Abstract: A method and composition are shown for capping exposed land areas, such as landfills, toxic waste sites, and mineral waste sites areas. A formulation is prepared by making a hydrated solution of a particular sodium Bentonite clay and mixing the solution with a specific proportion of sand material. The mixture is distributed over a prepared subgrade by means of a slurry pumping technique. A multilayer cap is created by overlaying additional layers having different clay formulations. The clay mixtures are optimized to be effective water and oxygen barriers while eliminating the problem of desiccation cracking. An anisotropic barrier is achieved as a result of interlayer hydraulic conductivity differences. This barrier together with the establishment of a 1-6% slope of the layer interface encourages lateral movement of water while inhibiting its downward penetration. The overall hydraulic conductivity of the composite cap is extremely low.