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t ~� 3. 0 ENVIRONMENTAL SETTING f 3. 1 SOILS ti The site lies in the center of Great Central Valley geomorphic -= province. The valley has been down-warped and filled with material eroded from the surrounding Mountains since pre.. r,, Cretaceous times. A; a result, the central part of the valley Ir. is filled with several hundred feet of consolidated and unconsolidated sediments. Important water-bearing units in the vicinity of Rainbo Bakei!y t include the Mehrten, Laguna, Tulare, and Victor Formations. The top of the Mehrten ranges fr-+m 820 to 1 ,000 ft deep in thIe EI central part of the valley and is about 590 €t thick. The !. nonvolcanic Laguna Formation overlies the andesitic Mehrten on the eastern side of San Joaquin Count y as does the site and ..•; varies from 430 to 1 , 000 ft in thickness. The Laguna unit' is a stream-laid unit of discontinuous sand and silt lenses; 'with it lesser amounts of clay and gravel. The Laguna sedimentj become finer grained toward the center of the basin wheie the formation merges with the equivalent west-side formation, the Tulare. The contact is near the San Joaquin River. A thick a confining layer, the Corcoran clay (usuallyto clay" by drillers ) , is found near the to g "blue p of thee TTulaulare , Formation. The thickness of the clay varies from �r approximately 0 €t to 160 ft. The Victor Formation lies over the Laguna. on the eastern side �\ of the"rcounty. It varies from 0 to 50 ft thick. The sediments are alluvial fan and interfan deposits, which are generally coarser than the Laguna Formation. However, the interfan areas contain large amounts of fine sand, silt, and cla;', making the two formations practically indistinguishable. The aquifer is considered semiconfined because of discontin- 3-1