Laboratory Investigation of Buoyancy-Induced Flow (Plume Sinking) During Two-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/1, corresponding to relative densities between 7.5 × 10−5 and 1.5 × 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/1. The occurrence of buoyancy‐induced flows at such low relative densities also suggest that the phenomenon may be more widespread than is generally recognized.