Physics of gravity fingering of immiscible fluids within porous media: An overview of current understanding and selected complicating factors

While gravity fingering of water in unsaturated sands has been demonstrated and studied under constrained laboratory conditions, its significance under field conditions in the vadose zone is still under debate. We review current understanding of the gravity fingering process as developed through linear stability analysis and laboratory experiments. With respect to many complicating factors inherent in the field, this understanding is deficient in determining the field conditions where gravity fingering can be expected and the behavior of the field-scale fingering process when it occurs. This deficiency is exemplified in the results we present of a field experiment conducted in a sandy alluvial deposit. While simple extrapolation of current understanding of the process predicted instability, fingers did not occur. To begin to understand the influence of complicating factors inherent in the field, we conducted several laboratory experiments that address three important complicating factors: uniform and non-uniform initial moisture content, media heterogeneity, and the presence of macropores and fractures. These factors can fundamentally alter the gravity fingering process, its scale of expression and, under many field conditions, suppress its occurrence entirely. While the significance of gravity fingering of water in a wettable vadose zone is still inconclusive, gravity fingering will likely play a significant role in water/NAPL (nonaqueous-phase liquid) or water/NAPL/air systems, as occur within the saturated and vadose zones, respectively, at many contaminated industrial sites. To demonstrate this role, the results of a simple experiment are presented.