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0 <br /> �LEARWATE <br /> G P O U P, 1 N C. <br /> i <br /> Number of Extraction Wells Required <br /> Once an approximate permeability.value has been obtained and is consistent with <br /> known subsurface ithology, the feasibility of reducing hydrocarbon contamination <br /> through vapor extraction in-a reasonable period of time (<5 years) can be estimated <br /> using the model presented by P.C. Johnson, et al (1990). : This model is based on the <br /> maximum contaminant vapor concentrations in extracted vapors, residual soil <br /> { contaminant composition, vapor pressure, assumed soil temperature and the ideal <br /> r gas law and will be determined with the aid of the computer program called <br /> . "HyperVentilate®" (Johnson, 1992). The number of vapor extraction wells needed. <br /> to remediate the''contaminated soil within five years. will be determined from the <br /> results of the pilot test and the feasibility calculations and is approximately equal to - <br /> the estimated area of contamination divided by the area of influence of one well. <br /> fl <br /> All calculationsperformed in the computer-program,-or-performed by hand .and any , <br /> assumptions used will be presented for review with the data and pilot test results. <br /> In addition to estimating the rnirdmum required number of extraction-wells using <br /> the radius of influence, the required number of wells will also be determined based <br /> on the required volume of air needed to achieve remediation through <br /> volatilization in a given length of time{Tg). This calculation presented by Johnson <br /> & Ettinger (1993) determines the number of wells required (N,) based on estimated <br /> saturated hydrocarbon vapor concentrations (1/a), an estimated___mass of <br /> contaminant -to be removed (Mcont), and feasible single well flow rate (Q-.ell - <br /> 1 determined from the step tests): <br /> Nw = (a Mcont) I (Qwell TR) <br /> The two estimates, determined using the radius of influence and using the above- <br /> referenced equation, have inherent limitations and sources of error. However, the <br /> results will be compared to estimate a reasonable number of vapor extraction wells <br /> and well spacing. Placement of the additional vapor extraction wells, if required, <br /> will depend on the location of existing utilities, buildings, and other site constraints. <br /> Flows and vacuums may need to be modified to ensure complete coverage of .the <br /> area to be remediated. It.must be recognized that the pilot tests and calculations are <br /> performed to provide a basis for the system design. The actual system configuration <br /> may be modified from the design based on continual evaluation of operating data <br /> once the systent is installed. On-going system evaluation will be an important part <br /> of the remedial!action. <br /> F <br /> October =, 1994 <br /> Soil Vapor Extraction Test Protocols Page 4 of 3 <br />