Open-File Report 01-335
Potentiometric Surface, Carbonate-Rock Province, Southern Nevada and Southeastern California, 1998-2000
Water-level contours shown on plate 1 represent the regional potentiometric surface of ground water in consolidated rocks of the carbonate-rock province. Assumptions made during construction of the contour lines are that: (1) in primarily carbonate rock, water levels represent the water table, (2) water flows nearly horizontally through the aquifer, (3) there are no hydrologic barriers between wells and springs, and (4) inferred flow paths are perpendicular to the potentiometric contours. The contour lines were generated initially by ARC/Info (a comprehensive Geographic Information System software package), then were refined according to known hydrogeologic conditions. Although these water-level contours are based on current conditions, uncertainties remain. First, water levels are based on approximate land-surface altitudes derived from topographic maps and global positioning system field measurements. Second, the potentiometric surface may be poorly defined in areas where wells are sparsely distributed.
The full extent of contour lines that can be shown on plate 1 is limited by lack of available data from wells completed primarily in carbonate rock. The dashed parts of the contour lines show areas that are similar to previous representations of regional ground-water levels (Winograd and Thordarson, 1975; Waddell and others, 1984; Thomas and others, 1986, 1996; Laczniak and others, 1996), and are supported only by current water-level measurements in alluvial wells that are thought to have a good hydraulic connection between carbonate rocks and basin fill.
In the study area, basin-fill deposits overlying carbonate rocks are the source of most of the ground water being pumped (Dettinger, 1989). Composed primarily of sand and gravel, basin-fill aquifers facilitate ground-water flow into and out of the carbonate rock. Together with carbonate-rock aquifers, these basin-fill aquifers support regional flow systems. Water levels in highly permeable basin-fill aquifers can represent water levels in carbonate aquifers where highly permeable carbonate rocks are overlain by basin-fill deposits. Ertec Western, Inc. (1981) concludes that in many places basin-fill deposits are hydraulically connected with adjacent and underlying carbonate rocks, resulting in one continuous ground-water flow system that is bounded by noncarbonate rocks or structural features. Thomas and others (1986) also notes that basin-fill deposits generally are more permeable than carbonate rocks and are capable of storing and transmitting vast quantities of water. Therefore, in areas in which carbonate and basin-fill aquifers are hydrologically connected, the interpolated water-level gradient between wells completed primarily in carbonate rock, and wells completed in basin-fill deposits may be less steep than contour lines indicate (pl. 1).
Springs are found where fractures in carbonate rock are exposed to the surface below the associated water table. Water-level altitudes at these springs, based upon the land-surface altitude of the spring orifice (table 3), were used to define regional water levels in the carbonate-rock aquifer (pl. 1). These water levels will be slightly lower than the vertically averaged water level because of the lack of bounded rock around the orifice of the spring.
Prudic and others (1995) described geologic barriers that affect regional ground-water flow in the carbonate-rock province. These barriers consist of isolated complexes of possible Mesozoic age, metamorphic rocks; Early Cambrian and Late Precambrian clastic sedimentary rocks; Paleozoic and Precambrian metamorphic core complexes; and Precambrian crystalline basement rocks (Coney, 1980). Isolated, Mesozoic-age metamorphic rocks generally are identified as mobile metamorphic-plutonic basement rocks, overlain by unmetamorphosed rocks that are deformed by low-angle extensional faults (Prudic and others, 1995). The thickness and subsurface distribution of these barriers at depth are not known. The region in which these rocks are known to exist is structurally complex; differing rock types potentially are more extensive at depth than is indicated by outcrops. Surface exposures of these geologic barriers (pl. 1, red-shaded areas) are shown as one unit. Where such barriers exist, geologic or otherwise, contours have been omitted.
