Influence of mineral precipitation and dissolution on hydrologic properties of porous media in static and dynamic systems

A critical component in determining the suitability of disposing glassified, low activity waste is the identification of key mineral assemblages affecting the porosity and permeability of both the glass and near- and far-field materials. In this study, two different classes of geochemical models are used to identify mineral precipitation and dissolution potentials for an immobilized low-activity waste (ILAW) disposal facility in Hanford, Washington. The first is a static geochemical model that does not consider the effects of transport. The second model is dynamic, and combines geochemical reactions with hydrogeological processes such as advection, diffusion and dispersion. This reactive transport model also includes an innovative application of a depositional film model for determining changes in permeability due to mineral precipitation and dissolution reactions. Although both models describe solid-aqueous phase reactions kinetically, the two models identify two different sets of mineral assemblages affecting the porosity and permeability of the media. These markedly different results are due to transport considerations, the most significant of which are the spatial variability in aqueous concentrations, and advection and diffusion of dissolved glass constituents into the backfill materials. This work shows that for the prediction of geochemical behavior of engineered systems, such as the ILAW disposal facility, the traditional reaction path modeling approach is not sufficient for an accurate assessment of the precipitation of key mineral assemblages and their effect on the geochemical and hydraulic behavior of the waste glass. Reactive transport modeling improves this assessment significantly. The static model is useful in identifying potential minerals to be included in the reactive transport simulations. The dynamic model, however, ultimately determines the key mineral assemblages affecting both the geochemical behavior and the hydraulic properties of the waste glass in the presence of a flowing aqueous phase. (C) 2002 Elsevier Science Ltd. All rights reserved.