Abstract: Water resources are essential for sustaining life, driving economic development, and protecting the environment. We investigated the hydrochemical variations of surface water and groundwater in the Shandian River Basin to identify the driving factors and the transformation relationship between each water body. Precipitation data was collected from May to October 2023, while surface water and groundwater samples were collected in both August (wet season) and October (dry season). We analyzed water samples by Piper triplex diagram, Gibbs diagram, ion ratio method, hydrogen and oxygen isotope analysis, and MixSIAR mixed model. The results indicated that both groundwater and surface water were generally weakly alkaline. The dominant type of groundwater chemical composition was HCO₃^--Ca²⁺·Na⁺.During the process of surface water transitioning from the wet season to the dry season, the predominant surface water chemistry type shifted from HCO₃^--Ca²⁺-Na⁺ to HCO₃^--Na⁺-Mg²⁺, as well as HCO₃^--Ca²⁺-Na⁺-Mg².Furthermore, the main hydrochemical types of surface water changed during the transition from wet season to dry season. The hydrochemical characteristics of different types of water were influenced by weathering of rocks, evaporation and concentration of water, as well as cation exchange. Additionally, significant enrichment of δD and δ¹⁸O isotopic values in surface water was observed during the wet season, and the ground-water remained in a depleted state during both wet and dry seasons. During the wet and dry seasons, the slope of surface water line was lower than that of the precipitation line, and the slope of groundwater line during the wet season was similar to that of the precipitation line and the surface water line, indicating the complexity of the conversion relationship between various water bodies during the wet season. The precipitation served as ~70% primary recharge source for surface waters, whereas underground aquifers contributed ~30%, being the secondary recharge source. During the dry season, groundwater replenished surface water. These findings would provide fundamental support for effective resource management and protection practices related to the basin.

Key words: Blue Flag section of Shandian River, stable isotopes of hydrogen and oxygen, hydrochemical characteristics, conversion of water bodies, MixSIAR model