| Karst groundwater is an important drinking water source in southwestern China, but its vulnerability to pollution poses a threat to water resource security. This study took the underground river basin in Maocun, Guilin as the research object. By collecting surface water, spring water and drilling water samples, combined with hydrochemical analysis, stable isotope technology, and Bayesian mixing model (MixSIAR), the hydrochemical characteristics and control factors of groundwater were explored, and the sources of nitrate pollution were quantitatively analyzed. The results indicate that the surface water in the study area is weakly acidic, while the spring water and drilling water are weakly alkaline; the main hydrochemical types of drilling water and spring water are HCO3-Ca?Mg and HCO3-Ca type, respectively. The hydrochemical composition of surface water is controlled by the weathering of silicate rocks. The hydrochemical composition of spring water and drilling water is controlled by the weathering of carbonate rocks, with Ca2+, Mg2+ and HCO-3 mainly derived from the dissolution of carbonate rocks during weathering. The water in the watershed is not significantly affected by cation adsorption. The MixSIAR model calculation results show that the main sources of nitrate in various water bodies in the study area are chemical fertilizers (30% -39%), manure and sewage (27% -30%), followed by soil organic nitrogen (16% -26%) and atmospheric precipitation (16% -17%). There is a significant difference in the sources of nitrate in drilling water between the dry season and rainy season. Chemical fertilizers are the main source in the dry season, while soil organic nitrogen is the main source in the rainy season. The research results provide a scientific basis for the protection and pollution control of groundwater resources in karst areas. |