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EM 1110-1-4001 3 Jun 02 Appendix F Overview of Rebound Test Procedures and Data Evaluation F-1 Introduction a Purpose This appendix lays out a theoretical and practical framework for the collection and evaluation of data during rebound testing As mentioned in Section 9-9, rebound tests (also known as "pulsing tests")are commonly used to assess the attainment of cleanup in the vadose zone However, little tnformatton is available to guide field practitioners through the data collection and its interpretation (Holbrook et al , 1998) The objective of this appendix is to fill this void by providing a step-by-step approach to planning and performing a rebound test along with straightforward mathematical techniques for evaluating the data to determine adequacy of cleanup The material presented in this appendix is intended to be an introduction to rebound testing and serves as a practical starting point for further development of this important procedure b Background The intent of rebound testing is to assess residual contamination remaining in soils after a penod of active remediation The underlying premise is that soils are inherently heterogeneous and may often be divided into two relative categories low permeability and high permeability During soil vapor extraction, vapors flow predominantly in the high permeability soils intersected by extraction wells and contaminant removal is primarily from these soils This extraction creates a difference,or gradient, in the concentration between more permeable and less permeable soils Contaminants residing in the less permeable soils are removed only after migrating into more permeable soils where soil vapors are flowing(DiGiulio et al , 1998) The migration occurs primarily by vaporous diffusion into active flow paths caused by the concentration gradient Similar contaminant transport occurs during bioventing During a period of rebound(i e ,no extraction or infection), the soil vapors are relatively dormant and contaminant concentrations equilibrate between phases and soil types In other words,the air is not moving and contaminant vapors diffuse from lesser to more permeable soils and the vapor concentration in the more permeable soils approaches that of the low permeability soils The vapor concentration in the more permeable soils can be measured periodically in the extraction wells to track the progress of rebound Hence,the goal of a rebound test is to measure the concentration remaining in the least permeable soils by allowing equilibration of contaminant vapors among all soil types at a site A measure of the concentration in less permeable soils is a strong indicator of the residual contamination at the site C Introductory Example Consider an SVE site with a single extraction well in the middle of a vapor plume within which no NAPL exists Assume a uniform extraction rate has been drawn from the well since the initiation of operation except for two periods of dormancy Measurements of the vapor concentration within the extraction well are illustrated in Figure F-1 F-1