The CRACKER Program and Modelling of Äspö Groundwater Chemistry

Abstract: A sufficiently accurate understanding of groundwater chemistry and interactions between minerals and groundwater is of fundamental importance for performance assessment in connection with a repository for nuclear waste.

Here, a model is described, which has been able to give agreement between observed and modelled values for more than ten element concentrations (including pH and pE values). The model makes use of a number of steady state waters that are mixed naturally, after which the mixtures react with minerals in the fractures. A fracture surface is represented by a distribution of minerals across the surfaces of a fracture through which water is moving.

The chemistry of the groundwater appears to be mostly a consequence of chemical reactions between water and minerals at the fracture walls. This has been studied in simulations where rain has been allowed to enter a fracture. The results found in this modelling are similar to the observed values although there are significant differences. This means that the fracture filling minerals alone are not enough to explain the observed groundwater chemistry. This should not be expected, since the old waters found hundreds of meters below the surface have been able to react with other mineral sets as well.

It has been found that the simulated waters are similar to observations with respect to redox properties. In addition to reactions with iron species, reactions in which dissolved hydrogen, carbonates and methane are participating, are important.