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EM 1110-1-4001 3 Jun 02 Several rounds of vapor sampling should be performed at normal intervals (e g ,monthly)to define the slow decay in the existing extraction configuration After several rounds of sampling yield consistent decay curves, the rebound test may be initiated This phase begins by collecting a final round of vapor samples along with measurements of total concentration using field portable instruments such as a PID or FID,as appropriate After collecting the samples and measurements,the extraction wells are closed and the extraction pumps are turned off Care should be taken to minimize the flow of atmospheric air through the wells and into the subsurface All extraction wells and monitoring points should be capped The site should then be left undisturbed except for vapor sampling as specified in the Work Plan Vapor sampling during the dormant period is best performed with a small sampling pump, if possible The well being sampled should be purged by extracting a volume equivalent to at least three volumes of the well casing and screen In addition,a field portable PID or FID should be used to monitor the extracted vapors to ensure a relatively steady concentration has been reached before the vapor sample for detailed analysis is collected If the vapor sampling during the dormant period yields a clear trend to an equilibrium concentration, the system may be re-started In past practice it was common to allow the rebound period to be "long" to ensure the site re-equilibrated before re-starting extraction No samples were collected during the rebound period since the only parameter of interest was the equilibrated concentration However, without sampling during the rebound period,no data will exist to demonstrate re-equilibration and a valuable assessment of the mass transfer rate between mobile and immobile soils is lost The final period of the rebound test is the system re-start Just prior to starting the extraction system, a round of vapor sampling should be performed to define the endpoint of the rebound The re-start of extraction should be in the same configuration and at the same rates as the steady period leading up to the rebound period With the same configuration, the extraction well vapor concentrations before and after the rebound period are directly comparable Vapor sampling during the period immediately following the re-start is critical The maximum rebound concentration may or may not appear at the extraction wells immediately after re-start Recall the previous discussion on the distribution of contaminants If the extraction wells are not located in the middle of the contaminant mass, the concentration may rise for a short period before the decay associated with the sweep of the mobile soils begins Therefore, the extracted vapors should be monitored with a field portable PID or FID during the initial hours of extraction A vapor sample should be collected for analyses at or just after a peak is observed If the initial concentration is the maximum and decay begins immediately, then the final rebound sample is representative of the initial extracted concentration The goal of the vapor sampling during the re-start is to define the decay trend associated with the sweep of the mobile soils and the subsequent slower decay as the removal becomes diffusion limited It is expected that the diffusion- limited trend will return to the decay rate observed just before the rebound period F-5 Data Evaluation As described in Section F-2, analyses of the early and late concentration trends can yield estimates for the total contaminated volume and the field mass transfer coefficient However, the objectives of a rebound test are to assess the mass of residual contamination and concentrations within low permeability materials This section presents mathematical relationships to estimate the residual contaminant mass at a site using the data from a rebound test Measures of vapor concentrations in the low permeability soils come directly from the rebound data The mathematical analysis assumes the following data are available after following the field procedures described in the previous section, F-11