Equilibrium geochemical modelling of Äspö groundwaters: a sensitivity study of thermodynamic equilibrium constants

An equilibrium model of the composition of granitic groundwaters, previously applied to the Stripa site, was tested on the Äspö groundwaters. A first application of this model to selected chemical groundwater data showed that the predicted concentrations could be sensitive to some of the model parameters, i.e., thermodynamic constants. A sensitivity study was realized, using the methodology of experimental design. The code NEMROD was used to set up the parameter values given in each calculation and to evaluate the influence of studied factors (mineral-solution equilibrium constants) on predicted values (pH, total concentration of elements). Geochemical calculations were done with the speciation code EQ3/6. In a first stage, a screening of the experimental domain identified parameters that had the strongest influence on EQ3/6 predictions. These four factors (equilibrium constants for chalcedony, kaolinite, albite and laumontite) were then studied in greater detail, in order to evaluate curvature effects and interactions.