The hydraulic conductivities of the Opalinus Clay are uniformly low and comparable among the studied sites. The low values are supported by independent evidence such as natural tracers in porewater (Section 4.6) and are the reason for diffusion being the dominant transport process (Section 5.9).
The Opalinus Clay fracture hydraulic properties as observed in in-situ tests in boreholes in Northern Switzerland are consistent with the range of values of the intact matrix. That is, fractures do not represent preferential flow paths. This is explained by self-sealing (Section 5.7).
Hydraulic conductivities in the confining units are low and will contribute to the retardation of radionuclides. Tectonic overprint on clay-poor lithologies may result in locally elevated transmissivities – but it remains a hypothetical assumption that such units may provide a continuous release path.
Hydraulic heads in the Opalinus Clay are lower than in the adjacent aquifers. This is probably caused by transient hydromechanical processes related to loading and unloading of the rock column. Such processes underpin the very low hydraulic conductivity.
