The chemical properties of groundwater entering a repository for nuclear wastes, have a major impact on the performance of natural and engineered barriers. Thus, accurate models for the chemical evolution of the groundwater are important in connection with performance assessments. Simple mixing of waters is inadequate for describing most chemical properties, like concentrations of iron, pH or redox potential.

In the present work, ground waters from Äspö at the Swedish east coast have been modeled. In the model it is assumed that ancient waters (end member waters) have been able to reach a steady state with respect to the minerals in the fracture walls. Due to groundwater flow, end member waters have been mixed to form intermediate waters. In the sampling process (by pumping), waters from a large number of small units are mixed too rapidly for mineral reactions to be of importance. After the sampling mixture has been formed, the water reaches equilibrium with respect to homogeneous reactions (in the aqueous phase) but not with respect to heterogeneous reactions.

The CRACKER program has been used to simulate groundwater formation and sampling process according to the model. It has been found that the end member waters are fairly stable with respect to observed mineral sets.

Conservative elements like Cl have been used to find the initial mixing ratios for intermediate waters. Evolution of the mixtures was simulated and finally mixing and speciation of waters from some 900 locations have been simulated to describe the final solution. It is found that the model is able to give a good agreement between observed and modeled concentrations for the ten most important elements as well as pH and redox potential.