On simulating reactive chemical transport with dominating precipitated species controlling chemical equilibrium

We present an approach developed to compute chemical equilibrium and its corresponding reactive chemical transport when dominating precipitated species (DPS) exist. In computing chemical equilibrium, most models take the concentrations or activities of component species and precipitated species as the master variables. However, when the amount of a precipitated species is much larger than those of other species, small computational errors on this DPS concentration might introduce large errors on the concentrations of other species and would cause non-mass-conserved numerical results. To deal with the existence of DPS, we pick as master variables the concentration change, rather than the concentration, of DPS to compute chemical equilibrium. Since the concentration changes of DPS will no longer be much larger than the concentrations of other species in determining equilibrium, our approach is able to provide correct numerical results. We also employ the modified total analytical concentrations, rather than the total analytical concentrations, of aqueous components as the dependent variables in presenting and solving corresponding transport equations. Several examples are given to reveal the numerical problems associated with DPS and to demonstrate the success of our approach.