Quantitative analysis of dissolved inorganic carbon sources in water bodies in the Lujiang river basin

The watershed carbon cycle is an important part of the global carbon cycle, playing an important role in seeking global carbon sinks that have been missed. Dissolved inorganic carbon (DIC) is one of the important indicators for studying the carbon cycle of water bodies in the basin. As an important component of terrestrial ecosystems, karst water bodies containing high DIC concentrations play an important role in the global carbon cycle. The Lujiang river basin, a typical development area of karst graben basin, is an applicable place for us to study the carbon migration and transformation of water bodies. In order to study the hydrochemical types and sources of DIC in the rivers of the Lujiang river basin, sampling data of hydrochemistry and carbon isotope from 14 surface water points, 8 underground river points, and 10 spring points during the rainy season were analyzed. It is believed that exploring the proportional contribution of DIC sources in different water bodies may help to understand the sources and cycles of groundwater DIC under the influence of human activities, providing scientific references for water resource management.

Firstly, this study determines the hydrochemical types of different water bodies through the Piper trilinear map and Schukalev classification. The results show that regardless of the type of water body in the study area, the hydrochemical type in the drainage basin is mainly HCO₃-Ca, belonging to the "type of carbonate rock weathering". Due to the long interaction time between water and rock, the rock weathering in groundwater is more obvious than in surface water. Secondly, the analyses of ion sources in water bodies by Gibbs plot indicate the consistency in the hydrochemical composition, changes, and origin mechanisms between groundwater and surface water. The controlled mechanism belongs to the "leaching type of rock weathering". Thirdly, due to the widespread distribution of carbonate rocks in the study area, an analysis has been conducted on carbonate rock dissolution in order to distinguish the relative contribution of limestone and dolomite dissolution to the chemical ion compositions of water bodies in the watershed. It is found that there are additional sources of Mg²⁺ and Ca²⁺ in the surface water of the Lujiang river basin, while other acids participate in the chemical weathering reaction in the water bodies of the underground river points and spring points. Furthermore, it is determined that ${\rm{SO}}_4^{2-}$ and ${\rm{NO}}_3^{-}$ are involved in the rock weathering in the basin, which has caused a large disturbance to the hydrochemical characteristics of water bodies in the basin. The Lujiang river basin is mixed with carbonate and silicate rocks. Therefore, the main sources of ${\rm{HCO}}_3^{-}$ in water are classified into three types: carbonate rocks weathered and dissolved by carbonic acid, carbonate rocks weathered by sulfuric acid/nitric acid, and silicate rocks weathered by carbonic acid. Based on the quantitative analysis by ion ratio method, carbonate rocks weathered by carbonic acid contribute average 68.8% to HCO$_3^{−}$, carbonate rocks weathered by sulfuric acid/nitric acid 27.2%, and silicate rock weathered by carbonic acid 3.9%. During the weathering process of water bodies in the Lujiang river basin, carbonate rocks are mainly weathered by carbonic acid. Finally, the verification by carbon isotope method has been compared with the measured values to show that the theoretical average values of underground river water and spring water δ¹³C_{DIC_Th} are relatively close, while the values of surface water are positive too, indicating that the source of DIC in surface water is not only controlled by rock weathering, but also by biogeochemical processes within river water bodies. In comparison, the areas of surface rivers are more significantly affected by human activities with severe urbanization and agricultural activities.