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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. |
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