Geology, selected geophysics, and hydrogeology of the White River and parts of the Great Salt Lake Desert regional groundwater flow systems, Utah and Nevada

Abstract

The east-central Great Basin near the Utah-Nevada border contains two great groundwater flow systems. The first, the White River regional groundwater flow system, consists of a string of hydraulically connected hydrographic basins in Nevada spanning about 270 miles from north to south. The northernmost basin is Long Valley and the southernmost basin is the Black Mountain area, a valley bordering the Colorado River. The general regional groundwater flow direction is north to south. The second flow system, the Great Salt Lake Desert regional groundwater flow system, consists of hydrographic basins that straddle the Utah-Nevada border, with a length of about 150 miles from north to south. The general regional groundwater flow direction is from south to north towards the Great Salt Lake Desert. For 15 years with support from the Southern Nevada Water Authority (SNWA), hydrologists, geologists, and geophysicists studied the basin connections and the groundwater resources in these and adjacent flow systems over an area of about 25,000 square miles. A major first part of the SNWA study was constructing a 3-dimensional digital hydrogeologic framework based on geologic maps and cross sections at 1:250,000 scale. This framework documents the presence of three major aquifers: (1) Paleozoic carbonate rocks, (2) Eocene to Miocene volcanic rocks, and (3) Miocene to Holocene basin-fill sediments, as well as confining units that constrain flow. We interpret that movement of most groundwater through and across basins is by fracture-dominated flow along faults/fractures, yet in most places flow is prevented or retarded across faults, so mapping structures gives a first approximation to conduits and barriers to flow. The most important structures by far are high-angle normal faults of the basin-range episode of east-west extensional deformation. This event began at about 20 Ma, although most deformation and the formation of the present topography took place between 10 Ma and present. This topography consists of north-trending basins (mostly grabens) that alternate with north- trending ranges (mostly horsts); erosion of the ranges filled the basins with clastic alluvial basin-fill deposits. Geophysics provides data on the third dimension (cross sections) of the hydrogeologic framework. Audiomagnetotelluric profiles and gravity inversion located faults and enabled us to estimate thicknesses of basin-fill deposits. To this framework, hydrologic studies addressed precipitation, surface water, and springs, as well as groundwater levels, volumes, geochemistry, water budgets, and monitoring. At nearly the same time as our study, the Utah Geological Survey (UGS) and U.S. Geological Survey (USGS) addressed the same issues in many of the same areas, and publication of the efforts by all three agencies reveals a surprising similarity of conclusions, with some critical exceptions, which therefore demonstrates the great value of many scientists independently studying the same complex scientific problem. The differences in conclusions include directions and volumes of some ground- water flow paths, such as one proposed by the USGS of unlikely groundwater flow from Steptoe Valley to southern Snake Valley, and another proposed by the UGS of unlikely significant groundwater recharge flow from the Snake Range to the Fish Springs complex.