A DUAL CONTINUUM FLOW AND REACTIVE TRANSPORT MODEL WITH N-TH ORDER SOLUTE TRANSFER TERM FOR STRUCTURED POROUS MEDIA: THEORY AND EVALUATION WITH SYNTHECTIC CASES

Abstract

Compacted bentonite is foreseen to be used as backfill and sealing material for high-level radioactive waste disposal. Experimental evidence indicates that bentonite exhibits two types of porous domains: macro-porous domain which contains free water and micro-porous domain mainly occupied by interlayer and sorbed water. Geochemical non-equilibrium between macro- and micro-domains calls for a fully coupled reactive transport Dual Continuum Model (DCM) to describe water flow and hydrochemistry of bentonites. Most DCM in the literature assume a lumped first- order solute transfer term between both domains. However, it is well known that such a term is not correct at all times. Here we present a formulation of flow and reactive transport for a dual continuum with an n-th order solute transfer term, derived from an approximation of an analytical solution for diffusion through a thin slab. Such formulation which has been implemented in a finite element code which solves both for the forward and inverse problem greatly improves the ability and accuracy of DCM to simulate reactive transport in bentonites. Solute transfer terms of different orders (n) are evaluated for several 1- and 2-D synthetic cases. The order and the empirical coefficients of the solute transfer term are estimated by inverse modelling. Optimum results for bentonite are achieved with n = 0.72 and a scale term of 2.5.