Key points:
- In the Opalinus Clay, the anion-accessible porosity fraction relevant for the transport of negatively charged species is well constrained and depends on porewater ionic strength. In JO it is about 30%, in NL and ZNO in the range of 40 – 50% of the total porosity.
- In the confining units, the anion-accessible porosity fraction is partly larger. This is explained by the partly lower clay-mineral contents and larger pore sizes.
- Opalinus Clay porewater chemistry can be robustly constrained based on complementary datasets and approaches. The porewater of the general Na-Cl type is of moderate ionic strength (0.15 – 0.37 molal), the pH is in the near-neutral range and the redox conditions are buffered by mineral equilibria in the reducing range.
- Porewater ionic strength is lowest in JO because of the different palaeo-hydrogeological evolution. For ZNO and NL, the similar data allow the same reference porewater model to be used to evaluate radionuclide mobility and the stability of the engineered barriers.