2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 4
Presentation Time: 1:30 PM-5:30 PM


HUNT, Andrew G., United States Geol Survey, Denver Federal Center, Bld 21, MS 963, Denver, CO 80225, NAUS, Cheryl A., Water Resources Division, U.S. Geol Survey, 5338 Montgomery Blvd, NE, Suite 400/300, Albuquerque, NM 87109, NORDSTROM, D. Kirk, U.S. Geol Survey, 3215 Marine St., Suite E127, Boulder, CO 80303 and LANDIS, Gary P., US Geol Survey, P.O. Box 25046, MS 963, Denver, CO 80225, AHUNT@usgs.gov

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-11CFC-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.