HR: 0800h
AN: H31D-0422    [Abstracts]
TI: The Cambric Ditch at the Nevada Test Site as a Long-term Vadose Zone Test Bed
AU: * Tompson, A F
EM: afbt@llnl.gov
AF: Environmental Sciences Division, L-208, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551 United States
AU: Hunt, J R
EM: hunt@ce.berkeley.edu
AF: Chemical Biology and Nuclear Science Division, L-231, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551 United States
AU: Hudson, G B
EM: hudson5@llnl.gov
AF: Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720 United States
AB: Atomic weapons testing at the Nevada Test Site introduced many tracers for quantifying hydrologic transport processes in arid climates. The particular experiment at the Cambric site in Frenchman Flat represents an ongoing 29-year field test that could never be repeated and continues to offer opportunities for vadose zone studies. The Cambric test released the energy yield equivalent of 0.75 kt of TNT when it was detonated 294 m below the land surface and 73 m below the water table in Frenchman Flat in May 1965. Beginning in 1975, groundwater was pumped steadily from a well located 91 m from the detonation point in order to elicit information on radionuclide migration. The pumping well effluent was monitored, discharged to an unlined ditch, and allowed to flow towards a dry lake 1.6 km away. Approximately one third of this flow was lost to infiltration. Over the next 16 years, pumped groundwater was shown to contain tritium, fission products (technetium-99, iodine-129) and activation products (chlorine-36), all of which can be used to trace water flow in the vadose zone. Bromide was also added as an additional tracer into the ditch. Multi-year records exist for water migration in the shallow vadose zone along with temperature profiles. Over the course of the pumping experiment, vegetation developed in and near the ditch, providing an additional pathway for water loss by transpiration and selective radionuclide transport. Significant water has not flowed in the ditch since 1991 and the site remains an ideal analog site for the studying drying in arid climates, the adaptability of vegetation under changing water conditions, and the use of helium-3 as a tracer of soil-atmosphere exchange and vadose zone - groundwater interactions. In addition, there is evidence that tritiated water and chlorine-36 have infiltrated over 200 meters vertically in the vadose zone and have become a source term for groundwater contamination. The Cambric Ditch remains as a field site ideally suited for further experimental investigations and the development and testing of transport models at spatial and temporal scales typical of contaminated, arid vadose zone sites. Data from this site has been useful in model development and additional field studies are suggested to further guide the application of models to contaminant migration in other deep vadose zones. Portions of this work were conducted under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract W-7405-Eng-48.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1866 Soil moisture
DE: 1875 Unsaturated zone
DE: 1040 Isotopic composition/chemistry
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting