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Surface contaminant plumes emanating from waste disposal facilities are often denser than the ambient groundwater. Under certain conditions these dense plumes may become unstable, contaminating larger regions of an aquifer. The behavior of contaminant plumes with different densities was examined in three flow containers packed with homogeneous porous media simulating unconfined aquifers. Glass beads and a medium sand were used as the porous media. A horizontal ambient groundwater stream was established in each flow container using deionized water. The contaminant plumes consisted of NaI or NaBr solutions introduced into each flow container from a line source located on top of the porous medium and extending over the total width of the container. Optical tracers were added to the salt solutions to allow flow visualization. Results show that, for a given porous medium, dense plumes were either stable or unstable depending on the magnitude of the horizontal flow velocity, the contaminant leakage rate, and the density difference between the contaminant solution and the ambient groundwater. When a dense plume becomes unstable, lobe-shaped gravitational instabilities develop which are unsteady and three dimensional. Our experimental results suggest that the behavior of dense contaminant plumes overlying a less dense groundwater stream in a homogeneous porous medium depends on the magnitude of certain nondimensional parameters. It appears that gravitational instabilities begin to develop when the values of these nondimensional parameters exceed certain critical values. |
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