Cation-exchange properties of the Mesozoic sedimentary sequence of Northern Switzerland and modelling of the Opalinus Clay porewater

Abstract

In Switzerland, radioactive waste management requires the safe disposal of nuclear waste within a deep geological argillaceous sequence that includes the Jurassic-age Opalinus Clay. The process of selecting a suitable site for the disposal is ongoing and involves a rigorous site selection process. As part of this process, site-specific physico-chemical data were collected, which include parameters such as cation exchange capacities (CEC) and exchangeable cation occupancies, in addition to other geochemical and mineralogical data. The mineralogy, CEC and exchangeable cation occupancies of rock samples collected from the various lithologies of cores from seven boreholes across the study areas, namely Zürich Nordost (ZNO), Nördlich Lägern (NL) and Jura Ost (JO) were investigated. Four different CEC methods were applied, and the results obtained are in good agreement. The general trend in the CEC data follows: Cs-CEC ≥ (Σcations) > Ni-CEC. The higher Cs-CEC values are due to the higher interlayer extraction yields of low hydration cations K+ and NH4+ in illite rich rock samples by the highly selective Cs+. A clear correlation between the 2:1 phyllosilicate content and the CEC values is observed over the entire sequence, so clay minerals primarily govern cation exchange processes and thus the retention of cations. Finally, for the Opalinus Clay of each of the three study areas, porewater chemistries were modelled based on a combination of mineralogical and physico-chemical data. The calculated porewater compositions are in good agreement with the compositions obtained from squeezing and advective displacement experiments in the laboratory.