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ATTACHMENT A SUMMARY OF PREVIOUS INVESTIGATIONS . Chevron 9-8264, 3775 Tracy Boulevard, Tracy, California 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 12 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 bonngs 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-I at 9 5') BTEX concentrations ranged from non-detect to 0 20 ppm Results of groundwater grab samples from bonngs B-1 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-I 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 Third quarter 2002 analytical data was collected on July 15, 2002 Four wells were sampled during this event TPHd was detected at concentrations of 69 µg/1 in the sample collected from MW-3 and 4,600 µg/l in the sample collected from MW-4 TPHg was detected at concentrations of 130 µg/1 in the sample collected from MW-3 and 25,000 µg/l in the sample collected from MW-4 Benzene was detected at 2 µg/l in the sample collected from MW-4 MtBE was detected at concentrations of 2,300 µg11 in the sample collected from MW-3 and 1,100 µg/1 in the sample collected from MW-4 Groundwater flow was to the northeast at a gradient of approximately 0 001 to 0 002 ft/ft, generally consistent with historical data