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ATTACHMENT A <br /> SUMMARY OF PREVIOUS INVESTIGATIONS <br /> Chevron 9-9840, 4344 Waterloo Road, Stockton, California <br /> In March 1998, three 10,000-gallon gasoline underground storage tanks (USTs), one 1,000- <br /> gallon waste oil UST, one 500-gallon waste oil UST, product dispensers, associated product <br /> lines, two hoists, and a sump were removed as part of station upgrade activities The highest <br /> initial hydrocarbon concentrations were found at the west end of the former UST pit, and <br /> below the former product lines These areas were subsequently over-excavated <br /> (approximately 725 cubic yards) <br /> On September 22 and September 25, 1998, three onsite groundwater monitoring wells (MW- <br /> 1, MW-2, and MW-3) were installed Soil analytical data from MW-3 showed no detectable <br /> hydrocarbon concentrations at depth Methyl-tertiary-butyl-ether(MtBE) was not detected in <br /> any of the soil samples collected during the advancement of the borings for the wells MW-1, <br /> MW-2, and MW-3 Soil analytical results from MW-1 and MW-2 reported total petroleum <br /> hydrocarbons quantified as gasoline (TPHg) at concentrations of 8580 mg/kg (MW-1@30') <br /> and 3 23 mg/kg (MW-2@60') Benzene was also reported in soil samples from MW-1 and <br /> MW-2 at concentrations of 12 1 mg/kg(MW-1@30'), and 0 995 mg/kg (MW-2@60') <br /> On July 31 through August 3, 2001, eight direct-push borings were advanced (CPT-1 through <br /> CPT-8) along the northern and eastern property boundaries to assess soil stratigraphy and <br /> collect discrete soil and groundwater samples CPT-1 was advanced to approximately 60 feet <br /> below ground surface (bgs), however, the CPT log for that boring showed no easily <br /> identifiable water-bearing zones Asa result CPT-2 was advanced to approximately 100 feet <br /> • bgs to collect deeper stratigraphic data Subsequent CPT borings were advanced to a total <br /> depth of approximately 70 feet bgs Soil samples from CPT-1 through CPT 5 and CPT-8 did <br /> not contain any of the analytes above the laboratory detection limits Soil samples from <br /> CPT-6 contained xylenes at a concentration of 13 milligrams per kilogram (mg/kg) at 15 feet <br /> bgs, and MtBE at a concentration of I mg/kg at 50 feet bgs Soil samples from CPT-7 <br /> contained TPHg at a concentration of 2 0 mg/kg at 45 feet bgs, benzene at 53 mg/kg at 45 <br /> feet bgs, ethylbenzene at 130 mg/kg at 45 feet bgs, xylenes at 550 mg/kg at 45 feet bgs, and <br /> MtBE at 32 mg/kg and 24 mg/kg at 45 feet bgs and 60 feet bgs, respectively TPHg was <br /> detected in three groundwater samples at concentrations ranging from 0 11 micrograms per <br /> liter (µg/L) in CPT-4 at 68 feet bgs to 0 26 µg/L in CPT-5 at 68 feet bgs Benzene was <br /> detected in three groundwater samples at concentrations ranging from 0 69 µg/L (CPT-6 at <br /> 68 feet bgs) to 3 0 µg/L (CPT-5 at 68 feet bgs) MtBE was detected in four groundwater <br /> samples at concentrations ranging from 0 88 µg/L (CPT-3 at 68 feet bgs) to 78 pg/L (CPT-6 <br /> at 68 feet bgs) TAME and TBA were detected in CPT-6 at 68 feet bgs at concentrations of <br /> 3 3 µg/L and 26 µg/L, respectively <br /> During the second quarter 2002 monitoring and sampling event (April 4, 2002 by G-R), three <br /> monitoring wells were sampled during this event TPHg was detected in the samples <br /> collected from MW-1 and MW-2 at concentrations of 53 µg/1 and 65 µg/1, respectively <br /> Benzene was detected in the sample collected from MW-2 at a concentration of 0 81 µg/1 <br />