GEOPHYSICAL INVESTIGATIONS TO SUPPORT THE REGIONAL GROUNDWATER STUDIES OF DEATH VALLEY IN INYO COUNTY, CALIFORNIA

Yucca Mountain is the site of the proposed high-level nuclear repository in the United States. The principal transporting mechanism for radionuclides is moving groundwater. In the 1960s the conventional wisdom in Nevada was that individual basins contained separate and distinct groundwater systems. Winograd, hypothesized that Paleozoic Carbonate rocks that underlie both eastern and southern Nevada integrated the groundwater flow into a much larger system. Winograd's prime examples were the major springs in Death Valley that he postulated were the discharge from the underlying Paleozoic Carbonates. Underlying the repository at a depth of approximately 4250 ft (1.3 km) is an extensive Carbonate Aquifer known to be highly permeable. Thus, a potential flow path exists between Yucca Mountain and the springs in Death Valley.

Inyo through a Cooperative Grant program funding by the US Department of Energy has been conducting a research program to prove groundwater movement through the Paleozoic Carbonates from the Amargosa Valley through the Southern Funerals to the major spring in Death Valley. Our study culminated in a MODFLOW groundwater model that is supported by our current program of: 1) monitoring well drilling in the Southern Funeral Mountains, 2) surface and subsurface geologic mapping in the area, and 3) geophysical surveys.

The goal of the geophysical survey program was to character the complex geological structure of the Southern Funeral range to assist with the siting of monitoring wells in the Paleozoic carbonate aquifer system. A total of 87 miles of gravity data and 66 TEM soundings were collected. The gravity data was modeled. The surface of the bedrock in the Amargosa Valley indicates that the basin and range fault blocks of the Southern Funeral Mountains project into the valley under the valley fill, but that the fault blocks are broken into isolated strands of the State Line Fault System. The result is a series of steep sided blocks of bedrock that rise close to the valley floor, surrounded by areas where the bedrock is in excess of 6,000 feet deep.

The geophysical survey program was successful in characterizing the subsurface structure configuration of the Paleozoic carbonate aquifer, and allowed the siting of the monitoring wells to penetrate the carbonate aquifer at an acceptable depth.