Thick sequences of carbonate rocks underlie most of the alluvial basins within the Basin and Range area in eastern Nevada and southeastern California (fig. 60); these rocks also extend into western Utah, northwestern Arizona, and southeastern Idaho. The carbonate rocks have been faulted, deformed, and eroded through geologic time; original thicknesses of up to 40,000 feet have been reduced by one-half or more. Consequently, most of these rocks are in isolated blocks that form individual aquifers with areal dimensions of only a few square miles. In Nevada, however, the carbonate rocks form a north-south section of aquifer, or "central corridor" (fig. 61), that is generally laterally continuous for more than 250 miles. The southern part of this corridor has been most studied, and two major flow systems have been identified. In both flow systems, ground water is recharged in east-central Nevada. In one system, ground water discharges at Ash Meadows and Death Valley and, in the other, primarily at Muddy River Springs (fig. 61).
These aquifer systems contain the closest approximation to regional flow systems in the Basin and Range area. Flow is transmitted along several valleys through the basin-fill and the carbonate-rock aquifers. However, an insufficient number of wells have been drilled in the carbonate rocks to permit a complete description of the flow system. Discharge from large springs in the alluvial basins is always in areas underlain by carbonate rocks.
The most complete description of the carbonate-rock flow system is from the ancestral White River/Muddy River Springs area, which consists of 13 valleys in southeastern Nevada (fig. 62). During Pleistocene time, when the climate was more humid than at present, the White River drained six of these valleys and left a wash along its drainageway that is the lowest point anywhere along the drainage system. Five of the remaining seven valleys--Long, Jakes, Cave, Dry Lake, and Delamar--are topographically closed and have no streams that flow out of them. The Garden and the Coal Valleys form a topographically closed unit, in which streams in the Garden Valley drain into the Coal Valley. The entire 13-valley drainage area is bounded by mountains that range in altitude from about 7,000 feet in the south to more than 9,000 feet in the north (fig. 62).
Consolidated rocks (fig. 63) of Paleozoic and Tertiary age form the boundaries of and underlie the basins, which are filled with unconsolidated deposits of Tertiary and Quaternary age. The principal water-yielding rocks of Paleozoic age are limestone and dolomite, which are bounded above and below by confining units of shale, sandstone, and quartzite. The rocks of Tertiary age are primarily tuff and welded tuff and generally transmit little water. However, in areas where the welded tuffs have been fractured, they can yield large quantities of water. The basin-fill deposits consist of coarse sand and gravel at the valley margins and grade to fine silt and clay near the center of the valleys. Valleys that were transversed by the White River during Pleistocene time contain channel deposits of sand and gravel in some places along the ancestral course of the river near the center of the valley.
Data used to determine the movement of ground water in the White River/Muddy River Springs area are from wells completed in basin-fill sediments and carbonate rocks, altitudes of spring orifices, and several mine shafts in carbonate rocks in the bordering mountains where water levels have been measured. The overall pattern of ground-water movement is shown in figure 64. Ground water moves southward from Long Valley to the White River Valley. The White River Wash defines the principal avenue of flow from the White River Valley to the upper Moapa Valley. Springs issue at several places along the White River Wash wherever the water table is at or near the land surface. Valleys that border the main drainageway of the White River--the Garden, the Coal, the Cave, the Dry Lake, and the Delamar Valleys--do not drain directly to the main drainage-way but rather to the south where the ground water leaves them.
The topography of the drainage area controls the movement of ground water at a regional scale. The flow is in a southerly direction along the axis of the valley as shown by the decrease in water levels along the profile in figure 65, and is generally within tens of feet of the land surface in the center of the valleys. Three areas along this profile show that water levels may be hundreds of feet below land surface. The deep water table in Jakes Valley is caused by the high altitude of the valley floor. Some interpretations of the flow system have identified Jakes Valley as the northern limit of the White River/Muddy River Springs flow system. The deep water table in the Pahroc Valley and at the end of the Pahranagat Valley is thought to be along faults where the fractured rocks have been either cemented to form a barrier to flow or partly dissolved downgradient of the faults so that the permeability is greater and water is more quickly transmitted down-valley from the faults.
