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[1] 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 advectiondispersion equation (ADE) and a Continuous Time Random Walk (CTRW) method developed for non-Fickian transport. The low b 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.

[1] A simple yet effective method is presented to include the effects of fracture aperture variability into the modeling of solute transport in fracture networks with matrix diffusion and linear sorption. Variable apertures cause different degrees of flow channeling, which in turn influence the contact area available for these retarding processes. Our approach is based on the concept of specific flow-wetted surface (sFWS), which is the fraction of the contact area over the total fracture surface area. Larsson et al. (2012) studied the relationship between sFWS and the standard deviation ln K of the conductivity distribution over the fracture plane. Here an approach is presented to incorporate this into a fracture network model. With this model, solute transport through fracture networks is then analyzed. The cases of S ¼ 0 and S ¼ 1 correspond to those of no matrix diffusion and full matrix diffusion, respectively. In between, a sFWS breakpoint value can be defined, above which the median solute arrival time is proportional to the square of sFWS. For values below the critical sFWS (more channeled cases), the change is much slower, converging to that of no matrix diffusion. Results also indicate that details of assigning sFWS values for individual fractures in a network are not crucial; results of tracer transport are essentially identical to a case where all fractures have the mean ln K (or corresponding mean sFWS) value. This is obviously due to the averaging effect of the network.

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