Geochemical Characteristics Of Cycling Of Substances In Karstic Ground Water System

This research studies the general geochemical characteristics of karstic surface/ground water systems of Guiyang and Zunyi cities, SW China, with main purposes to characterize exchange between surface water-ground water, water-rock interaction, and to understand human impact on karstic ground water system. The chemical and isotopic analyses of the river/stream and ground water show that the water chemistry of the studied water is dominated by Ca²⁺, Mg²⁺, HCO₃^-, SO₄^2-, which totally accounting for more 80% of total ions. This chemical compositional feature should be ascribed to dissolution mainly of carbonate rocks (or minerals) and sulfate evaporite in the aquifers. Most of water samples contain high SO₄^2- concentrations, isotope compositional variations of which indicate multiple origins of sulfur: main origins of sulfur include dissolution of gypsum, oxidation of sulfide minerals and input of acid rain as well as of municipal sewage. Analyses on stoichiometry and isotope signatures of the studied river/stream and ground water show that the sulfuric acid has been involved in dissolution of the rocks. The inputs by human activities are characterized by high concentrations of K⁺, Na⁺, Cl^-, SO₄^2-, NO₃^- ions, human input features of which are also supported by the stable isotope compositions. The SO₄^2-, NO₃^- may be the indicators for contamination of the ground water systems. Through comparison of geochemical compositions of the surface and ground water, it is shown that the ground water system shows quick responses in geochemistry to the surface water, suggesting that karstic ground water system is susceptible for human impact. In addition to enhancing our understanding of geochemical characteristics and their controlling factors and mechanisms of karstic ground water system, these results described above can be important scientific information for reasonable use and management of karstic ground water resources.