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ERI 205505 ROI Tosco Service Station 5886,Stockton Califorma December 28, 1999 Currently there are three on-site single-casing groundwater monitoring wells (MW1 through MW3), four on-site dual-casing groundwater monitoring wells (MW4S/D through MW7S/D), and nine off-site groundwater monitoring wells (MWBS/I/D through MW 10S/IID) consisting of three wells at each nested well location as shown on Plate 2 The boring Iogs and well designs for wells MW1 through MW IOS/IID are provided in Appendix A The purpose of these wells is to evaluate the lateral and ' vertical extent of dissolved hydrocarbons detected in groundwater beneath the site Based upon data collected during ongoing quarterly groundwater monitoring and sampling, dissolved hydrocarbons are effectively delineated both laterally and vertically in the vicinity of the site 1 However, dissolved hydrocarbons continue to be detected in groundwater beneath the site and are concentrated in the vicinity of the UST cavity (Plate 2) Cumulative groundwater monitoring and sampling data are provided in Appendix B (Gettler-Ryan, Inc , September 22, 1999) ' 222 Regional Settmg ' The site is located within the northern San Joaquin Valley, which comprises the southern portion of the Central Valley geomorphic province of California The Sacramento Valley comprises the northern portion of the Central Valley The Central Valley is bounded by the Coast Ranges to the west and the Sierra Nevada Range to the east, and measures approximately 400 miles (northwest to southeast) by approximately 50 miles wide (Norris and Webb, 1990) The Central Valley consists of an asymmetrical syncline with the fold axis located towards the western portion of the valley and formed by sediment deposition and downwarp within the basin The lithology of the Central Valley consists of Mesozoic basement granitic and metamophic rock overlain by Cretaceous marine sedimentary rock of the Panoche Formation The Panoche Formation is overlain by ' a thick sequence of older Tertiary marine and then younger Tertiary non-marine sedimentary rock The surface of the valley is overlain by Quaternary unconsolidated sedimentary deposits, which ' originated from the erosion of the surrounding highlands and subsequent deposition within the basin (Bartow, 1985, and Norris and Webb, 1990) The Central Valley drains the interior of California through the Sacramento River drainage in the north and San Joaquin River drainage in the south Combined, these rivers account fora pproximately 40 4