Three-dimensional Simulation of Coupled Groundwater Flow and Salt Transport in Connection with Radioactive Waste Repositories

Abstract

Some problems in subsurface hydrology involve coupled groundwater flow and transport of solutes due to changes in the fluid density. Recently, attention has been focused on long term salt dissolution and brine transport in connection with salt formations that are under consideration as radioactive waste repositories. For salt domes and bedded-salt formations where density variations exceed 20%, the existing numerical codes have emphasized the difficulty of solving strongly coupled flow and transport equations. Numerical simulation capability for flow and transport of strongly coupled variable-density brines is decisive for the performance assessment of such geological formations for isolating radionuclides from the biosphere. Part of the problems encountered in simulating these sites are related to the nonlinearities of the coupled problem, to the heterogeneous nature of regional aquifers, to the very large finite-element grids needed to avoid numerical instabilities and, to a large extent, also to the very large CPU times needed for three-dimensional simulations. In this paper we briefly review numerical approaches to the coupled problem including recent developed codes and discuss experiences gained from previous modelling studies. We introduce anew, fully three-dimensional, numerical code SoTraCoF developed for treating large problems. Finally, we present an example of the application of the code for a 3D generic model, based on the characteristics of a real site where a regional aquifer overlays a salt dome.