Abstract: Nine river water samples and 27 fissure water samples were collected to study the ion chemistry and geological genesis of natural water in Karst Region in Zhaotong City, Yunnan Province by means of statistical approaches such as Piper trilinear chart, Ion ratio and saturation index method. Results showed that the predominant anion and cation was HCO₃^-(relative concentration: 67.3% and 70.0%) and Ca²⁺ (relative concentration: 60.0% and 72.1%), respectively. Total hardness (TH) in 81.7% of natural water from the fluorosis area in Zhaotong was between 75 and 300 mg/L. The F^- concentration (mean value 0.1 mg/L) of natural water from the fluorosis area in Zhaotong was well below the standards for drinking water quality (1.0 mg/L). The hydro-chemistry type of fissure water, of which aquifer rock was carbonatite, was Ca-HCO₃ and Ca·Mg-HCO₃. It is concluded that aquifer lithology controlled the hydro-chemistry of fissure water. High quality water could be found in aquifer rock which is carbonatite and unitary lithology in the fluorosis area of Zhaotong City.

Key words: Zhaotong city of Yunnan, Karst, natural water, hydro-chemistry, fluorosis, geological genesis