Buoyancy‐Induced Flow of a Tracer in Vertical Conduits

Buoyancy‐induced flow (BIF) of both ¹⁸O‐enriched (δ¹⁸O, +160 to +5195‰) and ¹⁸O‐depleted (δ¹⁸O, −500‰) water has been studied in a 10‐cm‐diameter tube that simulates a section of a well. BIF was detected in all the experiments for a density contrast (Δρ) between 0.03 and 1.0 σ units (equivalent to a total dissolved solids contrast of 30−1000 mg/L). The magnitude of the Rayleigh number suggests that the natural convection flow is turbulent. A mathematical model is presented to predict vertical tracer propagation inside the cavity. The model assumes a local mixing length scale proportional to the tube diameter. A possible transition to the laminar natural convection regime, with a significant reduction in mixing rate compared to that predicted by the model, is detected when the density contrast is reduced to Δρ = 0.03 σ units. This smaller mixing rate is still orders of magnitude larger than diffusion alone. BIF may be an important groundwater transport and mixing mechanism in open conduits in aquifers. Its effects should also be considered when performing tracer experiments in the field.