Abstract: Study on the hydrogeochemical features of thermal groundwater is conducive to the exploitation of underground thermal water. Researches on the thermal groundwater in carbonate Reservoir in Triassic in Chongqing are expected to benefit Chongqing as the "Hot spring capital of the world" to achieving sustainable development of geothermal. In this study, samples of the thermal groundwater in carbonate reservoir in Triassic in Chongqing were obtained and tested in January and July in 2009. It was found that all the thermal water have similar features, such as being neutral in pH, with temperature ranging among 30℃ to 50℃, with high Electrical Conductivity (EC). These underground thermal water were defined as SO₄-Ca (Mg) type, which were identified to low-medium temperature geothermal water in carbonate reservoir. The major cations of these thermal water were Ca²⁺ and Mg²⁺ and the major anions were SO₄^2- and HCO₃^-. The concentration of SO₄^2-, Ca²⁺, and Mg²⁺ varied slightly with time but significantly among places, with an exception of two springs in Qingmuguan. These indicated that the hydrogeochemical properties of thermal groundwater in the study area are mainly controlled by the water-rock interaction, a process of water interaction with gypsum in carbonate rock. The calcite and dolomite of the underground thermal water are under the critical state of saturated to unsaturated, and there is no obvious phenomenon of travertine deposition. The Na-K-Mg-Ca diagram shows that the sources of the matters in thermal water in Carbonate Reservoir in Triassic in Chongqing are basically the same in each season and haven't reached the water-rock equilibrium.

Key words: carbonatein triassic, underground thermal water, hydrogeochemical feature, water-rock interaction