Geology and ground-water appraisal of the Naval Air Missile Test Center area, Point Mugu, California

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The Naval Air Missile Test Center at Point Mugu, Calif., is at the southeast edge of the Oxnard Plain in southern Ventura County. The Oxnard Plain is about 15 miles long and is traversed by the Santa Clara River near its northwest margin and by Calleguas Creek on the southeast. On the north and east the plain is bounded by highland areas underlain by rocks that are, for the most part, not water bearing. The water-bearing deposits beneath the plain are composed of floodplain and marine sedimentary deposits that yield large quantities of water to wells. The water body generally is semiconflned between lenses of clay and is contained in deposits of silt and sand of late Tertiary and Quaternary age. The semiconflned water body beneath the test center area extends continuously from beyond Hueneme Road on the north to beneath the Pacific Ocean on the south and west and to the base of the Santa Monica Mountains on the east. The chemical quality of the ground water varies with depth. Water in the deposits extending from land surface to about 150 feet below land surface is reported to be brackish or salty, but in the deposits from about 150 to about 1,000 feet, the water is generally relatively fresh and is predominantly a calcium sodium sulfate type. The water from about 1,000 to about 1,550 feet beT ow land surface is brackish or salty. Sea-water encroachment into the deposits containing fresh water has begun near Port Hueneme where water in some wells is reported to be salty. Owing to a landward hydraulic gradient, sea-water encroachment is a definite threat to the water supply at the test center. Fowever, conclusive evidence of sea-water encroachment into the deposits beneath the test center is lacking. Ground-water discharge (pumping and natural discharge) has exceeded recharge beneath the test center proper and the Oxnard Plain in general. Consequently, a decrease in the amount of ground water in storage has occurred. However, much of this decrease may have occurred in the area of urconflned aquifers to the north of the test center, and a lesser amount of this decrease has occurred in the semiconfined deposits of clay, silt, and sand urderlying the test center. An expanding need for water at the test center necessitates an orderly plan of development of the existing water supply in the area. To supply the ir^reasing demand for water, three possible sources for future use are: continued use SI S2 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES of deep, 500 to 750 feet, aquifers at the test center and expansio^ of that source, either by construction of new wells or by purchase of existing wells, or both; development or purchase of a supply of water from a source outside the test center and far enough inland so that the threat of sea-water encroachment would be greatly reduced; and possible development of water from the depth between 750 and 950 feet beneath the test center.