COUPLING GEOCHEMISTRY WITH A PARTICLE TRACKING TRANSPORT MODEL

Coupling geochemistry and transport appears unavoidable since it is rare that either of these two phenomena alone can account for the movement of solutes in groundwater. The chemical model is based on thermodynamic equilibrium. The method used is a Gibbs free energy minimization constrained by mass balances. The model calculates the aqueous speciation, the precipitation and the dissolution of pure minerals or solid solutions. The transport equation is solved by the random walk technique which avoids the problem of numerical dispersion for transport, but may be more time consuming than finite differences or elements if a large number of particles are necessary in order to get a sufficiently ''smooth'' solution. However, when the chemistry deals with a realistic number of elements (e.g., > 10), the cost of the chemistry computation largely dominates that of transport. Special techniques had to be developed in order to solve problems linked to the conditions present in some of the CEC CHEMVAL tests (boundary with fixed concentrations and very low Peclet numbers). The coupling consists of calculating the exchanges of chemical elements betweeen two populations. The first population is sedentary, constituted by a mesh of fixed cells representing the composition of the solid phase. The other population is nomadic, represented by a set of particles which are advected by groundwater flow. A vector of real numbers is associated with each mobile particle. This vector accounts for the mass of each element dissolved in the moving liquid phase. For this reason, the transport equation is only solved once for the whole set of elements. The main assumptions that were necessary to perform the coupling in a simple way are discussed. Two applications are presented: (1) a verification compared to an analytical solution; and (2) the simulation of a percolation experiment through a sandstone core.