Water geochemistry controlled by carbonate dissolution: a study of the river waters draining karst-dominated terrain, Guizhou Province, China

Concentrations of major ions and Sr, and Sr isotopic compositions of two major river systems (Wujiang River and Yuanjiang River) in karst-dominated regions, and during winter were measured. The rivers and their tributaries have high total dissolved solid concentrations, with TZ(+) ranging from 2.1 to 6.3 meq/l, which are significantly higher than that of the global river average. Calcium and HCO3-, followed by Mg2+ and SO4,2- dominate the chemical composition of major ions in the river waters. Both Na++K+ and Cl-+Si account for 5-10% of the cations and anions, respectively. The waters of the Wujiang River have high Sr2+ concentrations and lower Sr-87/Sr-86 ratios (0.7077-0.7110), while those of the Qingshuijiang River, one of two major tributaries of the Yuanjiang River system, show higher Sr isotopic ratios (0.7090-0.7145) and low Sr concentrations. The lithology and geography of the river basins largely control the chemical and isotopic compositions of these rivers. Analyses of the stoichiometry of the river waters show that the water chemistry is controlled by carbonate dissolution by both carbonic and sulfuric acid (especially of the Wujiang River), and suggest an origin of sulfate mainly from atmospheric input because of coal combustion and/or oxidation of sulfide minerals in the catchments. We identify three main weathering sources: limestone, dolomite and silicates. Chemical and Sr isotopic composition of Wujiang River waters are characterized mainly by limestone weathering, while those of the Qingshuijiang River waters mainly by dolomite and silicate weathering. Chemical weathering rates of carbonate and silicate are estimated, and the results indicate that the chemical weathering rates of carbonate in the catchments are remarkably high, significantly higher than most large rivers. It is considered that presence of sulfuric acid in the rock weathering, overuse of land and deterioration of vegetation are most likely the key factors responsible for the high weathering rates of carbonate in the catchments. (C) 2003 Elsevier B.V. All rights reserved.