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E , � <br /> 3 <br /> pressure differential created by a vacuum pump is measured, <br /> in the lateral and vertical planes. The capture radius was <br /> noted by recording the vacuum gradient in neighboring probes <br /> or monitoring wells in inches of water. This procedure was <br /> performed at various flow rates at several of the probes and <br /> monitoring wells. The detection limit for the vacuum <br /> gradient was 0. 005 inches of water. The effluent from the <br /> extraction points was routed through two cylinders in <br /> series, each containing 200 pounds of vapor phase activated <br /> carbon. The carbon adsorbs organic vapors from the <br /> extraction air stream, preventing discharge of contaminants <br /> into the atmosphere. <br /> After the investigation was completed, the vapor probes were <br /> removed. Upon removal of the probes, all locations were <br /> E sealed with bentonite and Portland cement (Figure B) . This <br /> procedure prevents the possibility of contaminants reaching <br /> the aquifer through the hole created by the probe. <br /> RESULTS <br /> Soil Gas Survey <br /> The total hydrocarbon concentration as vapor (TVH) recorded <br /> at the temporary vapor probes and monitoring wells at the <br /> depth intervals tested are given in Table 1. <br /> The highest TVH concentration, 19,425 parts per million <br /> (ppm) , was recorded at monitoring well MW-3 in the vicinity <br /> of the former tank complex. Hydrocarbon concentrations as <br /> vapor were recorded at depths greater than 28 feet only, <br /> with one exception. A vapor concentration of 250 ppm was <br /> recorded at 19 to 21 feet below grade at Probe J. Probe J <br /> is in the vicinity of the former tank complex; TVH <br /> concentrations were recorded through the 32 to 34 feet depth <br /> interval at this probe. <br /> Performance Test <br /> The Performance Test was conducted on 13 March 1990 at the <br /> sub3ect site. Extraction volumes from the probes and wells <br /> ranged from 14 to 26 cubic feet per minute (cfm) with <br /> vacuums ranging from 1. 5 to 8 . 5 inches of mercury (" Hg) . <br /> The results of the Performance Test are given in Table 2 . <br /> Influence was noted in both the vertical and horizontal <br /> planes. The influence in the horizontal plane was more <br /> Ipronounced. Capture radii of 11 feet and 23 to 41 feet were <br />