The Pahrump Valley is part of an intervalley ground-water flow system. The regional movement of ground water is generally southwestward to low areas adjacent to the Amargosa River. The major areas of ground-water discharge downgradient from the Pahrump Valley are between the towns of Tecopa and Shoshone, Calif., which are 10 to 15 miles southwest of the topographic boundary of the Pahrump Valley (fig. 35).
Mountain-building activity in southern Nevada has affected the ground-water flow system in the Pahrump Valley. Several large thrust faults are exposed in the Spring Mountains and at the northern end of the Nopah Range (fig. 36). In some places, low-permeability clastic rocks have been displaced by the faulting so that they are above or adjacent to water-yielding carbonate rocks and thus restrict ground-water movement in the carbonate rocks. Under some conditions, permeable zones of broken rock along the fault planes might be conduits for ground water. Springs and stands of mesquite along the northwestern sides of these faults, however, suggest that the faults are barriers to ground-water flow and that the ground water moves upward along the barriers until it emerges at the land surface. Folding, associated with the faults, produced joints and fractures in some of the rocks, resulting in significant secondary permeability.
Two distinct aquifers are in the Pahrump Valley--the carbonate-rock aquifer, formed of carbonate rocks that bound and underlie the valley, and the basin-fill aquifer, which consists of unconsolidated deposits that have accumulated in the structural depression of the valley (fig. 37). The carbonate rocks transmit water readily and carry significant ground-water flow from the Pahrump Valley into the adjacent Chicago and Amargosa River Valleys to the southwest. The carbonate-rock aquifer is virtually undeveloped and significant future development is improbable because it is necessary to drill wells to great depths in order to obtain adequate yields. The basin-fill aquifer is, therefore, the source of virtually all withdrawals.
The carbonate-rock aquifer consists primarily of carbonate rocks of Triassic to Cambrian age that crop out in the Spring Mountains (fig. 36) and underlie the basin fill of the Pahrump Valley (fig. 37). The aquifer extends westward and southwestward through the Nopah and the Resting Springs Ranges into the California and the Chicago Valleys (figs. 35 and 37). Because no well in the Pahrump Valley penetrates the carbonate-rock aquifer, the hydraulic properties of the aquifer are inferred from information obtained in other areas. Hydraulic continuity in the carbonate-rock aquifer is the result of an extensive network of interconnected fractures and, to a small degree, of localized solution openings. Estimates of transmissivity from nearby localities outside the valley ranged from 130 to 120,000 feet squared per day as determined from aquifer-test data from 10 wells. The greater the transmissivity, the more water the aquifer will yield. The wide range in estimated transmissivity might not be randomly distributed and might reflect variations that result from faulting as well as the number and size of solution openings.
The basin-fill aquifer consists of unconsolidated alluvial and lacustrine deposits that partly fill the structural depression of the Pahrump Valley. Coarse-grained materials have been deposited near the sides of the valley, and fine-grained lacustrine materials are in the central parts of the valley (fig. 37). The approximate areal extent of the basin-fill aquifer is 650 square miles, or about two-thirds of the total area of the Pahrump Valley. To the northeast, northwest, and southwest, the aquifer is bounded by consolidated rocks of the Spring Mountains and the Resting Springs, the Nopah, and the Kingston Ranges. To the southeast, the aquifer is bounded by a ground-water divide beneath a topographic high that separates the Pahrump and the Mesquite Valleys. Because no ground water flows across this divide, the ground-water flow systems of the two valleys are separate.
Wells drilled into the basin-fill aquifer range from several tens of feet to more than 1,000 feet deep. With the exception of one or two wells near the margin of the aquifer, the wells do not fully penetrate the basin fill. Therefore, the thickness of the basin-fill aquifer was estimated from geophysical meas-urements. The maximum thickness of the aquifer is about 4,800 feet in the central part of the valley (fig. 38). In general, the thickest accumulations of basin fill parallel the axis of the valley. The area of maximum thickness is offset slightly toward the south end of the valley, suggesting some faulting or folding in that area.
Estimates of the transmissivity of the basin-fill aquifer (fig. 39) are representative only of the upper 1,000 feet of the aquifer, which is the part penetrated by most wells. Variations in transmissivity are related to the deposition of the coarser materials and the position of the water table. Transmissivity values increase from the edge of the Spring Mountains, where the saturated materials are thin, toward the center of the valley, where the land surface is flatter, the water table approaches the land surface, and the aquifer is thickest. The increase in saturated thickness within the zone of coarse materials provides the highest transmissivity values; values are greater than 4,000 feet squared per day in the Pahrump and the Manse Fans. Transmissivity values decrease in nearly parallel bands across the valley to less than 1,000 feet squared per day as the sediments become finer and the saturated thickness lessens near the mountains on the southwest side of the valley.
