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0 u operation of a LFG collection system also have a soil cover of variable thickness. This soil cover can have substantially lower air permeability than the waste itself. Modifications to the above soil vapor extraction equations should be made when the equations are applied to landfills in order to account for internal positive pressure produced by the generation of LFG. 2.3 DEVELOPMENT OF LANDFILL GAS FLOW EQUATIONS At a landfill with no active extraction, LFG generated through the decomposition of the buried waste will migrate from the waste area laterally into native soils, and vertically through the soil cover to the surface. This migration is driven by the internal positive pressure (PL) generated by the LFG. A vertical pressure gradient, H, is developed between the internal PL and the ambient atmospheric pressure Palm. This vertical pressure gradient Hv can be defined as the difference between PL and Patin divided by the vertical distance, mv, between PL and Patm ; that is: H, = ( PL - Patin ) / my At landfills with a LFG collection system in operation, the LFG collection well controls the surface emission of LFG by generating a horizontal pressure gradient, Hh, between the internal LFG positive pressure PL and the negative pressure induced at the LFG extraction well, PW. This horizontal pressure gradient can be defined as the difference between PL and PW divided by the radial distance, rh, between PL and PW ; that is Hh = (PL - PW) / rh. This relationship is illustrated in the following figure. EXTRACTION Hh = (PL - P") / rh WELL Patin HORIZONTAL AND VERTICAL PRESSURE GRADIENT Ham= (PL- Pte) / m Surface and lateral emissions of LFG will be prevented within the area where horizontal pressure gradient created by the collection well is greater than or equal to vertical gradient to the surface. 5 Installation Work Plan to Remediate Landfill Gas Migration ):\Allied\Forward\Workplan\Forward Evaluation - rev4.doc:9/15/2004