Water-Resources Investigations Report 01-4239
Ground-Water Discharge Determined from Measurements of Evapotranspiration, Other Available Hydrologic Components, and Shallow Water-Level Changes, Oasis Valley, Nye County, Nevada
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Oasis Valley is one of only a few areas of natural discharge within a large, regionally extensive ground-water basin known as the Death Valley regional ground-water flow system (fig. 1). This flow system, as defined by Harrill and others (1988), extends hundreds of miles over a geologically complex, arid to semi-arid region of southern Nevada and adjacent California. Centrally located within the boundaries of this flow system is the Nevada Test Site (NTS), a Federal facility that for more than 40 years was used to test nuclear devices. This nuclear testing released significant quantities of radionuclides to the subsurface of parts of the NTS. Radionuclides in ground water beneath the NTS may have the potential to migrate from the NTS in the direction of ground-water flow. Ground water beneath the NTS generally moves southward and westward toward one of four areas of major natural ground-water discharge: (1) Oasis Valley, (2) Ash Meadows, (3) Alkali Flat, and (4) Death Valley (Winograd and Thordarson, 1975; Waddell and others, 1984; Laczniak and others, 1996).
Contaminants generated at the NTS are the subject of a long-term program of investigation and remediation by the U.S. Department of Energy (USDOE) under its Environmental Restoration Program (ERP). As part of this program, the USDOE is evaluating potential transport of radionuclides from the NTS to adjacent areas. This objective requires that the potential for contaminant migration be determined and the hydrologic factors controlling their transport be reasonably well known. Because the rate and direction of ground-water flow away from the NTS is controlled in part by the location and amount of water leaving the flow system, any accurate assessment of contaminant migration is predicated on having a sound understanding of ground-water discharge. Although the general locations of the discharge areas are known, much uncertainty exists as to the precise amount of water leaving the flow system at each of these locations. To reduce this uncertainty, the U.S. Geological Survey (USGS), in cooperation with the USDOE, began a series of studies in 1993 designed to refine and improve previous estimates of ground-water discharge throughout the region.
Oasis Valley is one of the discharge areas chosen for study based in part on (1) the area's proximity to past locations of underground testing (a distance of less than 17 miles, figs. 1 and 2); (2) the potential for rapid water and contaminant transport through the highly fractured volcanic aquifers that contribute water to the area (Fridrich and others, 1999); (3) the availability of data about Oasis Valley acquired by previous and ongoing studies; and (4) the use of water in the area by ranches in upper Oasis Valley and by residents of Springdale and Beatty, Nev. Related investigations to refine estimates of ground-water discharge at other major discharge areas influencing ground-water flow away from the NTS have been completed (Laczniak and others, 1999, 2001) or are in progress.
The purpose of the study is to refine and improve the current estimate of ground-water discharge from Oasis Valley. This report presents a new estimate of ground-water discharge computed from evapotranspiration (ET) rates, subsurface outflow, and ground-water withdrawal. ET rates were calculated from field measurements of localized meteorological information (referred to as micrometeorological data) and extrapolated over the study area on the basis of similarities in vegetation, soil-moisture characteristics, and depth to ground water. Subsurface outflow was estimated using Darcy's Law and estimates of hydraulic gradient, aquifer geometry, and hydraulic conductivity. Ground-water withdrawal was compiled from local public water supply records and estimates of non-municipal use. This report presents the results of the study and describes the general approach used to estimate ground-water discharge.
The method used to determine ET rates required the collection of micrometeorological data and water levels on a nearly continuous basis. This intense data-collection effort generated a substantial amount of climatic, ecological, and hydrologic data. This report is not intended to be a comprehensive data compilation, and presents only those data most pertinent to its final conclusions. Other data specific to the study can be found in previously published reports by Reiner and others (1999) and the USGS (1996-2000), or can be requested from the Las Vegas Subdistrict Office of the USGS.
