Abstract: This paper discusses the cause of the tufa in Badain Jaran Desert, throughout investigating the distribution of the tufa and analyzing the hydrochemical characteristics of water samples. Results indicates that the lake water is the type of Cl-Na, along with low concentration of Ca²⁺ and high concentration of CO₃^2-+HCO₃^- and groundwater are the types of Cl-SO₄-Na and Cl-CO₃-Na, with low concentration of CO₃^2-+HCO₃^- and high concentration of Ca²⁺. Saturation index of calcite and aragonite in some lake water is more than 0. Based on the simulation of PHREEQC, the mixing between lake water and groundwater results in SI (saturation index) increasing. Evaporation concentration effect in the lake and the mixing effect between fresh water and salt water are the main cause of calcium carbonate and tufa precipitation. The mixing ratio companied the maximum saturation index is mainly determined by the composition of end member mixing water, while the contribution of temperature is small compared with mixing effect.

Key words: Bardain Jaran Desert, tufa, mixing effect, hydrochemical characteristics