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ATTACHMENT A SUMMARY OF PREVIOUS INVESTIGATIONS Chevron 9-8632, 575 Grant Line Road, Tracy, CA i 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 below ground surface (bgs) and 24mg/kg at 6.5 feet bgs, respectively. Groundwater monitoring has been performed quarterly since September 1995. During the first quarter 2001 , total petroleum hydrocarbons quantified as diesel (TPHd) was detected in samples from all four wells at concentrations ranging from 88. 1 ppb (MW-1) to 293 ppb (MW-3). Total petroleum hydrocarbons quantified as gasoline (TPHg) were detected in thre t tour samples at concentrations ranging from 572—ppb—(MW-2 to ,720 ppb (MW-3) . Benzene was detected in three of four samples at concentrations ranging from 43 .3 ppb (MW-3) to 226 ppb (MW-4). Methyl-tertiary-butyl-ether (MtBE) was detected in all four samples at concentrations ranging from 20.0 ppb (MW-4) to 262 ppb (MW-3). Groundwater flow has been reported consistently to the northwest at gradients ranging from 0.005 to 0.02 ft/ft.