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) played pioneer role this regard. During last 25 years high-quality data from Opalinus Clay acquired. Notably, since early synthesis Pearson et al. (2003) considerable number newer borehole waters extracted drillcores generated. In study, borehole, squeezing, leaching cation exchange were critically evaluated order to derive consistent database across formation. results underline composition not constant but exhibits regular change towards formation boundaries. This explained by diffusive between Na–Cl type two bounding freshwater aquifers. Furthermore, constrained exchange, carbonate mineral celestite equilibria. Major solute obtained squeezed are broadly consistent, although latter exhibit somewhat more scatter. Overall, knowledge on porewaters at has significantly improved. particular, regards spatial profiles major elements besides Cl, better constraints exchanger pH/pCO 2 conditions. • A unique compiled. Consistency cores shown. conditions appear remain