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M WK, - 07 - 1.0 INTRODUC'T'ION The following report presents the findings of a soil and goundwater investigation performed by Pacific Environmental Group, Inc. (PACIFIC) at UNOCAL Bulk Plant 0788, located at 8203 East 11th Street in Tracy, California (Figure 1). Three groundwater monitoring wells designated U-=1, U-5, and U-6, and three exploratory soil _. borings designated U-A,U-B, and U-C,were installed on site on May 9, 1991,to further evaluate the extent of petroleum hydrocarbons in the soil and groundwater underlying the site. This report presents site background information, a regional geologic and hydrogeologic overview, sample collection and analytical procedures, findings, and recommendations. 1.1 Background The site referenced above is an operating bulk plant with two 19,500-gallon and R J two 20,000-gallon above-ground hydrocarbon storage tanks located in the western portion of the property (Figure 2). The 19,500-gallon tanks contain diesel and super ` - unleaded gasoline, and the 20,000-gallon tanks contain regular leaded gasoline and f unleaded gasoline. According to site personnel, these tanks were installed in 1918 to replace the tanks originally installed during construction of the facility. A water supply well, designated WSW-1 is located near the eastern properly boundary. According to site.personnel, a leak occurred in the underground piping between the -- ahove-ground storage tanks.and the loading rack during 1975 and 1976, In 1976, j hydrocarbons were noted during underground construction in the Southern Pacific right-of-wav to the north of the site. 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. Details of the excavation of this tank are described in a PACIFIC letter to UNOCAL dated March 20. 1996. Soil rt samples were collected from the eastern and western ends of the tank-excavation during tank removal activities, at a depth of 8 feet below grade. Laboratory analysis of these samples indicated the presence of 17 parts per million (ppm) diesel, 12 ppm toluene, --,-.- -------------an?d--4.9-ppm-ethylberrzene-in the eastern--end-of the-excavation.- The transmission-fluid -- --- -- - ----- ---- - E 3100101/REPORT'- 1 September 30.199! x - r