Impact of landscape topography and quaternary overburden on the performance of a geological repository of nuclear waste

The topographical driving forces for groundwater on different spatial scales in several ways influence the performance of a repository for nuclear waste located at large depth in crystalline bedrock. We show that the relation between local topographical characteristics (topographical steepness and wavelengths) in the area of a repository (kilometer scale) and the large-scale (hundreds of kilometers) surroundings, together with repository depth, are the primary controls of residence time distributions and the discharge pattern of radionuclides released from an underground repository. In addition, the topography affects the groundwater flow at repository depth and, therefore, influences the long-time degradation of the repository. In the areas studied, all located in Sweden, the local topography mainly controls the groundwater flow down to a depth of similar to 500 m, which is the suggested depth of the Swedish repository. The importance of the large-scale topography increases with depth but even at depth where local-scale topography is dominant, the continental-scale topography affects length and depth of flowpaths as well as groundwater velocities. The impact of large-scale topography is particularly clear in areas where the steepness of local-scale landforms is relatively small. The study also shows that quaternary deposits (bedrock overburden) may have a significant impact on the overall residence times in the underground because of their hydraulic and sorption properties. This effect is further enhanced by the fact that flow paths originating from repository depth generally emerge in topographical lows with relatively deep layers of quaternary deposits. The findings of this study underscore the need to consider multiscale topographical characteristics as well as bedrock overburden in assessments of radiological consequences of underground repositories.