Rock matrix, diffusion into

The assumptions behind this calculated example are a considerable oversimplification of the real migration behavior. Effects such as diffusion into the rock matrix, dilution with inflowing water, dispersion of the migration front, channeling of the host... 

The rates of metal and ligand adsorption and desorption involving surface sites within rock matrix pores are also diffusion limited. Thus, when ions must diffuse into the matrix of rock forming the walls of a groundwater-filled fracture, or into sand or gravel-sized stream sediments before adsorption can begin, slow ion diffusion and counter ion diffusion processes will limit the rate of attainment of equilibrium. 

Processes being studied in the modeling of the Kamaishi Test included groundwater and heat flow in the rock matrix, fractures, buffer, and their interfaces under varying unsaturated conditions. Before emplacement of buffer and heater, the inflow of water into the test pit was affected, not only by the presence of fractures, but also by the unsaturated condition of the rock near the test pit. Strong variation in the areal distribution of inflow was observed on the walls of the test pit. After the heater and bentonite were emplaced, diffusion of water into the bentonite from the rock occurred simultaneously with drying of the bentonite near the... 

For very long contact times the penetration by diffusion can reach far into the rock matrix whereas for short times the penetration depth will be small. From diffusion theory one can derive a simple expression for the penetration depth, here defined as the distance that leading edge of the solute has travelled. The concentration at the leading edge of the penetrating solute is here taken to be 1% of that at the fracture surface. The relation is... 

When a tracer is injected into the fracture, it will be advected and dispersed. The tracer will also be subject to mass transfer processes. In the modelling, the following mass transfer processes are considered sorption on the fracture surface, diffusion into the rock matrix and sorption in the inner surface of the rock matrix. The following assumptions are made concerning the transport ... 

The tracers are transported into the rock matrix only by diffusion. The diffusion into the rock matrix is one-dimensional (the diffusive flux is perpendicular to the fracture plane) ... 

The efficiency of a rock formation as a transport barrier depends on fluid flow and on radionuclide retention in the rock due to a variety of physical and chemical processes. Open fissures or fractures in the rock provide pathways through which water and radionuclides may travel. Although most radionuclides have a strong tendency to sorb to mineral grains in the rock, tracers first have to diffuse from fractures into the rock matrix in order to access the extensive pool of sorption sites (Neretnieks, 1980). Diffusion in turn depends on mass transfer properties of the rock matrix and on the hydrodynamics of fracture networks, emphasizing the interaction between water flow, advective transport and retention processes. Although models for reactive transport in discrete fracture networks have been around for some time, it is only recently that a theoretical framework is available for systematic studies of the hydrodynamic impact on retention (e.g., Cvetkovic et al., 1999, 2002). 

Steefel Lichtner (1994) highlighted the need to take flow geometries into account when assessing the effects of host-rock alterations. They modelled diffusive and advective transport processes along a hyperalkaline fluid-filled fracture in marl and also perpendicular to it between fracture and matrix. Dolomite dissolution was found to result in increased permeability parallel to the fracture, and diffusion was responsible for the precipitation of a calcite front in the wall rock, thus isolating the fracture physically and chemically from the rock matrix. This may reduce the effective buffering and sorption capacity of the rock. The mechanisms which affect the transport properties of a host rock are shown in this work to depend on many different factors and may be far more complex than can easily be modelled or simulated in a laboratory. 

The immobilization of dissolved chemical species by adsorption and ion exchange onto mineral surfaces is an important process affecting both natural and environmentally perturbed geochemical systems. However, sorption of even chemically simple alkali elements such as Cs and Sr onto common rocks often does not achieve equilibrium nor is experimentally reversible (l). Penetration or diffusion of sorbed species into the underlying matrix has been proposed as a concurrent non-equilibration process (2). However, matrix or solid state diffusion is most often considered extremely slow at ambient temperature based on extrapolated data from high tem-... 

Diffusion is a thermally activated process, whereby a chemical or isotopic species moves down a chemical potential gradient (usually from high to low concentration), at a rate dependent on the diffusion coefficient, D. This applies to the progressive decrease of growth zoning in a mineral as a rock heats, and to diffusive fluxes into or out of the surface of a mineral as processes in the matrix alter the rim composition relative to the mineral interior. The diffusion coefficient is formally defined as the proportionality constant between flux rate (7) and concentration gradient (VC) ... 

Radon is produced by radium decay in ore bodies and by recoil emanating from the solid matrix of the material into the air- or water-filled pore space, or into fractures in rocks. In underground mines radon is transported from the rocks to the galleries by diffusion and convection through water or air circulation. The importance of each one of these processes depends on the geological and tectonic fractures of the formation and the hydrologic behavior of the aquifer. 

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