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SECTION 3.0 SUMMARY OF PREVIOUS INVESTIGATIONS 3.1PREVIOUS SOIL AND GROUNDWATER INVESTIGATIONS Environmental investigations have been performed at Shop 10 since 1987 Historical soil analytical laboratory results are summarized in Table 3-1 Table 3-2 provides historical groundwater analytical results A summary of the previous environmental investigations is discussed below 0 The initial soil and groundwater assessment was conducted by Woodward-Clyde in 1987 Four groundwater monitoring wells (MW-1 through MW-4) were installed in the vicinity of the four 5,000-gallon USTs Groundwater sample results indicated that TPH-g concentrations in groundwater ranged from 154 to 231 milligrams per liter (mg/L), and benzene concentrations ranged from 14,700 to 25,700 micrograms per liter (µg/L) Twenty soil samples were collected and analyzed for TPH-g and BTEX TPH-g concentrations in soil samples were below detection limits in samples collected from MW-1 through MW-3 and ranged from non-detect to 1,100 milligrams per kilogram (mg/kg) in soil samples collected from MW-4 Benzene concentrations ranged from non-detect to 7 7 mg/kg Toluene concentrations ranged from non-detect to 29 5 mg/kg, ethylbenzene concentrations ranged from non-detect to 12 1 mg/kg, and xylenes concentrations ranged from non-detect to 55 2 mg/kg The highest BTEX concentrations were detected in MW-4 between 20 and 30 feet bgs A soil and groundwater evaluation report was prepared summarizing the assessment (Woodward-Clyde, 1987) • In 1989, the four USTs and a waste-oil UST were removed by Placer Tractor Service Soil samples were collected beneath the USTs TPH-g concentrations in soil from beneath the gasoline and diesel fuel USTs ranged from 2 2 to 3,700 mg/kg and TPH diesel range (TPH-d) concentrations in soil from beneath the diesel and waste-oil USTs ranged from 100 to 1,700 mg/kg, and 11 to 12 mg/kg, respectively • An additional site assessment was conducted by Woodward-Clyde between 1989 and 1990 Five additional groundwater monitoring wells (MW-5 through MW-9) were installed Twenty-four soil samples were collected and analyzed for TPH-g and BTEX BTEX were detected above laboratory detection limits in one soil sample collected from MW-7 at a depth of 30 feet bgs at the following concentrations 0 4 mg/kg, 0 2 mg/kg, 0 1 mg/kg and 0 2 mg/kg, respectively TPH-g concentrations in the soil samples from the five borings were not detected above the laboratory detection limit TPH-g concentrations in groundwater ranged from non-detect to 3 4 mg/L BTEX concentrations in groundwater ranged from non- detect to 200 µg/L, non-detect to 8 gg/L, non-detect to 0 7 Ag L, and non-detect to 170 µg/L, respectively S 1PRO.M1040100041SHOPt41SHOP10SA.DOC 3-1 URS Greiner Woodward Clyde