Modeling Methane Migration and Oxidation in Landfill Cover Materials with TOUGH2-LGM

Methane oxidation within a passive methane oxidation barrier (PMOB) and the downward migration of molecular O2, whose presence is necessary for the oxidation reaction to occur, were simulated using the finite element simulator TOUGH2-LGM. The goals of the study were to validate the use of TOUGH2-LGM by reproducing real field profiles obtained under different conditions and to evaluate the depth of O2 penetration under several conditions. TOUGH2-LGM handles both advective and diffusive gas fluxes. The oxidation reaction was simulated by imposing a Neumann condition, i.e. CH4 was extracted from pre-determined elements. The main variables of concern were the degree of water saturation of the PMOB, the pressure differential between its base and the surface, the position and thickness of the oxidation front and, finally, the oxidation rate, i.e. the rate at which CH4 was removed from the system. Other important variables, such as the gas permeability and diffusion coefficient were obtained in the laboratory. Inspection of the results shows that TOUGH2-LGM was able to quite accurately reproduce the field profiles. The simulator also makes it possible to predict the depth of O2 penetration as a function of pressure differential and humidity within the PMOB. This type of information is fundamental for the design of effective biocovers.