Controlling processes in a CaCO3 precipitating stream in Huanglong NaturalScenic District, Sichuan, China

Huanglong Scenic District is well known for its unusual and diversified lan dforms such as travertine pools, travertine falls and travertine flows. The se landforms, resulting from high-altitude surface cold-water CaCO3 precipi tation, were chosen by UNESCO in 1994 as an entry in The World's Nature Her itage. Huanglong is a pristine region where there are limited human activit ies. Water analyses and thin section (glass slide) precipitation experiment s were conducted to determine the aqueous processes controlling CaCO3 preci pitation and travertine landform formation. Results from the travertine how indicate that the concentrations of HCO3-, Ca+2, and H+ decrease regularly along the how paths. Chemical equilibrium modeling results demonstrate the importance of CO2 out-gassing and CaCO3 precipitation processes. CO2 out-g assing and CaCO3 Precipitation increase with increasing flow velocities. In the pool area, varying hydrodynamics are the primary factors which determi ne the extent of processes such as advection and diffusion, and hence also control CaCO3 precipitation and CO2 out-gassing. When the pool water circul ation is very slow, the pH of water flowing over the travertine dams increa ses significantly (approximately 0.15 pH units) downstream. When the circul ation is relatively fast, the pH of stream water initially decreases follow ed by an increase of approximately 0.21 pH units as it flows past the trave rtine pool dams. In both cases, the pn rise is caused by sudden changes in the hydrodynamics of thr pools, despite the different initial flow conditio ns. Pool development is a consequence of spatial variations in pH which pro vide different conditions for CaCO3 precipitation inside the travertine dam , where less precipitation or even dissolution occurs, compared to conditio ns at the top and downstream side of the dams. Precipitation experiments de monstrate that the top and downstream side of travertine darns are the loca tions of the most active precipitation, particularly for pools having faste r circulation. Precipitation experiments also reveal that vaterite, a rare polymorph of CaCO3, co-precipitates with calcite in milky opalescent water near the upstream input portion of the pool groups. Thin sections covered b y algae at the bottom of pools have 40% less CaCO3 precipitation than those not covered by algae. SEM photographs of the surface of natural travertine deposits show that biofilms with diatom minimize CaCO3 precipitation and t hat diatom-adhered calcite surfaces show signs of etching, suggesting that calcite dissolution may be aided by diatoms, (C) 2000 Elsevier Science B.V. All rights reserved.