USING BOREHOLE DATA AND 3-D LITHOLOGIC MODELS TO MAP THE GROUND-WATER AQUIFER SYSTEM IN THE AMARGOSA DESERT, NEVADA AND CALIFORNIA

Ground-water aquifers beneath the Amargosa Desert in southern Nevada and southeastern California are under increasing demands from agricultural, commercial, and residential users. The Cenozoic basin fill in the Amargosa Desert basin has great lithologic diversity and state of consolidation, and is thus hydrogeologically heterogeneous. Data from boreholes from a 20 km by 90 km area in the basin were compiled and interpreted in terms of lithology and stratigraphic facies, and then modeled in 3-D digital representations. The 3-D models extrapolate data radially from each borehole until a solid volume is created. The computed 3-D solid models assume that bedding was horizontal and faults were not explicitly included in the model. Despite these assumptions, the models compare well to resistivity data, aeromagnetic data, and geologic map data, lending confidence to the interpretation. The 3-D models delineate the complex aquifer geometry and assist in the analyses of the sustainability of ground-water resources. The basin fill includes consolidated, mostly Tertiary units and younger unconsolidated deposits, both of which interfinger to the north and south with extrusive Miocene and younger volcanic rocks. The northern part of the basin has a complex layering of primarily alluvial and eolian deposits, whereas the southern part of the basin is filled with an essentially uniform accumulation of fine-grained playa and palustrine deposits. These rocks record a basin that was intermittently internally drained and infrequently the location of regional ground-water discharge. Coarse-grained alluvial deposits and local deposits of continental limestone and spring deposits are capable of supporting large-capacity production wells. These aquifers are part of a complex system of interfingered aquifers and fine-grained confining units that constitute the basin fill. Typically, the aquifers respond as confined units to short-term loading and pumping stresses. Over the long term, the system responds to withdrawal by drainage through the confining units and the basin fill operates as an integrated flow system. With time, broad cones of depression develop in the potentiometric head around pumping centers and the temporal and water-yielding response approaches that characteristic of an unconfined system.