LABORATORY INVESTIGATION OF BUOYANCY-INDUCED FLOW (PLUME SINKING) DURING 2-WELL TRACER TESTS

The occurrence of buoyancy-induced vertical flow (sinking) of a bromide (Br-) tracer plume at small injection concentrations is investigated in transport experiments conducted in a large-scale physical aquifer model containing a homogeneous and isotropic sand pack. Two-well tracer tests are conducted using Br- at concentrations ranging from 50 to 1000 mg/l, corresponding to relative densities between 7.5 X 10(-5) and 1.5 X 10(-3). Analysis of three-dimensional solute concentration data indicates that the center of mass of the Br- plume was displaced downward as the denser tracer solution sank through the sand pack. Plume sinking occurred at all solute concentrations investigated; the magnitude of the vertical displacement increased with increasing Br- concentration of the injected tracer solution. The dynamic collapse of the Br- plume caused by buoyancy forces resulted in increased apparent transverse and longitudinal dispersivities. The results suggest that the possibility of buoyancy-induced flow must be considered when interpreting tracer tests conducted with anion concentrations as low as 50 mg/l. The occurrence of buoyancy-induced flows at such low relative densities also suggest that the phenomenon may be more widespread than is generally recognized.