Coupling Hydrological and Geochemical Simulations to Assess Spatial Heterogeneity and Chemical Evolution of Groundwaters at Two Candidate Repository Sites in Sweden

The chemical composition of groundwater surrounding a high level radioactive waste repository is of importance to many factors that affect repository performance. The geochemical characteristics of Swedish groundwater systems are governed by successive mixing events between several end-member waters during their paleogeographic evolution. An approach is proposed here to investigate the spatial and temporal evolution of groundwater geochemical conditions by coupling hydrogeological and geochemical models in a sequential way. The procedure combines hydrogeological results by others [1,2] of a discrete fracture network using CONNECTFLOW with a mixing and reaction-path simulation using PHREEQC. The hydrological results contain mixing proportions of four reference waters (a deep brine, glacial meltwater, marine water, and meteoric infiltration) at each time step and at every node of the 3D model domain. In this work, mixing fractions are fed into PHREEQC using software developed to build formatted input files and to extract the information from output files for subsequent plotting and analysis. The geochemical calculations included both chemical mixing and equilibrium reactions with selected minerals: calcite, chalcedony and an Fe(III) oxyhydroxide. Some results for the Forsmark site, about 170 km north of Stockholm, Sweden, are graphically presented. Cross sections, where each node is color-coded with respect to an important variable (pH, Eh or concentrations of main elements), are used to visualize the future evolution of the site. Sensitivity analyses were made to evaluate the effects of the different reactions and/or assumptions. The proposed methodology has proved useful for evaluating the future geochemical evolution of the repository sites and to increase the confidence in the site descriptions. (authors)