Geochemical mass balances of major chemical constituents in the watershed of the Changhuajian River in China

Based on the geological background, R-mode factor statistics, and the analysis of the stability diagram for the corresponding system, five weathering reactions controlling the surface-water chemical composition in the watershed of the Changhuajiang River are deduced. In the mass balance model, the precipitation accounts for some solute input, since the rainwater is dilute without pollution. Most of the Ca²⁺ and HCO₃^– ions are from the dissolution of calcite, K⁺, Na⁺, H₄SiO₄ and some of the Mg²⁺ and HCO₃^– come from albite and biotite weathering to kaolinite. The dissolution of dolomite and gypsum controls the mass balances of Mg²⁺ and SO₄^2–. The dissolution of calcite is the dominant chemical weathering reaction in the watershed because of its reactivity and high concentration. In the watershed in 1986, the chemical weathering rate was 0.073 (kg/m² a), and the mechanical denudation rate is 0.093 (kg/m² a). The chemical weathering mass output proportion of carbonate rocks to silicate rocks was about three to one.