A Study On The Hydro-geochemical Characteristics, The Major Weathering Processes And The Atmospheric CO₂ Consumption Budget For The Small Watersheds In The Subtropical Zone

The chemical and geochemical features of river water are a manifestation of the extent of surface weathering and denudation. The chemical elementary composition of the river water can show the environmental quality status of water body, the regional environmental chemical characteristics, the regularities of distribution and migration of the aquatic element to a large degree. At present, the small river basins are less selected to discuss the hydro-geochemical features. In this paper, the upper reaches of Ganjiang and rivers near Pingba county in Guizhou Province have been selected as the objects of study, combining with related data about the southern Jiangxi collecting from water year book. Based on the datas, the hydro-geochemical characteristic, the major weathering processes, and weathering rate of small drainage basins are analyzed intensively and systemically. The following conclusions have been drawn: (l)The Ganjiang river and its tributaries have low total dissolved solids concentrations and the chemical composition of river water is dominated by Na⁺, Ca²⁺, Cl^-, HCO₃^- and Si (the water has higher Si concentration), which represents that of the river water from the typical silicate rock areas. The rivers near Pingba county in Guizhou Province have much higher TDS than that of Ganjiang river and the mean value of the world rivers. The dominated ions are Ca²⁺, Mg²⁺, SO₄^2- and HCO₃^-, by the influence of cropping out the big proportion of the carbonate rock. (2)To critically evaluate wet fallout input to river poorly documented formerly, we have chosen the granite basin in the southern Jiangxi Province, more prone to "precipitation controlling" as Gibbs (1970) defined, and seawater correction approach (Cl" normalized seawater ratios) was used to estimate the contribution proportions of local precipitation on the solutes. In conclusion, the local precipitation contributed 9.25% dissolved substance to the watersheds in the southern Jiangxi Province. (3)On the basis of the principal composition analysis and factor analysis, the contributions of atmospheric CO₂ and three types of rock to the total dissolved solids are estimated quantificationally. The watersheds in the southern Jiangxi Province are influenced strongly by silicate weathering, but the dominant factors still are carbonates and evaporates, which are responsible for 42.8% and 29.2% of total ion equivalent concentration, and that of atmosphere CO₂ is 21.4% which is lower than the average value of the world. (4)The evaporate and calcite dissolution are the primary weathering processes in the watersheds of the southern...