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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5044

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Introduction

The potential for transport of radionuclides and other test-generated contaminants away from areas of past underground nuclear testing at the Nevada Test Site (NTS) is of great concern and interest to the U.S. Department of Energy and to State and certain Federal regulatory agencies. Currently, numerical models are being developed to simulate the flow of ground water and the transport of contaminants away from underground tests detonated in the subsurface of Rainier Mesa and Shoshone Mountain (RMSM). As part of this effort, geologic data and well information have been integrated spatially to create a three-dimensional hydrostratigraphic framework model (HFM) of the local hydrostratigraphy. The HFM portrays the ground-water flow system as a complex arrangement of aquifers and intervening confining units. This hydrostratigraphic framework serves as the foundation for the ground-water flow and transport models, which will be used to formulate decisions related to the remediation of contaminants introduced into the flow system as a consequence of underground testing.

The direction and rate of transport within the local aquifers is controlled in part by the hydraulic-head gradient. Hydraulic head defines the water potential at a given location and commonly is estimated by converting a measurement of depth to water in a well to a water-level altitude. The spatial distribution of water-level altitudes across the RMSM area has been portrayed historically by maps showing a single set of generalized water-level contours. These maps either are regional in scale (Fenske and Carnahan, 1975; Waddell and others, 1984; Laczniak and others, 1996; D’Agnese and others, 1998) or focus specifically on areas to the west (Blankennagel and Weir, 1973; O’Hagan and Laczniak, 1996), southwest (Robinson, 1984; Ervin and others, 1993; Tucci and Burkhardt, 1995), or east of the study area (Doty and Thordarson, 1983; Hale and others, 1995). Water-level maps showing separate sets of contours for rocks of Cenozoic and pre-Tertiary age in the areas surrounding, but not including, the study area were constructed by Winograd and Thordarson (1975).

Maps generalizing the water-level distribution with a single set of contours, by their very nature, ignore vertical flow components and represent the complex subsurface geology of the area as a single, continuous, regionally extensive flow system. Contrarily, as is indicated by published hydrostratigraphic framework models, the ground-water flow system is made up of multiple aquifers that are separated hydraulically by confining units. The degree of hydraulic interconnection between these aquifers varies depending on the permeability of the intervening confining rock. The hydraulic separation of the aquifers by low-permeability units creates multiple, semi-independent systems, in which the primary flow directions and rates are controlled by the head gradient within the aquifer. The directions of flow between two adjacent aquifers may or may not be similar. Successful and accurate simulation of the potential transport of test-generated contaminants requires a sound understanding of the rate and direction of ground-water flow within each aquifer. This understanding can be gained by a more thorough integration of hydrologic and geologic information and an accurate depiction of the water-level contours and corresponding hydraulic gradients within each of the major aquifers.

Purpose and Scope

The purpose of this report is to show the areal distribution of aquifers beneath the RMSM area and to develop maps of water-level contours that define the likely direction of ground-water flow within each of the major aquifers. These contour maps are intended to represent natural or predevelopment ground-water conditions. Predevelopment conditions are assumed to represent steady-state or near steady-state conditions prior to any human activities in the area, such as pumping and nuclear testing. The contour maps are designed to conceptualize and describe ground-water flow within and between aquifers in the multi-aquifer ground-water flow system. The maps and their companion water levels can serve as calibration targets for future flow models and help in determining likely ground-water flow paths.

Maps included in the report provide a generalized delineation of the spatial distribution of aquifers, major flow directions within these aquifers, and potential recharge areas. Maps also show areas of lateral inflow to and outflow from the major aquifers. The report provides well-construction and water-level data derived from boreholes drilled in the RMSM area. Open intervals in a well are associated with aquifers and confining units defined and delineated by the three-dimensional hydrostratigraphic model of the RMSM area (National Security Technologies, LLC, 2007). The well-construction, water-level, and hydrostratigraphic data can be displayed with interactive spreadsheets that accompany the report.

Description of Study Area

The study area is about 90 mi northwest of Las Vegas in Nye County, Nevada, on the NTS and encompasses Rainier Mesa and Shoshone Mountain (fig. 1). Together, these two topographic highlands form the eastern extent of an extensive volcanic plateau that spans most of the western half of the NTS. The topography of the area is defined by the many varying physiographic and topographic features including mesas, mountains, and valleys. Pahute Mesa and Timber Mountain bound the study area on the west and the Belted Range, Yucca Flat, and CP Hills on the east (fig. 1). Altitudes in the area range from about 4,500 ft in Mid Valley and Yucca Flat to about 6,800 ft at Shoshone Mountain and 7,600 ft at Rainier Mesa.

The Rainier Mesa and Shoshone Mountain areas were used to test underground nuclear devices, primarily in tunnel complexes (fig. 2) mined into low-permeability, zeolitized tuff. The complexes were mined westward into the steep eastern sidewall of the local highland that demarcates the transition from more typical basin and range to upland plateau. All underground tests in the RMSM area were detonated above the regional water table. Sixty-six of these tests were detonated in tunnel complexes and two were detonated in vertical shafts (U.S. Department of Energy, 2000).

The climate of the area is described as semiarid high desert and is characterized by low precipitation and humidity and large fluctuations in daily and annual temperatures. Annual precipitation ranges from less than 5 in. on the valley floor of Yucca Flat to nearly 12 in. on Rainier Mesa (Soulé, 2006). Precipitation occurs primarily in winter to early spring and in mid-summer. Precipitation falls primarily as rain and as snow at high altitudes during the winter months. Temperatures are cold in the winter and hot in the summer, and range from lows of near 0°F in mid-winter to highs of more than 100°F in mid-summer. Temperatures generally are 10 to 20°F cooler on the mesas and mountains than in the valleys and can fluctuate daily by more than 30°F.

