Abstract: Highfluoride groundwater has been found in the Niuxin Mountain in the north Liujiang basin. The Piper, Gibbs, Alkali indices and Ion ratio diagrams were used in this study to analyze the chemical characteristics and forming mechanism of shallow groundwater in the area, and the source and enrichment of fluoride in groundwater were discussed from the perspective of mineral dissolution, precipitation and ion exchange. The results showed that the major ions include slightly alkaline Ca2+, Na+ and HCO-3, with the hydrochemical types of Ca(Na)HCO3, Ca(SO4)HCO3 and Ca(Cl)SO4; high concentration of F- is present in Ca(Na)HCO3 type water, and the chemical composition of groundwater is mainly affected by weathering. Moreover, the chemical characteristics of groundwater and the origin and distribution of fluoride are controlled by hydrogeochemical and geological factors. Calcite and gypsum are dissolved into groundwater as Ca2+ source, affecting fluorite dissolution and precipitation. The cation exchange indirectly affects the concentration of F- by changing cations concentration in groundwater, and F- adsorbed on clay minerals is replaced by OH- in the alkaline environment. Based on the new findings, we suggest that dissolution equilibrium and ionexchange are the major factors controlling fluoride concentration in groundwater runoff.

Key words:  hydrogeochemistry, fluoride origin, distribution characteristics, Liujiang basin