Open-File Report 2006–1356
The U. S. Department of Energy (DOE) and the National Nuclear Security Administration (NNSA) at their Nevada Site Office (NSO) are addressing groundwater contamination resulting from historical underground nuclear testing through the Environmental Management (EM) program and, in particular, the Underground Test Area (UGTA) project. During 2005, the U.S. Geological Survey (USGS), funded by the DOE and NNSA-NSO, collected and processed data from twenty-six Magnetotelluric (MT) and Audio-Magnetotelluric (AMT) sites at the Nevada Test Site. Data stations were located in and near Rainier Mesa and Shoshone Mountain to assist in characterizing the pre-Tertiary geology in those areas. These new stations extend to the west the hydrogeologic study that was conducted in Yucca Flat in 2003. This work has helped to refine the character, thickness, and lateral extent of pre Tertiary confining units. In particular, a major goal has been to define the upper clastic confining unit (UCCU – late Devonian to Mississippian-age siliciclastic rocks assigned to the Eleana Formation and Chainman Shale(Bechtel Nevada, 2006)) in the Yucca Flat area and west towards Shoshone Mountain in the south, east of Buckboard Mesa, and onto Rainier Mesa in the north.
The Nevada Test Site magnetotelluric data interpretation presented in this report includes the results of detailed two-dimensional (2 D) resistivity modeling for each profile (including alternative interpretations) and gross inferences on the three dimensional (3 D) character of the geology within the region. The character, thickness, and lateral extent of the Chainman Shale and Eleana Formation that comprise the Upper Clastic Confining Unit (UCCU) are generally characterized in the upper 5 km. The interpretation is not well determined where conductive TCU overlies conductive Chainman Shale, where resistive Eleana Formation overlies resistive LCA units, or where resistive VTA rock overlies units of the Eleana Formation. The nature of the volcanic units in the west has been refined as are large and small fault structures such as the CP Thrust Fault, the Carpetbag Fault, and the Yucca Fault that cross Yucca Flat. The subsurface electrical resistivity distribution and inferred geologic structures determined by this investigation should help constrain the hydrostratigraphic framework model that is under development for the Rainier Mesa/Shoshone Mountain Corrective Action Unit and areas to the west and in understanding the effects on ground-water flow in the area.
Posted December 2006
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