30 years of porewater chemistry at Mont Terri: Method developments and key findings

The characterisation of porewaters in argillaceous rocks has long been a difficult task due their extremely low permeability and nanoporous character (NEA 2000). Over the past 30 years, the international Mont Terri Project has played a pioneering role in this regard by developing novel experimental methods and modelling approaches. Nuclear waste programmes from many countries in Europe and overseas have benefitted from these developments. In return, findings from other programmes, such as for example at the URLs in Bure (F) and Mol (B), or the recent Swiss deep drilling campaign (TBO), have contributed to method improvements of porewater characterisation at the Mont Terri (MT) rock laboratory. An important milestone was the synthesis report of Pearson et al. (2003), in which methods for characterising porewaters in the Opalinus Clay (OPA) established during the early years of the Mont Terri rock laboratory were discussed. Since then, numerous further studies have been conducted which have helped to refine process understanding and reduce uncertainties. In this context, the GD (Geochemical Data and Analysis) experiment has played an important role by 1) providing an interactive discussion forum and 2) carrying out dedicated experimental and modelling studies. Further important contributions are the syntheses of Wersin et al. (2022) and Fernández & De Cannière (pers. comm.). Here we present major developments and key findings on the characterisation of in-situ porewaters. We focus on major solutes and gas compositions as well as geochemical modelling, but do not discuss other aspects, such as solid phase characterisation methods, isotopic compositions or effects of biogeochemical perturbations.