title: Use of a variable-index fractional-derivative model to capture transient dispersion in heterogeneous media.
abstract: Field and numerical experiments of solute transport through heterogeneous porous and fractured media show that the growth of contaminant plumes may not exhibit constant scaling, and may instead transition between diffusive states (i.e., superdiffusion, subdiffusion, and Fickian diffusion) at various transport scales. These transitions are likely attributed to physical properties of the medium, such as spatial variations in medium heterogeneity. We refer to this transitory dispersive behavior as "transient dispersion", and propose a variable-index fractional-derivative model (FDM) to describe the underlying transport dynamics. The new model generalizes the standard constant-index FDM which is limited to stationary heterogeneous media. Numerical methods including an implicit Eulerian method (for spatiotemporal transient dispersion) and a Lagrangian solver (for multiscaling dispersion) are utilized to produce variable-index FDM solutions. The variable-index FDM is then applied to describe transient dispersion observed at two field tracer tests and a set of numerical experiments. Results show that 1) uranine transport at the small-scale Grimsel test site transitions from strong subdispersion to Fickian dispersion, 2) transport of tritium at the regional-scale Macrodispersion Experimental (MADE) site transitions from near-Fickian dispersion to strong superdispersion, and 3) the conservative particle transport through regional-scale discrete fracture network transitions from superdispersion to Fickian dispersion. The variable-index model can efficiently quantify these transitions, with the scale index varying linearly in time or space.
authors.affiliation: College of Mechanics and Materials, State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing, Jiangsu 210098, China. Electronic address: shg@hhu.edu.cn.
authors.forename: Hongguang
authors.initials: H
authors.lastname: Sun
authors.affiliation: Division of Hydrologic Sciences, Desert Research Institute, Las Vegas, NV 89119, USA. Electronic address: Yong.Zhang@dri.edu.
authors.forename: Yong
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Institute of Soft Matter Mechanics, Department of Engineering Mechanics, Hohai University, Nanjing, Jiangsu 210098, China. Electronic address: chenwen@hhu.edu.cn.
authors.forename: Wen
authors.initials: W
authors.lastname: Chen
authors.affiliation: Division of Hydrologic Sciences, Desert Research Institute, Reno, NV 89512, USA. Electronic address: mreeves@dri.edu.
authors.forename: Donald M
authors.initials: DM
authors.lastname: Reeves
mesh_terms: D060587:Groundwater; D008962:Models, Theoretical; D014872:Water Movements
keywords: Anomalous dispersion; Contaminant transport; Numerical solutions; Transient dispersion; Variable-index model