The coastal plain aquifer system is made up of as many as 11 locally named aquifers. Each consists of a distinct layer of water-yielding sand and gravel usually separated from other sand and gravel beds by clay and silt confining units (fig. 126). In many places, however, either the water-yielding sediments are in direct hydraulic contact or the intervening confining units contain sufficient sand and gravel to allow water to pass between adjacent aquifers.
A layer of clay and silty clay of marine and continental origin, which is at or near the land surface over most of the basin, is a competent confining unit where it does not contain large amounts of sand and gravel. This confining unit ranges from less than 1 foot to about 180 feet in thickness. Near Santa Monica and San Pedro Bays, the confining unit is not present, and ground water is under unconfined, or water-table, conditions.
Freshwater is contained within deposits that range in age from Holocene to late Pliocene. The main freshwater body extends from depths of less than 100 to about 4,000 feet. At greater depths, the water is saline and unpotable. The freshwater body is thickest near the axis of the troughs where water-yielding sediments reach their greatest thickness and thinnest where these sediments overlie anticlines or become thin at the margins of the aquifer system.