CFC degradation to microbial reduction of iron only. This study has important implications for the mechanism of CFC degradation in anoxic environments.