Geologic and Hydrologic Setting

The Rainier Mesa and Shoshone Mountain area forms a volcanic upland preserved in part by a dense cap of welded volcanic tuff. The caprock is underlain by a thick sequence of less-dense and less-resistive Tertiary-age bedded tuffs that are underlain by thousands of feet of massive bedrock of pre-Tertiary age. The older bedrock sequence consists of Precambrian and Paleozoic-age sedimentary rock deposited by ancient transgressing and regressing seas. The sedimentary bedrock is intruded locally by Cretaceous-age granites and granodiorites. The entire assemblage is overlain by the aforementioned Miocene-age volcanic rock and variably thick deposits of primarily Miocene-age and younger sedimentary rocks and partially consolidated to unconsolidated deposits of sand, gravel, and clay.

The sequence and position of the local rocks have been modified by structures associated with the complicated tectonic and volcanic history of the area. After deposition, the Paleozoic-age and older rocks in the area were subjected to compressive forces that warped and altered the bedrock by thrusting and folding. Following this period of compression, the rocks were subjected to extensional forces that pulled apart the bedrock and produced normal faults. Concurrent with the normal faulting, Cretaceous-age granitic magma intruded into the local area. This initial deformational episode lasted through the Mesozoic Era and was followed by a period of subdued tectonic activity; during this time the exposed bedrock surface was reshaped by erosion. This interlude was followed by a second period of extension when the rocks again were pulled apart along low-angle normal and strike-slip faults. The downdropping of bedrock blocks began the formative stages of the generally north-south trending mountain ranges and valleys that characterize the Basin and Range physiographic province of today. Successive volcanic eruptions in the late-Tertiary Period produced at least six large and partially overlapping calderas in the NTS area that make up the southwestern Nevada volcanic field (Sawyer and others, 1994). Rocks extruded by active volcanoes and local landslides and eroded sediments filled the local calderas. The volcanic rocks blanketed the surrounding region with extensive sheets of tuff and local lava flows. The relatively young, Tertiary-age tuff deposits form the uppermost layered sequence of rocks in the RMSM area. The Quaternary Period was dominated primarily by erosion and basin-filling processes that shaped the area into its modern topography.

The rocks of the study area form a complex interconnected series of aquifers and confining units, commonly dissected and offset by local faulting. The pre-Tertiary bedrock units are classified according to their hydrologic properties into two basic categories: carbonate aquifers and siliceous confining units. The siliceous confining units are composed of siliciclastic and granitic rocks. The Tertiary-age volcanic rocks form volcanic aquifers and volcanic confining units, and the Tertiary and Quaternary-age basin-fill deposits form alluvial aquifers and alluvial confining units. In the carbonate and volcanic aquifers, ground water moves primarily through secondary fracture openings that occasionally are enhanced by dissolution. In the alluvial aquifer, ground water moves through interstitial openings between grains. Geologic structures, such as faults, commonly influence the flow of ground water. Faults can impede flow by juxtaposing a less permeable rock against a more permeable rock. Alternatively, faults can enhance flow, primarily along strike because of the locally increased secondary permeability caused by intense crushing and fracturing of the rock within the fault zones.

Ground water generally flows through the aquifers in a southerly direction toward downgradient discharge areas south and southwest of the study area in Beatty, Amargosa Desert, and Death Valley (Winograd and Thordarson, 1975; Laczniak and others, 1996). Some ground water is discharged from the aquifers by the pumping of wells. Local pumping began in 1951, and through 2006, about 24 billion gallons of ground water have been pumped from the NTS, primarily from 16 wells (U.S. Geological Survey, 2008). Within the RMSM area, wells completed in four boreholes (WW-2, WW-8, UE-2ce, and UE-16d WW; fig. 2) have had significant (greater than 1 million gallons) amounts of water withdrawn for supply or investigative purposes. About 3.3 billion gallons of water were withdrawn from these wells from 1962 to 2006.

Much of the ground water flowing beneath the NTS region originates from precipitation falling on highlands at and to the north of the NTS. Locally, water recharges the ground-water flow system beneath upland areas in the western part of the NTS (fig. 2). This local recharge area generally is bounded on the east by Rainier Mesa and Shoshone Mountain. Precipitation falling on Rainier Mesa and other nearby areas of high precipitation collects in the fractures and openings that dissect the caprock. Some of this trapped water infiltrates downward through interconnected fractures or through the rock matrix to depths beyond the influence of active evaporation and transpiration (Russell and others, 1987). The less-permeable volcanic tuff present beneath Rainier Mesa and elsewhere beneath these upland areas impedes the downward movement of water through interconnected fractures, creating local zones of perched and semi-perched ground water (Thordarson, 1965). The term “semi-perched” serves to distinguish zones of shallow, elevated water that are underlain by saturated rocks from perched zones, which by definition are underlain by unsaturated rocks (Meinzer, 1923). The few springs that are present in the study area (fig. 2) are low flow and supported by perched and semi-perched water that moves laterally until it intersects the land surface and discharges.

The recognition and delineation of a regional saturated zone beneath the upland recharge areas is complicated by the presence of perched water. Recharge on eastern Pahute Mesa and Rainier Mesa has created a local water-level mound that influences ground-water flow directions in the perched and semi-perched zones and in the underlying shallow saturated flow system. Water within the unsaturated rock or in semi-perched and perched zones beneath the Rainier Mesa and Shoshone Mountain underground test areas may move test-generated contaminants downward into more regional, saturated, permeable rock. Here, transport is controlled primarily by ground-water flow—the rate and direction of which depends on the permeability of the host rock and on local and regional differences in hydraulic head.

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