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,1 + 45 East Harding Way, Stockton, California July 22, 1992 Chevron U S A Products Company Page 2 20 BACKGROUND The site, currently an empty lot, was previously a Chevron service station which dispensed gasoline from three underground storage tanks All underground tanks, piping and pumps have been removed and the structures demolished Seven separate subsurface investigations have been conducted at the site since November 1987 The first six investigations were conducted by EA Engineering, Science, and Technology The seventh subsurface investigation was conducted by Groundwater Technology in July 1991 On November 18, 1987, a soil-vapor contaminant assessment was conducted in which samples were collected at various depths up to 15 feet using a 518-inch diameter steel probe that was driven into the vadose zone The sample area included the tank pct and pump islands No concentrations of petroleum hydrocarbons greater than 1 part per million (ppm► were detected in the sampled soil vapors On February 24, 1988, four fiberglass underground storage tanks were excavated and removed Soil samples were collected from beneath individual tanks and analyzed Only one sample, from beneath the used-oil tank, contained 26 ppm high-boiling-point hydrocarbons However, samples from the soil pile contained up to 490 ppm low-boiling-point hydrocarbons, 28 ppm toluene, and 26 ppm xylenes On November 7-15, 1988, six boreholes were drilled, sampled, and completed as groundwater monitoring wells Soil samples collected from MW-4 (Figure 2) contained benzene concentrations up to 9 6 ppm and total petroleum hydrocarbons (TPH) concentrations up to 3,300 ppm TPH concentrations in soil samples from all other well borings were below the detection limit of 1 ppm Groundwater samples collected from wells MW-1 through MW-6 in November 1988 contained TPH concentrations as high as 9,300 parts per billion (ppb) and 6,800 ppb from MW-3 and MW-4, respectively i In January 1989, three additional underground storage tanks were removed One of the tanks, discovered while drilling MW-5 in November 1988, still contained product ��►--��l �i7JW GROUNDWATER LLj__ TECHNOLOGY