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EM 1110-1-4001 <br /> 3 Jun 02 <br /> f Hiller(1991)reviewed a number of full-scale SVE case histones and selected for analysis six well- <br /> documented successfully vented sites with TCE and/or PCE contamination which vaned somewhat in <br /> geologic setting and initial contammnant concentrations Vent gas PCFJTCE concentrations followed <br /> similar trends at the six sites,with an initial steep 80 to 90 percent decline lasting about 20 days, followed <br /> by a gradual asymptotic decrease to background concentrations During this latter phase,concentrations <br /> were similar among sites,falling from 20 ppm or less to about 2 ppm in the final stages after 6 months <br /> The data are shown in Figure 9-1 This was interpreted to reflect initial rapid evaporation of free product <br /> droplets, followed by a diffusion-controlled process of partitioning of contaminants previously dissolved in <br /> soil moisture into the gaseous phase and desorption from soil particles However,sharp declines in <br /> extracted vapor concentrations do not always indicate that advectmve removal has ended If only aggregate <br /> vapor concentrations are monitored(m e , from a common header),then the behavior of each well is masked <br /> Areas that contain high concentrations,but yield low air flowrates may be obscured by wells that yield <br /> higher flow rates This emphasizes the importance of monitoring individual wells <br /> ,aaa <br /> �d <br /> sflo <br /> s <br /> "400 <br /> n <br /> 0 <br /> 200 <br /> 0 <br /> 0 24 40 90 80 100 120 149 <br /> 3RE9 Er.ps.d Tim.(O.ya) <br /> A <br /> B <br /> G <br /> o <br /> E p <br /> F + <br /> Figure 9-1 Vent Gas VOC Concentrations at Six Sites Over Time <br /> 9-5 Evaluation of Results <br /> a Methods of evaluation for shutdown The results of the sampling and analyses described above <br /> must be carefully evaluated before deciding that the system is ready to be shutdown Typically,the criteria <br /> for determining when the system can be shut down include one or more of the following <br /> • Total amount of contammmant removed <br /> • Residual soil concentrations <br /> • Extraction well(s)vapor concentrations and composition <br /> • <br /> Soil gas contaminant concentrations and composition in monitoring points <br /> • <br /> 9-5 <br />