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EM 1110-1-4001 3 Jun 02 for selected target compounds rather than the full suite of site compounds Care must be taken to collect and analyze the samples consistently with the collection and analysis procedures used during prior phases Of the project to maximize comparability The SAP should include provisions for resampling should an unusual positive result be found during this stage The data should continue to be entered into the database if one had been implemented 9-9. Rebound As previously described,the rate of contaminant removal by soil venting is generally fast during early phases of operation and then becomes progressively slower until it approaches a relatively low value When a soil venting system is turned off and then on again,there is generally a spike in the rate of contaminant removal, as depicted in Figure 9-2 This phenomenon is usually described as rebound, that is the concentrations of the contaminants rebound toward their initial higher levels after soil venting has ceased for some period Rebound of vapor concentrations implies that rate-luruted mass transfer is occurring during soil venting For example, if air extraction rates exceed the rate of diffusive mass transfer from within the pore water to the air-water interface and then into the flowing air stream,contarmnant concentrations in the extracted air can dinvmsh without removing all of the contaminant from the pore water When extraction stops, the diffusion process continues and eventually the concentration(s)within the soil pores that are most conductive to air begin to rise If a soil gas sample is collected or extraction begins again,then the contaminant concentrations will have "rebounded" Rebound will be observed whenever air extraction occurs faster than the rate of contaminant diffusion from some sequestered location Contaminants can be considered sequestered if they reside outside of the air-filled pores that conduct the majority of the air that flows to the extraction well a What is the significance of rehound? At many sites,the objective of soil venting is to reduce soil contaminant concentrations to below a specific value Those portions of the subsurface that can readily be swept by air that flows due to soil venting may reach these target values However, if concentrations are observed to rebound,then there is most likely soil where contaminant concentrations have not been appreciably reduced to target clean-up levels Thus,even at sites where substantial contaminant mass is removed by soil venting, this"sequestered" contannnation can often cause the site to fail to meet cleanup objectives b How is rebound measured and assessed? Rebound in the simplest sense is observed when a system is shut down for a time, and then vapor phase concentrations are observed to rise in the treatment area Tlus phenomenon is observed at most sites,however,site specific conditions such as soil stratigraphy,moisture content,and historical pattern of contamination cause the rate at which contaminant concentrations rebound to vary dramatically from site to site There is rarely a systematic approach applied for measuring rebound or for evaluating the rate and extent of concentration rebound Instead,rebound tests are often used to qualitatively evaluate the progress of remediation, or sometimes quantitatively to compare the rebounded concentration at some designated time to a specified"standard" However,a properly executed rebound test can provide much greater insight into the status of remediation Appendix F provides a detailed approach for performing rebound tests and for interpreting the data collected before and during the test In addition,Appendix F provides the mathematical framework that is the basis of rebound data evaluation,as well as the basis for modeling SVEBV cleanup rates and predicting remediation endpoints and timeframes 9-10