Most usable ground water in the predominantly mountainous Central Coastal Subregion occurs in alluvium-filled valleys and coastal plains and in deeper aquifers of Quaternary and Tertiary age. The intervening mountainous areas are underlain by consolidated sedimentary, igneous, and metamorphic rocks, mainly of Mesozoic age. These older rocks contain only small quantities of recoverable ground water and, therefore, are not considered a major source of ground water.
In the Central Coastal Subregion, 24 basins have been identified as significant sources of ground water. The total area of the 24 basins is about 3,500 square miles. The water-bearing deposits range in thickness from about 200 to 4,000 feet. Depending on local conditions, recharge infiltrates at rates of less than 1½ feet per day to more than 10 feet per day in the upper part of alluvial fans and stream channels and at the outcrops of the deeper aquifers. The maximum measured depth to water in the water-bearing deposits is 568 ft. In several valleys there are flowing wells.
Total storage capacity of 16 of the basins is more than 20,000,000 acre-feet . The usable storage capacity of 18 of the basins is more than 7,600,000 acre-feet; the limiting factors are sea-water intrusion and high pumping lift. Ground-water temperature ranges from about 55° to about 75°F . The dissolved-solids content of the water is generally less than 800 parts per million, but locally is more than 11,000 parts per million. The predominant water type is calcium bicarbonate, but sodium, magnesium, sulfate, and chloride are present locally in significant quantities.
Properly constructed wells in some areas can yield 425 gallons per minute.