DEVELOPMENT OF NUMERICAL MODELS TO ASSESS GROUND-WATER FLOW PATTERNS IN THE GREAT BASIN OF SOUTHERN NEVADA

Development of numerical models to assess ground-water flow patterns in the Great Basin of Southern Nevada began in the 1970’s. Most of these models were used to assess basic patterns of ground-water flow and effects of pumping in individual basins such as Las Vegas and Pahrump Valleys (or) and to assess and quantify interbasin flow between sub-basins over large regions such as the Nevada Test Site and the carbonate-rock province in Utah and Nevada. Early numerical models were greatly simplified because of limited capabilities of computers and programs. Models did not exceed a few thousand cells and had no more than three layers. Basin-scale models usually focused only on the basin fill, whereas larger regional-scale models usually lumped basin fill, volcanic rocks, and sedimentary rocks into a single unit with varying hydraulic properties that depended on the relative percentage of each geologic unit within a cell. Larger, regional-scale models were well suited for assessing and quantifying interbasin flow. Rapid development of computers and availability of sophisticated programs have allowed for development of more detailed models. These models often exceed 100,000 cells, use more than ten layers and are linked to hydrogeologic databases that more realistically relate hydraulic properties to the complex geology of the Great Basin. Such models provide a basis for directing data collection and monitoring efforts and are more adept to simulating the complex flow paths needed to realistically predict contaminant migration. The regional-scale models are being used to supply boundaries for extremely detailed models of contaminant migration at the Nevada Test Site. The limited water availability within this sensitive desert ecosystem along with large projected increases in the population of nearby Las Vegas and Pahrump Valleys requires continued assessment of ground-water flow in the Great Basin of southern Nevada, especially considering the potential for radionuclide transport from the Nevada Test Site and Yucca Mountain.