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EM 1110-1-4001 <br /> 3 Jun 02 <br /> 1000 <br /> ❑ Measurements <br /> sa. <br /> Theoretical Fit <br /> 0 100 <br /> O <br /> U <br /> O 10 <br /> U <br /> U <br /> 0 1 <br /> .r <br /> U <br /> I <br /> W � <br /> 01 <br /> 0 100 200 300 400 500 <br /> Elapsed Days <br /> Figure F-1 Typical vapor concentration history during SVE with two rebound periods <br /> In the example of Figure F-1, the extraction well concentration decays rapidly during the first two weeks <br /> of operation following an exponential trend This period represents the time required to sweep the initial <br /> contaminant vapors found in the more permeable soil intervals Beyond the first two weeks, the <br /> concentration decay follows a much slower exponential rate (i e ,notice the change in slope in the <br /> theoretical fit around Day 15) This slower rate corresponds to the rate of mass transfer from less <br /> permeable soils to more permeable soils During this slower rate of decay, the process is often referred <br /> to as "asymptotic"with the asymptote being zero In this example,a rebound period of 30 days was <br /> undertaken after 200 days of extraction to assess a hypothetical cleanup goal of 10 parts per million by <br /> volume (ppmv)throughout the site Concentrations measured in the extraction well during this <br /> dormancy period revealed an increasing trend above the cleanup goal as contaminants diffused out of the <br /> less permeable soils and into the more permeable soils Extraction was re-started at Day 230 before an <br /> equilibnum concentration had been reached The concentration again decayed rapidly and the previous <br /> slow decay was re-established A second rebound period was initiated at Day 350 and lasted 60 days <br /> The 60-day period was sufficient to establish an equilibrium concentration less than the goal of 10 ppmv <br /> The hypothetical system was operated an additional 90 days while closure reports were prepared and <br /> accepted This example illustrates the simplest implementation and interpretation of SVE and rebound <br /> data Many complexities arise when multiple extraction wells are operated, multiple contaminants are <br /> present, or the actual center of mass of the initial vapor plume is not known The concepts in this <br /> appendix should provide a starting point for discussion and interpretation of results from more complex <br /> situations <br /> F-2 <br />