Factors controlling tufa deposition in natural waters at waterfall sites

Study on calcite precipitation has major implications for both the hydrochemical evolutions of river systems and the global carbon cycle. The precipitation of calcite generally requires the water to be 5-10 times supersaturated with respect to calcite, which is usually achieved by the removal of CO2. Formation of waterfall tufa has been often simply described as the result of water turbulence in fast-flowing water. In this paper, the formation mechanisms of waterfall tufa are discussed and a series of laboratory experiments are designed to simulate the hydrological conditions at waterfall sites. The influences of the air-water interface, the water flow velocity and the solid-water interface on CO2 outgassing and calcite precipitation are compared and evaluated quantitatively. The results show that the principal cause of waterfall tufa formation is the enhanced inorganic carbon dioxide outgassing resulted from the sudden hydrological changes occurring at waterfall sites, rather than organisms, evaporation or the solid-water interface. The air-water interface area and the water flow velocity are greatly increased at waterfall sites as a result of the aeration effect, low pressure effect and jet-flow effect, which greatly accelerate CO2 outgassing. Inorganic CO2 outgassing drives the waters to become highly supersaturated with respect, to calcite and, consequently, results in much calcite deposition. The solid-water interface is less important as the air-water interface in affecting calcite precipitation at waterfall sites. Field measurements showed that conductivity, Ca2+ and HCOT concentrations along Tianhe Creek and Hot Creek decrease downstream while pH rises. Field observations also showed that tufa deposition occurred mainly at waterfall sites. (C) 2004 Elsevier B.V. All rights reserved.