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EM 1110-1-4001 <br /> 3 Jun 02 <br /> The two-region model provided a good match to the historical manifold TCE concentration As <br /> described in Section F-5,the model was run varying the initial soil vapor concentration, the total <br /> contaminated volume,the percentage of the subsurface categorized as mobile, and the mass transfer <br /> coefficient between mobile and immobile zones The best-fit values are listed in Table F-1 The total <br /> characteristic volume was estimated to be 40,690,000 cubic feet (1,152,300 cubic meters) Of this total <br /> volume, 25%was represented as mobile and 75%as immobile For a vadose zone thickness of 80 feet <br /> (24 4 m), the characteristic radius was 400 feet(123 m) equivalent to a surface area of about 12 acres <br /> This total characteristic volume was consistent with the flow observations from PneuLoe and the <br /> vacuum responses at monitoring points The two wells influenced a very large area because they <br /> intersected a permeable sand channel near the bottom of the vadose zone In addition,usage history <br /> indicated a number of possible release points for contaminants existed across the site Using the initial <br /> estimated vapor concentration of 545 mg/m3,the best-fit total volume yields an estimated total original <br /> mass for TCE of 1,074 pounds (487 kg) The overall mass transfer coefficient between the mobile and <br /> immobile soils was estimated to be 0 108 day ' corresponding to a time scale of 9 days This suggests <br /> rebound during shutdown periods was relatively rapid <br /> The calibrated two-region model developed by matching the historical data for the combined extraction <br /> was used to forecast future performance Model predictions for the combined concentration and <br /> cumulative mass removed are presented in Figure F4 The model forecasts reaching an equivalent TCE <br /> concentration of MCL in the pore water of the immobile soils around day 800 <br /> 10000 -- - ---- 1000 <br /> 1000 _ -------- <br /> 100 <br /> }F <br /> ]nnnbik Zone 1�� FA <br /> Pore Water at MCL <br /> rj 10 + en <br /> C ++ O <br /> V ++++ C. <br /> 4 �. <br /> Mobile Concentration(nglnm3) Md <br /> 1 + Irrcmbde Concentration(ng/m3) -- <br /> - - - TCE Mass Removed(k6) <br /> t+ <br /> {+ <br /> 01 10 <br /> 0 100 200 300 400 500 600 700 800 900 1000 <br /> Elapsed Days <br /> Figure F-4 Forecast for continued operation of the existing SVE system <br /> F-17 <br />