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• G•ydrologic Consultants, Inc. fault, is the boundary used by most geologists to separate the Central Valley into Sacramento and San Joaquin sub-basins, (Thrifty, 1997). Holocene age flood-plain deposits extend from ground surface to a maximum depth of approximately 100 feet below the site. Oligocene to Holocene age alluvial deposits lie below the younger flood- basins deposits to a maximum depth of approximately 3,500 feet below ground surface (bgs). Continental and marine rocks of pre-Tertiary to Oligocene age underlie the alluvial deposits and are composed of shale, sandstone and conglomerate. Crystalline bedrock underlies the sedimentary formations(Thrifty, 1995). 2.1.2 Site Geology Previous subsurface investigations conducted by others encountered interbedded clayey silts and silty clays to the north at an approximate depth of 53 bgs. From north to south previous investigations encountered a continuous poorly sorted sand layer starting from 5 feet bgs and ending at a maximum depth of 25 feet bgs. Interbedded clayey silts and silty clays were encountered as shallow as 12 feet bgs and as deep as 53 feet bgs from (north to south and east to west)the USTs. From approximately 42 feet to the maximum depth investigated of 73 feet bgs,soils consist of poorly-sorted sand and silty sand. 2.1.3 Hydrogeology The deposits containing fresh groundwater are principally uncemented continental deposits of Pliocene to Recent age that extend to depths ranging from less than 100 to more than 3,500 feet below ground surface(bgs). Groundwater occurs under both confined and unconfined conditions in the Central Valley. Recharge to the groundwater reservoir is by infiltration of rainfall, infiltration from stream,canals and ditches, by infiltration of excess irrigation water and by underflow entering the valley from tributary stream canyons(Poland and Evenson, 1966). The hydrogeologically significant geologic units encountered in the area in order of shallowest to deepest, are the river-channel and flood-plain deposits(0-50 feet thick),the Victor Formation and related deposits(0-150 feet thick),the Laguna Formation(0-400 feet thick)and the Mehrten Formation (75-400 feet thick)(Poland and Evensen, 1966). In the Central Valley, great agricultural development has caused major subsidence of the land surface. Maximum subsidence ranges from 8 feet south of Bakersfield to 23 feet southeast of Los Banos. The subsidence is of the three types. In the lowlands of the Delta at the confluence of the Sacramento and the San Joaquin Rivers, subsidence has been caused chiefly by the oxidation of peat lands accompanying drainage and cultivation. In the largest area,between Los Banos and Wasco,and at the south end of the valley between Arvin and Maricopa, most of the subsidence has been caused by lowering of the artesian head in confined aquifer systems, due to the intensive pumping of groundwater. Locally,on the west and south flanks of the valley, a third type of subsidence has been caused by near-surface compaction of moisture-deficient alluvial fan deposits above the water table,after initial wetting by percolating irrigation water(Poland and Evenson, 1996). TOC 172 UST Removal Report Final Page 2