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' Site Background Information g ' Former BLUE STAR GASOLINE 4040 East Main Street, Stockton, California ' BACKGROUND ' The site is bounded on the north by Main Street and on the east by Oro Avenue A single structure occupied the central portion of the site, a concrete foundation currently remains Residential and commercial properties are located north beyond Main Street and south of the site A gasoline station ' and mini-mart previously occupied the site Three underground storage tanks (USTs) and two dispensers, removed by AGE in March 2002, operated on-site for approximately 40 years ' REGIONAL GEOLOGIC/HYDROGEOLOGIC CONDITIONS The site is situated within the southern portion of the Great Valley Geomorphic Province of California, a large, elongate, northwest trending, asymmetric structural trough, the northern and southern portions of the Province have been designated the Sacramento and San Joaquin Valleys, ' respectively The Province 3s bordered by the Coast Ranges to the west,the Klamath Mountains and Cascade Range to the north, and the Sierra Nevada to the east ' • The Great Valley has been filled with sediments derived from both marine and continental sources Thickness of the sedimentary fill ranges from thin veneers along the valley edges to more than 20,000 feet in the south central portion of the valley The sedimentary formations range in age from ' Jurassic to Recent,with the older deposits being primarily marine in origin and the younger deposits, continental Continental-denved sediments were typically deposited in lacustrme, fluvial, and alluvial environments with sediment sources being the mountain ranges surrounding the valley ' (Olmsted and Davis, 1961), the site itself is located on unconsolidated and semi-consolidated alluvium, lake, playa, and terrace deposits of Quaternary age (California Division of Mines and Geology, 1977) Rocks composing the basement complex of the Province have not been completely ' defined, but are believed to be metamorphic and igneous in ongin ' The Modesto, Riverbank, and Turlock Lake Formations and overlying Recent alluvium are the principal sources of domestic ground water in the 13,500-square mile San Joaquin Valley Ground Water Basin (Basin 5-22) The closest surface water feature to the site is the Mormon Slough and ' Stockton Diverting Channel, located approximately 1,500 feet south and 2,000 feet east of the site, respectively Water from canals drains into the San Joaquin River Deep Water Channel, which is primarily used for commercial and recreational boating ' Based on the General Soil Map from the San Joaquin County Soil Survey,published by the United States Department of Agriculture Soil Conservation Service in 1992, the site area is within the ' Jackson-Hallenbeck-Stockton (JHS) association The JHS soils are located within basins, and generally consist of moderately- to poorly-drained, fine-textured soils The soils are generally • derived from both marine and non-marine sediment, and are generally formed in alluvium derived Advanced GeoEnvironmental,Inc