Importance of hydrogeology

Because water is the main vector for return of radionuclides to the surface, hydrogeology is the prime consideration in the determination of a suitable site for

deep disposal. Geology is also important, of course, but mostly because of the need to avoid sites that may be unstable due to volcanic and seismic effects and for its potential to influence hydrogeology. The driver for groundwater movement is gravity so that infiltration at a topographic high will cause sub-surface water to flow towards a drainage point such as a spring line, a river or the sea (Fig. 18.4). Even when basement rock extends all the way to the surface, flow is likely to be concentrated in the upper levels for a number of reasons: the near-surface rock may be weathered and therefore more permeable; the deeper rocks are compressed by the overburden leading to lower permeability; and the flow path through the deep rock is longer so that it has higher flow resistance. These circumstances may lead to a situation in which deep groundwater moves very slowly so that salts, leached out of the rock, form a concentrated solution. This increases the density of the water and produces negative buoyancy that may be a further hindrance to flow at depth. In the absence of salt-laden water at depth, a borehole drilled directly from the surface into the repository may exhibit artesian flow.