The Opalinus Clay provides a stable geochemical environment for hosting a repository for radioactive waste.

In clay-rich rocks, only part of the porosity is accessible to anions. In the Opalinus Clay, the accessible fraction is well constrained and depends primarily on porewater salinity. For JO the accessible fraction is ca. 30% and for NL and ZNO it is in the range of 40 to 50% of the porosity. This fraction is partly larger in the confining units: This is explained by the partly lower clay-mineral contents and larger pore sizes.

The Opalinus Clay porewater chemistry can be robustly constrained based on complementary datasets and approaches. The model includes mineral and cation exchange equilibria and concentrations of free solutes such as chloride. The porewater of the general Na-Cl type is of moderate ionic strength (0.15 – 0.37 molal), pH is in the near-neutral range and the redox conditions are buffered by pyrite and siderite equilibria to the reducing range.

The porewater ionic strength of the Opalinus Clay is lowest in JO because of its different palaeohydrogeological evolution. The equivalent data from ZNO and NL allow the same reference porewater model to be used.