A new approach to account for fracture aperture variability when modeling solute transport in fracture networks

摘要

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 σ_(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= 0 and S=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 σ_(ln K) (or corresponding mean sFWS) value. This is obviously due to the averaging effect of the network.