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U.S. GEOLOGICAL SURVEY
Data Series 269

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Introduction

The Nevada Test Site (NTS) is located in Nye County, southern Nevada (fig. 1). The NTS was established in 1950 as a continental proving ground for the testing of nuclear weapons and alternative uses of nuclear explosions (U.S. Department of Energy, 2000, p. 48). Atmospheric nuclear testing at or above land surface at the NTS was conducted from 1951 to 1962. Underground nuclear testing at the NTS began in 1951 and continued until a worldwide moratorium on nuclear testing began in 1992. From 1951 to 1992, 828 underground nuclear tests were completed at the NTS (Laczniak and others, 1996, p. 21–22, table 4). Radionuclide contaminants may be introduced to ground water during underground nuclear tests if the depth at which the nuclear device is detonated or the depth of the shock cavity produced by the detonation is below the pre-test water table. Of the 828 underground nuclear tests conducted at the NTS, about 220 were detonated below or near the water table and are considered by Laczniak and others (1996, table 4) certain or probable sources of ground-water contamination.

A long-term program to investigate and remediate radionuclide contaminants generated at the NTS is a mission of the U.S. Department of Energy (USDOE), National Nuclear Security Administration Nevada Site Office, Environmental Restoration Program. Models of the ground-water flow system at and in the vicinity of the NTS are being developed to help the USDOE evaluate the risk that radionuclide contaminants may have on the public and the environment (Waddell, 1982; Belcher, 2004; Stoller-Navarro Joint Venture, 2006a, 2006b). Accurate ground-water temperatures are useful when developing ground-water flow models. Periodic ground-water temperature measurements can be used to detect temporal changes in ground-water temperature. Ground-water temperature depth profiles provide the data needed to correct measured water levels for temperature effects, to evaluate the direction and magnitude of ground-water flow within a well, and to calibrate hydrologic models with temperature targets.

Purpose and Scope

This report presents ground-water temperature data collected by the U.S. Geological Survey (USGS) at and in the vicinity of the NTS. Periodic ground-water temperatures were measured in 166 wells during calendar years 2000 through 2006 (pl. 1). In general, periodic ground-water temperatures were measured annually at about 5 and 55 feet below the water surface. Periodic ground-water temperature data are presented in appendix A.

Ground-water temperature profiles were collected in 73 wells during calendar years 2004 and 2005 (pl. 1). Ground-water temperatures were collected on one occasion in each well at multiple depths within the accessible water column. Ground-water temperature profile data are presented in appendix B.

Description of Study Area

The study area is the NTS and other areas in the vicinity of the NTS. The study area lies within the southern part of the Great Basin, an internally drained subdivision of the Basin and Range physiographic province (Fenneman, 1931). The predominant physiographic features of the Basin and Range are linear mountain ranges separating broad, elongated valleys. North-trending mountain ranges are elevated above the similarly trending sediment-filled valleys along large vertical fault displacements. Precambrian siliclastic and metamorphic rocks, Paleozoic carbonate and siliclastic rocks, and Tertiary volcanic rocks constitute the primary rock type of the hills, ridges, and mountain ranges in the study area. The valleys are filled with unconsolidated clastic sediment and semi-consolidated to consolidated clastic rocks, lacustrine limestone, and interbedded volcanic ash and lava flows (Reiner and others, 2002).

The climate at and in the vicinity of the NTS is arid. Climate information from Air Resources Laboratory, Special Operations and Research Division (2006), is summarized below. Annual precipitation ranges from about 2 in/yr at low altitudes (Laczniak and others, 1999, p. 7) to about 12 in. at high altitudes (Soule, 2006, p. vi). Precipitation is most common in winter, early spring, and mid-summer. Most precipitation is rain; however, snow is possible from autumn through late spring. Humidity at the NTS is fairly low, ranging from typically less than 35 percent in the summer to 70 percent in the winter. Average daily minimum and maximum temperatures at lower altitudes at the NTS are about -5°C to 15°C in the winter and about 15°C to 40°C in the summer. Average daily minimum and maximum temperatures at high altitudes are about -5°C to 5°C in the winter and about 15°C to 30°C in the summer.

More than 90 percent of the study area is contained within the Death Valley ground-water flow system, one of the major hydrologic subdivisions of the southern Great Basin (fig.1; Harrill and others, 1988, sheet 1). Ground water moves southward from recharge areas in central Nevada through carbonate and volcanic rock aquifers (Laczniak and others, 1996, p. 2). Precipitation at high altitudes along the flow path also recharges the carbonate and volcanic rock aquifers. Within the Death Valley ground-water flow system, ground water flows towards four major areas of surface discharge: Ash Meadows, Oasis Valley, Alkali Flat, and Death Valley (fig. 1). The remainder of the study area is contained within the Railroad Valley and South-Central Marshes ground-water flow systems.

The concept of ground-water subbasins helps explain regional ground-water flow at and in the vicinity of the NTS (fig. 1; Laczniak and others, 1996, p. 16–20). Ground-water subbasin boundaries are determined by the location of recharge and discharge areas, geology, hydraulic gradients, and ground-water chemistry. All ground water at the NTS flows within one of three ground-water subbasins of the Death Valley ground-water flow system: Ash Meadows, Oasis Valley, and Alkali Flat-Furnace Creek Ranch (fig. 1). Ground water in the vicinity of the NTS may flow either within these ground-water subbasins, in parts of the Death Valley ground-water flow system outside of these subbasins, or outside the Death Valley ground-water flow system. Ground-water levels at and in the vicinity of the NTS range in altitude from about 6,000 ft beneath the Kawich Range to below sea level at Death Valley (Laczniak and others, 1996, p. 16). The ground-water flow system or subbasin where ground-water temperature data-collection sites are located is provided in appendixes A and B.

