In this volume the authors describe a mathematical model for the solution of convection-diffusion flow of a mixture of gases in a multi-layer porous media, and apply this to the problem of the generation and migration of gases inside a landfill and their release into the atmosphere. They present two different boundary element formulations, based upon the dual reciprocity method (DRM), to transform the resulting domain integrals into surface integrals. Chapter 1 provides a brief introduction to the field plus numerical schemes previously applied to the problem. This is followed with introductory material about sanitary landfills and landfill gases, with an emphasis on the importance of the control of the emission of the gases. The foundations of the convection-diffusion flow of mixture of gases in a porous media, including a review of heat generation and transport theory, are then given and the BEM formulations for single-gas and multi-gas models are explained. Chapter 5 presents the domain decomposition concept and tests it on a variety of one- and two-dimensional problems. The DRM-MD concept is then described in more detail and the validity of the algorithm is verified through a number of numerical examples. Finally, the model is applied to the design of trenches in a multi-layer landfill.