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Frontier Transportation, Inc. B23502.01 October 16, 1996 Page 2 California. The Great Valley is a nearly flat northwest to southeast trending structural basin approximately 450 miles long by 50 miles wide. The Great Valley has been filled with a sequence of older to younger alluvium of Pliocene to Holocene age which overlie sedimentary rocks of Cretaceous to Tertiary age. These sedimentary units, in turn, overlie a crystalline basement of Paleozoic and Mesozoic metamorphic and igneous rocks. The shallow subsurface geology in the site vicinity is a heterogeneous mix of gravel, sand, silt, and clay (Hackel, 1966). 2.2.2 Hydrology±: Sediments containing fresh groundwater are largely unconsolidated silts and sands deposited as river channel, flood plain, and alluvial fan deposits of Pliocene to Recent age (Davis and others, 1959). During Twining's investigation, first encountered groundwater beneath the site occurs under unconfined (water table) conditions at a depth of approximately 10 feet below site grade (BSG). Local groundwater elevation and flow direction may be influenced by pumping of water wells and recharge from various sources. The effect of these influences on groundwater beneath the site is beyond the scope of this investigation. 2.3 Pro-ject History: On July 21, 1995 three gasoline and three diesel USTs were removed from the project site. Twining personnel were present during the removal of the gasoline USTs; however, the diesel USTs had been removed prior to the arrival of the Twining technician. The contents of the tanks and tank capacity are summarized in Table 1. TABLE 7.Summary of Underground Storage Tank (UST) Contents and Capacity UST UST Contents DtameteY Length': Vofutne Depth Number(1) feet;>. feet gallons(�) feet,(3) ' 39-2257-01 Gasoline 4 1S 1,413 6 39-2257-02 Gasoline 4 7 659 6 39-2257-03 Gasoline 4 7 659 6 39-2257-04 Diesel Unknown Unknown 12,000 10 39-2257-05 Diesel Unknown Unknown 12,000 10 39-2257-06 Diesel Unknown Unknown 12,000 10 UST = Underground Storage Tank (1) SdCPHS tank identification number as stated in November 7, 1995 correspondence (2) = Approximate (3) = Depth of bottom of tank below site grade Note = Diesel USr data based on information provided by K.S. Dunbar and Associates personnel Following tank removal, four soil samples were collected from approximately 3 feet below the bottom of the gasoline USTs (S-1-W, S-1-E, S2, and S3) and twelve soil samples were