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i� <br />7 <br />l <br />F11 <br />11 <br />0 <br />0 <br />u <br />0 <br />established, then the air permeability of the vadose zone material can be determined using <br />the steady-state radial flow equation: <br />Q — H�r�k 1 P <br />Where: <br />(1 — (Pat. / PW <br />In (Rw / Ri ) <br />Q = Volumetric flow rate from the collection well (cm3 <br />/s) <br />H = Length of well screen (cm) <br />p = Absolute viscosity (g/cm-s) <br />k = Permeability (cm2) <br />Patrn = Atmospheric pressure (g/cm-s2) <br />P, = Pressure applied at the collection well (g/cm-s2) <br />Ri = Radius of influence of the vapor extraction well (cm) <br />Rw = Radius of the gas collection well (cm) <br />Equation 2 <br />The air permeability (k) of a material can be defined as the material's capacity for gas flow. <br />Permeability is measured in darcy or cm2 (1 darcy = 1.013 x 10-$ cm2). The air permeability <br />is a function of particle size, porosity, uniformity, and moisture content of the material. <br />Because the value of k is a physical property, it should not change with different collection <br />rates or different pressure levels, as long as the structure of the in-place material is not <br />altered. <br />The absolute viscosity of air is Pair = 1.8 x 10-4 g/cm-s (at 30° Q. Viscosity is a function of <br />molecular weight, temperature and pressure. Based on the temperature, pressure, and <br />composition of the LFG measured during the test, the LFG viscosity was calculated. The <br />LFG viscosity is approximately p = 1.6 x 10-4 g/cm-s. <br />Both of the above equations are subject to the following boundary conditions 9: <br />P=Pw r=RW <br />P = Patm r = Rlatm <br />These conditions imply that at a radial distance greater than the atmospheric radius of <br />influence of a vapor extraction well, the pressure induced by the well is equal to <br />atmospheric pressure. Therefore, no flow of air towards the extraction well occurs beyond <br />the radius of influence. In addition, both equations assume that air flow to an extraction <br />well is entirely derived from the surface and passes through a single homogeneous and <br />isotropic material. <br />In a landfill, the generation of LFG through the anaerobic decomposition of the waste <br />produces an internal positive pressure, greater than atmospheric pressure. The flow of LFG <br />is derived from within the waste, and the intrusion of ambient air from the surface is not <br />required to generate LFG flow. In addition, most landfills requiring the installation and <br />4 <br />Installation Work Plan to Remediate landfill Gas Migration <br />J:\Allied\Forward\Workplan\Forward Evaluation - rev4.doc:9/15/2004 <br />