This paper shares key lessons and findings from 30 years of geochemical research at Mont Terri. It discusses experiments on the characterization of undisturbed Opalinus Clay, pore water, and organic matter, as well as on the behavior of naturally occurring (N₂, CO₂, CH4, …) or injected (H₂) gases and related disturbances. It also covers issues related to microbial activity development, alkaline plumes from cement barriers, and iron-clay interactions caused by steel corrosion. Significant progress in understanding Opalinus Clay geochemistry has been made over the past 30 years, following Pearson et al. (2003), who published the first synthesis report of geochemical data collected since 1996, developed proper sampling techniques and characterization methods, and laid solid foundations for further exploration of the topic. The chloride profile identified in the Mont Terri anticline remains strong evidence of solute transport limited by diffusion at large scale and paved the way for the determination of natural tracer profiles in other national programs (Mazurek et al. 2011). Although in-situ experiments are challenging and may be affected by unexpected disturbances or technical failures, they are essential for maintaining pristine, undisturbed conditions in deep clay formations. Parameters such as pCO₂, pH and Eh, and strictly anoxic conditions are best preserved in situ, while the high buffer capacity of solid phases stabilizes pore water properties.
Cannière et al. (Thu,) studied this question.