Hydrogeology of the unsaturated zone, North Ramp area of the Exploratory Studies Facility, Yucca Mountain, Nevada

Yucca Mountain, in southern Nevada, is being investigated by the US Department of Energy as a potential site for a repository for high-level radioactive waste. This report documents the results of surface-based geologic, pneumatic, hydrologic, and geochemical studies conducted during 1992 to 1996 by the US Geological Survey in the vicinity of the North Ramp of the Exploratory Studies Facility (ESF) that are pertinent to understanding multiphase fluid flow within the deep unsaturated zone. Detailed stratigraphic and structural characteristics of the study area provided the hydrogeologic framework for these investigations. Shallow infiltration is not discussed in detail in this report because the focus in on three major aspects of the deep unsaturated-zone system: geologic framework, the gaseous-phase system, and the aqueous-phase system. However, because the relation between shallow infiltration and deep percolation is important to an overall understanding of the unsaturated-zone flow system, a summary of infiltration studies conducted to date at Yucca Mountain is provided in the section titled Shallow Infiltration. This report describes results of several Site Characterization Plan studies that were ongoing at the time excavation of the ESF North Ramp began and that continued as excavation proceeded