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ATTACHMENT A SUMMARY OF PREVIOUS INVESTIGATIONS Chevron 9-8264, 3775 Tracy Boulevard, Tracy, CA In November 1995, Touchstone Developments oversaw the excavation and removal of the USTs, hydraulic hoists, sump, product dispensers and associated product lines. Soil samples were collected from beneath the USTs, hydraulic hoists, sump, product piping, and dispensers. The highest concentrations of hydrocarbons (830ppm and 420 ppm TPHg) were reported near the center and northeast corner of the former UST pit at depths of 9.5 feet to 12.5 feet bgs. TPHg concentrations of 1.2 ppm to 66 ppm were reported beneath the product lines. Concentrations of oil and grease in soil beneath the hoist and sump ranged from 91 ppm to 120 ppm, and were not detected beneath the used oil UST. In February 1996, Touchstone Developments advanced eight borings onsite and installed four monitoring wells in four of the eight borings to an approximate depth of 22 feet bgs. Laboratory analysis of soil samples reported TPHg in two samples at concentrations of 2.6 ppm (B-1 at 5') and 13 ppm (B-1 at 9.5'). BTEX concentrations ranged from non-detect to 0.20 ppm. Results of groundwater grab samples from borings B-I through B-4 indicated TPHg concentrations ranging from non-detect to 890 ppb, and MtBE concentrations ranging from non-detect to 1,500 ppb. BTEX concentrations ranged from non-detect to 150 ppb, and TPHd concentrations ranged from non-detect to 580 ppb. Initial groundwater sampling of wells MW-1 through MW-4 indicated non-detect concentrations of all analytes with the exception of MtBE at a concentration of 140 ppb in the sample collected from MW-3 and 91 ppb in the sample collected from MW-4, and 1.8 ppb of toluene in the sample collected from MW-4. First quarter 2001 analytical results reported MW-1 and MW-2 as non-detect for all analyzed constituents. MW-3 reported MtBE at 11,000 ppb. MW-4 reported TPHd at 21,000 ppb, TPHg at 26,000 ppb, benzene at 170 ppb, and MtBE at 950 ppb. Groundwater flow at the site has been reported to the northeast, east, and southeast and is currently to the southeast at a gradient of 0.002 to 0.003 ft/ft.