The magnitude of the vertical hydraulic gradient across the host rock and the entire aquitard sequence influences the advective water flux. In this section, two bounding cases are discussed: In the first one, the hydraulic gradient is controlled by the difference in hydraulic head between the aquifers above and below the low-permeability sequence. This case would be relevant for water flux along a hypothetical steep fault connecting these aquifers. The second case discusses the effects related to mechanical loading and unloading of the host rock by a significant ice cover. Note that while the hydraulic heads in the aquifers will quickly react to changing conditions on the surface, the low-permeability host rock (and confining units) may need substantial time to re-equilibrate to new boundary conditions (see also Section 5.6.5).

Changes in aquifer hydraulic heads can be caused mainly by erosion and by recharge below glaciers (Section 6.5.1.3). In the absence of significant glaciations, the magnitude of expected changes is limited by the expected overall moderate evolution of topography over the period under consideration. In the case of significant (warm-based) glaciations, hydraulic heads in the aquifers and consequently also the hydraulic gradient across the aquitard sequence may be more affected. However, for the siting regions NL and ZNO, maximum ice thickness is estimated at 400 m and the duration of the ice coverage at a few thousand years (Section 6.3.2). Therefore, the increase in gradient is limited in magnitude and to a comparably short duration. The overall effect on advective flow in the host rock is small to negligible.

Mechanical loading and unloading of the host rock due to glaciation and deglaciation can affect the pore pressure in very low-permeability units such as the Opalinus Clay. The pore pressure and therefore also the hydraulic gradient are influenced by transient hydromechanical processes (Section 5.6.5). The currently measured underpressures in the Opalinus Clay (e.g. in the Benken borehole ca. 70 m subhydrostatic, Fig. 5‑44) indicate vertical hydraulic gradients directed from the confining units towards the centre of the Opalinus Clay. During glaciation periods, the additional loading can result in an increase in hydraulic head in the Opalinus Clay relative to the confining units. During these episodes, the hydraulic gradient may therefore change direction from the centre of the Opalinus Clay towards the confining units. Conversely, a glacial retreat will again induce low pore pressures which will remain low for a longer time in the Opalinus Clay than in the more permeable confining units. Therefore, the hydraulic gradient after a deglaciation will again be directed from the confining units to the Opalinus Clay. The temporary increase in advective flux due to glacial loading was already studied in earlier work and was considered insignificant (Nagra 2002, Kosakowski 2004).

The effects of changing porewater pressures and hydraulic gradients are considered small at all sites, but the probability of a significant ice thickness above the repository is smaller in JO compared to the other sites.