ISSUES IN SINGLE-FRACTURE TRANSPORT MODELING - SCALES, ALGORITHMS, AND GRID TYPES

Transport in single fractures has recently been intensely examined, Po tential applications of these studies include nuclear waste storage an d infiltration of rainwater into soil desiccation cracks. We modeled h ydrodynamic dispersion in single fractures, using a variable-aperture model and particle-tracking techniques. We examined issues of scale of heterogeneity, particle-tracking method, and grid topology. Hydrodyna mic dispersion tends to zero as the scale of the transport path increa ses in relation to the scale of heterogeneity. Since this is not obser ved in nature, it implies either that fractures have fractal structure or that hydrodynamic dispersion alone does not account for all the di spersion that occurs in fractures. Dispersion and retardation as simul ated using a node-to-node or mixing type algorithm are greater than wh en they are simulated using an interpolation algorithm, and the differ ence cannot be attributed to molecular diffusion. Differences in condu ctivity and dispersion between different grid types (serial, parallel, square, and random field) are related to the coordination number (deg ree of connectedness) of the grid, with lower coordination number grid s having higher dispersion.