Conceptualizing of the Pre-Developed Groundwater Flow System and Transient Water-Level Responses in Yucca Flat, Nevada National Security Site, Nevada

Contaminants introduced into the subsurface of Yucca Flat, Nevada National Security Site, by underground nuclear testing are of concern to the U.S. Department of Energy and regula- tors responsible for protecting human health and safety. The potential for contaminant movement away from the under- ground test areas and into the accessible environment is great- est by groundwater transport. The primary hydrologic control on this transport is evaluated and examined through a set of contour maps developed to represent the hydraulic-head dis- tribution within the two major aquifer systems underlying the area. Aquifers and confining units within these systems were identified and their extents delineated by merging and analyz- ing hydrostratigraphic framework models developed by other investigators from existing geologic information. Maps of the hydraulic-head distributions in the major aquifer systems were developed from a detailed evaluation and assessment of avail- able water-level measurements. The maps, in conjunction with regional and detailed hydrogeologic cross sections, were used to conceptualize flow within and between aquifer systems.