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

Abstract

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. This work shows that the predictions are sensitive to two parameters that are generally not well known and not well defined: the equilibrium constants for kaolinite and laumontite. The use of such equilibrium models could thus be strongly limited, although complementary data on groundwaters, like Al concentration in solution, could help to constrain the interpretation. Further investigations are thus needed on actual processes controling the fluxes of elements, especially alkalis and alkaline earths, between solid and solution in the case of rapidly mixing waters. The influence of ion-exchange processes could in particular be investigated by the use of reactive tracer tests along fractures.