Strontium Isotope Characterization of Flow Systems in Southern Nevada, USA

Abstract

An international scientific consensus argues for the long-term isolation of nuclear waste in deep geologic repositories. Among the many geological criteria for selecting suitable sites, the hydrological setting is especially critical because release of radionuclides to the biosphere would most likely occur through the movement of ground water. Characterizing the hydrological setting entails understanding the present-day rates and paths of ground water flow, how these may have varied in the past, and how they are likely to vary in the future. Among the sites being evaluated worldwide, Yucca Mountain in southern Nevada is unique because the potential respository would be constructed several hundred metres above the water table within a thick, unsaturated zone. Yucca Mountain is a broad ridge of Miocene ash-flow tufts in the Basin-and-Range physiographic province of the western United States, where north-trending bedrock ranges alternate with alluvium-filled valleys. Mesozoic compression folded and faulted Upper Proterozoic and Palaeozoic clastic and carbonate rocks which had been deposited on the eastern Cordilleran miogeoeline, and a thick sequence (c 1000-3000 m) of Miocene silicic ash-flow tufts subsequently was deposited on this structurally complex terrane. Late Cenozoic extension faulted and