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i <br /> ppm. <br /> 4.2 Removal of Aromatic Volatile Organic Compounds <br /> Table 4 shows that low concentrations of aromatic VOC' s are <br /> I presently being removed by the VES. The present VOC concentrations <br /> are less than 2% of TPH concentrations. Because VOC' s are <br /> considerably more volatile than TPH, low concentrations suggest <br /> that most of the VOC ' s have already been removed by the VES. <br /> 4.3 Initial Volume of Hydrocarbons in soil <br /> Prion to construction of the VES, GeoAudit estimated the original <br /> volume of contaminated soil at 1, 182 cu yds (see GeoAudit Workplan <br /> for In-situ Vapor Extraction, 28 May 1991) . This estimate was <br /> based on an assumption that the contamination plume was cylindrical <br /> in shape, with a diameter of 25 feet and a height of 65 feet (the <br /> assumed depth to groundwater) . Using maximum TPH concentrations <br /> for 10-foot depth intervals, average concentrations for TPH and <br /> I BTE&X were also calculated and used to calculate the total weight <br /> and volume of hydrocarbons in the soil . <br /> Subsequently, vapor extraction well VE-1 was installed to a depth <br /> of 70 feet, and slightly contaminated soil was recovered at this <br /> depth. The estimate of the contaminated volume should therefore be <br /> revised upward to approximately 1, 275 cu. yds. However, because no <br /> I TPH was detected between 45 and 70 feet in VE-1, including this <br /> 1 deeper interval in the calculations decreases the average <br /> concentrations for each contaminant (see below) : <br /> HYDROCARBON PREVIOUS CONC. REVISED CONC. <br /> Benzene 168 mg/kg 110 mg/kg <br /> Toluene 615 405 <br /> Ethylbenzene 139 92 <br /> Xylene 779 513 <br /> ITPH 5 , 780 3 , 757 <br /> Decreasing the average concentration of all contaminants by this <br /> amount offsets the increase in the thickness of the contaminanted <br /> interval from 65 to 70 feet. The revised volume of hydrocarbons is <br /> 2 , 680 gallons. <br /> 4. 4 Volume of Extracted Hydrocarbons <br /> The total volume of hydrocarbons that have been extracted from VE--1 <br /> can be estimated from Figure 3 . The operational period can be <br /> divided into several segments, based on curve shape. An average <br /> . PID reading for each segment can be determined visually, and <br /> I related to the actual concentration of removed hydrocarbons by <br /> multiplying by the ratio determined in section 4 . 1. This <br /> concentration can then be multiplied by the number of hours that <br /> I 3 <br />