Analysis of hydrochemical characteristics and evolutionary patterns of shallow groundwater in transition zone of the middle-lower Yellow River

The distribution characteristics, hydrochemical types, and variation trends of major chemical components in shallow groundwater within the transition zone of the middle-lower Yellow River (Jili-Mengjin reach) were analyzed using mathematical statistics, Piper diagrams and Schoeller diagrams. The controlling factors and primary sources of these components were identified through the Gibbs model, ionic ratio analysis and factor analysis, and the hydrochemical evolutionary patterns of shallow groundwater were verified through the profile method. The results show that there is a close hydraulic connection between shallow groundwater in the transition zone of the middle-lower Yellow River and Yellow River water. The hydrochemical composition of groundwater is predominantly controlled by rock weathering (primarily carbonate rocks), and is also influenced by evaporation and human activities to a certain extent. Cation exchange processes and mineral exploitation activities lead to a dynamic saturation state of carbonate minerals in groundwater within the river valley plain. The hydrochemical types of shallow groundwater in the transition zone of the middle-lower Yellow River are relatively complex, evolving from the HCO₃-Ca·Mg type in the loess hilly area to the HCO₃·SO₄-Ca·Mg and HCO₃·Cl-Ca·Mg types in the Yellow River floodplain area, and further transitioning to the HCO₃·SO₄·Cl-Ca·Na·Mg, HCO₃·Cl-Ca·Mg·Na and HCO₃·Cl·SO₄-Mg·Ca·Na types.