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Remedial Action Plan _. RE: 425 W. Larch Street, Tracy, CA October 1 , 2008 W&A Project No. FTIM-08-001 Page 2 of 11 2.2 Geologic The site is located on the boundary of the Sacramento Valley and the San Joaquin Valley within the west-central portion of the Great Valley Geomorphic province of California. The Great Valley has been filled with a sequence of older to younger alluvium of Pliocene to Holocene age which overlay sedimentary rocks of Cretaceous to Tertiary age. These sedimentary units, in tum, overlay 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. BEII encountered predominantly clays with traces of inter-bedded sands from 0 feet to 25 feet bsg during the well construction that has taken place at the site. 2.3 Hydrologic First subsurface water was encountered at depths from 11 feet to 12 feet bsg during tank removal and monitoring well installation at this site in 1995 and 1996. Since then, first subsurface water has been encountered beneath the site between 6 feet and 10 feet bsg. The groundwater flow is generally a western direction beneath the site. Historic first subsurface groundwater data identified a west or southwest flow beneath the site. Groundwater was also encountered at approximately 35 feet bsg during installation of deep monitoring wells MW- 12 through MW- 16 and MW- 18 . The potentiometric groundwater levels in these deep wells, however, were similar to the first subsurface water levels. i Groundwater levels beneath the site appear to be affected by seasonal recharge. In general, the groundwater is at its yearly low in the winter and yearly high in the spring. During the recent sampling event, groundwater depths were measured between 8.38 feet and 9 .60 feet from the top of the casing. 3 SITE HISTORY Six USTs were removed from the site in 1995 . Since that time there have been soil investigations (both soil and soil gas investigations), groundwater investigations, and remedial activities and testing. The details of those activities are provided below. 3. 1 UST Removal On July 21 , 1995 , six underground fuel storage tanks (USTs) were removed from the site. Three of the USTs had been used to store diesel-fuel and had been located northeast of the terminal (Figure 3) . Three of the USTs had been used to store gasoline and had been located southeast of the terminal. As part of the excavating and removal activities, soil samples were collected for laboratory analyses. The laboratory reported concentrations of TPHg up to 9,800 milligrams per kilogram (mg/kg) in and concentrations of benzene up to 84 mg/kg the samples collected from the excavation in which the USTs that had stored gasoline had been located (Table 1 ). The laboratory reported no significant concentrations of TPHg, TPHd, or BTEX in the samples collected from the excavation in which the USTs that and gasoline-related compounds in the samples collected from the excavation that held the USTs that hat had stored gasoline had been located. WINEFIELD £ ASSOCIATES. INC. tIVINCMFUTF1 Axe SAFETY CONSULTANTS