The southern eight-ninths of the San Joaquin Valley, about 10,000 square miles, is a broad structural trough bordered by the Sierra Nevada on the east and by the Coast Ranges on the west. In that area, fresh ground water is contained in principally unconsolidated continental deposits of Pliocene to Holocene age that extend to depths ranging from less than 100 to more than 3,500 feet. Because of widespread pumping of ground water, water levels have declined rapidly beneath extensive areas of the valley. Consequently, it becomes necessary to delineate the base of fresh water so that changes in thickness of the fresh-water zone can be monitored, and limiting depths of water wells can be determined.
For the purpose of this study, a specific conductance of 3,000 micromhos per centimeter (about 2,000 milligrams per liter dissolved solids) was selected as a measure of the base of fresh groulid water. The base of fresh water was determined from data from electrical logs of wells, chemical analyses of ground water, and from numerous hydrologic and geologic reports. Because of the estimations of various factors used to interpret salinity from electrical logs, the base of fresh water as shown in this report must be considered to be only approximate.
The base of fresh water is commonly in the continental deposits of Tertiary and Quaternary age, but in places along the eastern and western boundary of the valley it occurs in marine and continental sedimentary rocks of Tertiary age. In addition, it occurs locally in the igneous and metamorphic rocks of pre-Tertiary age of the Sierra Nevada.
The configuration of the base of fresh water in places reflects underlying structure, but in other places, particularly along the eastern boundary of the valley, the base transgresses the structure. Furthermore, along and near the southwestern boundary of the valley, the base transgresses the major confining bed in the valley.
Locally, the fresh-water body is overlain by water with a dissolved-solids` content that approaches and exceeds 2,000 milligrams per liter. And nearly everywhere in the valley, the fresh water is underlain by a basal, salinewater body of a sodium chloride type.
