Evaluation of diffusive gas flux through covers with a GCL

The main purpose of geosynthetic clay liners (GCLs) used in cover systems is to limit the infiltration of water to the wastes disposed underneath. In many situations however. the cover system should also be able to limit gas flux, so that undesirable products emitted from the wastes, like radon or methane, will not escape to the atmosphere. In other situations, covers may have to prevent oxygen from the atmosphere to come into contact with reactive materials, such as sulphidic tailings that could otherwise generate acid. It is thus important for cover design to evaluate gas flux through the GCL used in the system. This gas flux is usually controlled by diffusion through the porous media, because such highly saturated fine grained materials have a very low gas permeability. In this paper, the authors briefly review the basic theory used to calculate diffusive gas flux F-g, and introduce an experimental procedure to evaluate, in the laboratory, the effective diffusion coefficient D-e which controls this flux. Experimental results obtained on a nonwoven needlepunched GCL are shown and compared to values ensuing from a predictive model that relates D-e to porosity and degree of saturation. Sample calculations on gas flux in cover systems are finally presented and discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.