Geochemical characterization and modelling of the Toarcian/Domerian porewater at the Tournemire underground research laboratory

For safety evaluation of hazardous waste repositories in clay-rocks, a thorough assessment of porewater chemistry and water–rock interactions is required. However, this objective is a challenging task due to the low hydraulic conductivity and water content of such rocks, which subsequently renders porewater sampling difficult (without inducing perturbations). For this reason, an indirect approach was developed to determine porewater composition of clay-rocks, by a geochemical model of water–rock interaction using some properties of the rock and the solution. The goal of this paper is to obtain the porewater composition of the Toarcian/Domerian argillaceous formation at Tournemire (South of France), for which a reliable model is still lacking. The following work presents a comprehensive characterization of the geochemical system of the Tournemire clay-rock, including mineralogy, petrology, mobile anions, cation exchange properties, accessible porosity and CO₂ partial pressure. Perturbation corrections from fracture water sampling were also computed. These water were found in sealed fractures (Beaucaire et al., 2008) and their radiocarbon apparent age is estimated at 20 ka. Their age together with their equilibrium situation allow considering these fracture waters as representative of the formation porewater. The model developed to calculate the Tournemire porewater composition is essentially based on cation exchange by a multi-site approach, but equilibrium with some mineral phases (calcite, quartz and pyrite) is also considered. Different exchange sites of different affinities towards cations are used, which proportions are given by the mineralogy. Exchange on illite is performed with a three-sites model, while one site is considered for smectite phases. Multi-site model results are compared with corrected fracture water data and two other models: a model only based on mineral equilibrium and a model using cation exchange on one global site. The best results were obtained with the models that take into account cation exchange and particularly with the multi-site model. The interest in considering a model with exchange sites of different affinities is particularly obvious for a satisfactory representation of the K⁺ content in solution. A dependence of K⁺ content to the amount of high affinity sites was observed, leading to an improvement of its simulation when uncertainty on mineralogical data is considered. Once validated, the multi-site model was applied at different levels of the Tournemire argillaceous formation to obtain a profile of the porewater composition.