INTRODUCTION

Ash Meadows is one of only a few areas of natural discharge within a large, regionally extensive ground-water basin known as the Death Valley 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 parts of California. Centrally located within the boundaries of this flow system is the Nevada Test Site (NTS), which historically has been the primary continental-based location for testing nuclear devices. As a consequence of about 40 years of nuclear testing at this facility, significant quantities of radioactive and other chemical contaminants have been released into the subsurface at depths whereby many contaminants are in contact with ground water. Once within the ground-water system, contaminants are subjected to local flow conditions, and although retarded by chemical and physical processes, can begin moving in consonance with ground water. Ground water beneath the NTS generally moves southward and westward toward one of four areas of major natural ground-water discharge: (1) Ash Meadows, (2) Oasis Valley, (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 under its Environmental Restoration Program (ERP). As part of this program, the U.S. Department of Energy will evaluate the risk that these contaminants pose to the public. To accomplish this objective, the potential for contaminant migration must be determined and the hydrologic factors controlling their transport must 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 downgradient 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 the uncertainty in the current estimates of ground-water discharge, the U.S. Geological Survey, in cooperation with the U.S. Department of Energy, began a series of studies in 1993 designed to refine and improve previous estimates of ground-water discharge throughout the region.

The discharge area chosen for study first, was Ash Meadows. This selection was based in part on (1) the close proximity of the area to past locations of underground testing (less than 50 mi, fig. 1); (2) the potential for rapid water and contaminant transport through the highly fractured carbonate-rock aquifers contributing water to the area (Winograd and Pearson, 1976); (3) the availability of data acquired by previous and ongoing studies; and (4) the significance placed on the area as a National Wildlife Refuge and as the sole habitat for many threatened and endangered plants and animals native to the region. Additional investigations to refine estimates of ground-water discharge at other major discharge areas influencing ground-water flow away from the NTS are planned or are in progress as "follow ups" to the Ash Meadows study.

Purpose and Scope

The purpose of the study is to refine and improve the current estimate of ground-water discharge from the Ash Meadows area. The estimate of ground-water discharge presented in this report is computed from evapotranspiration rates determined from field measurements of micrometeorological data and extrapolated over the study area on the basis of similarities in vegetation, soil-moisture characteristics, and depth to ground water. This report presents the results of the study, describes the general approach used to determine ground-water discharge from evapotranspiration estimates, and documents and describes the methods used to measure evapotranspiration and extrapolate these measurements throughout the Ash Meadows region. The methods employed required the collection of micrometeorological data and water levels on a nearly continual basis. This intense data-collection effort generated a substantial amount of climatic and hydrologic data during the period of study (October 1993 through September 1997) that may be of value to other investigations of the region's climate, ecology, and hydrology. 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 Nichols and Rapp (1996), Nichols and others (1997), and the U.S. Geological Survey (1994-98), or can be requested from the Las Vegas office of the U.S. Geological Survey.

Acknowledgments

The authors express their appreciation to all agencies that cooperated in this study. These agencies include the U.S. Department of Energy, U.S. Fish and Wildlife Service, National Park Service, and Bureau of Land Management. The authors also thank the many individuals who contributed to the completion of the study. In particular, the authors extend thanks to David Ledig, and Beth and David St. George, U.S. Fish and Wildlife Service, who not only provided valuable assistance with access to many sensitive areas controlled and maintained as part of the Ash Meadows National Wildlife Refuge, but more importantly, taught us much about the ecology of the area's plants and wildlife. The authors also acknowledge William D. Nichols of the U.S. Geological Survey for his valuable insight into the techniques and instrumentation applied to measure evapotranspiration in arid environments of the southwest. And lastly, the authors wish to thank and express their appreciation to the many private landowners in the area that openly provided access to their property and extended their hospitality to project personnel having the opportunity to visit this unique and interesting part of the world. The genuine interest expressed by all involved in this effort was most greatly appreciated.

Location and Jurisdiction

Ash Meadows is in southern Nye County, Nev. (figs. 1 and 2), about 40 mi east of the Death Valley National Park headquarters near Furnace Creek Ranch, Calif., and 90 mi northwest of Las Vegas, Nev. The boundaries of Ash Meadows are not well established, but where defined loosely, the general area covers about 50,000 acres of desert uplands and spring-fed oases (Sada, 1990). Most of Ash Meadows is within southern Nevada, but some acreage, depending on boundary definition, may extend across the state line into California (fig. 3). About 23,000 acres of this total make up the Ash Meadows National Wildlife Refuge (U.S. Fish and Wildlife Service, 1988). The U.S. Fish and Wildlife Service controls most of the land within the refuge, with some acreage held by the Bureau of Land Management. Together these agencies manage the refuge under a plan to restore and maintain the area as a natural ecosystem -- the intent being the preservation of the local flora and fauna. Devils Hole (fig. 3) and a surrounding 40-acre tract are part of Death Valley National Park maintained and managed by the National Park Service. A few small-acreage parcels within the refuge remain in private holding.

