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t particular) after other extractions stems have reached their effective limits due to lowered <br /> Y Y <br /> ' hydrocarbon concentrations. These systems operate at temperatures of under 1,000°F, <br /> requiring less supplemental fuel than either thermal oxidizers or internal combustion <br /> engines. Other requirements and limitations are similar to those for internal-combustion and <br />' thermal oxidizer systems. <br /> 6.1.2.2.2 Required Soil Conditions <br /> Operation of the blower in a vapor extraction system creates a vacuum in the subsurface, <br /> inducing air currents through pore spaces. Vapors move by convection toward the area of <br /> lowered air pressure (the extraction point). The efficiency of this process is proportional to <br />' soil porosity and permeability. Neither of these parameters has been measured at the Art <br /> & Artist's site, but qualitative information has been gained through examination of soil <br />' samples and the performance of a vapor extraction pilot test (see section 6.1.2.2.5). The <br /> information indicates that soil conditions at the site are adequate for operation of a vapor <br /> extraction system. <br />' 6.1.2.2.3 Extraction Wells <br />' At least one extraction well is required in a vapor extraction system. Well RW-1 was <br /> installed for this purpose. This well was drilled at a 15° angle in order to penetrate the <br /> center of the plume of contaminated soil. This well was screened through the contamination <br /> plume (see section 2.7). A blank section of PVC was installed between 40 and 45 feet,where <br /> a dense silty clay bed is present. This bed is less suitable for vapor extraction, and might <br /> require the installation of a second extraction well screened only through this interval after <br /> ' the more permeable sand above and below it have been remediated. The results of the <br /> vapor extraction test indicate that one well should be adequate to reach the perimeter of <br /> the hydrocarbon plume (see below). <br /> ' 6.1.2.2.4 Monitoring Activities <br /> ' Monitoring of vapor extraction systems involves weekly measurement of vapor <br /> concentrations at the inlet to the destruction unit and periodic service and repair. Systems <br /> can be equipped with automatic fax communication to alert the operator of system <br /> ' malfunction or failure. Vapor samples should be collected quarterly to track the efficiency <br /> of the remediation program. When the concentration of extracted vapors decreases and <br /> stabilizes after several months of operation, borings are drilled to collect soil samples for <br /> quantitative analysis in the laboratory. <br /> 6.1.2.2.5 Vapor Extraction Test <br /> An Environmental Techniques Vaclean 1000 vapor extraction unit was used in a vapor <br /> ' extraction pilot test conducted on December 17, 1993. This unit is approved by the San <br /> Joaquin Valley Air Management District. Figure 11 shows the configuration of the system. <br /> —bP'l-d„s–i-.h, 30 <br /> ARA PARR-29-WI.47 <br /> 1 <br />