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EM 1110-1-400'1 3 Jun 02 b Case Study Site 2 Site 2 is located in the Central Valley of California The subsurface at Site 2 readily meets the conceptual model of mobile and immobile zones for air movement during SVE PneuLogo flow profiles revealed only about 20%of the vadose zone allowed effective air flow These mobile zones consisted of interbedded sand layers within an immobile zone predominantly made up of moist silts and fine sands Historical concentration and flow data were available from both the manifold (representing the cumulative extraction rate from all wells and the mixed contaminant concentrations), individual extraction wells, and vapor monitoring wells The history shows the concentrations rising during the first week of operation After this initial period,both the TCE and PCE concentrations exhibited the classic exponential decay The initial 90 days of extraction were performed with a single extraction well (VW-01)near the center of the suspected source area This 90-day penod was followed by additions and modifications to the extraction configuration using three other surrounding wells and two rebound periods The two-region model was applied using soil properties measured at the site and a detailed extraction history employing 193 constant flow segments over a four-year period to fit the observed PCE concentrations in the manifold and from well VW-01 The procedure followed to fit the data is outlined in Section F-5 The resulting model fits are illustrated in Figures F-5 and F-6 Similar results were achieved for TCE data The initial concentration in the mobile phase was assumed equal to the early (day 9)peak concentration observed in the manifold The other best-fit values for PCE are listed in Table F-3 The total characteristic volume of contaminated soil was estimated to be 1,099,400 cubic yards (840,600 m3)with roughly 18 % characterized as mobile and 82 %immobile For a depth to groundwater of 105 feet this yields an equivalent radius of 300 feet for the contamination This radius correlated well with the overall dimensions of observed PCE contamination at the start of remediation The characteristic diffusion length estimated for the immobile zone was about 1 15 feet(0 35 m) corresponding to an average total thickness of about 2 3 feet for each immobile zone region This characteristic length was calculated using equation(F-22)and the fit mass transfer coefficient In addition,this length is consistent with the observations from the PneuLog®flow profiles The best-fit immobile zone initial vapor concentration(565 ppmv)was equivalent to the mobile zone (650 ppmv) indicating the mobile and immobile regions were in equilibrium The values differ because differing soil properties yield differing partitioning components Using the estimated initial mobile and immobile zone PCE vapor concentrations and the best-fit total volume yielded an estimated total initial mass for PCE of 4,130 pounds (1,873 kg) F-18