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0 0 Work Plan for Additional Subsurface investigation @ Former Unocal Station 40187 November 12, 1999 waste oil underground storage tanks (USTs) in a common excavation. All station facilities have been removed and the site is currently an unpaved vacant lot. Pertinent features of the former service station are shown on the Site Plan (Figure 2). PREVIOUS ENVIRONMENTAL WORK Two 1,000-gallon gasoline and one 550-gallon waste oil USTs were removed from the southeast corner of the subject site in 1967. Data related to soil conditions beneath the USTs at the time of removal is not available. These USTs appear to have been replaced with two 10,000-gallon gasoline and one 280-gallon waste oil USTs situated in a common excavation in front of the station building. The two 10,000-gallon gasoline and one 280-gallon waste oil USTs, associated product piping, hydraulic lifts,and oil/water separatorwere removed and the station facilities demolished in January, 1992. Petroleum hydrocarbons were not detected in soil samples collected beneath the USTs, hydraulic lifts, or oil/water separator. Total Petroleum Hydrocarbons as gasoline(TPHg)and benzene were detected in one soil sample collected from the product piping trench between dispenser islands D-I and D-3 at concentrations of 150 and 0.81 parts per million(ppm),respectively. This trench area was overexcavated,and aconfirming soil sample collected from the base of the excavation. Petroleum hydrocarbons were not detected in this soil sample. A second 130-gallon waste oil UST was removed from the eastern portion of the property in May 1992, Petroleum hydrocarbons were not detected in the soil beneath this UST. Results of the UST removal investigation were presented in GeoStrategies (GSI) Report No. 782701-1, Tank Removal Observation Report, dated September 11, 1992. Based on the data collected during this investigation, there does not appear to be hydrocarbon-impacted soils in the vicinity of the former UST pit south of the station building, waste oil tank,hydraulic lifts, or dispenser islands. In October 1993, GSI drilled eight on-site exploratory soil borings(U-A through U-H) in the locations of the former USTs,the former product dispenser islands,and the former hydraulic lifts in anticipation of site closure. During this investigation,TPHg were detected in soil samples collected at 16.5 and 31.5 feet below ground surface(bgs)from boring U-C,drilled within the former gasoline UST pit in the southeastern comer of the site, and from boring U-D, drilled adjacent to former dispenser island D-4. TPHg concentrations ranged from 1,600 to 2,900 ppm. Benzene was not detected in these soil samples. TPHg or benzene were not detected in soil samples collected from the other borings. Results of this subsurface investigation are presented in GSI Report No. 782702-3,Exploratory Soil Boring Report, dated December 15, 1992. In July 1994, GSI drilled five additional on-site soil borings(U-I through U-M) and installed three on-site groundwater monitoring wells(U-I through U-3). This work was performed to assess the extent of impacted soil in the southeast corner of the site and evaluate whether groundwater beneath the site had been impacted by petroleum hydrocarbons. TPHg were detected in soil samples analyzed from borings U-J(1.7 ppm at 26.5 feet bgs),U-K(130 ppm at 41.5 feet bgs),U-3(330 ppm at 46.5 feet bgs),U-L(19,000 ppm at 45.5 feet bgs), and U-M (5 10 ppm at 45.5 feet bgs). Results of this subsurface investigation are presented in GSI Report No. 4827.703-5, Well Installation and Exploratory Soil Boring Report,dated September 21, 1994. GSI performed a limited environmental site assessment(ESA)of the properties in the vicinity of the subject 240042.05 2