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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Recent studies have shown that cationic surfactants can be used to increase the organic carbon content of aquifer materials, creating enhanced sorbent zones for hydrophobic organic contaminants (HOCs) migrating in groundwater. The coupling of nonionic surfactant-enhanced solubilization of HOCs to the cationic surfactant-enhanced sorption zone concept is examined as a possible groundwater remediation scheme. The partitioning behavior of 1,2,4-trichlorobenzene (TCB) and Igepal CO 730 (CO 730, a nonionic surfactant), both singly and jointly, were determined in batch hexadecyltrimethylammonium (HDTMA, a cationic surfactant) chloride-modified aquifer material/water systems. The apparent CO 730 critical micelle concentration (cmc) was found to decrease by a factor of 17 due to nonideal mixed micelle formation in the presence of low aqueous phase HDTMA concentrations that resulted from exposure to HDTMA-modified aquifer material. A greater portion of CO 730 will be present in the micellar psuedophase (the HOC solubilizing phase) as a result of the lowered cmc. The TCB partitioning behavior in the HDTMA-modified aquifer material/water systems was found to be dependent upon the aquifer material organic carbon content, the apparent CO 730 cmc, and the partitioning of TCB between the micellar pseudophase and the bulk aqueous phase containing surfactant monomers. The batch experiments indicate that the micellar pseudophase would be a more favorable partitioning medium for TCB relative to the solid phase organic carbon within an enhanced sorbent zone. Onedimensional column experiments were performed using HDTMA-treated and untreated aquifer materials to determine the transport behavior of CO 730 and to assess the feasibility of using CO 730 to solubilize and recover TCB bound within a cationic surfactant-enhanced sorbent zone. Complete removal (>99%) of the bound TCB was achieved using a 12-column pore volume flush of 50 g/L CO 730. The column experiments demonstrated that a nonionic surfactant can effectively remove an HOC from a cationic surfactant-enhanced sorbent zone.