COUPLING GEOCHEMISTRY WITH A PARTICLE TRACKING TRANSPORT MODEL

Coupling geochemistry and transport appears unavoidable since it is ra re that either of these two phenomena alone can account for the moveme nt of solutes in groundwater. The chemical model is based on thermodyn amic equilibrium. The method used is a Gibbs free energy minimization constrained by mass balances. The model calculates the aqueous speciat ion, the precipitation and the dissolution of pure minerals or solid s olutions. The transport equation is solved by the random walk techniqu e which avoids the problem of numerical dispersion for transport, but may be more time consuming than finite differences or elements if a la rge 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 computatio n largely dominates that of transport. Special techniques had to be de veloped 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 ex changes of chemical elements betweeen two populations. The first popul ation is sedentary, constituted by a mesh of fixed cells representing the composition of the solid phase. The other population is nomadic, r epresented by a set of particles which are advected by groundwater flo w. A vector of real numbers is associated with each mobile particle. T his vector accounts for the mass of each element dissolved in the movi ng liquid phase. For this reason, the transport equation is only solve d once for the whole set of elements. The main assumptions that were n ecessary to perform the coupling in a simple way are discussed. Two ap plications are presented: (1) a verification compared to an analytical solution; and (2) the simulation of a percolation experiment through a sandstone core.