A mathematical model was developed to quantify the environmental impact produced by the gases emission from sanitary landfills. The stages of gas generation and diffusion were modeled using waste and cover materials placed in a landfill over an isotropic porous medium, while the dispersion stage was modeled for the atmosphere using a Gaussian model. The United States Environmental Protection Agency (USEPA) criteria were adopted for the estimation of greenhouse gases emissions. The MATLAB computer program was used to prepare simulations of a proposed sanitary landfill to serve the municipalities of Temuco and Padre Las Casas in Chile, considering a lifetime of 20 years. The simulated results show that the conditions of confinement have a greater incidence on the rate of gas emission than does the quantity of waste disposed. It was also concluded that the level of environmental impact varies considerably according to the evaluation scenario and the project design.