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When can the local advection–dispersion equation simulate non-Fickian transport through rough fractures?
- Zheng, Lizhi, Wang, Lichun, James, Scott C.
- Stochastic environmental research and risk assessment 2019 v.33 no.3 pp. 931-938
- diffusivity, dispersibility, equations, models, solutes
- Non-Fickian solute transport is observed across many scales, which has motivated development of numerous non-Fickian-based 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 (DTₐyₗₒᵣ) or the molecular diffusion coefficient (Dₘ). The assessment was based on how well the local ADE compared to particle-tracking solutions for solute transport across a range of Péclét 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 DTₐyₗₒᵣ to the local ADE, it extended the applicability of the local ADE to a threshold of Pe < 450; using Dₘ, the local ADE was only accurate when Pe < 70. No differences were observed for small Pe. Therefore, our recommendation is to always use DTₐyₗₒᵣ in the local ADE to capture non-Fickian transport so long as the Pe threshold is not exceeded.