Redox‐controlled multiple‐species reactive chemical transport: 1. Model development

Abstract

A redox‐controlled multiple‐species, multidimensional reactive chemical transport model, DYNAMIX, is developed. The model includes advection, diffusion‐dispersion, transport of oxygen, oxidation‐reduction and, as a consequence, acid‐base reactions, aqueous complexation, precipitation‐dissolution, and kinetic mineral dissolution. A partial equilibrium condition is incorporated into the model which accounts for both thermodynamic equilibrium and kinetic chemical interaction between aqueous and solid phases. The model solves for aqueous and mineral phase compositions as function of time and space. A correct mineral assemblage is automatically searched for based on the principle of minimized Gibbs free energy. The mass of chemicals is conserved in both aqueous and solid phases. A two‐step dynamic mixing algorithm is used to solve the coupled transport and reaction equations. At each time step, the transport equation is first solved by the explicit difference method. The chemical equilibrium submodel is then called to calculate the distribution of chemical species under thermodynamic partial equilibrium conditions. By splitting the transport and reaction equations, the two‐step method significantly reduces computational time and memory storage requirements, which allows DYNAMIX to efficiently handle two‐ or three‐dimensional problems.