[Water Chemical Isotope Characteristics and Water Transformation Relationship in Mongolian Section of the Yellow River Basin].

The Yellow River in Inner Mongolia was selected as the study area in this study. In July (wet season) and October (dry season) of 2021, the acquisition of seasonal rivers, the Yellow River tributaries and precipitation, the Yellow River, Wuliangsuhai, Lake Hasuhai, Lake Daihai, an irrigation canal system, and underground water and sea water samples were collected to test the water chemical composition and hydrogen and oxygen isotopic values of different water types. Using the Piper triplot, Gibbs plot, ion ratio, and MixSIAR model methods, the evolution of water chemistry in the Mongolian section of the Yellow River Basin was analyzed, and the transformation relationship between precipitation, surface water, and groundwater was revealed. The results showed that both groundwater and surface water in the study area were slightly alkaline; the dominant anion in water was Cl, and the dominant cation was Na. The main hydrochemical types of surface water were Cl·SO-Na·Mg and SO·HCO-Na·Mg, whereas those of groundwater were Cl·SO-Na·Mg and SO·HCO-Na·Ca. Groundwater Ca and Mg were primarily derived from the dissolution of silicate and evaporite, and surface water Ca and Mg were primarily derived from carbonate karst dissolution and carbonate and sulfuric acid in water participating in the dissolution process of carbonate and sulfide minerals. Na and Cl in different water bodies were all affected by anthropogenic pollution sources. Owing to the seasonal effect, D and O of surface water and groundwater were higher in the wet season than in the dry season. The results showed that surface water was affected by evaporative fractionation after receiving precipitation recharge, and the groundwater recharge sources were complex. The MixSIAR model revealed that surface water was the main recharge source of groundwater, accounting for 52.4%-62.2% of the total recharge, and atmospheric precipitation was the main recharge source of surface water, accounting for 85.4%-97.1% of the total recharge.