Infiltration in Unsaturated Layered Fluvial Deposits at Rio Bravo: Macroscopic Anisotropy and Heterogeneous Transport

and Yeh, 1996; Roth and Hammel, 1996; Birkholzer and Tsang, 1997) suggest that in unsaturated media, An infiltration and dye transport experiment was conducted to the macrodispersivity for the equivalent homogeneous visualize flow and transport processes in a heterogeneous, layered, medium, which is in general anisotropic, increases with sandy-gravelly fluvial deposit adjacent to Rio Bravo Boulevard in Albuquerque, NM. Water containing red dye followed by blue-green a decrease in saturation. The results of all these studies dye was ponded in a small horizontal zone (about 0.5 by 0.5 m) above suggest that accounting for anisotropy in the modeling a vertical outcrop (about 4 by 2.5 m). The red dye lagged behind the of flow and transport within the vadose zone may be wetting front due to slight adsorption, thus allowing both the wetting important. However, most of these studies have focused front and dye fronts to be observed in time at the outcrop face. on a theoretical analysis of the flow and transport behavAfter infiltration, vertical slices were excavated to the midpoint of the ior in synthetic media where the media is considered infiltrometer, exposing the wetting front and dye distribution in a quasi simply composed of distinct individual macroscopic laythree-dimensional manner. At small scale, wetting front advancement ers within a stratigraphic sequence. was influenced by the multitude of local capillary barriers within the Natural sedimentary sequences can be quite complex deposit. However, at the scale of the experiment, the wetting front with layers that are highly variable in thickness and latappeared smooth with significant lateral spreading, twice that in the vertical, indicating a strong anisotropy due to the pronounced horieral extent. Often, macroscopic layers, or units, are idenzontal layering. The dye fronts exhibited appreciably more irregularity tified though sedimentological mapping, with differences than the wetting front, as well as the influence of preferential flow between the units reflecting changes in the local deposifeatures (a fracture) that moved the dye directly to the front, bypassing tional environment, origin of the sediments, stream dythe fresh water between. To illustrate the ability of equivalent homonamics, or subsequent superposition of pedogenic progeneous media models to capture the behavior of the wetting front, cesses. As an example, refer to Fig. 1a, where a sequence we performed numerical simulations using equivalent homogeneous of macrounits have been identified using sedimentologimedia with isotropic, anisotropic, and moisture-dependent anisotropic cal mapping methods within an outcrop of fluvial origin properties. Those containing anisotropy matched the experimental at the Rio Bravo site in Albuquerque, NM. A close indata best. spection of individual macroscopic units within the stratigraphic sequence reveals each unit to be composed of a hierarchy of many subunit layers, as can best be seen I the past two decades, numerous studies have inin Fig. 1b. Such subunit layers are typical of deposits vestigated the effects of flow and solute transport in where eolian and fluvial processes have deposited the variably saturated media. Theoretical work (e.g., Yeh initial sequence of strata. At small scale, the bedding is and Gelhar, 1983; Mualem, 1984; Yeh et al., 1985a, composed of alternating fine and coarse layers obviously 1985b, 1985c; Mantoglou and Gelhar, 1987; Green and truncated by local erosion and redeposition of sediments Freyberg, 1995), numerical simulations (e.g., Yeh, 1989; resulting in the superimposition of a variety of longer Desbarats, 1998; Wildenschild and Jensen, 1999b; Baglength scales that increase the variability within the tzoglou et al., 1994; Polmann et al., 1991; Ababou, 1988; macrounit. The now classic work of R.A. Bagnold spanAbabou et al., 1991; Khaleel et al., 2002), and experining from the 1940s to the 1970s (e.g., Bagnold, 1941, mental studies (e.g., Stephens and Heerman, 1988; Yeh 1973) has elucidated much of the physics of such sediand Harvey, 1990; McCord et al., 1991; Wildenschild mentological processes and the deposits they create. and Jensen, 1999a) indicate that if stratified sediments During the past few decades, numerous field infiltraare regarded as an equivalent homogeneous medium, tion and tracer experiments (e.g., Wierenga et al., 1986; the effective hydraulic conductivity (K) tensor for the Sisson and Lu, 1984; Gee and Ward, 2001; Brainard, equivalent medium can exhibit moistureor tension1997; Brainard et al., 2004a) have been conducted to dependent anisotropy. That is, the anisotropy (ratio of study infiltration and solute transport processes in the K parallel to bedding to K perpendicular to bedding) heterogeneous vadose zone. Monitoring of the processes increases with decreasing saturation (increasing tension). in these field experiments has mainly relied on measureFor solute transport, several studies (e.g., Mantoglou ments of capillary pressure, moisture content, and tracer and Gelhar, 1985; Polmann, 1990; Russo, 1993; Harter concentration, using a limited number of tensiometers, neutron access tubes, and suction lysimeters over a large R.J. Glass and J.R. Brainard, Flow Visualization and Processes Laboarea. As a consequence, these sparse measurements deratory, Sandia National Laboratories, Albuquerque, NM; T.-C. Jim pict moisture and solute distributions only at low resoluYeh, Department of Hydrology and Water Resources, The University tion and provide temporal changes of the processes only of Arizona, Tucson, AZ. Received 29 Dec. 2003. Original Research Paper. *Corresponding author ([email protected]). at the measurement locations and not beyond. Recent advanced geophysical surveys (e.g., cross-borehole ground Published in Vadose Zone Journal 4:22–31 (2005). penetrating radar and cross-borehole electric resistivity © Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA tomography) are allowing the imaging of these distribu-