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A16 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA grained sediments." Below the upper few hundred feet everywhere, they consider the aquifer to be virtually confined. These two concepts of the aquifer system in the Central Valley are shown in figure 10. Lithologic studies described in chapter C (Page, 1986) show that the aquifer system contains many isolated lenses of sand, silt, and clay. The fine-grained lenses, although limited in lateral extent, constitute more than 50 percent of the system and have an aggregate thickness of as much as several thousand feet. In contrast, the Corcoran Clay Member, a confining unit, ranges in thickness from zero to 160 ft and has an average thickness of 55 ft (Williamson and others, 1989). Vertical head differences are present nearly everywhere in the Central Valley. Head differences, as much as 400 ft, were observed in wells of different depths in some areas on the west side of the San Joaquin Valley. These large head differences result from very heavy pumpage in the lower zone combined with the resistance to vertical flow provided by the fine-grained lenses in the aquifer system. Although some head difference in wells is observed across the Corcoran Clay Member, an even greater head difference occurs in wells that tap the intervals above and below the Corcoran. In addition, numerous wells that contain perforated sections both above and below the Corcoran Clay Member show little vertical head differ- ence. Thus, the Corcoran is much less important than the combined effect of the many fine-grained lenses in controlling vertical flow. In summary, the concept of a single heterogeneous aquifer system is supported by the presence of numerous fine-grained lenses and the hy- draulic response of the system to pumping. The concept of a single, mostly confined, heterogeneous aquifer system was used as the basis for the computer simulation of regional ground-water flow described in WEST EAST WEST EAST Unconfined to semiconfined aquifer '. '., water-bearing zone) .-? .' SACRAMENTO VALLEY Principal confining unit ^orcoran Clay Member ^iSSeTrTlimit of confined Xaquifer of the Tulare Formation) M SAN JOAQUIN VALLEY WEST EAST Numerous overlapping, discontinuous clay beds B FIGURE 10. Concepts of Central Valley aquifer system. A, Concept of aquifers used in many hydrologic reports of the Central Valley; Sacramento Valley (adapted from Bloyd, 1978); and San Joaquin Valley (adapted from Poland and Lofgren, 1984). B, Concept of single heterogeneous aquifer with varying vertical leakance and confinement (adapted from Williamson and others, 1989).