Infiltration into an Analog Fracture: Experimental Observations of Gravity-Driven Fingering

and homogeneity. In turn, those differences fundamentally alter the balance between capillary, gravity, and The infiltration of water into unsaturated geologic media is an viscous forces that controls the occurrence and behavior immiscible displacement process that is unstable with respect to gravity and can thus lead to the formation of gravity-driven fingers. Where of gravity-driven fingers. The two-dimensional nature the geologic media (e.g., rock, soil) is fractured, gravity-driven fingers of fractures forces infiltration to occur at an arbitrary within the fractures may lead to extremely rapid vertical migration angle with respect to gravity and places constraints on of waterborne contaminants. We designed analog fractures to facilitate accessibility that lead to greatly enhanced phase entrapthe competition between viscous, gravity, and capillary forces that is ment over porous media. Finally, granular porous media expected to control finger behavior, then conducted an extended commonly exhibits micro-rough grain surfaces and closely experimental investigation to observe and measure finger behavior. spaced intergranular contacts. In fractures, contact points Results show that the spatially variant two-dimensional nature of fracare likely to be widely spaced with respect to the fracture ture geometry leads to different behavior than is reported for the aperture, and in many instances fracture surfaces will related problem of gravity-driven fingers in porous media. Observabe locally smooth. These differences will affect both tions of finger behavior are presented, along with a simple scale analysis used to relate the key measures of finger velocity, finger capillary properties along the air–water interface and width, and fingertip length. We also present a series of illustrative the nature of residual moisture content following gravexperiments designed to guide future research. ity drainage. The occurrence of gravity-driven fingers will have a substantial influence on infiltration within an individual I is an immiscible displacement process in fracture. For a given infiltration event, fingers will move which water seeps downward into unsaturated soil much faster and further than would be predicted for a or rock, displacing the resident air phase from pores flat (stable) displacement front. In addition, fingers will and fractures. Simple linear stability theory suggests that occupy a much smaller cross-sectional area than a flat gravity will act to destabilize infiltration, while viscous front, making them difficult to detect and greatly reand capillary forces will provide a stabilizing influence stricting contact between infiltrating water and the frac(e.g., Saffman and Taylor, 1958; Chouke et al., 1959). ture walls. This latter characteristic is critical, as imbibiGravity-driven fingers will form in situations where intion of water into the adjacent rock matrix and film flow filtration is unstable. The occurrence and subsequent along the fracture walls would slow or perhaps halt behavior of gravity-driven fingers in granular porous advancement within the fracture. Contact with the fracmedia (i.e., sands of various textures) has received conture walls also facilitates processes that inhibit the misiderable attention (see reviews in Chen et al., 1995; gration of waterborne contaminants, such as adsorption Chen and Neuman, 1996; Glass and Nicholl, 1996; Scanand chemical or biological degradation. Thus, for infillon et al., 1997; de Rooij, 2000; Eliassi and Glass, 2002). tration into otherwise low permeability units such as Conversely, few investigations have considered gravitythat shown in Fig. 1, the formation of gravity-driven driven fingers within individual fractures (Nicholl et al., fingers in fractures may lead to transport velocities that 1992, 1993a, 1993b, 1994; Glass and Nicholl, 1996; Su are orders of magnitude more rapid than would be exet al., 1999, 2001, 2004). Gravity-driven fingers can also pected for capillary (i.e., matrix) dominated flow. Gravoccur in free-surface flows on large aperture fractures ity-driven fingers may also be a more ubiquitous occur(e.g., Benson, 2001); however, we restrict our discussion rence in fractures than in natural porous media, where to those that locally saturate the fracture aperture. the presence of fine materials and initial moisture conThe need to consider gravity-driven fingers in fractent can act to suppress fingering. tures as separate from those formed in granular porous Here, we present laboratory experiments designed to media rises from the basic differences in topology of the explore gravity-driven fingers formed during infiltration void spaces between the two. Fracture void space differs into single fractures. Insight from related areas (Backfrom that found in granular porous media in terms of ground and Theoretical Network) was used to develop dimensionality (two vs. three), connection, size, isotropy, a systematic investigative approach. The experimental design and analog fracture used to control the balance M.J. Nicholl, Geoscience Dep., Univ. of Nevada, Las Vegas, NV between viscous, capillary, and gravitational forces are 89122-4010; R.J. Glass, Flow Visualization and Processes Lab., Sandia presented in Experimental Design. Our results begin National Laboratories, Albuquerque, NM. Received 27 Aug. 2004. with experimental observations of fingers formed during *Corresponding author ([email protected]). the redistribution of flow that occurs after ponded infilPublished in Vadose Zone Journal 4:1123–1151 (2005). tration (Experimental Observations: Multiple Fingers as Original Research Generated via Redistribution Following Ponding Events). doi:10.2136/vzj2004.0110 © Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: DNAPLS, dense nonaqueous phase liquids. 1123 Published online November 16, 2005