When can the local advection-dispersion equation simulate non-Fickian transport through rough fractures?

Non-Fickian solute transport is observed across many scales, which has motivated development of numerous non-Fickianbased models. Assuming that local fluid flow was estimable from the Modified Local Cubic Law, this study determined whether the local ADE better simulated non-Fickian transport through rough (3-D) fractures when local dispersion was described using either the Taylor dispersion coefficient (D-Taylor) or the molecular diffusion coefficient (D-m). The assessment was based on how well the local ADE compared to particle-tracking solutions for solute transport across a range of Peclet numbers (Pe) through two simulated fractures. Even though the local ADE is based on local Fickian transport processes, it was able to reproduce non-Fickian transport characteristics through these heterogeneous fractures. When supplying D-Taylor to the local ADE, it extended the applicability of the local ADE to a threshold of Pe < 450; using D-m, the local ADE was only accurate when Pe < 70. No differences were observed for small Pe. Therefore, our recommendation is to always use D-Taylor in the local ADE to capture non-Fickian transport so long as the Pe threshold is not exceeded.