Potentiometric contours also have been omitted in areas where measured water levels, between wells, did not provide enough gradient for interpretation. For example, ground water has been described previously to flow through carbonate rocks in a south-southeasterly direction from Coyote Spring Valley to Moapa Valley (Winograd and Thordarson, 1975) (pl. 1). Deuterium mass-balance ratios were used (Thomas and others, 1996) to identify the source of water flowing into Moapa Valley. These ratios suggest that 62 percent of the discharge from the Muddy River Springs in Moapa Valley originated in Coyote Spring Valley, and that the remaining 38 percent originated in the Sheep Range (Thomas and others, 1996). Water levels measured in Moapa and Coyote Spring Valleys define a potentiometric surface that is relatively flat throughout Moapa Valley. The potentiometric contour drawn in Moapa Valley (pl. 1) can be substantiated only by the altitudes of the Muddy River Springs. As noted, water-level altitudes at the Muddy River springs are based upon land-surface altitudes at the spring orifices. The inherent error associated with these water levels at the Muddy River springs and relatively similar ground-water altitude in wells surrounding the Moapa Valley are factors that make interpretation of the potentiometric surface difficult.
Geologic structural features may further inhibit ground-water flow in Moapa Valley. Winograd and Thordarson (1975) described the Las Vegas Valley shear zone (pl. 1) as a barrier to flow. Plume (1996) identified the combined effects of the crystalline basement and thin clastic sedimentary rocks in the valley as factors that inhibit ground-water flow and force water to the surface as spring discharge. Furthermore, the Sevier Thrust Belt, which is composed of Mesozoic-age compressional features, could be a barrier to ground-water flow and support a mechanism for its discharge at the Muddy River Springs. In addition, the carbonate rocks from Coyote Spring Valley to Moapa Valley appear to be more transmissive than surrounding carbonate rocks (pl. 1). This interpretation is supported by the finding that ground-water gradients are less than 1 ft/mi (2.04 m/km) near wells 18 and 16, compared to gradients of more than 10 ft/mi (20.39 m/km) upgradient of well 18 and downgradient of Muddy Springs. Transmissivity of the Paleozoic carbonate rocks from Coyote Spring Valley to Moapa Valley probably has been enhanced by secondary permeability from Late Tertiary extensional fracturing and faulting (Schmidt and Dixon, 1995).
Figure 1, figure 2, and figure 3 indicate the general trend of regional water-levels from 1985 to 2000. Although these graphs are relatively incomplete, there is some indication of fluctuation in water levels that is probably caused by seasonal changes and by variations in recharge. However, these data show relatively little change within the period of record (1985 to 2000). The influence of earth tides and barometric pressure on ground-water levels was not considered when these graphs were compiled. These global and atmospheric influences do not significantly affect long-term (10 year and greater) trends in water levels (Fenelon, 2000). Bright and others (2001) noted that water-level measurement errors at Desert Rock station at the Nevada Test Site near Mercury were greater than the rate of long-term changes in barometric pressure, therein concealing any barometric-pressure influences. Evaluation and explanation of trends in water levels and spring discharge as shown in these graphs are beyond the scope of this report. However, analysis of water-level trends within the central Great Basin (Dettinger and Schaefer, 1995) suggests that upward trends in water levels may be the result of long-term variations in regional precipitation.
Current (1998-2000) water-level values were obtained from the USGS National Water Information System (NWIS). Additional ground-water levels and spring altitudes were obtained from the Bureau of Land Management and the Southern Nevada Water Authority, and through field checking and updating of data from existing wells and springs. Water levels and spring discharges shown in figure 1, figure 2, and figure3 are based on quarterly ground-water measurements and periodic spring measurements by the USGS and on periodic and continuing ground-water measurements by the Nevada Division of Water Resources. Two criteria were used in choosing these hydrographs -- that they provide: (1) a distribution of wells and springs that convey a good regional representation of water-level trends, and (2) a selection of wells and springs with the most complete period of record from 1985 through 2000. Well parameters were taken from drillers' logs, the USGS NWIS database, and an International Technology Corporation data documentation package (1996).
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