The intervalley ground-water flow pattern and the estimated water budget for the flow system are shown in figure 66. The estimated recharge to the valleys was based on the relation of altitude to annual precipitation (table 2). Because of the orographic effect of the mountains, the amount of precipitation increases as the altitude increases. Conversely, the amount of evapotranspiration decreases as altitude increases, as a consequence of lower temperatures at higher altitudes. Because of higher altitudes and lower temperatures in the northern one-half of the White River/Muddy River Springs drainage area, about 70 percent of the recharge is estimated to be in this one-half of the area. From measurements of spring discharge, about 62 percent of the discharge of the system has been determined to be in the Pahranagat and the upper Moapa Valleys in the southern one-half of the area. This distribution of recharge and discharge is additional evidence of interbasin flow. The substantial amount of ground water that moves from the northern to the southern one-half of the area must flow through the carbonate rocks.
Interbasin flow within the carbonate rocks also is indicated by the water chemistry of perennial springs. Three classifications of the types of flow systems that feed the springs have been defined in the White River/Muddy River Springs area--regional, large local, and small local. Chemical analyses of water that discharges from perennial springs indicates that increased concentrations of dissolved constituents associated mostly with carbonate rocks are observed with regional systems characterized by long flow paths. Gypsum, anhydrite, halite, and scattered clay minerals in the carbonate rocks are dissolved more readily than carbonate minerals and they contribute sodium, potassium, chloride, and sulfate ions to ground water. Concentrations of these ions increase the longer the ground water is in circulation through the carbonate rocks. At each perennial spring that issues from the carbonate rocks, an indication of the relative distance the water has traveled can be determined by the concentration of these ions. Also, the temperature of the water discharged from a spring indicates the probable depth of ground-water circulation; higher temperatures are associated with the deep circulation of regional systems.
The concentration of tritium in water that issues from perennial springs has been used to determine places where ground water has had a short residence time in an aquifer. Before 1954, tritium, a hydrogen isotope, was scarce in the atmosphere. Beginning in 1954, detonation of thermonuclear bombs released large concentrations of tritium to the atmosphere in the northern hemisphere. Springs with water that contains tritium in concentrations of 200 units or more (fig. 67) are likely to be discharging a large percentage of ground water that has been in the aquifer for only a short time, perhaps only a few months.
A plot of concentrations of sodium and potassium ions against chloride and sulfate ions in water from the carbonate rocks in Nevada (fig. 68) shows the relation of these ions to the regional, large local, and small local flow systems. Regional flow systems are characterized by interbasin flow, long flow paths, and one or more local systems that feed the regional system. Springs connected to these systems have large perennial discharges and small seasonal ranges in discharge. Discharge waters contain relatively large concentrations of sodium, potassium, chloride, and sulfate ions but have small concentrations of tritium (fig. 68). Some springs discharge thermal water (greater than 80 degrees Fahrenheit).
Large local flow systems are characterized by predom-inantly interbasin flow and flow paths that are typically confined to one basin. Springs connected to these systems have moderate to large discharges and moderate seasonal ranges in discharge. Discharge waters contain moderate concentrations of sodium plus potassium and chloride plus sulfate (fig. 68) and no significant concentrations of tritium. Discharge waters have temperatures that typically range from 50 to 60 degrees Fahrenheit.
Small local flow systems are generally characterized by very short flow paths, usually no more than a few miles in length. Springs connected to these systems have highly variable annual ranges in discharge. Discharge waters have small concentrations of dissolved sodium plus potassium and chloride plus sulfate, large concentrations of tritium, and water temperatures that commonly approach average air temperatures.
The evidence of regional flow in the carbonate rocks can be used to evaluate the water-supply potential of the White River/Muddy River Springs Basin. Even if the carbonate-rock aquifer, as yet undeveloped, can supply additional water, the effect of such development upon the aquifer remains uncertain. The effect could be minimal if development can merely capture ground water currently being lost to evapotranspiration in the southern part of the White River/Muddy River Springs area. More information about the aquifer system, particularly data that pertains to aquifer boundaries, is needed in order to accurately determine the potential effects of development
