Precipitation/dissolution calculations in complex multicomponent reactive transport problems based on mixing ratios

Mixing of waters in perfect chemical equilibrium with a given mineral leads indefectibly to local disequilibrium. A reaction takes place then in order to re-equilibrate the system. This reaction can be either precipitation or dissolution (in this latter case limited by mineral availability. Both reactions can lead eventually to changes in porosity and permeability at the local scale. The fate of solutes in natural systems, such as rivers and aquifers, are thus controlled by mixing, which in groundwater is a consequence of local diffusion/dispersion. The presentation will discuss a new methodology for computing exactly reaction rates on complex multicomponent reactive transport problems involving precipitation-dissolution of minerals. We start from the simple problem of the evaluation of reaction rates at the local scale when reactions are in equilibrium. Then we move to the problem of kinetic reactions on one hand, and to upscaling reaction rates on the other. All the solutions are exemplified by means of relatively simple flow set-ups, which allow obtaining analytical solutions even for quite complex geochemical set-ups. These solutions provide some insight to multispecies reactive problems, and more, can be used for benchmarking.