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ATTACHMENT A SUMMARY OF PREVIOUS INVESTIGATIONS Chevron 9-8632, 575 Grant Line Road, Tracy, CA In 1964 the product lines and an unspecified number of steel underground storage tanks (USTs) were installed at the site In 1984, the steel USTs and product lines were removed They were replaced with three 10,000-gallon capacity gasoline USTs, and one 1,000-gallon capacity waste oil UST, all of single-wall fiberglass construction In May 1994, the USTs, product lines, hoists, sand/grease trap, and approximately 400 cubic yards of impacted soil generated during excavation activities were removed from the site As approved by San Joaquin County Public Health Services (SJCPHS), slightly impacted soil was aerated to non-detect levels and reused onsite Groundwater was encountered during excavation activities at approximately 8 feet below ground surface (bgs) In August 1994, the waste-oil UST and sump excavations were over-excavated until soil exhibiting low to non-detect levels of oil and grease were encountered Approximately 150 to 200 cubic yards of oil and grease impacted soil was removed and hauled offsite In May 1995, wells MW-1 through MW-4 were installed at the site Maximum concentrations of TPHg and benzene in wet soil were reported in the area of the former pump islands (boring MW-3) at 7,600 mg/kg at 6 5 feet bgs and 24 mg/kg at 6 5 feet bgs, respectively On May 22 through 24, 2001, SECOR supervised Gregg Drilling & Testing, Inc (Gregg) of Martinez, California as they advanced 15 direct push bonngs (CPT-1 through CPT-15) and the collected depth-discrete soil and groundwater samples TPHd was detected in all eight soil samples collected with concentrations ranging from 14 mg/kg in the sample from CPT-1 at 23 feet bgs to 14 mg/kg in the sample taken from CPT-2 at 7 feet bgs The sample collected from CPT-2 at 7 feet bgs also contained TPHg and BTEX at 400 mg/kg, 2 3 mg/kg, 1 1 mg/kg, 10 mg/kg, and 5 mg/kg, respectively MtBE was not detected in any of the soil samples collected TPHg was detected in eight groundwater samples at concentrations ranging from 55 ggfL in the sample from CPT-12 at 16 feet bgs to 3900 gg/L in the sample from CPT-7 at 9 feet bgs Benzene was detected in ten groundwater samples at concentrations ranging from 0 51 gg/L in the sample from CPT-I1 at 18 feet bgs to 31 gg/L in the sample from CPT-7 at 9 feet bgs Toluene was detected in five groundwater samples at concentrations ranging from 0 58 gg/L in the sample from CPT-4 at 18 feet bgs to 14 gg/L in the sample from CPT-10 at 18 feet bgs Ethylbenzene was detected in thirteen groundwater samples at concentrations ranging from 0 66 gg/L in the sample from CPT- 2 at 15 feet bgs to 110 gg/L in the sample from CPT-6 at 23 feet bgs Xylenes were detected in twelve groundwater samples at concentrations ranging from 0 68 gg/L in the sample from CPT-12 at 34 feet bgs to 76 gg/L in the sample from CPT-14 at 16 feet • bgs MtBE was detected in seven groundwater samples at concentrations ranging from 4 8 gg/L in the sample from CPT-10 at 24 feet bgs to 56 gg/L in the sample from CPT-9