AN EFFICIENT TIME SCALING TECHNIQUE FOR COUPLED GEOCHEMICAL AND TRANSPORT MODELS

In earlier work using coupled geochemical and transport models such as CHEQMATE, HYDROGEOCHEM, PHREEQM and other codes, we have found that, when the mass of mineral phases in the system is large compared with t he solubilities of the minerals in the water, computing times become p rohibitively large. These problems have been found for several systems of interest for radionuclide migration. In a study of the coupled for mation and movement of redox fronts, hydrolysis fronts and dissolving and precipitating secondary minerals, scaling of the amounts of the mi nerals could be made without influencing the results. This is not poss ible in general. In these circumstances the computations would typical ly need millions or many millions of time steps to have the fronts mov e through one ''cell'' in a finite difference based code. It has been noted, however, that this also leads to stationary states, developing in the system. This has been pointed out and utilised by Lichtner. We have developed a code, CHEMFRONTS, which utilises this property. The d rawback is that it is not easy to incorporate diffusion/dispersion usi ng this approach. It can only handle advection dominated transport. Fo r such cases however it is extremely fast compared with conventional c odes. We have now developed a technique whereby the stationary state a pproximation can be superimposed on and incorporated in finite differe nce and finite element codes. This speeds up the computations consider ably. So far we have used it successfully in CHEQMATE, HYDROGEOCHEM an d PHREEQM. In this paper we show the method and give some examples of its application using previously addressed problems. (C) 1997 Elsevier Science B.V.