The authors express their appreciation to all agencies that cooperated in this study. These agencies include the U.S. Department of Energy (USDOE), Bureau of Land Management (BLM), Beatty Water and Sanitation District (BWSD), Beatty General Improvement District, and Nevada Division of Water Resources (NDWR). The authors also thank the many individuals who contributed to the completion of the study. In particular, David L. Berger, Donald H. Schaefer, and Armando R. Robledo of the USGS provided valuable assistance with geophysical surveys and logging. The authors also thank the many private landowners and residents in the area, including Mr. and Mrs. Glenn L. Coffer, Mr. David Spicer, Ms. Sharon Patton-Bailey, and Mr. Ed Peacock, who allowed access to their property and extended their hospitality to project personnel. The genuine interest expressed by all involved in this study is greatly appreciated.
Oasis Valley is in southern Nye County, Nev. (figs. 1 and 2), about 40 mi north of the Death Valley National Park headquarters near Furnace Creek Ranch, California, and 120 mi northwest of Las Vegas, Nev. The boundaries of the Oasis Valley Hydrographic Area1, as established by Rush (1968), encompass about 300,000 acres of desert uplands and spring-fed oases (fig. 2). About 40,000 acres of this area overlies a valley-fill aquifer; the valley floor contains about 3,800 acres of phreatophytes that discharge ground water by ET (Malmberg and Eakin, 1962). The BLM administers most of the land within the area, and the remaining acreage is held by private citizens and local governments.
Oasis Valley 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, formed in response to a long and still active period of crustal extension. Large vertical displacements along faults offset bedrock blocks that topographically isolate north-trending mountain ranges from similarly trending sediment-filled valleys (fig. 2). Most of the ranges in the general region are composed of pre-Cenozoic rocks of diverse age and lithology. Paleozoic carbonate and siliceous rocks and Tertiary volcanic rocks constitute the primary rock type of the hills, ridges, and mountain ranges in the area. The intermontane basins are filled with (1) unconsolidated clay, silt, sand, gravel, and boulders; and (2) semi-consolidated to consolidated conglomerate, sandstone, siltstone, claystone, lacustrine limestone, and interbedded volcanic ash and lava flows.
The Oasis Valley Hydrographic Area borders Gold Flat to the north, Sarcobatus Flat to the west, Amargosa Desert, Crater Flat, Yucca Mountain to the south, and Timber Mountain and Pahute Mesa to the north and east (Rush, 1968; fig. 2). The Oasis Valley Hydrographic Area does include the western part of Gold Flat and parts of western and central Pahute Mesa (Laczniak and others, 1996).
Oasis Valley is located in the south-central and southwestern part of the Oasis Valley Hydrographic Area and is generally bounded by Pahute Mesa to the north and northeast, Springdale Mountain and the Bullfrog Hills to the west, Bare Mountain to the south, Yucca Mountain to the southeast, and Timber Mountain to the east (fig. 2). The Oasis Valley discharge area is located between Oasis Mountain and Bare Mountain on the floor of Oasis Valley (fig. 3). The valley floor is typified by a gently southward-sloping terrain ranging in altitude from 3,900-4,000 ft above mean sea level at the northernmost part of the discharge area to approximately 3,200 ft at the lowermost discharge area south of Beatty. The Oasis Mountain Hogback (fig. 3), a rectangular bedrock exposure east of the Hogback Fault (fig. 4), starkly contrasts with the surrounding valley floor. The Oasis Mountain Hogback protrudes upward by as much as 400 ft, forming fairly steep volcanic-rock outcrops.
