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45 E Harding Way, Stockton, California March 14, 1994 • Chevron USA Products Company Page 2 2.0 BACKGROUND The site, currently an empty lot, was formerly a Chevron service station which dispensed gasoline from three underground storage tanks Nine separate subsurface investigations have been conducted at the site since November 1987 sic investigations were conducted by EA Engineering, Science, and Technology (EA) from November 1987 to April 1991, and three subsurface investigations were conducted by Groundwater Technology from July 1991 to June 1993 On November 18, 1987, EA conducted a soil-vapor contaminant assessment In which samples were collected at various depths down to 15 feet below grade, using a 5/8-inch diameter steel probe that was dnven into the vadose zone The sample area included the tank pit and pump Islands No concentrations of petroleum hydrocarbons greater than 1 part per million (ppm) were detected in the sampled sod vapors On February 24, 1988, EA supervised the removal and excavation of four fiberglass underground storage tanks Soil samples were collected from beneath individual tanks and analyzed Only one sample, from beneath the used-oil tank, contained 26 ppm high-boding-point hydrocarbons Samples from the sod pile contained up to 490 ppm low-boding-point hydrocarbons, 28 ppm toluene, and 26 ppm xylenes On November 7-15, 1988, EA conducted an environmental assessment involving the drilling and installation of sic groundwater monitoring wells (MW-1 through MW-6) Soil samples collected from MW-4 contained benzene concentrations up to 9 6 ppm and total petroleum hydrocarbons-as-gasoline (TPH-G) concentrations up to 3,300 ppm TPH-G concentrations in soil samples from all other well bonngs were below the detection limit of 1 ppm TPH-G concentrations in groundwater samples ranged from 6,800 parts per billion (ppb) in MW-3 to 9,300 ppb in MW-4 (Figure 2) In January 1989, three addirtional underground storage tanks were removed One of the tanks, discovered while drilling MW-5 in November 1988, still contained product Seven soil borings were dniled and sampled on January 23 through February 7, 1989 Two of the borings were converted to groundwater monitoring wells, and the remaining five borings were converted to vadose monitoring wells Hydrocarbon-impacted sods were encountered in V-3, V-4, and V-5 I!'_�' GROUNDWATER 4e955UM APT ��` �'� TECHNOLOGY