The Opalinus Clay acts as an effective chemical buffer because of its mineralogical-chemical properties (Wersin et al. 2020). Thus, the very large cation exchange capacity (in the range of 100 meq/kg) compared to the porewater inventory helps to keep the exchanger composition nearly constant and thus also the dissolved cation ratios over a wide range of salinities. The buffering of pH to near-neutral values is provided by rapid calcite dissolution/precipitation and surface protonation/de-protonation reactions. Silicate weathering reactions occur at much lower rates but may be effective in buffering pH over timescales of years and longer (Wersin et al. 2020). It should be noted that, under alkaline conditions (e.g. induced by cementitious materials in the repository), silicate reaction rates are enhanced and the contribution of these minerals to buffering becomes more important.

The Opalinus Clay has a large reducing capacity because of its ferrous- and sulphide-bearing minerals (siderite, ankerite, pyrite) and organic matter. The reducing character of the porewater was confirmed by earlier measurements (Wersin et al. 2020, Mäder & Wersin 2023).