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EM 1110-1-4001 3 Jun 02 zones Because of this uncertainty,the composition of the extracted vapors is usually monitored as well as the concentrations (2) Holbrook et a] (1998)explain that if the composition of the extracted gas stream from an SVE system at a petroleum fuel site reflects higher and higher boiling compounds,then the extraction system can be considered to have successfully remediated the site relative to the more volatile compounds The system may then be focussed on bioventmg the less volatile components This approach is applicable to sites where the SVE system is affecting all of the contaminated soil If there are pockets of contamination that are subject to rate-limited mass transfer and are not being remediated by advective flow,or are only slowly being remediated,then changes in extracted gas composition will NOT allow evaluation of how close the system is to achieving specific cleanup goals Rather,compositional analysis provides insight into how well the system is performing relative to its ability to treat the part of the problem it can"reach" advect vely e Rebound of concentrations in extracted vapors An alternate method of applying this approach involves the use of pulse extraction,where an area is alternately subjected to a vacuum and then allowed to return to"atmospheric"conditions This method may be employed by using the same vacuum pump to treat two(or more)areas of a site, and cycling over two-week(or other)operating pulse times When an area is brought back under vacuum,the initial concentrations of VOC are measured in the extracted airstream and compared with the initial readings for previous operating cycles The initial concentrations at each cycle are plotted versus time to demonstrate a drop in the"equilibrium"soil air concentrations An example of this graph is shown in Figure 9-1 When the initial cycle concentration approaches zero for the compounds of concern,consideration should be given to entering the shutdown phase Further information regarding rebound testing is provided in section 9-9 and Appendix F The use of rebound testing based only on extracted vapors is subject to the same uncertainties and caveats discussed above f Soil gas concentrations in monitoring points Soil gas concentration and composition in strategically placed monitoring probes can be the most effective indicators of the progress toward cleanup As stated in Chapter 5,monitoring points must be installed in areas that will be the most difficult to remediate and placed in geologically representative strata If soil gas concentrations decline in areas between extraction(and injection)wells where air throughput or oxygen delivery is the least effective,then the system is probably being effective Soil gas concentrations are less expensive to collect,and generally represent more integrated(i e , from a larger area)data Adequate purging must be conducted before sampling,however Although soil gas sampling conducted during remediation is often done,care must be taken to not allow clean air entry into the monitoring probe before purging since the subsurface is usually under vacuum relative to the atmosphere Sampling conducted during remediation also represents the dynamic condition where clean air is being drawn into the treatment areas,and the diffusion-dnven release of contaminant vapors is diluted by the entry of clean air Soil gas sampling of monitoring points following temporary shutdown is a more reliable means to assess progress Some rebound of soil gas concentrations in monitoring points is likely following shutdown and rebound may take weeks When significant rebound is not observed,available mass has probably been removed Remaining mass is likely in low-permeability zones that are unlikely to allow significant leaching of mass to occur If regulatory requirements permit, soil gas concentrations from monitoring points are the preferred data to use for establishing clean closure g For BV systems,the focus of evaluation of treatment progress should be on contaminant degradability Oxygen respiration measurements conducted routinely(e g,quarterly)should be used as an indicator of when system shutdown should be considered Only when respiration rates drop to background levels(i e ,those observed in uncontaminated soil of the same type)would confirmatory soil core samples 9-7