A comparison of fracture mixing models, 2. Analysis of simulation trials

This paper describes the application of a transfer function approach developed to model mass transport in a network of fractures subject to advection and dispersion within the fractures, and matrix diffusion. The model is able to treat mass mixing at fracture intersections by either flowline routing or complete mixing. This feature of the model is exploited in examining here the question of whether the choice of a particular model significantly influences the pattern of mass transport in a fractured medium. Previous studies appeared to have produced somewhat contradictory results. Our simulation experiments with the model involved a series of 12 trials with case of uniform flow imposed by constant-head boundaries and strongly radial flow created by an injection withdrawal doublet. The results showed that the case of uniform flow most emphasized the difference in the two mixing models, while in the case of strongly radial flow there was no difference in mass distributions measured through normalized breakthrough curves. These results can be interpreted in light of the possible patterns of inflow and outflow at a node, which show that mathematically the complete mixing model is the same as the stream-flow routing model for three of the four possible flow conditions. Thus, the differences between the mixing models are only evident when flow conditions emphasize mixing via the one inflow/outflow pattern that is different.