USING BOREHOLE DATA AND 3D LITHOLOGIC MODELS TO REFINE THE CONCEPTUAL HYDROGEOLOGIC MODEL OF THE ALLUVIAL AQUIFER SYSTEM IN AMARGOSA DESERT, NEVADA AND CALIFORNIA

The Amargosa Desert in southern Nevada and southeastern California is a critical part of the regional ground-water flow system that includes Death Valley, CA, the Nevada Test Site and the proposed high-level nuclear waste repository at Yucca Mountain, NV. A USGS ground-water flow model of this region was difficult to calibrate for alluvial basins, such as Amargosa Desert and Pahrump Valley, likely because of a lack of characterization of the basin-fill sediments. The geologic framework for the alluvial aquifer system in Amargosa Desert has been improved by using borehole data to construct 3D lithologic and interpreted facies models. The revised hydrogeologic framework of the basin fill will lead to refinements in ground-water flow modeling of this complex and important region, enabling more useful analyses of alluvial groundwater quantity and quality issues, such as land subsidence, effects of ground-water withdrawals on habitats, and sustainability of ground-water resources.

To improve the framework, 3D models of basin-fill lithology were created using data from 505 boreholes from a 20 km by 90 km area. Lithologic data were reduced to a limited suite of descriptors based on geologic knowledge of the basin and distributed in 3D space using interpolation methods. The resulting lithologic model of the Amargosa Desert portrays a complex system of interfingered coarse- to fine-grained alluvium, playa and palustrine deposits, eolian sands, and interbedded volcanic units. Lithologic units could not be represented in the model as a stacked stratigraphic sequence because of the complex geology and absence of time markers, but were instead grouped into interpreted genetic classes, such as playa or alluvial fan. 3D facies models computed from these data portray the alluvial infilling of a tectonically formed basin that is intermittently internally drained and infrequently the location of regional ground-water discharge. The lithologic and interpreted facies models compare well to resistivity data, aeromagnetic data and geologic map data, lending confidence to the interpretation.