The climate in Oasis Valley is typical of many desert regions that are characterized by short mild winters, long hot summers, and low annual rainfall. Long-term climatic data specific to Oasis Valley can be inferred from information available for the National Weather Service station Beatty 8N (station number 260718-4) located near Beatty, Nev. (Desert Research Institute, Western Regional Climate Center, electronic data accessed at <http://www.wrcc.dri.edu/summary/climsmnv.html> on June 17, 2001) (pl. 1). Mean annual precipitation during the station's period of record (1972-2000) was 6.33 in. The maximum and minimum recorded annual precipitation was 12.62 in. in 1998 and 2.43 in. in 1989. The maximum and minimum average monthly precipitation occur in March (1.10 in.) and June (0.22 in.), respectively. Annual precipitation during this study was 5.6 in. in 1996, 6.6 in. in 1997, and 12.6 in. in 1998. Precipitation collection at the weather station was incomplete in 1999 and 2000. However, precipitation was determined to be 4.7 in. in 1999 and 6.8 in. in 2000 based upon available weather station data and supplemental rainfall data collected at multiple evapotranspiration stations in Oasis Valley.
Mean annual temperature at weather station Beatty 8N, for its period of record, was 59.0°F in 1973. Mean monthly temperatures ranged from 78.8°F in July to 41.3°F in January. Temperatures ranged between 112°F on July 7, 1989, and 2°F on December 22, 1990. Mean annual temperatures during the study were 61.2°F in 1996, 60.2°F in 1997, 57.5°F in 1998, 59.7°F in 1999, and 58.8°F in 2000. Mean annual temperature during this study was 59.5°F.
In contrast to most desert basins, Oasis Valley has a high concentration of springs (fig. 4). Structurally controlled conduits and changes in rock unit lithology and thickness produce the more than 70 springs and seeps located in Oasis Valley. Although long-term spring discharge measurements are unavailable for these springs or seeps, some periodic measurements are in Malmberg and Eakin (1962), Thordarson and Robinson (1971), White (1979), and McKinley and others (1991).
A diverse community of plants depends on water provided by the numerous springs scattered throughout Oasis Valley. This community includes many varieties of grasses, reeds, shrubs, and trees, with denser growths concentrated along spring pools and drainages. Areas influenced by spring flow include groves of desert ash, cottonwood, and desert willow; expansive meadows of saltgrass, bunchgrass, and wire grass; and open marshland of cattails, reeds, and bulrush. Sparse to moderately dense covers of greasewood, rabbitbrush, and wolfberry are found in areas peripheral to those influenced by spring flow. The densest population of trees is found in the southernmost part of Oasis Valley along the Amargosa River drainage. Upland areas not influenced by spring discharge are dominated by flora more typical of the Mojave Desert, primarily sparse covers of creosote bush, saltbush, and desert holly.
The riparian and desert aquatic environments of Oasis Valley provide food and shelter to numerous birds, insects, fish, reptiles, amphibians, and mammals. Some animals benefiting from local aquatic desert and riparian habitats are Neotropical migratory birds, endemic snails, Oasis Valley speckled dace, and the Amargosa Toad. A population of wild burros also is found in the area.
Within Oasis Valley, the primary drainage is the intermittent Amargosa River (figs. 1, 2, and 3), which seldom flows through its entire extent except following infrequent storms. Short reaches of the river, directly downgradient from major springs, flow throughout the year. The length of reach flowing and the amount of flow vary during the year; longer, more continuous, and greater flows typically occur in winter. Precipitation is more likely in the winter, during which water losses through evapotranspiration are reduced by cooler temperatures and the dormancy of the vegetation. Beatty Wash and Thirsty Canyon (fig. 4), located in central and northern Oasis Valley, respectively, do not flow except during and after more intense storms. Numerous small unnamed channels, which may exhibit seasonal fluctuations in flow similar to the Amargosa River, drain many of the larger local springs and wetlands. A few impoundments near springheads and irrigation ditches have been constructed to support human activities in the area.
The many springs and a shallow water table in Oasis Valley are maintained primarily by ground water that moves into the area through a regional volcanic-rock aquifer system. This system is made up of a series of interlayered aquifers and confining units. Ground water in Oasis Valley originates in areas to the north and northeast. Its recharge area includes Pahute Mesa in the NTS (Laczniak and others, 1996). Geologic structures such as faults and caldera boundaries affect the flow path of the southward-moving ground water. Springs typically occur in these areas where ground water encounters faults.
