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' property boundary. Specifically gas monitoring probe GP -9, which is located on the western property boundary, has yielded methane concentration readings in excess of Federal regulatory limits (40CFR subsection 258.23). ' It was initially assumed that control of off-site migration in this area may be achieved by installing gas extraction wells at a spacing of 100 feet on center. The effectiveness of these ' wells in controlling off-site migration in this area will depend upon a variety of factors, including the subsurface soils, the well design, and well operation. The gas wells are proposed to be installed in native soil outside the waste footprint. Once installed, the wells are intended to create a continuous "vacuum curtain" along the property boundary, capturing any LFG migrating west. Landfill gas extraction wells are typically installed within the waste prism. Well operational parameters at the Forward Landfill for LFG extraction wells in waste is well documented through years of operational data gathering. However, well operational parameters for LFG extraction wells in native soil is less well known, and these operational parameters are expected to be significantly different from those in waste. 2.2 RADIUS OF INFLUENCE THEORY According to Johnson et. al (1990)', the radius of influence of a soil vapor extraction well tcan be estimated by fitting radial pressure distribution data obtained from an air permeability test into the steady-state radial pressure distribution equation: 1In( r ) z 2 z 2 RW P Pw = (p"2 Pan„ — Pw ) In( RI ) ' Equation 1 w 1-1 Where: Pr = Absolute pressure measured at a distance r from the collection well (g/cm-s2) Patr„ = Atmospheric pressure (g/cm-s2) P, = Absolute pressure applied to the collection well (g/cm-s2) RI = Radius of influence of the vapor extraction well (cm) Rw = Radius of the gas collection well (cm) r = Distance from the collection well to pressure monitoring probe (cm) The values needed to solve this equation are readily obtained during the performance of an air permeability test. The derivation of the above equation can be found in Johnson, Kemblowski and Colthart (1990)2. These authors also indicated that once steady-state flow conditions are ' A Practical Approach to the Design, Operation and Monitoring of In Situ Soil -Venting Systems, Johnson, P.C., Stanley, C.C., Kemblowski, M.W., Byers, D.L., and Colthart, J.D., Ground Water Monitoring Review, Vol. 10, No. 2, pp. 159-178, 1990. z Quantitative Analysis for the Cleanup of Hydrocarbon -Contaminated Soils by In -Situ Soil Venting, Johnson, P.C., Kemblowski, M.W., and Colthart, J.D., Groundwater, Vol. 28, No. 3, pp. 413-429, May -June 1990. 3 Installation Work Plan to Remediate Landfill Gas Migration J:\Allied\Forward\Workplan\Forward Evaluation - rev4.doc:9/15/2004