General Description and Setting

Ash Meadows lies within the southern part of the Great Basin, an internally drained subdivision of the Basin and Range physiographic province. The dominant physiographic features 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 isolate north-trending mountain ranges from similar 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 clastic rocks constitute the primary rock type of the hills, ridges, and mountain ranges in the area. The intermontane basins are filled with sedimentary and volcanic rocks (valley fill), including sandstone, siltstone, lacustrine claystone and limestone, and volcanic ash and lava flows.

Ash Meadows sits at the southern end of the Amargosa Desert within the Amargosa Desert Hydrographic Area ¹ (fig. 2). The valley is not a typical Basin-and-Range valley in that it is oriented northwest-southeast as a consequence of right-lateral movement along strike-slip faults bounding the valley to the north and south. Although positioned on the floor of the Amargosa Desert valley and typified by a gently, southwesterly sloping terrain that ranges in altitude from about 2,100 to 2,400 ft above land surface, Ash Meadows' easternmost extent includes a series of low carbonate-rock hills referred to by Carr (1988) as the Amargosa Ridges. These local hills, although thousands of feet lower in altitude than the major mountain ranges that rim the valley, starkly contrast with the surrounding valley floor and protrude upward by as much as 900 ft forming fairly steep carbonate-rock outcrops.

Located in the north-central part of the Mojave Desert (fig. 1), the Ash Meadows area is typical of most other desert regions in that it is characterized by short mild winters, long hot summers, and low annual rainfall. Long-term climatic data specific to Ash Meadows are lacking, but estimates of mean annual values can be inferred from information available for nearby National Weather Service stations at Amargosa Farms, Nev. (latitude 36°34´ N., longitude 116°28´ W., altitude 2,450 ft); at Beatty, Nev. (latitude 37°00´ N., longitude 116°43´ W., altitude 3,550 ft); at Pahrump, Nev. (latitude 36°12´ N., longitude 115°59´ W., altitude 2,670 ft); and in Death Valley near Furnace Creek Ranch, Calif. (latitude 36°28´ N., longitude 116°52´ W., altitude 194 ft below sea level). Mean annual precipitation at the four National Weather Service stations ranges from about 2 to 6 inches. On the basis of these values, a reasonable estimate of the mean annual precipitation at Ash Meadows is between 2.5 and 4.25 inches. Sada (1990, p. 3) describes average annual rainfall for Ash Meadows at less than 2.75 inches. Mean annual temperature at these same weather stations ranges from about 60 to 77°F. A reasonable estimate of the mean annual temperature for Ash Meadows is about 65°F. Annual precipitation determined from rainfall data collected in Ash Meadows as part of this study was 4.3 inches in 1995, 2.4 inches in 1996, and 4.0 inches in 1997. The annual mean temperature measured at a weather station, maintained as part of this study at the National Wildlife Refuge headquarters (fig. 3), was 65°F in 1995 and 66°F in 1996. The minimum temperature recorded at the weather station during this 2-year period was 19°F and the maximum temperature was 112°F.

Unlike most desert communities, Ash Meadows has a high concentration of springs. More than 30 springs and seeps are aligned in an approximate linear pattern spanning about 10 mi. Springflow varies substantially across the area with a maximum measured discharge of nearly 3,000 gal/min at Crystal Pool (fig. 3). The combined measured discharge has been estimated at about 10,500 gal/min, equivalent to about 17,000 acre-ft/yr of water (Walker and Eakin, 1963; Winograd and Thordarson, 1975; Dudley and Larson, 1976). More than 80 percent of the measured springflow discharges from nine of the springs. Although long-term discharge measurements are not available at every spring and seep, periodic measurements made at many of the major springs indicate that springflow has been fairly constant throughout recent history (Tim Mayer, U.S. Fish and Wildlife Service, written commun., 1997). The only exception was in the late 1960's and early 1970's when local agricultural interests pumped large quantities of ground water to irrigate local fields (Dudley and Larson, 1976). During this period of extensive pumping, local springflows decreased and water levels declined throughout much of the area. One major consequence of ground-water depletion was a drop in the pool level in Devils Hole, a shaft-like opening into the ground-water system through carbonate (limestone and dolomite) bedrock created by a collapse into a steeply dipping fault-controlled fissure. The ground-water pool provides the sole remaining natural habit for the endangered Devils Hole pupfish (Cyprinodon diabolis). The potential effects of pool decline on the continued existence of the pupfish compelled the U.S. Supreme Court to establish a minimum pool level for Devils Hole, essentially prohibiting any significant pumpage from the local area. Shortly after the mandate, all agricultural and development interests in the area faded, water levels began recovering, and springflows returned to nearly the previously measured rates (Westenburg, 1993).

