A consistent compositional formulation for multiphase reactive transport where chemistry affects hydrodynamics

Multiphase reactive transport formulations usually decouple flow (i.e., phase conservation) from reactive transport calculations (i.e., species conservation). Decoupling is not appropriate when reactions affect flow controlling variables (such as the partial pressure of gaseous components or the activity of water). We present a consistent compositional formulation that couples the conservation of all components. No explicit conservation of phases mass is required since they result from the conservation of all species in each phase. The formulation acknowledges that constant activity species do not affect speciation and can be eliminated, which reduces the number of unknowns. We discuss the formulation, the numerical solution, and the implementation into an object oriented code. The advantages of the formulation are illustrated by simulating the effect of mineral dehydration (including invariant points) on the hydrodynamic processes in an unsaturated column that reaches extremely dry conditions.