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02 September 2003 <br /> AGE-NC Project No. 98-0468 <br /> ' Page 4 of 11 <br /> the soil samples collected from the four sidewaIls of the excavation detected benzene(59 milligram <br /> per kilogram:mg/kg),toluene(46 mg/kg),ethylbenzene(23 mg/kg),total xylene(150 mg/kg),TPH-' <br /> g (3,800 mg/kg), and TPH-d (45 mg/kg). The majority of the reported detections of petroleum <br /> hydrocarbon compounds were from the east and west sidewalls of the excavation. Subsequently,the <br /> ' City of Stockton backfilled and compacted the excavation. <br /> 4. Site Characterization <br /> ' In 1996,The Twining Laboratories,Inc. cdnducted a site investigation to assess the lateral extent of <br /> petroleum hydrocarbon impacts to soil and ground water at the site. The site characterization <br /> activities consisted of the advancement of seven soil probes(SB-1 through SB-7)and the collection <br /> of soil and in-situ ground water samples for laboratory analysis (Figure 2). <br /> Laboratory analysis of soil samples detected BTEX,TPH-d, TPH-g and methyl tertiary-butyl ether <br /> ' (MTBE) for samples collected from soil probe boring SB-5. In addition, relatively low <br /> concentrations of petroleum hydrocarbon compounds were detected for soil samples collected from. <br /> SB-4 and SB-7. The analytical results of ground water samples collected from the soil probe borings <br /> ' suggested that ground water has been extensively impacted by BTEX, TPH-d, TPH-g and MTBE. <br /> The greatest petroleum hydrocarbon impact to ground water,were reported for soil probe boring <br /> ' locations SB-5 and SB-7 located north of the former diesel UST location. Laboratory analysis of <br /> ground water samples collected from soil probe SB-5 detected benzene (6,500 µg/1), toluene <br /> (35,000µg/1),ethylbenzene(4,500 µg/1),total xylenes(25,000µg/1),TPH-d(320,000 µg/l),TPH-g <br /> ' (93,000 µg/l), and MTBE (560 µg/1). <br /> In April 1999,,five soil borings (B-1 through B-5) were advanced at the site. Borings B-4 and B-5 <br /> were subsequently converted to monitoring wells MW-4 and MW-5;respectively,at locations down <br /> ' gradient (north) of previous soil probe boring SB-7 and the former UST locations, and soil boring <br /> B-1 was advanced at a location up gradient(south)of the former UST locations. Soil boring B-2 was <br /> ' located east of the former. UST area. This location was chosen to evaluate previously reported. <br /> anomalous groundwater analytical results in the vicinity of monitoring well MW-1 and soil probe <br /> .boring 5134. Soil boring B-3 was located west of previous soil probe boring SB-6 and the former <br /> ' UST area. Soil borings B-1 through B-3 were advanced to a total depth of 15 feet bsg. Soil borings, <br /> MW-4 and MW-5 were advanced to depths of 21 feet bsg and 17 feet bsg, respectively. During the <br /> soil boring activities, ground water samples were collected from soil borings B-1 through B-3 for <br /> ' laboratory:analysis. <br /> There were no reported detections of BTEX, TPH-g, or TPH-d in soil samples collected from,the <br /> ' five soil borings.Fuel oxygenating compound MTBE was detected in two samples at concentrations' <br /> ' Advanced GeoEnvironmental,Inc, <br />