Oktay Güven

Behavior of dense aqueous phase leachate plumes in homogeneous porous media

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.

Experimental investigation of dense solute plumes in an unconfined aquifer model

Unstable dense aqueous phase contaminant plumes may contaminate larger regions of an aquifer than stable plumes. To learn more about the behavior of variable density contaminant plumes, experiments were conducted in a flow container packed with homogeneous sand. Salt (NaI) solutions with different densities were introduced at different rates from a source, located on top of the sand, into the unconfined aquifer model where horizontal ambient groundwater streams were established with different horizontal velocities. Salt concentrations were measured with an automated, nonintrusive gamma radiation system. Results show that dense plumes were either stable or unstable. Important factors determining the stability of the dense plumes were the magnitude of the horizontal Darcy velocity and the relative density difference between the dense plume and the ambient groundwater. Longitudinal and transverse dispersivities, estimated from breakthrough curves and concentration profiles determined during the development of stable dense plumes, were small, indicating advection as the main mode of transport. Concentration profiles of stable dense plumes, taken along vertical transects, showed a rapid drop in relative concentration at the bottom of the plumes. When dense plumes became unstable, salt concentrations fluctuated considerably over time and concentration profiles of unstable plumes showed large mixing zones.