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Site Background Information PremDor Page 4 of 5 from surface grade to 6 feet below surface grade (bsg), clays, clayey silts and clayey sands from 6 feet bsg to 21 feet bsg, fine to coarse sand and silty sand from 21 feet to 29 feet bsg (35 feet at MW-2),clay with medium to high toughness and plasticity from 29 feet bsg to 36 feet bsg, silt from 36 feet bsg to 49 feet bsg, and fine sand from 49 feet bsg to 52 feet bsg or beyond Since initiation ofgi.ound water monitoring in October 2003,depth to ground water measurements have fluctuated between approximately 18 5 feet and 21 feet below the top of the casing(btoc) The rinferred ground water flow direction generally has been towards the northeast, four times, at gradients ranging between 0 005 ft/ft and 0 022 ft/ft, ground water flow was towards the east- northeast once at a gradient of 0 002 fdit Releases of petroleum hydrocarbons occurred from the former on-site USTs Once released to the subsurface,the contaminants migrated through the fine grained unit directly below the UST area to a vertical depth of approximately 20 feet bsg, at which point the contaminants encountered a more porous sand unit and the water table and began to migrate laterally as well as vertically,the direction of lateral migration should be towards the northeast, in the preferred ground water flow direction, however, maps of contaminant distribution suggest a more uniform spread away from the former UST location The vertical migration of soil contamination appears to be slowed by the clay layer • encountered from 29 to 36 feet bsg except at the location of boring B3, where significant concentrations are detected at this depth Significant petroleum hydrocarbon-impact to soil in the vadose and smear zone is present in borings B 1 and B2,in the area of the former USTs Total petroleum hydrocarbons quantified as gasoline and diesel(TPH-g,TPH-d)and benzene are present at maximum concentrations of 22,000 mg/kg,7,500 mg/kg and 1 1 mg/kg, respectively, all from sample B2-15 Most of the contamination appears to reside above the top of the clay layer,at approximately 29 to 36 feet bsg,except at B3,where TPH-g was detected at 35 feet bsg at a concentration of 900 mg/kg ' Contamination in ground water was resent in all four soil boring locations and in the up-gradient gr' p g well MW-2, the ground water sample from B4, collected between 53 feet and 57 feet bsg, in the Isecond sand unit, indicated elevated amounts of contamination at a depth of 57 feet bsg Concentrations have not been detected in down-gradient well MW-1 and cross-gradient well MW-3 only had one very small detection of toluene and xylene (January 2004), both below State of ICalifornia Department ofHealth Services(DHS)Maximum Contaminant Levels(MCL)for drinking water �k ' Lateral hydrocarbon-impact to soil is not defined toward the north, southeast and southwest of the former UST The vertical extent of soil contamination does not appear to exceed 40 feet bsg, with ' the exception of B3 at 35 feet bsg, and the low concentrations of hydrocarbons detected in samples collected at 35 feet from borings B1, B2 and MW-2 The lateral extent of ground water P contamination is not defined toward the north, southeast and southwest of the former UST, and is Advanced GeoEnwo=ental,Inc ` 0� �� ,y