Porewater chemistry of Opalinus Clay revisited: Findings from 25 years of data collection at the Mont Terri Rock Laboratory

The characterisation of porewater chemistry in nanoporous clayrocks is a difficult task. Appropriate extraction methods that have been developed fairly recently and the Mont Terri Rock Laboratory (Switzerland) have played a pioneer role in this regard. During the last 25 years high-quality data from the Opalinus Clay have been acquired. Notably, since the early synthesis of Pearson et al. (2003) a considerable number of newer data from borehole waters and waters extracted from drillcores have been generated. In this study, borehole, squeezing, leaching and cation exchange data were critically evaluated in order to derive a consistent porewater chemistry database across the formation. The results underline that the porewater composition is not constant but exhibits a regular change towards the formation boundaries. This is explained by diffusive exchange between the Na-Cl type porewater and the two bounding freshwater aquifers. Furthermore, the porewater is constrained by cation exchange, carbonate mineral and celestite equilibria. Major solute data obtained from borehole waters and squeezed waters are broadly consistent, although the latter exhibit somewhat more scatter. Overall, the knowledge on porewaters at the Mont Terri Rock Laboratory has been significantly improved. In particular, this regards the spatial profiles of major elements besides Cl, and better constraints on exchanger composition and pH/pCO2 conditions.