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'b ,9 s 2. PREVIOUS INVESTIGATIONS Six separate subsurface investigations have been conducted at former Chevron SS 9-1918 since November 1987. In addition,a vapor extraction and treatment system(VETS) was installed to remediate petroleum hydrocarbons in the soils,and groundwater has been sampled and analyzed quarterly. 2.1 SVCA, 18 NOVEMBER 1987 On 18 November 1987,concentrations of petroleum hydrocarbon vapors in the subsurface at the site were measured during a soil vapor contaminant assessment(SVCA)conducted by EA. The SVCA was conducted by collecting samples of soil vapor at various depths through a 518-inch diameter steel probe that was driven to depths of up to 15 feet below the ground surface at 10 locations throughout the site. The samples were analyzed onsite using a Photovac 10S50 inte- grating gas chromatograph equipped with a photoionization detector. Six points were located at the ends of the three pump islands and two points were located at the sides of the tank field not adjacent to the pump islands. No concentrations of petroleum hydrocarbons greater than 1 ppm were detected in soil vapors at the site. A value for TVH of 8 ppm (benzene <1 ppm) was measured in the headspace of the monitoring well in the southeast corner of the tank field. 2.2 TANK REMOVAL AND SOIL SAMPLING,24 FEBRUARY 1988 On 24 February 1988, four fiberglass underground storage tanks were excavated and removed from SS 9-1918. Soil samples were collected from beneath the individual tank locations by Blaine Tech Services, Inc. One soil sample, collected from beneath the waste oil tank location, contained 26 mg/kg high-boiling-pcint hydrocarbons. No concentrations of benzene, toluene, xylenes,ethylbenzene(BTXE),chlorinated solvents, or PCBs greater than method detection limits were measured in any of the soil samples. Two soil samples,collected from stockpiled soil in the fuel tank excavation area,contained up to 490 mg/kg low-boiling-point hydrocarbons,28 mg/kg toluene,and 26 mg/kg xylenes. 2.3 INSTALLATION AND SAMPLING OF SIX GROUNDWATER MONITORING WELLS,7-15 NOVEMBER 1988 On the basis of the petroleum hydrocarbon concentrations measured in the soil excavated from the storage tank pit,six groundwater monitoring wells were installed on the site to further inves- tigate soil quality and to permit the collection of groundwater samples. _- Six-boreholes were drilled,sampled,and completed as groundwater monitoring wells byEAon__ 7-11 November 1988. The well locations for MW through MW6 are shown in Figure 3. Soil samples collected from borehole MW4,located near the former tank field,contained benzene