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i <br /> . Protect No S8100-06-18 <br /> June 21, 1995 <br /> Page 3 <br /> • The direction of groundwater flow between 1987 and May 1994 has ranged from southwest <br /> to northeast <br /> • The depths to groundwater beneath the site in May 1994 ranged from approximately 64 to <br /> 66 feet bgs, reflecting a decrease in groundwater levels of approximately 25 feet since 1987 <br /> The maximum concentration of detectable TPHg in groundwater in the vicinity of the former <br /> USTs in May 1994 was 2 57 mg/l, which represents a 98% decrease in the initial 1987 TPHg <br /> concentrations <br /> • Monitoring data from May 1994 indicates that detectable TPHg in groundwater is delineated <br /> and bounded by wells MW-12, MW-24, MW-19, MW-6, MW-9 and MW-13 <br /> • Monitoring data from May 1994 indicates that the majority of detectable benzene in <br /> groundwater is limited to the vicinity of and to the north of the former USTs and is <br /> delineated to the west and south The source of the low concentrations of benzene detected <br /> in wells MW-11 (6 6 ug/1) and MW-12 (1 4 ug/1) may be from UST releases that have been <br /> reported to have occurred at the adjacent Caltrans Stockton Yard facility located directly <br /> • across South B Street The estimated lateral limits of TPHg and benzene in groundwater <br /> beneath the site are depicted on Figure 3 <br /> • With the exception of 350 mg/kg of TPHg detected in boring SB-17 at 45 feet bgs, soil <br /> analytical data indicates that TPHg in soil is generally laterally limited to the vicinity of the <br /> former USTs extending to the current groundwater surface <br /> • The volume of vadose zone and capillary fringe soil impacted with TPHg is estimated to be <br /> 1,500 cubic yards <br /> • The amount of TPHg mass in vadose zone and capillary fringe zone soil beneath the former <br /> USTs is estimated to be approximately 1,100 pounds <br /> O The limited vapor extraction pilot test performed by Alton resulted in the extraction of TPHg <br /> vapor from the vadose and capillary fringe zones at a measured concentration of 150,000 <br /> mgltn3 At the pilot test extraction flowrate of 48 actual cubic feet per minute, the calculated <br /> mass extraction rate for TPHg would be approximately 647 pounds per day The relatively <br /> high concentration of TPHg vapor measured from former well MW-10 further supports the <br /> fact that the majority of the TPHg mass in vadose zone and capillary fringe soil resides <br /> beneath and in the vicinity of the former USTs <br />