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16 ERI 205505 R02 Tosco Service Station 5886 Stockton,Califorma September 25, 2001 <br /> Based upon data collected during ongoing quarterly groundwater monitoring and sampling, dissolved <br /> hydrocarbons are effectively delineated both laterally and vertically in the vicinity of the site <br /> However, dissolved hydrocarbons continue to be detected in groundwater beneath the site and are <br /> ' concentrated in the vicinity of and downgradient of the UST cavity (Plate 2) Cumulative groundwater <br /> monitoring and sampling data are provided in Appendix C <br /> 2.2 Regional Settm <br /> The site is located within the northern San Joaquin Valley, which comprises the southern portion of the <br /> Central Valley geomorphic province of California The Sacramento Valley comprises the northern <br /> portion of the Central Valley The Central Valley is bounded by the Coast Ranges to the west and the <br /> Sierra Nevada Range to the east, and measures approximately 400 miles (northwest to southeast) by <br /> approximately 50 miles wide (Norris and Webb, 1990) <br /> The Central Valley consists of an asymmetrical syncline with the fold axis located towards the western <br /> portion of the valley and formed by sediment deposition and downwarp within the basin The lithology <br /> of the Central Valley consists of Mesozoic basement granitic and metamophic rock overlain by <br /> Cretaceous marine sedimentary rock of the Panoche Formation The Panoche Formation is overlain by <br /> a thick sequence of older Tertiary marine and then younger Tertiary non-marine sedimentary rock <br /> The surface of the valley is overlain by unconsolidated Quaternary sedimentary deposits, which <br /> originated from the erosion of the surrounding highlands and subsequent deposition within the basin <br /> (Bartow, 1985, and Norris and Webb, 1990) <br /> The Central Valley drains the interior of California through the Sacramento River drainage in the north <br /> and San Joaquin River drainage in the south Combined, these rivers account for approximately 40 <br /> jpercent of the total river discharge in California These rivers converge west of Stockton, California, <br /> to create the San Joaquin-Sacramento Delta region, which drains through the Carqinez Strait into San <br /> Francisco Bay (Norris and Webb, 1990) <br /> 4 <br />