Four major hydrogeologic units make up the regional volcanic-rock aquifer system in Oasis Valley. These units are the alluvial aquifer, nonwelded-tuff confining unit, welded-tuff aquifer, and basement confining unit. The alluvial aquifer, which overlies the volcanic units, consists primarily of valley-fill deposits and has a relatively high effective porosity and moderate matrix permeability (Fridrich and others, 1999). The valley-fill deposits typically consist of Quaternary sand, silt, clay, and gravel in the Oasis Valley discharge area and Tertiary sand and gravel in other parts of Oasis Valley. The aquifer usually is unconfined except where locally overlain by low-permeability deposits. The thickness of valley-fill deposits typically ranges from 10 to 100 ft in the Oasis Valley discharge area and exceeds 1,000 ft in the area east of the Hogback fault (fig. 4).
Throughout much of Oasis Valley, a nonwelded-tuff confining unit separates the alluvial aquifer from the underlying welded-tuff aquifer. This confining unit has fairly high matrix porosity, but low permeability (Fridrich and others, 1999), is fairly continuous, and extends laterally to the southwest from a concealed area near Pahute Mesa. The unit terminates in the south at the Hot Springs Fault and in the west at the Hogback Fault. The thickness of the nonwelded-tuff confining unit typically ranges from 100 to 1,000 ft and reaches a maximum in the area east of the Oasis Mountain Hogback (fig. 4).
The regional welded-tuff aquifer consists of numerous subhorizontal layers of Tertiary-age welded tuffs, lavas, and bedded tuffs. The welded-tuff aquifer generally has low effective porosity and moderate fracture permeability (Fridrich and others, 1999); however, the permeability of the intra-unit layers varies. The aquifer typically is confined either by the overlying non-welded tuff confining unit or by the low-permeability intra-unit layers (Fridrich and others, 1999). The thickness of the welded-tuff aquifer decreases south and west from the southwestern Nevada volcanic field (SWNVF) toward Oasis Valley (fig. 4). Welded-tuff aquifer thickness, according to gravity data from Hildenbrand and others (1999), averages about 10,000 ft within the SWNVF central caldera complex, about 5,000 ft in the area between the central caldera complex and the eastern edge of the Oasis Valley discharge area, about 2,500 ft beneath the discharge area, and less than 1,600 ft west of the discharge area, and thins to extinction at the southern end of Oasis Valley.
Paleozoic sedimentary rocks and Miocene intrusive rocks underlie the welded-tuff aquifer and form the basement confining unit beneath Oasis Valley. This very-low-permeability confining unit may locally include some high-permeability carbonate rocks. Within this unit, the carbonate rocks are subordinate to the very-low-permeability clastic rocks and granitoids and lack the continuity to host any substantial regional ground-water flow (Fridrich and others, 1999).
Geologic structures found throughout the area act both as conduits and barriers to ground-water flow. These geologic structures include, but are not limited to, faults and caldera boundaries. Conduits generated by these geologic structures often create preferred pathways, typically along the strike or at the intersections of multiple faults, where permeability is enhanced by faulting. Barriers typically are perpendicular to strike and most often are caused by juxtaposition of less-permeable against more permeable rock or by low permeability associated with fault gouge (Fridrich and others, 1999). Major geologic structures in Oasis Valley (fig. 4) most likely to influence ground-water flow are (1) the Thirsty Canyon fault zone, a northeast-striking fault zone/lineament, (2) the north-striking Hogback, Bare Mountain, and Beatty faults, (3) the east-west striking Colson Pond, Fleur-de-Lis, and Hot Spring faults, and (4) the Fluorspar Canyon-Bullfrog Hills detachment fault.