Virtually all the ground water in the Pahrump Valley is derived from precipitation. Most ground-water recharge occurs in the mountains, where percolating water moves through bedrock fractures to the zone of saturation, and on the upper slopes of the alluvial fans, where streamflow percolates through the unsaturated basin fill downward to the zone of saturation. The general slope of the ground-water surface in the Pahrump Valley before development (before 1913) is shown in figure 40. This map was constructed by using the earliest measurements available and shows the approximate configuration of the potentiometric surface of the basin-fill aquifer. Ground-water flow was generally from the principal recharge areas adjacent to the Spring Mountains, southwestward across the valley towards the Nopah Range. Water left the valley by evapotranspiration in the areas of shallow ground water and by subsurface outflow beneath the Nopah Range. The contours in figure 40 suggest that as ground water flowed southwest across the northwestern part of the Pahrump Valley, it moved into and through outcrops of carbonate rocks. The final discharge area for this water is not known with certainty. The 2,600-foot contour in figure 40 indicates that the hydraulic gradient in the northwestern part of the valley is toward the Ash Meadows discharge area in the Amargosa Desert north and west of the Pahrump Valley (fig. 35). However, the majority of the ground-water flow probably moved to a discharge area along the Amargosa River between the towns of Shoshone and Tecopa, which are southwest of the Pahrump Valley.
Ground water has been developed to support agriculture in the Pahrump Valley for many years. Two large springs, Bennetts and Manse Springs (fig. 35), provided water to early travelers and were soon developed as a source of supply for irrigation. In the late 1800's, Bennetts Spring reportedly discharged about 7.5 cubic feet per second (5,430 acre-feet per year), and Manse Spring, about 6 cubic feet per second (4,340 acre-feet per year); most of this water was diverted to agriculture. Spring flow decreased dramatically in 1913 when ground-water withdrawals began. Bennetts Spring eventually ceased to flow as the water table declined in response to withdrawals ( fig. 41), and Manse Spring ceased to flow during the 1975 irrigation season.
Ground water provides the water supply for virtually all uses in the Pahrump Valley. The first well was drilled in the valley in 1910 in an unsuccessful attempt to obtain a flowing well. However, in 1913, three flowing wells were successfully completed, and, by 1916, a total of 28 wells had been drilled, 15 of which were flowing. The number of new wells drilled and the annual withdrawals increased slowly until the mid-1940's when large-capacity wells were installed. From the mid-1940's through 1962, the annual discharge from wells increased from an estimated 4,000 to about 28,000 acre-feet (fig. 42).
From 1962 to 1975, population growth caused significant change in land use in the Pahrump Valley. Ground-water withdrawals increased rapidly between 1962 and 1968 (fig. 42) but decreased after 1968 as agricultural land was taken out of service and subdivided for residential use. Population of the valley increased from about 250 in 1962 to nearly 1,500 in 1975, and, when this land becomes fully developed, ground-water withdrawals could surpass those of 1968.
Between 1913 and 1975, nearly 700,000 acre-feet of ground-water withdrawals and about 550,000 acre-feet of spring flow had been discharged from the basin-fill aquifer in the Pahrump Valley. The two most apparent effects of the discharge were large ground-water level declines and a cessation in spring discharge. Water levels have been declining since the first wells were constructed in 1913. Variations in the annual rate of decline and the net change between predevelopment and 1975 water levels (fig. 43) among different locations depend on the distribution of withdrawal, the hydraulic properties of the basin fill, and the depth of the well being measured. Hydrographs of six wells are shown in figure 44 to illustrate the typical response of water levels to withdrawals in various parts of the valley. Generally, the greatest water-level declines (about 100 feet) were along the terminal parts of the Pahrump and the Manse Fans. Two wells located in the central part of the valley, away from the major concentration of withdrawal, showed substantially less water-level decline.
Withdrawals in the Pahrump Valley are distributed to capture ground-water flow downgradient of the Pahrump and Manse Fans and capture the discharge of Bennetts and Manse Springs in an effective manner (fig. 45). Consequently, spring discharge began to decrease shortly after withdrawals began and continued to decrease until 1975, when discharge of both springs ceased entirely during the irrigation season (fig. 41). Bennetts Spring ceased to flow in 1959; as of 1975, Manse Spring was dry during the summer irrigation season but recovered to discharge about 200 acre-feet during the winter months. The changes in spring discharge from 1875 through 1975 are shown in figure 41. The amount by which spring discharge has decreased is the amount of water captured by withdrawals. Thus, as of 1975, about 9,800 acre-feet per year of spring discharge had been captured by ground-water withdrawals.
