Chemical ions source analysis and stable isotope implications of different water bodies in large karst underground river system: A case study of Poxin groundwater basin in Guangxi

To explore the characteristics, sources and main controlling factors of water chemical ions in surface or groundwater of Poxin underground river system and the implications of carbon stable isotope, the authors collected hydrochemical and carbon stable isotopic data from 38 water sites in the Poxin underground river basin and did analysis with stoichiometry and isotopic method. The results show that spatially, the chemical ions in the mainstream of underground river vary significantly due to the effect of the regional rock types and the dilution of the tributaries. Ion proportional analysis shows that the precipitation has great influence on Cl^- and Na⁺ in some springs, and the main type of carbonate dissolution seems to be limestone in the study area. The Mg²⁺/Ca²⁺ molar ratios of surface water and water in ground river skylight are negatively correlated with HCO₃^-, indicating that, at the macroscopic scale, the dissolution will become more intense with the higher HCO₃^- value, and H₂SO₄ and HNO₃ will participate during the weathering of carbonate rocks actively. Silicate rocks weathering has contributed to Ca²⁺, Mg²⁺, Na⁺ and K⁺ of surface water and groundwater. Mining activities and agricultural activities have a great impact on the generation of SO₄^2- and NO₃^-.The forward model of mass balance shows that, owing to regional rock types and hydrological conditions, water in surface and underground river skylights is mainly dominated by carbonate dissolution, whereas silicate dissolution and atmospheric input also contribute some components, and hence the ratios of three sources vary greatly spatially. The dissolved inorganic carbon (DIC) in the water is mainly derived from the dissolution of carbonate rocks and the soil CO₂.DIC and the δ¹³C_DIC values are obviously different between surface water and groundwater, and are negatively correlated with δ¹³C_DIC, implying that the more DIC from the soil CO₂, the more intense the dissolution capacity of the carbonate minerals. Based on the data from this study area and previous study in upper, middle and lower reaches of Xijiang River, the authors detected that both of data support the argument that the aquatic algae can apply DIC transport to organic carbon, and forms a stable carbon sink.