CO₂ Outgassing and Precipitation of Calcium Carbonate in a Karst Stream: A case study of Baishuwan Spring in Nanchuan, Chongqing

Key words: 
• karst stream /
• CO₂ outgassing /
• precipitation of calcium carbonate /
• Baishuwan Spring in Nanchuan

Abstract: PH, conductivity, dissolved oxygen and concentration of HCO^₃-, Ca²⁺ , as well as other anion and cation, was monitored by certain space to explain the evolution of stream water chemistry along the flow path of Baishuwan Spring downstream in Nanchuan, Chongqing, China. After surfacing, stream water chemistry was changed regularly by CO₂ outgassing, water became increasingly supersaturated with calcium carbonate and the obviously precipitation occurred. The result shows that, concentration of HCO₃^-, Ca²⁺, conductivity(EC) and CO₂ partial pressure declined along the flow path, caused by CO₂ outgassing, while pH value and saturation index of common carbonate minerals increased. The concentration gradient between atmosphere and water is the main factor affecting the degassing rate, while the creek drainage gradient also has a certain influence. Extent of degassing at the spring vent is decisive factors controlling the carbonate deposition along the flow path. The small flow during dry season, a great degree of degassing, high level of Ca²⁺ concentration, and the hydraulic disturbance which accelerates CO₂ outgassing, as well as the physical adsorption of litter and other substances in the creek drainage, factors above all play  active role in deposition of calcium carbonate.