Fluvial geochemistry of rivers draining karst terrain in Southwest China

摘要

Samples of water from rivers draining karst areas within Guizhou Province, China (the Wujiang, Qingshuijiang, and Wuyanghe rivers), were analyzed for dissolved major element concentrations (HCO_3~-. NO_3~-. SO_4~(2-). Cl~-,Ca~(2-), Mg~(2-). K~-. Na~-), Sr~(2+), and ~(87)Sr/~(86)Sr to quantify the rates of chemical weathering of rocks and associated atmospheric CO_2 consumption in a typical karst area. The analyzed water samples in rivers draining karst areas typically show a dominance of Ca~(2+), Mg~(2+), and HCO_3, and are rich in SO_4~(2-).rnThe waters of the Wujiang River contain high Sr~(2+) concentrations (0.61-7.19 μmol/1) and low ~(87)Sr/~(86)Sr values (0.7074-0.7115), while the waters of the Qingshuijiang River, which drains silicate strata, yield high Sr isotopic ratios (0.7088-0.7155) and low Sr~(2+) concentrations (0.18-1.45 μmol/1). The chemical and isotopic compositions of these waters are largely controlled by the lithology of the basin. Stoichiom-etric analyses show that the water chemistry is controlled by carbonate dissolution under the influence of carbonic and sulfuric acid.rnThe rates of chemical weathering of rocks within the studied basin were estimated based on the chemical budget calculated for each of the three analyzed rivers. The weathering rates estimated for silicate rocks were 6.7-10.7 t/km~2/yr, while those for carbonate rocks were 67-116 t/km~2/yr. We also calculated the rates of chemical weathering of carbonate rock and related rate of CO_2 consumption by carbonic and sulfuric acid combined, and by carbonic acid alone. The results show that the involvement of sulfuric acid in weathering processes strongly enhances the rate of carbonate weathering but lowers the rate of CO_2 consumption. The catchment of the Wujiang River is dominated by a coal-bearing formation, and a coal mine is operated within the catchment. The consumption of CO_2 shows a marked decrease from 844 × 10~3 mol/km~2/yr in the case of carbonate weathering by carbonic acid alone to 546 × 10~3 mol/ km~2/yr in the case of carbonate weathering by both sulfuric and carbonic acid. No atmospheric CO_2 is consumed during weathering of the carbonate rocks by sulfuric acid; consequently, this form of weathering represents a net source of CO_2 to the atmosphere in the Guizhou karst area.