A CRITERION FOR GRAVITATIONAL-INSTABILITY IN MISCIBLE DENSE PLUMES

Miscible subsurface contaminant plumes emanating from waste disposal s ites and landfills can be denser than the ambient groundwater. These d ense plumes may be either stable or unstable. Since unstable plumes co ntaminate larger regions of an aquifer, it is important to predict whe n 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 pi(1) number and a modified Peclet number. The experimental results of this study, together with those o f previous research, seem to show that the proposed criterion is usefu l for prediction purposes.