A hydrogeologic unit is an assemblage of rocks and deposits of varying age, lithology, and structural properties (Laczniak and others, 1996, p. 10; Sweetkind and others, 2004, p. 34). Each hydrogeologic unit has distinct hydrologic properties based on this assemblage. Five significant regional hydrogeologic units are at and in the vicinity of the NTS (Laczniak and others, 1996, p. 10–16). These five regional hydrogeologic units are, from oldest to youngest, (1) the basement confining unit, (2) the carbonate-rock aquifer, (3) the Eleana confining unit, (4) the volcanic aquifers and confining units, and (5) the valley-fill aquifer.

The basement confining unit is a low-transmissivity confining unit composed of Eocambrian to Cambrian quartzite, micaceous quartzite, and siltstone. This hydrogeologic unit acts as the hydrologic basement for most of the study area.

The carbonate-rock aquifer consists of Cambrian to Devonian dolomite, interbedded limestone, and thin layers of shale and quartzite. Secondary openings from fractures and ground-water dissolution in the carbonate rocks make this regional hydrogeologic unit highly transmissive (Winograd and Thordarson, 1975, p. C74; Dettinger, 1989, p. 5). An upper carbonate-rock aquifer of Pennsylvanian age is in parts of the study area but is not considered a regional hydrogeologic unit. The upper and lower carbonate-rock aquifers are separated from each other by the Eleana confining unit; where the Eleana confining unit is absent, the upper and lower carbonate-rock aquifers act as a single hydrogeologic unit.

The Eleana confining unit, although limited in areal extent, is considered a regional hydrogeologic unit. The unit is low permeability, Mississippian to Devonian age siltstone, sandstone, and conglomerate.

Tertiary age volcanic-rock aquifers and confining units form a regional hydrogeologic unit. Highly fractured, dense volcanic rocks with abundant fractures, such as welded tuffs or lava flows, make up the volcanic aquifers whereas confining units are formed generally by nonwelded tuff that may be zeolitized (Winograd and Thordarson, 1975, p. C44). In general, this hydrogeologic unit forms localized aquifers; however, deep fractured volcanic-rock units may be considered a regional aquifer (Blankennagel and Weir, 1973, p. 6).

The valley-fill aquifer regional hydrogeologic unit consists of unconsolidated coarse- to fine-grained sediments with localized carbonate, volcanic, and sedimentary rock units (Sweetkind and others, 2004, p. 39–44). Ground-water flow within this unit is dependent on physical properties of the sediment, including grain size and amount of cementation (Belcher and others, 2006). In general, transmissive units within the valley-fill aquifer consist of coarser grained sediments like gravel and sand which, when saturated, may have high porosity and permeability (Laczniak and others, 1996, p. 15). Finer grained sediments, like silt, clay, and fine-grained sand, form confining units within the valley-fill aquifer.

The principal regional hydrogeologic unit contributing water to a well in which ground-water temperatures were measured is provided in appendixes A and B. Localized granitic stocks of Cretaceous age are not considered a regional hydrogeologic unit (Laczniak and others, 1996, p. 14, 25). However, granitic stocks are identified in appendixes A and B as a hydrogeologic unit when it is the principal rock type contributing water to a well.

Underground nuclear testing at the NTS (fig. 1) can affect local ground-water conditions. Any hydraulic property or water level measured in an underground nuclear test area may not be the same as pre-nuclear testing values (Laczniak and others, 1996, p. 42). Fractures resulting from underground nuclear testing can change aquifer permeability and storage properties. Interstitial fluid pressure in aquifers and ground-water temperature may be affected. Underground nuclear tests have changed water levels for months or years (Garber, 1971, p. C207; Thordarson, 1987, p. 12–16) and may affect ground-water flow rates and direction.

Ground-Water Temperature Data-Collection Sites

Periodic ground-water temperature data were collected in 166 wells located between latitude 36 and 39 degrees north and longitude 115 and 118 degrees west within Nye, Clark, and Lincoln Counties, Nev. (pl. 1; table 1, at back of report). A well, for the purpose of this report, is a bored, drilled, or dug hole with a depth greater than its largest surface dimension and a discrete open interval (U.S. Environmental Protection Agency, 1997; Fenelon, 2005, p. 6). Temperature data may be collected from multiple wells within a single borehole because each well has a distinct open interval. Ninety-two of the 166 wells at which periodic ground-water temperatures were collected are located within the boundaries of the NTS. About 95 percent of the wells with periodic ground-water temperature data are located in the Death Valley ground-water flow system (fig. 1). The remaining 5 percent are located either in the Railroad Valley or South-Central Marshes ground-water flow systems.

Ground-water temperature profiles were collected in 73 wells located between latitude 36 and 38 degrees north and longitude 115 and 117 degrees west within Nye County, Nev. (pl. 1; table 1). Of the 73 wells in which ground-water temperature profiles were collected, 52 are located within the NTS boundaries and 21 are located outside the NTS boundaries. Forty-one of the wells with ground-water temperature profile data are in the Ash Meadows ground-water subbasin, 16 are in the Oasis Valley ground-water subbasin, and 16 are in the Alkali-Flat-Furnace Creek Ranch ground-water subbasin.

All wells in which ground-water temperatures were measured are part of a USGS water-level monitoring network. This network is supported by the USDOE National Nuclear Security Administration Nevada Site Office, Environmental Restoration Program.

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