Simulation of contaminant transport in fractured permeable formations by multiporosity modeling

This paper concerns contaminant transport in aquifers comprising fractured porous formations. It is considered that the aquifer subject to contaminati on is composed of macro-blocks, which embed two sets of macro-fractures, Ea ch macro-block incorporates numerous micro-blocks of low permeability, whic h embed micro-fractures. Therefore, the basic conceptual model, used in thi s study, is a triple-porosity two-dimensional model. It is shown that five dimensionless parameters govern contaminant transport in the triple-porosity domain. However, a group of eight so-called practic al parameters is convenient to be used for consideration of possible scenar ios. From this group, major effects of contaminant diffusion into the micro -blocks are attributed to the density of micro-fractures and porosity of th e microblocks. Coefficient of diffusivity of contaminant into the micro-blo ck is also a significant parameter. Its effective value increases due to th e presence of vertical fractures, which do not transfer contaminant by adve ction, and it is dependent on the tortuosity of the micro-block material. Simulations of various possible scenarios were carried out by solving the b asic dimensionless equations developed in the present study. The solutions were obtained by a combination of analytical and numerical solutions. The s imulations indicate that at a comparatively high porosity of the micro-bloc ks, of the order 10(-1), the effect of contaminant diffusion into the micro blocks can be approximated as a retardation phenomenon, similar to contamin ant adsorption. An increase of the fracture density reduces that retardatio n effect. If the micro-block porosity is comparatively low, of the order 10 (-3), then contaminant diffusion into the micro-blocks changes the shape of the breakthrough curves, which are ended with very long tails. This phenom enon is reduced by the increase of the fracture density. (C) 1999 Elsevier Science B.V. All rights reserved.