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i. beg- — - : May 14, 1990 Project 310-01.02 r� Page 2 t The facility is currently operational and has four above-ground storage tanks: two 19,500-gallon tanks and two 20,000-gallon tanks, located in the northwest portion of the site. The 19,500-gallon tanks contain diesel and super unleaded gasoline, and the 20,000-gallon tanks contain regular and unleaded gasoline. A 10,000-gallon tank formerly existed on the eastern portion of the site, but was removed in 1989. PACIFIC documented the results of the excavation activities of a iL partially buried 5,000-gallon transmission fluid tank in a letter to Unocal dated March 20, 1990. The tank was formerly located in the southwest portion of the site and was removed on December 13, 1989. The tank was underlain by a concrete pad which was not removed. Two soil samples, collected from beneath each End of the tank at a depth of approximately 8 feet were J. analyzed for diesel #2, kerosene, motor oil, and benzene, t . toluene, ethylbenzene and xylene (BTEK) compounds. Diesel 42, toluene, and ethylbenzene were detected in the sample collected at the east end of the tank at concentrations of 17 parts per million (Ppm) , 12 ppm, and 4.9 ppm, respectively. None of the i other compounds mentioned above were detected in either of the soil samples_. Groundwater-was :note3: in` the excavation pit."at`a r depth of approximately 5 feet. s REGIONAL HYDROGEOL.OGY The site is located in San Joaquin County, in the northern portion of the San Joaquin Valley. The San Joaquin Valley is a broad, alluvium-filled structural basin bounded to the east by the Sierra Nevada Mountains and to the west by the Coast Range. The total thickness of alluvial fill in the valley is on the order of thousands of feet. The alluvial sediments generally of poorly sorted silt and fine sand with less extensive lenses of medium to coarse-grained sand and gravel: Quaternary lacustrine deposits also occur in the central and western t portions of the valley, and consist of clay, silt and sand. In the vicinity of the Tracy Bulk Plant, surficial deposits are characterized as low alluvial plain and fan deposits. These deposits generally are less than to feet thick and consist of silts, fine sands, and less extensive coarse sand and gravel lenses. Underlying deposits are referred to as older alluvium and reach a thickness of up to 400 feet. These deposits consist