Infiltration in unsaturated layered fluvial deposits at Rio Bravo: Macroscopic anisotropy and heterogeneous transport

An infiltration and dye transport experiment was conducted to visualize flow and transport processes in a heterogeneous, layered, sandy-gravelly fluvial deposit adjacent to Rio Bravo Boulevard in Albuquerque, NM. Water containing red dye followed by blue-green dye was ponded in a small horizontal zone ( about 0.5 by 0.5 m) above a vertical outcrop ( about 4 by 2.5 m). The red dye lagged behind the wetting front due to slight adsorption, thus allowing both the wetting front and dye fronts to be observed in time at the outcrop face. After infiltration, vertical slices were excavated to the midpoint of the infiltrometer, exposing the wetting front and dye distribution in a quasi three-dimensional manner. At small scale, wetting front advancement was influenced by the multitude of local capillary barriers within the deposit. However, at the scale of the experiment, the wetting front appeared smooth with significant lateral spreading, twice that in the vertical, indicating a strong anisotropy due to the pronounced horizontal layering. The dye fronts exhibited appreciably more irregularity than the wetting front, as well as the influence of preferential flow ( a fracture) that moved the dye directly to the front, bypassing fresh water between. To illustrate the ability of equivalent homogenous media models to capture the behavior of the wetting front, we performed numerical simulations using equivalent homogeneous media with isotropic, anisotropic, and moisture-dependent anisotropic properties. Those containing anisotropy matched the experimental data best.