Assessment of Ground-Water Hydrology Along the Western Slopes of the Spring Mountains

Federal land-management agencies and Nye County are concerned that increased ground-water pumping from existing and new wells in southern Amargosa and Pahrump Valleys could impact nearby water-dependent ecosystems by reducing the amount of natural ground-water discharge and decreasing shallow water tables. Ground-water discharges at Ash Meadows National Wildlife Refuge, at numerous springs and seeps, and along parts of the Amargosa River. Additionally, ground water is intersected at Devils Hole, a fissure in the regional carbonate aquifer. These areas support habitat for numerous species of plants and animals, including several federally-listed threatened and endangered species. The USGS proposes to evaluate potential impacts from pumping to water levels and discharge within these areas of concern by developing and applying a numerical ground-water flow model. A recently calibrated numerical model of the Death Valley regional ground-water flow system (DVRFS), which includes Amargosa and Pahrump Valleys, will be used to develop a finer-resolution flow model. The finer-resolution flow model will be embedded in the DVRFS model to more accurately simulate water levels within the areas of concern.

Objectives

The study has three principal objectives: (1) construct and calibrate a numerical flow model of the southern part of the Amargosa Desert that has appropriate resolution to address local-scale water-resource issues. The finer-resolution model will be applied to evaluate (2) the influence and control of known geologic structures on ground-water flow and (3) potential impacts of present and future pumping in the southern part of the Amargosa Desert and Pahrump Valley on Federal and Nye County areas of concern.

Strategy and Approach

Hydrologic and geologic data collected within the hydrologic basin after the DVRFS model was calibrated and model databases were completed in 2004 will be compiled. Hydrologic data (such as water levels and pumpage) will be quality assured and incorporated into existing databases. Geologic data will be incorporated into a digital, three-dimensional hydrogeologic framework model constructed with geologic modeling software. The new hydrologic and geologic data will be incorporated into the flow model. The DVRFS model was constructed with MODFLOW-2000 to simulate transient ground-water flow with a finite-difference method. The model has 15 layers and is discretized into 1,500-square-meter cells. The DVRFS model will be re-calibrated to reflect the addition of the new data.

A variety of pumping scenarios will be conducted with the re-calibrated DVRFS model to simulate the effects of increasing ground-water withdrawals on the flow system. The model also will be analyzed to identify additional data gaps in hydrogeologic information and develop a prioritized list of needs to help land management agencies develop strategies for acquiring future scientific data needed to monitor the condition of water-dependent ecosystems as regional growth continues. Additional monitoring wells will be installed in areas along the western slopes of the Spring Mountains to obtain hydrogeologic data needed to calibrate and refine the flow model. Well boreholes will be drilled using air rotary, mud rotary, or similar methods. Boreholes will be drilled and wells constructed with materials and methods that comply with regulations for water well and related drilling promulgated by the Nevada State Engineer. Work will be completed over two fiscal years to allow sufficient time to complete environmental assessments, well permitting activities, and field work. A BLM hydrologist will periodically measure water levels in the wells to obtain baseline.

Additional Project Information

Additional information about the Death Valley Regional Flow model is available from the Death Valley Regional Flow System (DVRFS) map viewer or the Death Valley Regional Flow System Project web site.

Relevance and Benefits

Land-management agencies have been collaborating for many years on efforts to evaluate and respond to proposed regional ground-water withdrawals from Federal lands in undeveloped areas of Clark County. This project will enhance and expedite results related to these efforts. This study also will require collaboration between the USGS and land-management agencies and promote ongoing efforts to develop science-based solutions for challenges posed by rapid population growth in southern Nevada. Study results also will contribute to efforts to develop approaches for ecosystem management of the Amargosa Desert by various offices of the Bureau of Land Management, National Park Service, and U.S. Fish and Wildlife Service in Nevada and California.