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equilibration with VOC vapors remaining within fine-grained materials; (2) further <br /> volatilization of VOCs in liquid phase within the soil matrix above the water table in the <br /> capillary fringe; and (3) volatilization of VOCs dissolved in shallow groundwater. <br /> Post-initial shutdown monitoring will be conducted at various frequencies, as indicated <br /> below, (1) to measure VOC concentrations and (2) assess whether rebound is occurring. <br /> Soil gas samples will be collected from all monitoring and extraction wells to assess the <br /> occurrence of rebound. The first rebound assessment will occur immediately after blower <br /> shutdown. The undisturbed/undiluted soil gas data will be evaluated to assess rebound <br /> and whether soil gas concentrations are approaching soil gas cleanup goals established <br /> for the specific sites. Monitoring will follow to assess whether rebound has occurred. <br /> Monitoring events will include collection of "static" soil gas samples (i.e. undisturbed <br /> samples) from each of the extraction wells. Extraction wells will be monitored at biweekly <br /> intervals during early rebound assessment. If rebound is not observed for three <br /> consecutive assessment events, monthly monitoring will be conducted. Vapor monitoring <br /> wells will be monitored monthly. <br /> Although the occurrence of rebound has been reported by other investigators at different <br /> sites (McCullough Morabito, 1994; EPA, 1994 and 1995; Wilson, 1995), these references <br /> have no specific criteria defining soil gas rebound. It has been reported that the extent of <br /> rebound of soil gas VOC concentrations under static conditions provides a useful method <br /> for assessing the completeness of SVE cleanups (Wilson, 1995). <br /> If rebound is judged to have occurred, a period of intermittent, or pulsing operations, will <br /> be initiated. Other investigators that have conducted SVE studies located at other sites <br /> (McCullough and Morabito, 1994; EPA, 1995; and Wilson, 1995) have observed that the <br /> efficiency of mass removal may be improved by cycling the SVE system on and off. <br /> Pulsing would involve the periodic shutdown and startup operation of the system to allow <br /> the subsurface environment to come to equilibrium (shutdown) and then begin extracting <br /> vapors again (startup) (EPA, 1995). <br /> A shutdown/startup cycle of one week on and one week off is proposed. SVE monitoring <br /> will occur with each weekly startup. SVE samples will be collected from extraction wells <br /> after one hour of operation following the restart. The monitoring data collected during the <br /> pulsing operations will be evaluated with the same criteria defined for rebound <br /> determinations. In addition, the change in rebound will be assessed. It is anticipated that <br /> rebound levels will begin to converge, or develop asymptotic behavior similar to that <br /> observed when the stabilization period is reached. <br /> If the rebound does not occur after three successive restarts during pulsing operations, the <br /> system will be shutdown again. Rebound will again be assessed using the system <br /> stabilization and soil gas cleanup goal criteria described above. Weekly to monthly <br /> monitoring of extraction wells and monthly monitoring of vapor wells will continue during <br /> the rebound assessment period. <br />