Using borehole data and three-dimensional models to map the ground-water system in the Amargosa Desert Basin, Nevada and California

The ground-water system beneath the Amargosa Desert in southern Nevada and southeastern California (Figure 1) is 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. However the configuration and continuity of basin-fill aquifers and confining units that underlie the Amargosa Desert are poorly known, leading to difficulties in estimating the available exploitable ground-water resource; understanding and responding to water-level declines created by ground-water withdrawals; and evaluating issues of groundwater quantity and quality. Better prediction of the effects of ground-water development on valued habitats and on the sustainability of water resources can come only from improved understanding of the hydrogeologic system. Knowledge of the geologic framework for the basin-fill aquifer system has been improved by using drill hole data from a 20 km by 90 km area to construct a three-dimensional lithologic model (Figure 2). Lithologic data were reduced to a limited suite of descriptors based on geologic knowledge of the basin and distributed in 3D space using gridding methods. Drill hole data were extrapolated radially from each hole using a cellbased modeling approach where solid model cell nodes were sequentially assigned properties by looking outward horizontally from each borehole in search circles of ever-increasing diameter. Cell dimensions for the modeling were 1000 m in the horizontal dimensions and 10 m in the vertical dimension. The resulting lithologic model portrays a complex system of interfingered coarseto fine-grained alluvium, playa and palustrine deposits, eolian sands, and interbedded volcanic units (Figure 2). Sixty-six additional boreholes were added in the vicinity of the US Ecology site (Figure 3), a low-level nuclear waste site, to compare the regional model to a more detailed study area. The lithologic models compare well to resistivity data, aeromagnetic data and geologic map data, lending confidence to the interpretation. Lithologic units could not be represented in the model as a stacked stratigraphic sequence because of the complex geology and absence of time markers. Instead, lithologic units were grouped into interpreted genetic classes, such as playa or alluvial fan. Three-dimensional models computed from these interpreted facies data portray the alluvial infilling of a tectonically formed basin that is intermittently internally drained and infrequently the location of regional ground-water discharge.