MINERALOGY AND GEOCHEMISTRY OF THE ALLUVIUM IN THE FORTYMILE WASH REGION SOUTH OF YUCCA MOUNTAIN, NEVADA

The U.S. Department of Energy (DOE) is preparing a license application for Yucca Mountain, Nevada, as a potential geologic repository for high-level nuclear waste. The U.S. Nuclear Regulatory Commission (NRC) is responsible for determining whether the DOE’s license application for the proposed repository complies with licensing regulations. A portion of the likely ground water flow path to the south of Yucca Mountain consists of the saturated alluvial sediments of Fortymile Wash. The alluvium may provide a barrier to the transport of radionuclides that might be released from a repository. The water chemistry and mineralogy of the alluvium are the most important factors influencing its potential radionuclide retardation capabilities.

As part of the independent oversight activities of the NRC, we conducted detailed studies of the mineralogy and chemistry of cuttings from two Nye County Early Warning Drilling Program (EWDP) wells. The wells, NC-EWDP-02D and NC-Washburn-1X, are located in the southern part of Fortymile Wash along the likely groundwater flow path from Yucca Mountain. The cuttings were analyzed using semi-quantitative XRD, thin section petrography, scanning electron microscopy, XRF, and ICP-MS. Results of analyses show that alluvium mineralogy does not change significantly at the water table. An increase in calcite and dolomite at depth in well NC-EWDP-02D is likely associated with the occurrence of sediments and sedimentary rocks that pre-date the tuff-derived alluvium. Zeolite and smectite minerals occur as grain coatings and interstitial material and may play an important role in alluvium retardation processes in Fortymile Wash.

This abstract was prepared to document work performed by the Center for Nuclear Waste Regulatory Analyses (CNWRA) for the NRC under Contract No. NRC-02-02-012. This abstract is an independent product of the CNWRA and does not necessarily reflect the view or regulatory position of the NRC.