Geological and Anthropogenic Factors Jointly Influence Hydrochemical Interactions between Groundwater and Surface Water in the Middle and Lower Yellow River.

The interaction between groundwater and surface water plays a crucial role in regulating water resource availability and maintaining ecosystem stability, particularly through hydrochemical exchange processes. However, at the regional scale, the driving factors governing hydrochemical interactions between groundwater and surface water remain inadequately understood. This study aims to identify the dominant controlling factors and ion sources influencing hydrochemical interactions in the middle and lower reaches of the Yellow River. A comprehensive literature review was conducted on the hydrochemical characteristics of groundwater and surface water across China. Ultimately, 150 representative sampling sites located in the middle and lower Yellow River Basin were selected, encompassing 28,351 groundwater samples and 15,487 surface water samples. Sampling was conducted during both the rain and dry seasons, covering different hydrological periods. Hydrochemical characteristics were analyzed by using a combination of hydrogeochemical statistical methods, Piper diagrams, ion ratio relationships, Gibbs plots, and Principal Component Analysis (PCA). Furthermore, the Absolute Principal Component Scores-Multiple Linear Regression (APCS-MLR) model and PHREEQC geochemical simulations were applied to quantify the contributions of different ion sources and assess mineral dissolution and precipitation processes. The results show that the main ions in groundwater and surface water mainly come from rock weathering, evaporation, and human activities. The dissolution of carbonates and gypsum is the source of Ca