Geochemical modelling to assess the consequences of an increase in pH for an in-situ experiment at the Mont Terri Rock Laboratory, Switzerland

At the Mont Terri Rock Laboratory in the Opalinus Clay, the in-situ Bitumen-Nitrate-Clay interaction experiment investigates the influence of oxidising components that could leach from radioactive waste on the reducing capacity of claystones at a near-neutral pH. Future tests are planned to examine the additional impact of an increased pH, mimicking the degradation of concrete-based engineered barriers used in radioactive disposal systems. However, substantial clogging phenomena due to mineral precipitation must be avoided in the experimental setup to ensure the continuous circulation of fluids in water lines and their contact in the borehole with the surrounding clay through filter screens. In this work, geochemical modelling was performed for prognostic purposes to assess the potential precipitation of minerals and formation of solid solutions at an increased pH, while also considering cation exchange with the surrounding clay. The modelling results indicated precipitation of negligible amounts of calcite (at relatively low pCO2) and cementitious phases, which should be insufficient to cause significant clogging. However, substantial brucite precipitation was predicted above pH ~ 10.4. Cation exchange enhanced brucite precipitation, as magnesium from the clay was exchanged primarily with potassium from alkaline solutions. Such brucite precipitation (or calcite at higher pCO2) could clog small-diameter installations near the clay since it has fast kinetics and only little oversaturation is needed for crystal nucleation. Furthermore, the geochemical model was designed to be easily adapted to investigate other clogging-related aspects in radioactive waste repositories and serves as a basis for upcoming laboratory experiments to verify clogging phenomena. To examine the limits of the microbial activity in the in-situ experiment, a pH increase is necessary. Clogging of water lines and filter screens caused by precipitates formed at high pH could jeopardise the in-situ experiment. A geochemical model was developed to calculate the quantities of precipitates needed to assess the clogging risk. The precipitation of cementitious phases and calcite at lower pCO2 is negligible, but small-diameter water lines or filter screens could become clogged by brucite. The release of Mg2+ from clay minerals through cation exchange enhances brucite precipitation.