A geothermal density current

Summary

Geothermal fluid from the Wairakei bore field, which contains high concentrations of dissolved salts, is discharged into Lake Aratiatia and is significantly denser than the lake water when both are at the same temperature. The geothermal fluid enters the lake at a temperature of 59 °C, which is, depending on the season, approximately 40–50 °C warmer than the lake water. On entering the lake, the fluid initially floats, but as it cools to a temperature approximately 10 °C warmer than the lake water it sinks, forming a density current that rapidly mixes with the lake water as a result of interface instabilities (at high lake-water current velocities) or by turbulent diffusion (at low lake- water current velocities). Because of the density differences of the two waters and resulting density currents, flow of geothermal water into Lake Aratiatia does not result in the formation of a buoyant overflow.