2011 GSA Annual Meeting in Minneapolis (912 October 2011)
Paper No. 17-3
Presentation Time: 8:35 AM-8:50 AM

USING DISSOLVED GASES TO OBSERVE THE EVOLUTION OF GROUNDWATER AGE IN A MOUNTAIN WATERSHED OVER A PERIOD OF THIRTEEN YEARS

MANNING, Andrew H., U.S. Geological Survey, P.O. Box 25046, Mail Stop 973, Denver, CO 80225-0046, amanning@usgs.gov, CLARK, Jordan F., Earth Science, Univ of California, 2114 Webb Hall, Santa Barbara, CA 93106, DIAZ, Stephanie H., Department of Earth Science, University of California at Santa Barbara, 1001 Webb Hall, Santa Barbara, CA 93106, RADEMACHER, Laura K., Department of Earth and Environmental Sciences, University of the Pacific, 3601 Pacific Avenue, Stockton, CA 95211, and EARMAN, Sam, Department of Earth Sciences, Millersville University, PO Box 1002, Millersville, PA 17551

Baseflows in snowmelt-dominated mountain streams are critical for sustaining ecosystems and water resources during periods of greatest demand. Future climate predictions for mountainous areas throughout much of the western U.S. include increasing temperatures, declining snowpacks, and earlier snowmelt periods. The degree to and rate at which these changes will affect baseflows in mountain streams remains unknown, largely because baseflows are groundwater-fed and the relationship between climate and groundwater recharge/discharge rates in mountain watersheds is uncertain. We use groundwater age determinations from multiple dissolved gas tracers (CFCs, SF6, and 3H/3He) to track changes in groundwater age over a period of thirteen years in the Sagehen Creek watershed, Sierra Nevada Mountains, CA. Data were collected from springs and wells in 2009 and 2010 and combined with those obtained in prior studies from 1997 to 2003. Apparent ages range from 0 to >60 years. Comparison between variations in age and variations in snow water equivalent (SWE) and mean annual air temperature reveals the degree of correlation between these climate variables and recharge rate. Further, comparison of apparent ages from individual springs obtained at different times and using different tracers helps constrain the age distribution in the sampled waters. The age data are generally more consistent with an exponential age distribution than with piston-flow. However, many samples, even those with relatively old mean ages, must have a disproportionately large very young fraction that responds directly to annual SWE variations. These findings have important implications for how future baseflows may respond to decreasing SWE.

2011 GSA Annual Meeting in Minneapolis (912 October 2011)
General Information for this Meeting
Session No. 17
Gas Bubbles and Dissolved Gases in Groundwater
Minneapolis Convention Center: Room M101AB
8:00 AM-12:00 PM, Sunday, 9 October 2011

Geological Society of America Abstracts with Programs, Vol. 43, No. 5, p. 57

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