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October 4, 1994 Page 2 • The site hes at an approximate elevation of 10 feet above mean sea level. The topography in the vicinity is essentially flat with a gentle slope to the west toward the San Joaquin River, which is located 3 mules west of the site The site is currently a vacant lot Unocal originally leased the site in January 1947 The station was completely remodeled in 1967 The station was demolished between February and April 1988 During station demolition, two 10,000-gallon underground gasoline tanks were removed from a common excavation along with a separate 550-gallon waste oil tank The former tank locations, along with the product islands and station building, are shown on Figure 2. During tank removal, 1,100 cubic yards of soil were removed from the site Soils within the tank excavation were removed to a depth of 30 feet Soils at the base of the excavation contained a maximum of 18 parts per million(ppm) total petroleum hydrocarbons calculated as gasoline (TPH-g) and 1.7 ppm benzene. Soils beneath the product lines were found to have a maximum TPH-g concentration of 6 ppm No benzene was detected in the product line samples No hydrocarbons were detected beneath the waste oil tank Klemfelder, Inc installed five soil borings (KB1 through KB5) at the site in January 1988 These.borings were installed around the former tank complex, adja- cent to the product islands, and adjacent to the former waste oil tank (Figure 2) Between June 1988 and February 1992, Resna Industries,Inc. (Resna) installed a total of seven monitoring wells (MW-1 through MW-7) and three soil vapor extrac- tion (SVE)wells (VW-1 through VW-3) Resna also installed 11 soil borings (B-1 through B-11) Most of these borings were converted to either groundwater monitoring wells or SVE wells The existing monitoring well and SVE well locations are shown on Figure 2 A SVE test was performed in March 1992 by Resna The results of this test show that with a wellhead flow rate of 43 cubic feet per minute (cfin), and an effective radius of influence of approximately 25 feet can be achieved for one vapor extrac- tion well. These results were documented in a report by Resna dated January 4, 1993 ` All site groundwater monitoring wells are currently being monitored and sampled on a quarterly schedule Soil Conditions The site is underlain by primarily fine-grained deposits consisting of interlayered clays and silts to an approximate depth of 50 feet. Some surficial silty sands occur to a depth of approximately 25 feet Occasional interlayered silty sands (up to 50 feet thick) were found to the total depth explored 31004002/MWINSrAL