Description
Yucca Valley is a small desert community located in the northern foothills of the Little San Bernardino Mountains, San Bernardino County, California. The population of about 10,000 is entirely dependent on local ground water resources from the Warren Ground Water Basin for domestic water supply. Continuing declines in ground water elevations indicate that the basin is in an overdraft condition which eventually may deplete the ground water resources of the area. The purpose of the study is to compile and analyze hydrogeologic data for the basin, estimate future trends in ground water availability and quality, and evaluate possible mitigating alternatives. The basin consists of unconsolidated alluvial deposits which form the main unconfined aquifer for the area. The aquifer is bounded by relatively impermeable metamorphic and granitic rocks. The dominant structural feature in the valley is the east-west trending left-lateral Pinto Mountain Fault. Splays of the Pinto Mountain Fault have displaced older alluvium and act as partial barriers to ground water flow. The alluvial aquifer covers an area of approximately 6,000 acres and has a maximum saturated thickness of about 600 feet. Subsurface outflow from the basin is restricted in a zone of limited aquifer cross-sectional area known as the Yucca Barrier. Aquifer test data indicate transmissivities ranging from about 2,000 to 7,000 square feet per day. The average specific yield of the aquifer is estimated to be between 10 and 12 percent. Ground water quality is excellent, with TDS ranging between 100 and 400 milligrams per liter. Degradation of ground water quality from septic effluent has occurred since 1975 in two wells, however, analyses of samples from most wells indicated little change in water quality since 1960. The 70 square-mile drainage area of the basin receives annual average rainfall of 6 inches (at 3000 feet elevation) to 9 inches (at 5000 foot elevation). Average annual recharge to the basin is approximately 500 acre-feet per year. Production demand has increased from about 650 acre-feet per year in 1958 to 3300 acre-feet per year in 1985, amounting to a total of 50,000 acre-feet during the 1958-1985 base period. During the same period, 42,700 acre-feet of water was removed from storage in the basin. Reconciliation of the hydrologic budget suggests that during the base period, less than 15 percent of the applied water has reached the relatively deep aquifer through deep percolation. The remainder is thought to be lost by evaporation or stored in the unsaturated zone. Projection of historic trends in water production and aquifer dewatering suggests that the estimated 225,000 acre-feet of ground water remaining in storage in the Warren Basin in 1985 will be depleted by the year 2014. However, increasing benefits of decreased basin outflow and increased percolation of sewage effluent may extend the basin resources until the year 2057. Percolating effluent is expected to have an increasing effect on degradation of ground water quality in the long term. Possible partially mitigating methods include induced infiltration of natural runoff, alteration of pumping patterns, and importation of water from the Joshua Basin or the California Aqueduct. Preliminary estimates of 30-year amortized capital costs for water resulting from these measures range from 25 to 275 dollars per acre-foot.
