SOME GEOLOGIC CONTROLS ON TRANSPORT IN THE LOWER CARBONATE AQUIFER, YUCCA FLAT, NEVADA

Underground nuclear testing via deep vertical shafts was conducted at the Nevada Test Site (NTS) from 1951 until 1992. The Yucca Flat area of the NTS was used for 35 years. The Underground Testing Area (UGTA) Project is currently conducting correction action investigations to ensure the protection of the public and the environment.

Underground nuclear testing was primarily done in volcanic and alluvial units in Yucca Flat that are semi-isolated from the regional flow system in the Lower Carbonate Aquifer (LCA), and only 4 nuclear tests were done in proximity to the LCA. Laczniak et al. (1996) indicated that most of the groundwater flowing beneath Yucca Flat passes through the LCA and is the only subsurface pathway by which groundwater leaves the basin.

The Yucca Flat basin was extended during Basin and Range formation, and is broken by numerous north-south striking normal faults. Field observations of the LCA geology in Yucca Flat reveal areas of intensely shattered, or brecciated, carbonate rock. Down hole televiewers also show this characteristic, which is associated with high-flow zones in flow logs. It has been suggested that normal faults formed by extension have less fault gouge, and are more likely to be high-permeability channels than barriers. Yucca Flat also lies at the edge of terrain described as “extended” by the USGS in its 2004 geologic model of the area.

A multiple-well interference test in the LCA indicates a discrete fracture or high-permeability channel flow (e.g., non-radial flow) regime without apparent hydraulic double-porosity effects (SNJV, 2005). At other locations single-well tests show a possible double-porosity response with generally lower hydraulic conductivity. At the basin scale, these results suggest that extensive fault zones, channel flow north to south. This is evident because of the lack of a response at wells across major fault zones versus the rapid response of wells along the strike of the faults. This conceptual model is similar to that of Rojstaczer (1987), who saw effects of groundwater pumpage in Ash Meadows clearly influenced by large-scale fracture and conduit features in the LCA.