Solute transport in saturated fractured media

Abstract

The accidental release of radiochemicals from a repository designed for the storage of high-level radioactive wastes within saturated geologic media is modeled by applying a Discrete-State Compartment (DSC) model to the advection-dispersion (AD) equation. The DSC model is applied to problems concerning matrix diffusion, variable time-dependent sorption and desorption processes, heat transport, and chemical and radioactive decay of radiochemicals. The parameters required for the operation of the DSC model are related to aquifer parameters, such as diffusion, dispersion, heat capacity and the specific heat conductivity. It is shown that by the selection of compartment sizes and an appropriate time step, the DSC solution closely approximates the AD solution. The DSC solution is more powerful than a standard cells-in-series (CIS) solution because variable parameters can be incorporated into the DSC model. A CIS solution allows for only uniform cell attributes and sizes throughout the system. Matrix diffusion is shown to penetrate several metres into a rock matrix when solvent transport is along a fracture. Sorption and desorption of the radiochemicals was modeled by using a thermodynamic equilibrium constant with the option for allowing for transient conditions before equilibrium. The transient condition was modeled using a first-order linear differential equation.