Impact of multiscale anisotropy on flow and transport in three-dimensional fracture networks

Abstract Anisotropy, as an intrinsic characteristic of naturally fractured rocks, is present across multiple length scales and can significantly affect hydrogeological processes within subsurface rocks. In this paper, the impact of multiscale anisotropy on flow and transport in fractured rocks is investigated using a three-dimensional discrete fracture network model. Anisotropy at the single fracture scale is governed by the ratio of correlation lengths ( λ ) of the aperture field, while the network-scale is controlled by the Fisher constant ( κ ) of fracture orientations. Steady-state flow and conservative solute transport is computed for model ensembles with different combinations of the anisotropic parameters, i.e., λ = 0.5, 1, 2, 3, and κ = 1, 25, 50. Results show that the key flow and transport parameters such as median travel time, equivalent permeability, effective porosity and flow channeling are significantly influenced by variations in λ and κ . Notably, for the same effective permeability, the median travel time can vary by three orders of magnitude. More isotropic systems tend to have longer travel times, while the least variability in travel times is observed at the extremes of permeability. Additionally, λ and κ exhibit a complementary effect, where high values of both significantly enhance flow connectivity, reducing median travel times by nearly two orders of magnitude. In contrast, low λ and κ significantly reduces flow connectivity, leading to longer and more tortuous flow paths and delayed solute breakthrough times. These findings indicate the importance of accounting for multiscale anisotropy to better capture complex hydrogeological processes in fractured media.