A criterion for gravitational instability in miscible dense plumes

Miscible subsurface contaminant plumes emanating from waste disposal sites and landfills can be denser than the ambient groundwater. These dense plumes may be either stable or unstable. Since unstable plumes contaminate larger regions of an aquifer, it is important to predict when gravitational instabilities will occur. In this paper, we propose a new criterion to predict gravitational instabilities in the transverse mixing zones at the bottoms of dense plumes in porous media. This new criterion is based on the existing π1 number and a modified Péclet number. The experimental results of this study, together with those of previous research, seem to show that the proposed criterion is useful for prediction purposes.