2011:22 Infiltration of dilute groundwaters and resulting groundwater compositions at repository depth

The planned Swedish concept for final disposal of spent nuclear fuel includes copper canisters placed in deposition holes at about 500 m depth in granitic bedrock. The copper canisters will be surrounded by bentonite buffer with the objective of inhibiting groundwater flow adjacent to the canister. It has been discovered that dilute glacial melt-water may induce erosion of the buffer material. A very dilute groundwater chemical condition promotes colloidal stability, which may aggravate erosional processes. It is therefore of interest to analyze chemical and physical processes, which affect groundwater chemical composition from the infiltration at the surface to a level in the bedrock corresponding to the repository depth. The infiltrating groundwater from a melting glacier may be affected by mixing with older more saline water and matrix water, as well as weathering reactions related to minerals in the bedrock. This report provides a preliminary assessment of two different modeling approaches to account for the reactions between dilute groundwater and primary silicates as well as secondary minerals in the bedrock. The first modeling approach focus on kinetic rate expression for main mineral components of the bedrock, and the other on local equilibrium conditions.

The objective of this assignment was to evaluate geochemical processes that may affect groundwater conditions at repository depth for the case with infiltrating dilute glacial melt-water at the surface above the repository. This report focus on the chemical reactions that affect alkali and alkaline earth cation concentrations in groundwater, of which the latter have the most pronounced effect on colloidal stability.

Most modeling runs suggest that concentration of Ca2+ increases relative to concentration of Na+ giving groundwater ionic strengths compatible with currently available criteria for colloidal stability (Σ [M2+]> 10-3 M). However, certain cases such as infiltration of very dilute Norwegian glacier water suggested that mineral reaction in the bedrock would be insufficient to prevent the dilute water conditions associated with buffer erosion. Some of the modeling runs were accompanied by pH increases that can be regarded as unrealistic. Modeled pH conditions can thus be used as one indicator of model performance.