Experiments of bentonite-water interactions using groundwater sampled from Horonobe area. Results of batch-type experiment and geochemical modeling

As a matter of consideration for applicability of porewater model to the actual geological environment, experiments of bentonite-water interactions using groundwater sampled from Horonobe underground laboratory were conducted. In order to test the applicability of model, comparison between the data obtained by experiments and modeling results by porewater model based on chemical equilibrium was performed. For the experiment, batch-type experiments with different solid-liquid (bentonite-water) ratio were conducted using the groundwater sampled from the depth of GL -500 to -550 m in the borehole HDB-10. Regarding to bentonite samples, Kunipia F and Kunigel V1 were used in the experiments. Results are summarized below. As the results of experiment, the measured pH of solution contacted with bentonite increased (comparing with the pH of solution for lower solid-liquid ratio) as a function of the solid-liquid ratio. On the contrary, modeling results suggested that the calculated pH of solution contacted with bentonite decreased as a function of the solid-liquid ratio, thus the modeling results could not follow the measured pH appropriately. With regard to the components of solution contacted with bentonite, the modeling results were approximately consistent with the experimental results for Na, K, Cl concentration. Additionally, the observed variation of Mg, SO42- and HCO3- in the solution with solid-liquid ratio was also reproduced by porewater model calculation. On the other hand, for Ca concentration, the modeling results were inconsistent with the experimental results. In this connection, the amount of exchangeable cations (in the interlayer of smectite) related to the solution chemistry showed the similar behavior to the solution composition. To consider the possible reasons for difference between experimental and modeling results, several case studies were performed to evaluate the effect on porewater chemistry in bentonite. The case studies considered the effect by dissolution of accessory minerals and precipitation of soluble impurities, the effect by coexistent ions (e.g., exchangeable ion such as NH4+) and the effect by degassing of CO2(g) from the porewater solution. Furthermore, future subjects to evaluate the applicability of porewater model more confidently were summarized based on the results of present consideration. (author)