HR: 08:00h
AN: H41I-01    [Abstracts]
TI: Identifying Groundwater Discharge in the Merced River Basin, California Using Radon-222
AU: * Shaw, G D
EM: gshaw@ucmerced.edu
AF: University of California, Merced Division of Engineering, 4225 N. Hospital Rd. , Atwater, CA 95301 United States
AU: Hudson, G B
EM: hudson5@llnl.gov
AF: Lawrence Livermore National Laboratory Chemistry and Material Science Directorate, 7000 East Avenue, Livermore, CA 94550 United States
AU: Moran, J
EM: moran10@llnl.gov
AF: Lawrence Livermore National Laboratory Chemistry and Material Science Directorate, 7000 East Avenue, Livermore, CA 94550 United States
AU: Conklin, M
EM: mconklin@ucmerced.edu
AF: University of California, Merced Division of Engineering, 4225 N. Hospital Rd. , Atwater, CA 95301 United States
AB: Groundwater flow in fractured granite of the Sierra Nevada is poorly characterized, in particular, contributions of mountain block recharge are not known. Using a combination of water quality and isotopic analyses, groundwater inputs to the Upper Merced River were characterized. Between November 2003 and July 2004, monthly water quality samples were taken from Happy Isles to the inlet of Lake McClure, a 75 km reach. These samples demonstrated the expected dilution due to snowmelt in the spring. In the fall, the spatial profile matched the geology with anion concentrations increasing downstream of the transition from the Sierra Nevada batholith to the country rock, suggesting significant groundwater inputs. From July 19 to 21, 2004, radon-222 and other noble gases (He, Ne, Ar, Kr and Xe abundances and 3He/4He ratio) were measured along a 37 km reach of the Merced River, extending from the top of Yosemite Valley to the confluence of the South Fork of the Merced River. All radon samples were extracted into mineral oil immediately in the field and counted using liquid scintillation; noble gas samples were collected in copper tubes. Radon-222 activity varied from about 1 to 100 pCi/L (at collection time) indicating significant, spatially variable groundwater discharge into the Merced River. Two one-mile reaches of the Merced River were sampled for 222Rn on a fine scale. Large fracture sets in these two locations and previous temperature measurements suggested that groundwater discharge was higher relative to other locations along the river. Radon-222 activity was low upstream and downstream of large fractures observed in the bedrock; whereas, 222Rn activity was high at large fracture zones. Degassing is rapid downstream of fractures where no groundwater discharge is observed. For a representative groundwater end-member, radon-222 activity measured in Fern Spring, Yosemite Valley was about 1200 pCi/L. Excess 4He from U and Th decay is observed in samples with elevated 222Rn, however the ratio of 4He to 222Rn is significantly different between the two fracture sets.
DE: 1829 Groundwater hydrology
DE: 1040 Isotopic composition/chemistry
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting