BUOYANCY-INDUCED FLOW OF A TRACER IN VERTICAL CONDUITS

Buoyancy-induced flow (BIF) of both O-18-enriched (delta(18)O, +160 to +5195 parts per thousand) and O-18-depleted (delta(18)O, -500 parts p er thousand) water has been studied in a 10-cm-diameter tube that simu lates a section of a well. BIF was detected in all the experiments for a density contrast (Delta rho) between 0.03 and 1.0 sigma units (equi valent to a total dissolved solids contrast of 30-1000 mg/L). The magn itude of the Rayleigh number suggests that the natural convection flow is turbulent. A mathematical model is presented to predict vertical t racer 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 w hen the density contrast is reduced to Delta rho = 0.03 sigma units. T his smaller mixing rate is still orders of magnitude larger than diffu sion alone. BIF may be an important groundwater transport and mixing m echanism in open conduits in aquifers. Its effects should also be cons idered when performing tracer experiments in the field.