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Regional channelized transport in fractured media with matrix diffusion and linear sorption

A regional‐scale solute transport model with long‐range flow channeling is used to study the effect of matrix diffusion and linear sorption on channelized transport. We start from a fracture‐network‐based block model to build up a large‐scale flow and transport model with regional flow channeling, and then incorporate the processes of matrix diffusion and linear sorption. Regional‐scale solute transport is then studied by applying the model to the fracture data set from Sellafield, England. The results demonstrate the significant impact that matrix diffusion has on regional‐scale solute travel times for different degrees of long‐range channeling. With no channeling, a relatively sharp and significantly delayed arrival can be observed, which is the well‐known retardation effect of matrix diffusion and linear sorption. However, with increasing regional channeling the delay becomes much smaller while the spread of transit times becomes much larger. The solute breakthrough curves obtained are analyzed with both the traditional advection‐dispersion equation (ADE) and a Continuous Time Random Walk (CTRW) method developed for non‐Fickian transport. The low β values obtained from the CTRW model indicate an extremely non‐Fickian transport, which is also confirmed by the low Peclet numbers (much less than 1) required for the best fit to the ADE model. In particular, the times for the first arrival of solute are much earlier when regional channeling occurs. In other words, the degree of large‐scale channeling is a crucial parameter for determining the first arrival of particles, and it becomes even more important when matrix diffusion and linear sorption are included in the model.