HR: 1340h
AN: H23E-1481    [Abstracts]
TI: A Comparison of Hydrogeologic Models, Tritium/helium-3, and Deliberate Tracer Experiments to Understand Ground Water Residence Time
AU: * McDermott, J
EM: jeni_mcdermott@umail.ucsb.edu
AF: University of California, Santa Barbara, Department of Geological Sciences, Santa Barbara, CA 93106 United States
AU: Clark, J
EM: jfclark@geol.ucsb.edu
AF: University of California, Santa Barbara, Department of Geological Sciences, Santa Barbara, CA 93106 United States
AU: Avisar, D
EM: droravi@post.tau.ac.il
AF: Tel Aviv University, Department of Geography and Environmental Sciences, Tel Aviv, 69978 Israel
AU: Hudson, G
EM: hudson5@llnl.gov
AF: Lawrence Livermore National Lab, Chemical Biology and Nuclear Science Division, Livermore, CA 94550 United States
AB: Assessment of subsurface residence time of ground water, used for determining flow paths and travel times, is an important criterion for understanding and monitoring water quality and in situ biogeochemical reactions. Established methods of determining travel time include hydrogeologic modeling, transient tracers and deliberate tracers. This paper presents a comparison of these three methods near the Montebello Forebay recharge site in Los Angeles County, California. Sulfur hexafluoride (SF6) gas tracer was injected into the Rio Hondo and San Gabriel recharge basins in early 2003. During this two-year experiment, SF6 was detected at nine monitoring wells, indicating that SF6 was successfully transferred from surface water to groundwater during percolation at spreading basins. SF6 tracer was detected at 11 of the 18 production wells sampled during this study, indicating travel time for recharge water to some of the wells is less than two years. All tritium/3He ages are greater than 10 years for the production wells, indicating mixing of young and old ground water. Because tritium/3He ages do not mix linearly - the mixed age is weighted by each flow path's initial tritium content - this technique leads to over-estimation of ground water age. At four of the eleven wells with SF6 detections, the hydrogeologic travel times determined in a previous study by Bookman-Edmonston Engineering were less than 10 weeks and are in basic agreement with the SF6 travel times determined in the present study. However, at the other seven wells, estimated hydrogeologic travel times were greater than 200 weeks, significantly longer than indicated by the tracer data. Leakage through low permeability layers leading to earlier tracer arrival provides a likely explanation. At all wells with no detected tracer, the hydrogeologic times were greater than three years.
DE: 1800 HYDROLOGY
SC: Hydrology [H]
MN: Fall Meeting 2005