Bentonite porewater chemistry

Ochs, M.*; Lothenbach, B.*; Shibata, Masahiro ; Yui, Mikazu

Porewater composition in a compacted bentonite is dependent on the composition of the surrounding groundwater, and on the characteristics of the bentonite itself. Two mechanisms through which bentonite influences the respective porewater composition are distinguished: sulface chemical reactions (ion exchange, surface complexation) on smectite and dissolution of minerals and soluble impurities included in bentonite. This report provides the results of different activities related to the definition of porewater chemistry through the use of geochemical models: (1) review of thermodynamic model on ion exchange reaction, (2) modeling of bentonite-water interactions under aerobic conditions, (3) performance of sensitivity analyses of key parameters in the bentonite model, (4) model simulation of bentonite porewater chemistry in the engineered barrier system under repository conditions. Experimental information of bentonite-water interaction allowed the determination of soluble impurities in the bentonite, and the knowledge of these impurities is important for predictive modeling. For the impurities of Kunigel-V1, 0.38% of CaSO, 0.0011% of NaCl and 0.0044% KCl were determined. The sensitivity analyses resulted in that the presence of calcite, CaSO and pyrite strongly influences the pH in the compacted bentonite, and the pH in compacted bentonite is buffered by the acid/base equilibria at the Na-smectite surface as well. Through the model calculations, some remarks on the expected trends for the long term behavior can be made like that the pH in compacted bentonite is expected to increase with increasing number of water exchange cycles, as long as CaCO contributes to the pH buffering capacity, due to slow depletion of the soluble impurities in the bentonite. The pH of the porewater, however, lies in all cases (but in the presence of CaCO) between 5.6 and 9.5. Based on the findings discussed above, a large number of calculations were carried ...