An eficient time scaling technique for coupled geochemical and transport models

Abstract

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 the solubilities of the minerals in the water, computing times become prohibitively large. These problems have been found for several systems of interest for radionuclide migration. In a study of the coupled formation and movement of redox fronts, hydrolysis fronts and dissolving and precipitating secondary minerals, scaling of the amounts of the minerals could be made without influencing the results. This is not possible in general. In these circumstances the computations would typically need millions or many millions of time steps to have the fronts move 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 drawback is that it is not easy to incorporate diffusion/dispersion using this approach. It can only handle advection dominated transport. For such cases however it is extremely fast compared with conventional codes. We have now developed a technique whereby the stationary state approximation can be superimposed on and incorporated in finite difference and finite element codes. This speeds up the computations considerably. So far we have used it successfully in CHEQMATE, HYDROGEOCHEM and PHREEQM. In this paper we show the method and give some examples of its application using previously addressed problems.