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

Authors

  • Martin Larsson,

    Corresponding author
    1. Air, Water and Landscape Sciences, Department of Earth Sciences, Uppsala University, Uppsala, Sweden
    2. Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden
    • Corresponding author: M. Larsson, Air, Water and Landscape Sciences, Department of Earth Sciences, Uppsala University, Uppsala SE-75236, Sweden. (martin.larsson@hyd.uu.se)

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  • Magnus Odén,

    1. Air, Water and Landscape Sciences, Department of Earth Sciences, Uppsala University, Uppsala, Sweden
    2. Swedish Nuclear Fuel and Waste Management Company (SKB), Stockholm, Sweden
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  • Auli Niemi,

    1. Air, Water and Landscape Sciences, Department of Earth Sciences, Uppsala University, Uppsala, Sweden
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  • Ivars Neretnieks,

    1. Division of Chemical Engineering, School of Chemical Science and Engineering, KTH Royal Institute of Technology, Stockholm, Sweden
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  • Chin-Fu Tsang

    1. Air, Water and Landscape Sciences, Department of Earth Sciences, Uppsala University, Uppsala, Sweden
    2. Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
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Abstract

[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 σln 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 σln K (or corresponding mean sFWS) value. This is obviously due to the averaging effect of the network.

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