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

Abstract

The chemical composition of groundwater surrounding a high level radioactive waste re-pository is of importance to many factors that affect repository performance. The geochemical characteristics of Swedish groundwater systems are governed by successive mixing events be-tween several end-member waters during their paleogeographical evolution. An approach is pro-posed here to investigate the spatial and temporal evolution of groundwater geochemical condi-tions by coupling hydrogeological and geochemical models in a sequential way.

The procedure combines hydrogeological results by others of a discrete fracture net-work using CONNECTFLOW with a mixing and reaction-path simulation using PHREEQC. The hydrological results contain mixing proportions of four reference waters (a deep brine, gla-cial 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 devel-oped 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 equilib-rium 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 graphi-cally 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.