The question of uniqueness in geochemical modeling

Contrary to prevailing opinion, geochemical models that predict the distribution of species in aqueous fluids do not always give unique results. To constrain calculations of this type, geochemical modelers commonly set the activities of certain dissolved species or fugacities of certain gases and many times assume equilibrium between the fluid and one or more minerals. In such cases, there may be distinct geochemical systems that satisfy equally well the conditions posed by the modeler. The modeling software may locate any of the roots to the governing equations, depending on the point at which iteration begins. Modeling algorithms may favor discovery of one of the roots, but not necessarily the root that is most geochemically meaningful or most appropriate to the problem at hand.