ABSTRACT: Convection and diffusion are the transport mechanisms for dissolved contaminants in a liquid phase moving through solids. Diffusion is approximately represented by the ideal Fick Laws, while convection can be considered as a percolation process described by the Darcy Law. The unsteady transport of active sanitary landfilling leachates through impermeable bottom liners can be described by a distributed parameter model using simultaneously both mechanisms, while only diffusion is used for a post-closure situation. Both models, using typical values for the relevant parameters, are applied to clay liners, to synthetic geomembranes and to double liners. Space and time evolution of contaminant concentration is obtained and breakthrough times are estimated. Model allows for design of liners and simulates space-time evolution either in operating circumstances, either after final sealing.