Biogeochemical Processes in a Clay Formation In-Situ Experiment: Part E - Equilibrium Controls on Chemistry of Pore Water from the Opalinus Clay, Mont Terri Underground Research Laboratory, Switzerland

F.J. Pearson (1) , Christophe Tournassat (2) , Eric C. Gaucher (2)

1 Ground-Water Geochemistry
2 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)

The chemistry of pore water (particularly pH and ionic strength) is an important property of clay rocks being considered as host rocks for long-term storage of radioactive waste. Pore waters in clay-rich rocks generally cannot be sampled directly. Instead, their chemistry must be found using laboratory-measured properties of core samples and geochemical modelling. Many such measurements have been made on samples from the Opalinus Clay from the Mont Terri URL. Several boreholes in that URL yielded water samples against which pore water models have been calibrated. Following a first synthesis report published in 2003 (Pearson, F. J., et al.., 2003. Mont Terri project - Geochemistry of water in the Opalinus clay formation at the Mont Terri Rock Laboratory. Geology Series No. 5 Swiss Federal Office for Water and Geology, Bern.), this paper presents the evolution of the modelling approaches developed within Mont-Terri URL scientific programs through the last decade (1997-2009). Models are compared to the composition of waters sampled during dedicated borehole experiments. Reanalysis of the models, parameters and database enabled the principal shortcomings of the previous modelling efforts to be overcome. The inability to model the potassium concentrations correctly with the measured cation exchange properties was found to be due to the use of an inappropriate selectivity coefficient for Na-K exchange; the inability to reproduce the measured carbonate chemistry and pH of the pore waters using mineral-water reactions alone was corrected by considering clay mineral equilibria as done by Gaucher et al. (Gaucher, E. C., Tournassat, C., Pearson, F. J., Blanc, P., Crouzet, C., Lerouge, C., and Altmann, S., 2009. A robust model for clayrock porewater chemistry. Geochimica & Cosmochimica Acta 73, 6470-6483). Re-examination of the measured calcium/magnesium activity ratios and consideration of the mineralogical composition of the Opalinus Clay suggested that calcium/magnesium cation exchange rather than dolomite saturation may control the ratio of these ions in solution. This re-examination also suggests that the calcium/magnesium ratio decreases with increasing pore water salinity. Several possible reasons for this are proposed. Moreover, it is demonstrated that feldspar equilibria must not be included in Opalinus Clay modelling because feldspars are present only in very small quantities in the formation and because sodium/potassium ratios measured in pore water samples are inconsistent with feldspar saturation. The principal need to improve future modelling is additional or better data on rock properties, in particular: (i) a more detailed identification of phases in the Opalinus Clay that include redox-sensitive elements together with evaluation of their thermodynamic properties; (ii) an improved understanding of the distribution of celestite throughout the Opalinus Clay for Sr/sulphate concentrations control; (iii) improvements in analytic and thermodynamic data for Ca-Mg rock cation exchange and mineral chemical properties and (iv) the measurement of composition and stability constants of clay minerals actually present in the formation.

Domaines: Géochimie; Milieux et Changements globaux

F.J. Pearson, Christophe Tournassat, Eric C. Gaucher. Biogeochemical Processes in a Clay Formation In-Situ Experiment: Part E - Equilibrium Controls on Chemistry of Pore Water from the Opalinus Clay, Mont Terri Underground Research Laboratory, Switzerland. Applied Geochemistry, 2011, 26, pp.990-1008. ⟨10.1016/j.apgeochem.2011.03.008⟩. ⟨hal-00597132⟩