The active fracture model (AFM) (Liu et al., 1998) has been widely used in modeling flow and transport in the unsaturated zone of Yucca Mountain, Nevada, a proposed repository of high-level nuclear wastes. This study presents an in-depth evaluation of the AFM, based on both theoretical arguments and field observations. We first argue that flow patterns observed from different unsaturated systems (including the unsaturated zone of Yucca Mountain) may be fractals. We derive an interesting relation between the AFM and the fractal flow behavior, indicating that the AFM essentially captures this important flow behavior at a subgrid scale. Finally, the validity of the AFM is demonstrated by the favorable comparison between simulation results based on the AFM and 14C age and fracture coating data collected from the unsaturated zone at Yucca Mountain. These data sets independently provide important insight into flow and transport processes at the Yucca Mountain site. Potential future improvements of the AFM include expanding it to consider film flow and multifractal concepts.

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