Radwaste disposal: medium permeability shales as a geological barrier

One of the major criteria of site selection for low radwaste disposal is hydrogeological. A common practice is to locate sites on a geological barrier, whose minimum thickness depends on national regulations. However, it is generally considered that the minimum permeability of the geological barrier must be less than 10 -9 m/s with a thickness of the impervious material of at least 20 m. In general, shale has a permeability approaching 10 -7 m/s. Therefore, by strict application of the above permeability criteria, they would be unsuitable for use as a natural barrier for a waste repository. Any contaminant movement can be avoided if between an underlying aquifer and the bottom of the waste disposal an ascending water flow exists. Since it is hard to prove a continuous and perennial ascending water flow below the radwaste repository, one could imagine creating an artificially controlled ascending water flow. This is the case where a deeper shale formation is overlain by an open-jointed surface shale layer containing a shallow aquifer. Their use as a natural barrier to a radwaste repository can be accepted by drawing down the water table. This has the effect of creating an upward flow below the repository. Using mathematical modeling, different conditions were analyzed such as the drawdown of the phreatic aquifer, water level differences between shallow and underlying aquifers, repository width, permeability of the shale mass and the existence of thin sandstone beds inside the shale body. Modeling showed that for this particular layout migration of pollutants can only occur by diffusion. Contaminant migration time can be evaluated and compared with migration time calculated from analytical equations and the migration time through a thick clay layer. A promising site has been field investigated. Results confirm the suitability of the shale as a geological barrier.