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i Thrifty Oil Co Station # 172 Groundwater monitoring and sampling has occurred at the site since June 6, 1991 A summary of historical groundwater laboratory analytical results are included in Table 1, and a benzene isoconcentration map for the May 28, 1997 data is provided as Figure 8 3.0 GEOLOGY AND HYDROGEOLOGY 3.1 Geology The site is located in the San Joaquin Valley of the Central Valley geomorphic province Geologically,the Central Valley is a large elongate northwest-trending asymmetric structural trough that has been filled with a tremendously thick sequence of sediments ranging in age from Jurassic to Recent The basin has a regional southward tilt, which is interrupted by two significant cross- valley faults The northernmost fault,the Stockton fault, is the boundary used by most geologists to separate the Central Valley into the 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-basin deposits to a maximum depth of approximately 3,500 feet bsg 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) 3.2 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 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 mayor 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, 6