Study on the hydrochemistry character and carbon sink in the middle and upper reaches of the Xijiang River basin, China

In order to explore the change process and law of carbon sinks flux in the middle and upper reaches of the Xijiang River basin, the research workers discuss the influencing factors of carbon sinks flux in the study area under natural process and impact of human activity. The hydrologic station at Wuzhou was taken as the research object. River water was sampled and measured regularly from April 2011 to March 2012 three times per month (intensive measurements and sampling were carried out during the heavy rainfall period), and then river water samples were analyzed. The following results are shown: (1) The chemistry of water revealed that the middle and upper reaches of the Xijiang River basin comprised the HCO₃ ⁻Ca type; in total anions, both HCO₃ ⁻ and Ca²⁺ comprised 69 % and 75.9 %, respectively, as a result of carbonate rock weathering. (2) The content of NO₃ ⁻ and SO₄ ²⁻ was high, which might be the result of human activities, such as agriculture and industry. Moreover, sulphuric acid is involved in dissolution of carbonate minerals, and missing sink that is caused by exogenous acid is indispensable. (3) The annual total carbon flux was 6664.45 × 106 kg CO₂; the carbon flux in flood season and non-flood season accounted for 58.25 and 41.75 % of carbon sinks of karst water, respectively. Correspondingly, the intensity of carbon sinks was 1694.48 and 1214.53 kg/mon km², respectively. (4) Discharge was the primary factor of the intensity of carbon sinks and carbon flux; other influential factors, such as inorganic carbon and biological pump, had less influence on the intensity of carbon sinks and carbon flux. These conclusions are theoretically important and practically significant for estimating the carbon sink in Xijiang basin accurately. This study is a scientific basis for exploiting and protecting water resources.