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Site Assessment Report <br /> Former Texaco Service Station <br /> Chevron Site ID 309075 <br /> March 18, 2010 <br /> are unavailable for the site. The soil samples collected from boring P3 (40 feet below ground <br /> surface [bgs]) in 2002 had a maximum total petroleum hydrocarbons as gasoline (TPH-g, also <br /> known as TPH-GRO) concentration of 6,300 milligrams per kilogram (mg/kg) and maximum <br /> toluene, ethylbenzene, total xylenes concentrations of 500 mg/kg, 250 mg/kg and 1,600 mg/kg, <br /> respectively. Concentrations of petroleum hydrocarbon constituents in soil samples collected <br /> from adjacent borings B1 and B2 (located approximately 20 feet northwest and southeast of P3) <br /> were below the California Regional Water Quality Control Board (RWQCB) Environmental <br /> Screening Levels(ESL) for deep soils (RWQCB, 2008). Soil samples collected from 65, 70, 75, <br /> and 85 feet bgs in borings B1 and B2 had no detections above the laboratory reporting limits. <br /> A detailed summary of the site history and previous investigations is included in Attachment A. <br /> Historical analytical soil data is included in Table 1. Historical groundwater monitoring and <br /> analytical data tables are included in Attachment B. Soil boring and well construction details are <br /> included in Table 2. <br /> 2.2 REGIONAL GEOLOGY/HYDROGEOLOGY <br /> '-" The Stockton area is part of the Great Valley physiographic province and is underlain by <br /> Holocene age alluvium and the Victor Formation to a depth of approximately 150 feet below <br /> grade. The formations are comprised of unconsolidated Quaternary age deposits. These <br /> formations are heterogeneous consisting of laterally discontinuous lenses of sand and gravel <br /> with varying amounts of sand and silt interstratified with units of clay and silt. <br /> Surface water bodies present in the vicinity of the site include Morman Slough, located <br /> approximately 6,000 feet to the north, and Walker Slough located approximately 10,000 feet to <br /> the south. <br /> 2.3 SITE GEOLOGY <br /> Site-specific soil borings indicate silty clays and sandy silts underlie the site. Boring and well <br /> logs from current and previous investigations are included in Attachment C. Boring logs for <br /> MW-1, P1, P2, P3, and P4 are unavailable in historical record. A historical groundwater flow <br /> direction rose diagram is included in Attachment D and indicates that predominate groundwater <br /> flow direction is towards the southwest. <br /> 3.0 SITE ASSESSMENT ACTIVITIES <br /> V <br /> Between November 9 and 13, 2009, Cascade Drilling, LP (Cascade), under the supervision of <br /> ARCADIS, installed shallow and deep monitoring wells (MW-6 and MW-6D) using hollow stem <br /> augers (HSA) to evaluate the downgradient extent of petroleum hydrocarbon impacts to soil and <br /> groundwater in the locations shown on Figure 2. The monitoring wells were developed on <br /> November 17, 2009 and surveyed on November 23, 2009. <br /> .r <br /> .r <br /> \\CA5FP\Data\Aproject\ENV\CHEVRON\309075\6 Deliverables\Reports\DG Wells Assessment\Assessment Report_4741010950.doc 2 <br />