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system approaches stabilization, the monitoring frequency will be weekly to biweekly <br /> depending upon site - specific SVE system evaluation and response. Data collected <br /> during this time will be used to assess the decline in mass removal rates. As noted above, <br /> it is anticipated that the decline will exhibit asymptotic behavior, and a stabilization of mass <br /> removal rates will occur. Throughout SVE operation, the SVE system removal efficiency <br /> will be optimized as possible. Optimization techniques may include 1) increasing flow <br /> rates for extraction wells with higher vapor concentrations and shutting down wells with <br /> lower VOC concentrations or 2) adjusting the blower/total extraction rates to maximize the <br /> rate of contaminant removal. The soil gas concentration will be measured at different flow <br /> rates to determine the optimum flow rate. <br /> The variation of the mass removal rate of VOCs during the full scale SVE operation will be <br /> documented throughout the duration of the SVE operations to allow a relative evaluation <br /> of each system's effectiveness. The pilot test initial mass removal rate will be confirmed <br /> by initial testing at the start of the full scale SVE operation. Both estimates will provide a <br /> baseline for evaluating comparative mass removal effectiveness. This will allow evaluation <br /> of the cost effectiveness and practical limits of each SVE system operation. <br /> 4.3 SVE System Mass Removal Rate Stabilization <br /> Stabilization of the VOC mass removal rate will be determined by comparing consecutive <br /> mass removal rates and VOC concentrations in extracted vapors. The stabilization period <br /> will be defined as occurring when mass removal rates do not vary by more than two <br /> percent (2%) during four consecutive weekly monitoring events, and VOC concentrations <br /> in extracted vapor do not vary by more than 20 percent (20%) over four consecutive <br /> weekly monitoring events. The 20% VOC concentration variance is considered technically <br /> practical and achievable with laboratory methods used for soil vapor analysis, although <br /> VOC concentrations variances of less than 20% during the four consecutive weekly <br /> sampling events may not necessarily be representative of stabilized vapor extraction <br /> concentrations due to a variety of technical factors such as laboratory tolerance for <br /> analytical error and field sampling technique repeatability. <br /> VOC concentrations in extracted vapor refer to soil gas samples collected from the SVE <br /> extraction wells. To monitor the concentration variation, the first soil gas sample(s) will be <br /> collected within the first hour after the pressure at the extraction well is stabilized. <br /> Subsequent soil gas data will be collected on a weekly basis. Soil gas samples will be <br /> collected at the blower suction point to avoid pump contamination. Once stabilization is <br /> confirmed, the SVE system will be shut down. A post-SVE operation monitoring period will <br /> then begin during which VOC concentration rebound, if any, will be assessed. <br /> 4.4 SVE System Performance Criteria <br /> Two criteria will be used to determine SVE system performance. One criterion is based <br /> on stabilization of vapor concentrations and the other criterion is based on actual soil gas <br /> 5 <br />