In order to reveal the formation mechanism of hydrochemical characteristics in Tongchuan City, we collected and detected 39 samples of underground water. Correlation analytical method, hydrochemical method and ion ratio coefficient method were employed to investigate the hydrochemical characteristics, influencing factors and principles of changing. Results demonstrate that the main factors, controlling the shallow water salinization, are the SO42-, NO3-, Cl- and K+. The contents of SO42-, NO3- and Cl- are in great differences and other indexes are in relatively stable. The values of γNa/γCl demonstrate that the Na+ is released from the aquifer of soil in the process of runoff. There is an exchange between the Ca2+ in water and Na+ in soil, which leads to γNa>γCl. As the exchange time in deep water is longer than that of shallow water, the exchange and adsorption of ions are more sufficient. The value of γNa/(γNa+γCl) demonstrates that with the increasing of ground water depth, the level of cation exchange enhances, which leads to the dominant cation turns to Ca2+ from to Na+. The value of γHCO3+γSO4/γCa+γMg demonstrates the shallow water is mainly from atmospheric rainfall and the influence of cation exchange is more obvious on deep water. The types of shallow water in Tongchuan City are mainly HCO3?SO4-Ca. Its classification is relatively complex. Whereas, the deep water is mainly HCO3-Na and the category of underground water is simple. As for shallow water, the contents of alkaline earth metal is larger that of alkali metal. For the deep water, the content of weak acid group is larger than that of strong acid group. In the region of upper reaches, the feature of both shallow water and deep water are mainly weak acid and alkaline earth metal. The chemical composition mainly consists of carbonate leaching. With the flowing of underground water, the shallow water turns to be strong acid and alkaline earth metal and the deep water turn to be weak acid and alkali metal.

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