Water geochemistry of rivers draining karst-dominated regions, Guangxi province, South China: Implications for chemical weathering and role of sulfuric acid

Abstract

Guangxi province, located in South China, is a typical karst region affected by acid depositions for a couple of decades. Water samples from the major river basins in this region were collected and the chemical compositions were measured with a main purpose to quantify chemical weathering rates and associated CO2 budget. The average total dissolved solids (TDS) of the river waters (232 mg/l) are comparable with other karstic rivers. The major ion compositions of the river waters are characterized by the dominance of Ca2+, Mg2+, HCO3−, and are significantly rich in SO42−. The chemical compositions of the river waters indicate that four major reservoirs (atmospheric, anthropogenic, carbonate and silicate inputs) contribute to the dissolved loads. The carbonate chemical weathering rates range from 61.4 t/km2/yr for the Qingshuihe river basin to 93.3 t/km2/yr for the Zuojiang river basin. The silicate chemical weathering rates range from 2.9 t/km2/yr for the Qingshuihe river basin to 7.9 t/km2/yr for the Liujiang river basin. The CO2 consumption rates by carbonate weathering range from 559 × 103 mol/km2/yr for the Qingshuihe river basin to 817 × 103 mol/km2/yr for the Zuojiang river basin. The CO2 consumption rates by silicate weathering range from 19 × 103 mol/km2/yr for the Zuojiang river basin to 85 × 103 mol/km2/yr for the Liujiang river basin. Sulfuric acid is involved as a proton donor in weathering reactions, and about 9–15% of the chemical weathering rates of the river basins are sulfuric acid-induced. The CO2 consumption rates are only 85–91% of the value when assuming carbonic acid provides all the protons in the weathering reactions. The CO2 released from sulfuric acid-induced carbonate weathering would exceed the CO2 consumed by silicate weathering, making the river basins in Guangxi net CO2 sources on geological timescales. The total net CO2 releasing flux is approximately 15.7 × 109 mol/yr (0.19 × 1012 gC/yr). This result quantitatively highlights the fact that CO2 budget by chemical weathering will be largely modified by sulfuric acid, especially in seriously acid deposition affected region.