Hydraulic gradients based on regional water-level data indicate that ground water discharging at Oasis Valley originates from areas to the north and northeast of the valley (Laczniak and others, 1996). Precipitation on local highlands and subsurface flow from areas to the north are primary sources of discharged water. Recharge occurs at higher elevations in western Pahute Mesa, Timber Mountain, the Bullfrog Hills, and Bare Mountain (figs. 1 and 3). Only a minimal amount of ground water flows into Oasis Valley from the west (Malmberg and Eakin, 1962). The water table between upland recharge areas and the Oasis Valley discharge area typically is several hundred to several thousand feet below the land surface (Malmberg and Eakin, 1962).
Most of the ground water flowing south-southwestward into Oasis Valley through the welded-tuff aquifer is diverted upward along faults (fig. 5). These diversions are a consequence of enhanced permeability along faults, contrasts in water-transmitting properties caused by the juxtaposing of hydrogeologic units along faults, a general thinning of the welded-tuff aquifer approaching Oasis Valley, and the termination of the welded-tuff aquifer against nearly impermeable siliciclastic rock at the southern end of Oasis Valley.
Springs occur throughout Oasis Valley where upward diversions coincide with areas in which the potentiometric surface is above land surface. Ground water entering Oasis Valley through the welded-tuff aquifer that is not discharged as springflow either flows upward and recharges the valley-fill deposits, is withdrawn for human uses, or flows laterally out of the valley as subsurface outflow. The most likely pathway for this subsurface outflow is to the south through the alluvium-filled Amargosa River channel into the Amargosa Desert. Other potential but less likely pathways for subsurface outflow are to the southeast under a ridge separating Oasis Valley and Crater Flat (fig. 4) (Fridrich and others, 1999) and to the south from the Bullfrog Hills into the Amargosa Desert (fig. 3).
About 75 springs and seeps are mapped throughout Oasis Valley. Flow rates range from less than 1 gal/min to more than 200 gal/min. Water temperatures range from about 60°F to more than 100°F (White, 1979; McKinley and others, 1991). Although flow and temperature characteristics vary, most of the springs in Oasis Valley can be grouped according to their hydrogeologic setting (fig. 4):
(1) Colson Pond group: Includes springs located along the Colson Pond fault. These springs probably form as a result of a transmissivity change across the Colson Pond fault. Their likely source is water flowing from the north and northeast beneath Pahute Mesa.
(2) Oasis Mountain Hogback group: Includes springs located west of the Hogback fault. These springs probably form as a result of an abrupt west-ward thinning of the welded-tuff aquifer across the Hogback fault. Their likely source is water flowing from Pahute Mesa.
(3) Amargosa River group: Includes springs along the Amargosa River north of Beatty. These springs probably form as a result of a transmissivity change and a disruption in aquifer continuity across the Beatty fault. Their likely source is a mixture of the water flowing into Oasis Valley from the east, west, and north.
(4) The Hot Springs group: Includes springs located in the central part of the Oasis Valley discharge area along the east-west-striking Hot Springs fault. Elevated water temperatures of about 105°F indicate probable upward flow along the fault from deeper parts of the flow system. Their likely source is flow from the east and north, possibly Timber Mountain and/or Pahute Mesa.
(5) Lower Amargosa River group: Includes springs issuing from channel-fill deposits along the Amargosa River south of Beatty. Their primary source probably is water flowing from the north through Oasis Valley.
(6) Upper Amargosa River group: Includes springs located in the northwest fork of the Oasis Valley discharge area. These springs probably form as a result of a transmissivity change and disruption in aquifer continuity across the Beatty fault. Their likely source is inflow from the north and northwest (White, 1979).
(7) Bullfrog Hills group: Includes springs located west of the Amargosa River channel. These springs probably form as a result of permeability changes within the welded-tuff aquifer caused by hydrothermal alteration. Their likely source is local recharge to nearby highlands.
1 Formal hydrographic areas in Nevada were delineated systematically in the late 1960's by the U.S. Geological Survey and Nevada Division of Water Resources for scientific and administrative purposes (Cardinalli and others, 1968; Rush, 1968). The official hydrographic-area names, numbers, and geographic boundaries continue to be used in Geological Survey scientific reports and Division of Water Resources administrative activities.
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