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Description
The San Onofre Basin of the United States Marine Corps Base at Camp Pendleton, California covers 42.3 square miles and is the northwesternmost watershed in San Diego County. This basin was studied to determine the safe yield potential for producing groundwater. The results were gained through study of general geology and hydrogeologic characteristics of the various rock units within the basin. The study of the geology resulted in identification and mapping of eight geologic formations, in addition to the identification of the major structural and geomorphic features. The hydrogeologic investigation, from a three-phase drilling program, sedimentological research, and well production data indicates two economic aquifer systems within five potential reservoirs. These data permit calculation of a long-term safe yield as well as selection of recommended drilling sites at which maximum economic development of the groundwater resource can take place. The drilling program of small- and large-diameter shallow wells and one deep well indicates that economic quantities of high quality recoverable groundwater is restricted to the San Mateo Sandstone and the alluvial systems. Recharge is primarily from precipitation, with a minor addition of water from two sewage treatment plants. These two aquifers have a storage capacity of approximately 6,850 acre-feet to an arbitrary limiting depth of sea level. The study resulted in recommendations that total water withdrawals from the basin remain below 680 acre-feet per year. The adherence to this safe yield production rate will not result in mining of the groundwater resource or in the degradation of the quality of the groundwater. Sites for future production of water will result in wells which will produce water at rates of at least 1,000 gallons per minute with less than 20 feet of drawdown for extended periods within the recommended safe yield figure. The continued proper watershed management--phreatophyte control, maintenance of spreading structures for artificial recharge, and production limits within safe yield amounts-- will insure the protection of the San Onofre water resources.
