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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 contaminant concentrations Vent gas PCE/TCE 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 advectrve 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 /> ,000 <br /> 8Oo <br /> 600 <br /> "400 <br /> e <br /> c <br /> 200 <br /> 0 <br /> 0 20 40 60 60 100 120 140 <br /> WES El.p..d Tim.(Dy.) <br /> A Q <br /> 6 <br /> C 00 <br /> D <br /> E 0 <br /> F <br /> swR Hw.test <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 shut down Typically,the criteria <br /> for determining when the system can be shut down include one or more of the following <br /> • Total amount of contaminant removed <br /> • Residual soil concentrations <br /> • Extraction well(s)vapor concentrations and composition <br /> • Soil gas contaminant concentrations and composition in monitoring points <br /> 9-5 <br />