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March 18, 1993 • Page 2 '110 0 noted during underground construction in the Southern Pacific right-of-way to the north of the site. Geology/Hydrogeology: The site lies at an elevation of approximately 47 feet above mean sea level (MSL) and is located in San Joaquin County, in the northern portion of the San Joaquin Valley. The San Joaquin Valley is a broad, alluvium- filled basin bounded to the east by the Sierra Nevada Mountain Range and to the west by the Coast Ranges. The total thickness of alluvial fill in the valley is esti- mated to be thousands of feet. The alluvial sediments generally consist 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 portions of the valley, and consist of clay, silt, and sand. In the vicinity of the site, surficial deposits are characterized as distal alluvial fan and flood plain deposits. These deposits generally are less than 10 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 of interbedded clay, silt, sand, and gravel. Geomorphology and regional drainage patterns suggest a northerly direction of groundwater flow. Site Investigations: A partially buried 5,000-gallon tank was removed from the southwestern portion of the site on December 13, 1989. The tank was primarily used for transmission fluid storage,but it had also been used to contain heating oil in the past. The transmission fluid tank was underlain by an underground concrete pad,which was not removed during excavation. Analytical results indicated that low levels of petroleum hydrocarbons remained in the soils adjacent to the excavation. Details of the excavation of this tank are described in a PACIFIC letter to Unocal dated March 20, 1990. PACIFIC installed groundwater monitoring Wells U-1 through U-3 on October 24 and 25, 1990. Details of this phase of investigationare described in PACIFICs Soil and Groundwater Investigation Report dated February 13, 1991. PACIFIC installed groundwater monitoring Wells U-4 through U-6, and exploratory soil Borings U-A through U-C, on May 9, 1991. Details of this inves- tigation are described in PACIFICs Soil and Groundwater Investigation Report dated September 30, 1991. On March 10 through 12, 1991, PACIFIC installed groundwater monitoring Wells U-7 through U-10. Details of this investigation are described in PACIFICs Soil and Groundwater Investigation Report dated June 11, 1992. 3100104/1anP