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i ' Historical Background & Site Conceptual Model • AGE-NC Project No 01-0827 ' Page 2 of 9 UST#3 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 wall However, the only sample with reported hydrocarbons was collected at the dispenser island (P 1) Sample P-1 had ' reported detections of total petroleum hydrocarbons as gasoline (TPH-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 I 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 I contained TPH-g at a maximum concentration of 3 50 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 ofwell 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 BI 3 was completed as remediation well RW-1,at a location 20 feet north of well MW-1 and the former UST excavation Boring B12 was completed as groundwater observation well PZ-1 ' between well RW-1 and the former UST excavation Additionally, boring 139 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 1 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 BI 7 was completed as dual-screened vapor well VW-5 During January 1994,a second SVE remediation ' feasibility 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-I Based on the pilot ' Advanced GeoEnvironmental,Inc 1