A 3D HYDROGEOLOGIC FRAMEWORK OF THE GREAT BASIN BASIN-FILL AND CARBONATE-ROCK AQUIFERS FOR REGIONAL WATER AVAILABILITY STUDIES

The U.S. Geological Survey (USGS) is conducting a water availability study involving more than 100,000 mi² in the eastern Great Basin to quantify current ground-water resources, evaluate how resource use has changed over time, and develop tools to assess system responses to stresses from future uses and climate variability. This study updates and refines USGS regional aquifer system studies of the Great Basin conducted in the 1980s. Numerical ground-water flow models developed for those studies used a simple 2-layer construction to approximate basin-fill and bedrock units.

A greater-resolution 3D hydrogeologic framework is being developed to provide an improved conceptual understanding of the geologic controls on ground-water flow within the system. The hydrogeologic framework is a fundamental input to subsequent steady-state and transient numerical ground-water flow models.  The hydrogeologic framework includes four pre-Cenozoic hydrogeologic units (Proterozoic siliciclastic rocks and all metamorphic and plutonic rocks; Lower Paleozoic carbonate rocks; Mississippian siliciclastic rocks; and Upper Paleozoic carbonate rocks and Mesozoic rocks) and two Cenozoic hydrogeologic units (volcanic rocks and unconsolidated alluvial fill). Grids of each unit are constructed from geologic maps, cross section data, and borehole lithologic logs; offset on Cenozoic faults is incorporated into each gridded surface.  The geometry of the Cenozoic basins is defined by a surface representing the top of pre-Cenozoic rocks, based on gravity-inversion models. The thickness of Cenozoic volcanic rocks is derived from geologic maps, cross sections, and a regional isopach map of volcanic-rock thickness.

The hydrogeologic conceptual model and 3D framework are based on decades of hydrologic studies in the Great Basin by the USGS and others that have emphasized the following points: (1) the Paleozoic carbonate rocks have sufficient permeability to act as regional bedrock aquifers; (2) the regional aquifer may be connected to the basin-fill and volcanic-rock aquifers within basins; (3) juxtaposition of hydrogeologic units is the primary geologic control on the ground-water system; and (4) geologic structures may be important local modifiers of the regional flow pathways.