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' Historical Background & Site Conceptual Model AGE-NC Project No 01-0827 ' Page 2 of 9 ' UST43 was a 500 gallon diesel UST These two USTs serviced the western dispenser on the fuel island Contamination was reportedly evident on the southern excavation wail However, the only sample with reported hydrocarbons was collected at the dispenser island (Pl) Sample P-1 had reported detections of total petroleum hydrocarbons as gasoline (TPI -g), TPH-diesel (TPH-d) toluene, ethylbenzene and xylenes at concentrations of 1,600 mg/kg, 4,900 mg/kg, 0 33 mg/kg, 4 9 mg/kg and 56 mg/kg, respectively 22 SITE ASSESSMENT MILESTONES ' In December 1988 the assessment of the petroleum hydrocarbons release from the USTs began with the installation of soil borings B 1 through B3, completed as ground water monitoring wells MW-1 ' through MW-3 Soil samples from boring B 1 contained TPH-g at a maximum concentration of 350 mg/kg(B 1-25) Benzene,toluene,ethylbenzene and xylenes(BTEX)were detected in the same sample at concentration of 4 7 mg/kg,21 mg/kg, 18 mg/kg and 110 mg/kg,respectively Monitoring ' well MW-1 was installed at the western edge of the former UST cluster area, well MW-2 was installed 100 feet east of well MW-1, and well MW-3 was installed 90 feet south of well MW-1 and ' the former UST excavation The wells were first sampled on 12 December 1988 In May 1990,five additional soil borings,B4 though B8 were established on the site Boring B8 was ' completed as monitoring well MW-4,at a location 140 feet north of well MW-1 and the former UST excavation In August and September 1992,five additional soil borings,B9 though B 13,were established on the site Boring B 13 was completed as remediation well RW-1,at a location 20 feet north of well MW-1 and the former UST excavation Boring B 12 was completed as ground water observation well PZ-1 ' between well RW-1 and the former UST excavation Additionally, boring B9 was completed as vapor well VW-1, boring B 10 was completed as vapor well VW-2 and boring B 11 was completed as dual-screened vapor well VW-2 During September 1992, a set of ground water and soil vapor extraction(SVE)remediation feasibility pilot studies were conducted on the site One ground water extraction(GWE)pilot test was conducted at well RW-1 and one SVE pilot test was conducted on wells VW-2 and VW-3 In September 1993, four additional soil borings, boring B14 thought B17 were established on the site Boring B 14 was completed as monitoring well MW-5 at a location 100 feet west of well MW-1 and the former UST excavation Boring B15 was established inside the truck bay of the building, north of the former UST excavation Boring BI 6 was completed as vapor well VW-4 and boring B 17 was completed as dual-screened vapor well VW-5 During January 1994,a second SVE remediation ffeasibility pilot study was conducted on the site, on wells VW-1 and VW-5 During October 1994, a third SVE remediation feasibility pilot study was conducted on well VW-1 Based on the pilot Advanced GeoEnvironmental,Inc I