Waste materials are often disposed in facilities constructed from natural earth materials. Frequently, the natural materials, especially clays, are emplaced to meet specifications designed to limit the migration of the wastes or their leachates from the disposal facility. However, as waste leachates permeate through earth materials, chemical reactions can occur which may cause the emplaced materials to no longer meet the disposal facility design specifications. A test procedure has been utilized to assess the long-term effects of waste leachates upon earth materials, that is, permeant-porous media interactions. The test consists of compacting representative specimens of the porous media to represent its field hydraulic conductivity, determining a reference hydraulic conductivity, based on a 0.01 N calcium sulfate (CaSO₄) solution, site groundwater or tap water, and then changing the permeant to the anticipated waste leachate. Field-time conditions are accelerated by permeating the waste fluid through the porous media under pressure. By monitoring the time, flow, and chemistry of the permeant effluents and with assessments of hydrologic and geochemical parameters, it is possible to evaluate long-term characteristics of flow and mass transport through the porous media. Evaluation of the performance of waste disposal facilities, including changes in the hydraulic conductivity of the porous media due to chemical reactions with the permeant and calculation of retardation factors for solutes of interest, has been the primary concern of our testing. Retardation factors are defined as the dimensionless ratio of the average linear water velocity to solute migration velocity, and can be determined by monitoring the solute front advancement within the porous media in relation to a tracer that does not react with the porous media. The testing methods and evaluation of the results are presented.