A large diversity of plants, fish, and local wildlife are dependent on water provided by the numerous local springs scattered throughout the area. Many plants and animals are native to the area, which is distinguished as having the largest concentration of endemic species of any locale in the continental United States (U.S. Fish and Wildlife Service, 1988). Spring pools and associated drainages provide habitat to several species of fish and a few rare aquatic insects. Vegetation throughout the area is diverse with denser growths concentrated along spring pools and drainages, and poorly drained bottomland. The vegetation provides food and shelter to numerous birds, insects, reptiles, and small mammals. Plant assemblages and species are numerous and include many varieties of grasses, reeds, shrubs, and trees. Areas influenced by local springflow include groves of ash (Fraxinus velutina var. coriacea), cottonwood (Populus fremontii), willow (Salix exigua), and mesquite (Prosopis glandulosa torreyana and P. pubescens); thick stands of saltcedar (Tamarix aphylla, T. parviflora, and T. ramosissima); expansive meadows of saltgrass (Distichlis spicata var. stricata), wire-grass (Juncus balticus, J. cooperi, and J. nodosus), and bunch grass (Sporobolus airoides); and open marshland of cattails (Typha domingensis), reeds (Phragmites australis), and bulrush (Scirpus robustus). More typical Mojave Desert flora, primarily sparse covers of healthy creosote bush (Larrea tridentata), saltbush (Atriplex canescens and A. polycarpa) and desert holly (A. hymemelytra), dominate upland areas not influenced by local spring discharge.

The primary drainage within Ash Meadows is Carson Slough (fig. 3), a local tributary of the Amargosa River (fig. 2). Carson Slough is intermittent and seldom flows through its entire extent, except after infrequent storms. Short reaches of the slough, directly downgradient from major springs, flow throughout the entire year. The length of reach flowing and the amount of flow varies during the year with longer, more continuous, and greater flows in winter, when temperatures are cooler and vegetation is dormant, thus reducing local water losses through evapotranspiration. Numerous small, unnamed channels, which exhibit seasonal fluctuations in flow similar to Carson Slough, drain many of the larger local springs. A few irrigation ditches and impoundments (Crystal and Peterson Reservoirs, fig. 3) constructed to support past human activities in the area remain. Most other manmade structures have been removed as part of the management plan being implemented by the U.S. Fish and Wildlife Service (USFWS) to return the area to a natural ecosystem.

General Hydrology

The abundance of water available to the Ash Meadows area, when compared with that of other desert environments, is attributable to the area's unique hydrogeology (Winograd and Thordarson, 1975; Dudley and Larson, 1976). The many springs and shallow water table of the area are maintained primarily by ground water that moves into the area from the north and northeast through thick, semi-continuous rock units composed of fractured limestone and dolomite (figs. 2, 3, and 4). Together these carbonate-rock units make up what is referred to as the "regional carbonate-rock aquifer" (Dettinger and others, 1995; Prudic and others, 1995). Ground water moving through this aquifer originates from precipitation falling on the higher mountain ranges and mesas throughout an area that extends hundreds of miles to the north and east (Winograd and Thordarson, 1975; Waddell and others, 1984; Laczniak and others, 1996). Throughout much of the area between Ash Meadows and the highlands at which the water originates, the carbonate-rock units carrying most of the ground water are buried by thick accumulations of basin fill, and the water table typically is several hundred to several thousand feet below the land surface. Along this journey, ground water moves primarily through interconnected fractures, possibly dissolving some of the host carbonate rock and enhancing the pathways through which it flows. Ground water approaching Ash Meadows from the northeast is thought to be channeled between two occurrences of impermeable rock (Winograd and Pearson, 1976) -- one beneath the northwestern part of the Spring Mountains and the other beneath the western part of the Specter Range (fig. 2).

Upon entering Ash Meadows, ground-water flow is impeded by the presence of one or more buried faults that down drop the bedrock block (carbonate?) underlying that part of the Amargosa Desert valley beneath and west of Ash Meadows (figs. 2, 3, and 4). Collectively, these faults are referred to as the Ash Meadows fault system (previously termed the "gravity" fault by Winograd and Thordarson, 1975, pl. 1). The contrast in water-transmitting properties between the more permeable faulted and fractured carbonate rock and the juxtaposed, less permeable lacustrine, palustrine, and alluvial valley-fill deposits hinders southwestwardly flowing ground water forcing it upward to the surface (Winograd and Thordarson, 1975, p. 82). Some of the water being pushed upward from the regional carbonate-rock aquifer discharges from springs emerging directly from faults in the bedrock along the margins of some of the carbonate ridges. Some of the water discharges from springs emerging from alluvium, which likely are fed by water from or associated with faults in the underlying carbonate bedrock (fig. 4). The remainder of the water in the regional carbonate-rock aquifer beneath Ash Meadows either seeps slowly upward into the alluvial cover or continues flowing southwestward as underflow across the Ash Meadows fault system into the central part of the Amargosa Desert.

¹ Formal hydrographic areas in Nevada were delineated systematically by the U.S. Geological Survey and Nevada Division of Water Resources in the late 1960's (Rush, 1968; Cardinalli and others, 1968) for scientific and administrative purposes. 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.