DIFFUSION AND REACTION IN ROCK MATRIX BORDERING A HYPERALKALINE FLUID-FILLED FRACTURE

Multicomponent diffusive and advective-dominant transport-reaction calculations are used to analyze water-rock alteration in rock matrix adjacent to a hyperalkaline fluid-filled fracture. The calculations indicate that rock alteration resulting from diffusive transport may be fundamentally different from that observed in the case of advective-dominant transport. Since advective-dominant and diffusive transport can result in differing reaction products as a function of time and space, the two transport processes may modify the chemical and physical properties of the rock at different rates. We apply reactive transport calculations to an analogue of the Cretaceous and Tertiary marls proposed as host rocks for the Swiss low-level nuclear waste repository. Diffusion-reaction calculations predict that the rock matrix bordering high-pH fluid-filled fractures could be completely cemented within 10 to 500 years. The bulk of the porosity reduction occurs because of the precipitation of calcite resulting from the interdiffusion of Ca2+ and CO32-. In contrast, advective-dominant transport results in the precipitation of calcite only as a replacement of dolomite. Both advective-dominated and diffusive transport result in a porosity increase within millimeters or less of the fracture wall, an effect which could widen the fracture and thus increase the rate of radionuclide transport along the fracture. Because solute diffusion is coupled to porosity and tortuosity change in the rock matrix, cementation causes the fracture to become physically and chemically isolated from the rock matrix if no expansion of the rock occurs. As a consequence, reaction-induced porosity reduction may potentially decrease the buffering and sorbing capacity of a fractured host rock, thus reducing the physical and chemical retardation of contaminants migrating along fractures. These effects may occur within the time required for radionuclides in the repository to decay to environmentally safe levels.