## 3 Results
### Influence of CO2 Transport
Figure 13: Simulated profiles of dissolved Cl\({}^{-}\) concentration after 1 year, 5 years, 18 years of heating: (**a**) Case C1; (**b**) Case C2. The concentration evolution of other dissolved species over the bentonite showed a different trend. As an example, Ca\({}^{2+}\) concentration profiles are presented in Figure 14. These solutes were transported by advection-dispersion as well as Cl\({}^{-}\), but they also participated in cation exchange reactions and mineral dissolution/precipitation. The overall impact of those chemical processes resulted in the solute distribution over the barrier different than that for Cl. Based on the results presented in Figure 14, different boundary conditions in terms of pCO\({}_{2}\) at the interface affected Ca\({}^{2+}\) concentration distribution in the bentonite barrier, especially at longer times. Following the simulated Ca\({}^{2+}\) concentration, a faster decrease in the concentration over the bentonite under the impact of a higher CO\({}_{2}\) flux from the interface would be expected. Under these conditions, the modelled concentration of Ca\({}^{2+}\) near the heater was more than twice as low by the end of the simulation. The transported Ca\({}^{2+}\) towards the interface reacted with the dissolved CO\({}_{2}\) forming the calcite and lowering the calcium concentration in the porewater. Ca\({}^{2+}\) cations are also incorporated in anhydrite, which tend to precipitate in the region closer to the heater. The modelled concentration of sulphate SO\({}_{4}\)\({}^{2-}\) was also significantly affected by the interface boundary condition. The model predicted a higher concentration than the initial in a particular location in the bentonite layer (Figure 15) even after 1 year of heating. Such results demonstrate that the resulting concentration distribution of reacting species was not determined solely by the transport processes. Peak concentrations were not observed near either of the interfaces (bentonite-granite or bentonite-heater) but within the barrier. Based on the modelling results, in time, the location of the peak concentration of sulphate was closer and closer to the heater. The interplay of a number of chemical processes was also reflected in the pH evolution as presented in Figure 16. The modelled pH evolution over the bentonite with a low pCO\({}_{2}\) at the interface was similar to that obtained in Case B without consideration of CO\({}_{2}\) gas transport. A pH increase to 9.5 was observed close to the bentonite-granite interface, and then it gradually reduced to the initial pH of the bentonite. This suggests that if the CO\({}_{2}\) influx to the bentonite is low, CO\({}_{2}\) gas diffusion representation could be omitted in the numerical model, and therefore, computational resources could be saved. Meanwhile, in the case with an imposed high pCO\({}_{2}\) (Case C2), a non-linear decrease in the pH was observed in the direction further away from that fixed on the interface. By the end of simulation, the pH in the outer part of the barrier was higher than the initial, but in the region close to the heater it was lower than the initial pH of the bentonite (-7). The higher imposed CO\({}_{2}\) influx resulted in an increased pH over a larger part of the bentonite in comparison to the fixed low CO\({}_{2}\) flux. Figure 14: Simulated evolution of dissolved Ca\({}^{2+}\) concentration after 1 year, 5 years, 18 years of heating: (**a**) Case C1; (**b**) Case C2. Observed differences in the concentration profiles of solutes (except Cl\({}^{-}\)) clearly indicate the importance of proper assessment of the conditions on the bentonite from the surrounding environment. While the solute concentration distribution was affected throughout the bentonite barrier, the mineral precipitation/dissolution and porosity were affected only close to the bentonite-granite interface.