Document Type

Technical Report

Publication Date

1965

Abstract

The Las Vegas ground-water basin as described in this report includes the southern part of Indian Spring Valley, Three Lakes Valley, the northern half of Ivanpah Valley, and Las Vegas Valley. These valleys in part are inferred to form a hydrologic unit that includes an area of about 3,000 square miles in the southern part of Clark County, Nev.

The valleys forming the Las Vegas ground-water basin are broad structural depressions surrounded by mountains. The climate of the region is arid, and precipitation in the basin lowlands rarely exceeds 5 inches per year. Drainage is interior except for occasional Hood runoff and waste from the industrial plants at Henderson; the occasional flood runoff and waste flow through the Las Vegas Wash to Lake Mead. The mountain ranges are composed of indurated rocks that impede the movement of ground water from the basin and form the boundary of the ground-water reservoir.

The valley fill that forms the principal ground-water reservoir is composed of a thick sequence ranging from Miocene (?) through Recent in age. Most of the ground water is in a large leaky artesian system comprising four principal zones of aquifers—the deep, middle, and shallow zones of artesian aquifers and the near-surface zone of water-table aquifers. To facilitate quantitative analyses of recharge, discharge, and yield, the aquifers have been divided arbitrarily into an artesian system, which includes the three principal artesian aquifers, and the near-surface water-table system.

Natural recharge to the artesian system is from precipitation in the mountains
within the drainage area of the basin. As ground water moves from areas of
recharge toward areas of discharge in the lower parts of Las Vegas Valley, it
becomes confined between relatively impermeable beds. Nearly impervious
barriers caused by faulting of the valley fill impede the lateral movement of the ground water. Artesian pressure causes the water to leak upward along the
faults into shallow aquifers.

The average annual natural recharge to the Las Vegas artesian system was estimated by the following methods: (1) Estimation of consumptive use by phreatophytes under natural conditions prior to ground-water development; (2) estimation of ground-water underflow to the Las Vegas area during a period of near-stability in the ground-water reservoir; and (3) study of the relationship between precipitation and recharge during periods of equilibrium in the ground-water reservoir. Estimates of the natural recharge based on these methods suggest that the average annual natural recharge to the basin is on the order of 25,000 acre-feet. The total draft on the artesian aquifers in 1955 was approximately 48,000 acre-feet, of which about 42,000 acre-feet was from wells and springs and about 6,000 acre-feet was from upward leakage. Therefore, overdraft on the artesian reservoir in 1955 was about 23,000 acre-feet.

Recharge to the near-surface reservoir in 1955 was derived from upward leakage from the underlying artesian aquifers and by infiltration of waste water. The total estimated recharge to the near-surface reservoir in 1955 was about 25,000 acre-feet. The draft on the near-surface reservoir in 1955 was about 25,000 acre-feet, of which about 24,000 acre-feet was discharged by phreatophytes and about 1,000 acre-feet was discharged by wells. During this study, the near-surface reservoir was in a state of approximate dynamic equilibrium.

The above data indicate that the overdraft on the entire Las Vegas ground-water basin in 1955 was approximately equal to the overdraft on the artesian aquifers—that is, about 23,000 acre-feet.

As the artesian heads continue to decline in Las Vegas Valley, the quantity of
ground water lost through upward leakage and subsequent transpiration will
decrease. Conditions of optimum development of the artesian system will be
achieved when the artesian heads have been lowered to about 50 feet below land surface, because most of the upward leakage that is currently discharged by phreatophytes will have been eliminated. At the present rate of decline of the artesian head and with the present distribution and amount of withdrawals, the artesian head will be lowered sufficiently to prevent most natural discharge in the Las Vegas area in about 40 years and in the Paradise Valley area in about 75 years. The maximum sustained yield that can be developed from the artesian aquifers when the artesian head has been lowered below the root zone of phreatophytes will be limited to the approximate average annual natural recharge to the artesian system plus the amount of nonconsumptively used ground water and imported water which becomes available for reuse through downward leakage or artificial recharge.

The accumulated annual discharge from the Las Vegas artesian aquifers has
exceeded the accumulated recharge since the development of the first successful well in 1906, and as a result the artesian pressure has declined almost uninterruptedly from year to year since that time. The artesian head in selected observation wells in the vicinity of Las Vegas and Paradise Valley declined about 30 feet between 1941 and 1956. During this time the approximate cumulative overdraft amounted to about 300,000 acre-feet. The amount of overdraft per foot of lowering of artesian head within the area of approximately 40 square miles represented by the observation wells during this period was about 10,000 acre-feet.

The chemical quality of the ground water in Las Vegas Valley is, in general,
better in the northern than in the southern part of the valley. In the northern
part of the valley, water from the shallow and middle zones of aquifers is of better quality than water in the deep zone of aquifers. As the ground water migrates southward into the Paradise Valley and the Whitney-Pittman areas, the water in the middle and shallow zones becomes more mineralized than the water in the deep zone of aquifers.

The lowering of artesian head in the Las Vegas area between 1935 and 1950
has caused about 180 millimeters of local land subsidence in the vicinity of the Bonanza Street underpass, where the sediments are predominantly clay and silt. In the western part of the valley, where the sediments are predominantly sand and gravel, subsidence during the same period of time was negligible.

Keywords

Artesian wells; Geological surveys; Groundwater; Groundwater flow--Measurement; Nevada--Clark County; Nevada--Las Vegas; Nevada--Lincoln County; Nevada--Nye County; Water conservation; Water-supply; Water-supply--Analysis

Disciplines

Desert Ecology | Environmental Monitoring | Environmental Sciences | Fresh Water Studies | Hydrology | Natural Resources and Conservation | Water Resource Management

Language

English

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