Geochemistry of surface and ground water in Guiyang, China: Water/rock interaction and pollution in a karst hydrological system

The chemical compositions of the surface/ground water of Guiyang, the capital city of Guizhou Province, China are dominated by Ca2+, Mg2+, HCO3-andSO42-, which have been derived largely from chemical weathering of carbonate rocks (limestone and dolomite). The production of SO42- has multiple origins, mainly from dissolution of sulfate evaporites, oxidation of sulfide minerals and organic S in the strata, and anthropogenic sources. Most ground water is exposed to soil CO2 and, therefore, the H2CO3 which attacks minerals contains much soil C. In addition, the H2SO4 produced as a result of the oxidation of sulfides in S-rich coal seams and/or organic S, is believed to be associated with the chemical weathering of rocks. The major anthropogenic components in the surface and ground water include K+, Na+, Cl−, SO42-andNO3-, with Cl− and NO3- being the main contributors to ground water pollution in Guiyang and its adjacent areas. The seasonal variations in concentrations of anthropogenic components demonstrate that the karst ground water system is liable to pollution by human activities. The higher content of NO3- in ground water compared to surface water during the summer and winter seasons, indicates that the karstic ground water system is not capable of denitrification and therefore does not easily recover once contaminated with nitrates.