Evaluation of Groundwater Flow in the Southern Part of the Amargosa Desert, Nevada and California

USGS has revised the regional-scale Death Valley regional groundwater flow system (DVRFS) numerical model with new data and interpretations since the original numerical flow model was published in 2004 and republished in 2010. Since the original publication of the DVRFS flow model, additional data have been collected and interpretations have been made. Cooperators such as the Bureau of Land Management, National Park Service, U.S. Fish and Wildlife Service, the Department of Energy, and Nye County, Nevada, recognized a need to update the existing regional numerical flow model to maintain its viability as a groundwater management tool for regional stakeholders.

The Death Valley regional groundwater flow system (DVRFS) is located in the southern Great Basin physiographic province in southern Nevada and eastern California.  Because of the arid nature of this region and its increased urbanization, there are many varying and competing interests involving the regional ground-water resources. 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 groundwater discharge and decreasing shallow water tables. Groundwater discharges at Ash Meadows National Wildlife Refuge, at numerous springs and seeps, and along parts of the Amargosa River. Additionally, groundwater is intersected at Devils Hole, a fissure in a regional carbonate-rock aquifer. These areas support habitat for numerous species of plants and animals, including several federally-listed threatened and endangered species. To evaluate potential impacts from pumping to water levels and discharge within these areas of concern, USGS is 

1.  developing a ground-water flow model embedded in the DVRFS model that has sufficient resolution to address local-scale water-resource issues within the study area,
2.  evaluating the influence and control of known geologic structures on ground-water flow, and
3.  evaluating 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. These areas may include Ash Meadows National Wildlife Refuge, selected reaches of the Amargosa River, Devils Hole, or the hydraulic connection between Pahrump and southern Amargosa Desert at Stewart Valley.

As part of this study, the existing DVRFS numerical flow model (v. 1.0) was converted to MODFLOW-2005, updated with the latest available data, and recalibrated. The updated model is now referred to as the DVRFS numerical flow model version 2.0 (v. 2.0). Five main data sets were revised:

1.  recharge from precipitation varying in time and space,
2.  pumping data,
3.  water-level observations,
4.  an updated regional potentiometric map, and
5.  a revision to the digital hydrogeologic framework model.

The DVRFS model is intended to meet the long-term needs of a number of Federal, State, and local entities that require a hydrogeologic and hydrologic conceptualization and simulation of the regional flow system. The updated regional numerical model simulates the groundwater conditions of the model domain through time and may be used to

1.  inform definition of boundary conditions for the Underground Test Area (UGTA) Corrective Action Units (CAUs) on the Nevada National Security Site (NNSS),
2.  evaluate the effects of changes in groundwater-system fluxes, regardless of whether the changes are natural or human-induced,
3.  provide a technical basis to inform evaluations and decisions on groundwater availability to meet water demands on and off the NNSS,
4.  address concerns about potential declines in groundwater levels as a result of increased pumping in local areas,
5.  provide a framework for determining effective contaminant source plume, ambient trend, and point-of-use groundwater-quality monitoring locations, and
6.  test alternative conceptual models of the regional groundwater system.

Detailed information about the updates to the DVFRS model is available in USGS Scientific Investigations Report 2016-5150.