Fluvial geochemistry of rivers draining karst terrain in Southwest China

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 (HCO3-,NO3-,SO42-,Cl-,Ca2+,Mg2+,K+,Na+), Sr2+, and 87Sr/86Sr to quantify the rates of chemical weathering of rocks and associated atmospheric CO2 consumption in a typical karst area. The analyzed water samples in rivers draining karst areas typically show a dominance of Ca2+, Mg2+, and HCO3-, and are rich in SO42-.The waters of the Wujiang River contain high Sr2+ concentrations (0.61–7.19 μmol/l) and low 87Sr/86Sr 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 Sr2+ concentrations (0.18–1.45 μmol/l). The chemical and isotopic compositions of these waters are largely controlled by the lithology of the basin. Stoichiometric analyses show that the water chemistry is controlled by carbonate dissolution under the influence of carbonic and sulfuric acid.The 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/km2/yr, while those for carbonate rocks were 67–116 t/km2/yr. We also calculated the rates of chemical weathering of carbonate rock and related rate of CO2 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 CO2 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 CO2 shows a marked decrease from 844 × 103 mol/km2/yr in the case of carbonate weathering by carbonic acid alone to 546 × 103 mol/km2/yr in the case of carbonate weathering by both sulfuric and carbonic acid. No atmospheric CO2 is consumed during weathering of the carbonate rocks by sulfuric acid; consequently, this form of weathering represents a net source of CO2 to the atmosphere in the Guizhou karst area.