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Alb REGIONAL AQUIFER-SYSTEM ANALYSIS—CENTRAL VALLEY, CALIFORNIA grained sediments." Below the upper few hundred differences result from very heavy pumpage in the lower feet everywhere, they consider the aquifer to be zone combined with the resistance to vertical flow virtually confined. These two concepts of the aquifer provided by the fine-grained lenses in the aquifer system. system in the Central Valley are shown in figure 10. Although some head difference in wells is observed across the Corcoran Clay Member, an even greater head Lithologic studies described in chapter C (Page, 1986) difference occurs in wells that tap the intervals above and show that the aquifer system contains many isolated below the Corcoran. In addition, numerous wells that lenses of sand, silt, and clay. The fine-grained lenses, contain perforated sections both above and below the although limited in lateral extent, constitute more than Corcoran Clay Member show little vertical head differ- 50 percent of the system and have an aggregate thickness ence. Thus, the Corcoran is much less important than the of as much as several thousand feet. In contrast, the combined effect of the many fine-grained lenses in Corcoran Clay Member, a confining unit, ranges in controlling vertical flow. In summary, the concept of a thickness from zero to 160 ft and has an average thickness single heterogeneous aquifer system is supported by the of 55 ft (Williamson and others, 1989). Vertical head presence of numerous fine-grained lenses and the hy- differences are present nearly everywhere in the Central draulic response of the system to pumping. Valley. Head differences, as much as 400 ft, were The concept of a single,mostly confined,heterogeneous observed in wells of different depths in some areas on the aquifer system was used as the basis for the computer west side of the San Joaquin Valley. These large head simulation of regional ground-water flow described in WEST EAST WEST EAST 7. ° Unconfined to semi confined aquifer ° . • upper water-bearing zone) Unconfined aquifer I ° Principal confining unit (Corcoran Clay Member °Eastern limit of confined aquifer • of the Tulare Formation) SACRAMENTO VALLEY SAN JOAQUIN VALLEY A WEST EAST 0 ° . •.. o: ° e Numerous overlapping, discontinuous clay beds . .. •. . .Qom: 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).