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EM 1110-1-4001 <br /> 3 Jun 02 <br /> • F-3 Planning and Evaluation of Historical Data <br /> r <br /> The mathematical techniques presented in the previous section are only valid if the site is adequately <br /> characterized and the remedial system is appropriately designed Before proceeding with a rebound test, <br /> answer the following questions <br /> I Does the current site conceptual model or does existing site characterization data allow an <br /> estimate for the vadose zone heterogeneities in terms of the fraction which is mobile (e g, sands) <br /> and the fraction which is immobile (e g, silt or clays)9 Have site-specific soil physical <br /> properties been measured? <br /> 2 Do concentrations measured in extraction wells and/or vapor monitoring points allow an estimate <br /> for the total volume of contaminated soth Are the edges of the vapor plume defined? Is a <br /> reasonable estimate for the vertical distribution of contaminants available9 <br /> 3 Has the concentration history in extraction wells exhibited a relatively rapid decay followed by a <br /> slow decay? <br /> If the answer to (1)or(2) is generally"No" then additional investigative work may be warranted before <br /> or simultaneous with a rebound test If the answer to (3)is"No"then it may be too early for a rebound <br /> test or the site may have additional complexities For example, if very little decay occurs or a high <br /> concentration persists for a long period then NAPL may be present or the extraction wells may not be <br /> within the original source area The impact of NAPL existence is to add another mass term to (F-1)and <br /> . additional mass transfer terms to equations (F-3) and (17-4) As a result,the extraction well concentration <br /> would persist at a higher concentration than described previously and the decay would be much slower <br /> If these types of complexities are suspected but not identified, then further investigative work should be <br /> undertaken prior to a rebound test Methods of investigation are described in Chapter 3 of the manual If <br /> the answers to all three are generally"Yes" then it is probably time for a rebound test <br /> Planning for a rebound test starts with an evaluation of historical data The data of most interest are the <br /> extraction rates and the concentration histones at extraction wells If these data are available,then <br /> estimates for the rebound period and vapor sampling frequency can be determined using the techniques <br /> described in the previous section The steps are summarized below <br /> 1 Plot the concentration on a log scale versus time <br /> 2 Fit a line through the initial decay using equation(F-S) The slope corresponds to Q/RmNm <br /> while the line is moved up and down by varying Cm,O <br /> I Fit a curve through the later slow decay using equation (F-14)and the fitted parameter <br /> Q/Rm/Vm The general trend corresponds to El/Ri while the curve is moved up and down by <br /> varying Ci,O <br /> 4 Estimate the minimum rebound period using equation(F-17) <br /> 5 Specify vapor sampling times by plotting the anticipated concentration rebound using equation <br /> (F-16) A vapor sample collected about halfway between the final extracted concentration and <br /> the equilibrium rebound concentration is highly desirable to validate the fitted mass transfer <br /> parameter D/Ri This validation will provide strong evidence for an understanding of the site <br /> during the preparation of the closure report <br /> • <br /> i <br /> F-9 <br />