This study provides an in-depth analysis of the hydrochemical characteristics and their controlling factors in the lower reaches of the Yellow River. Through water quality sampling and analysis over two hydrological periods within a year, combined with hydrochemical methods and machine learning techniques, the study reveals the joint impact of natural factors and human activities on the spatiotemporal variations in hydrochemical constituents. The findings indicate that the water in the lower reaches of the Yellow River exhibits weak alkalinity (the pH is between 7 and 8), with the primary hydrochemical type being HCO₃·SO₄—Ca·Na·Mg. The temporal variation in the hydrochemical constituents is mainly influenced by rainfall, where nitrate levels are higher during the flood season due to the flushing effect of rainfall, whereas other hydrochemical constituents show an opposite temporal pattern due to the dilution effect of rainfall. The spatial variation in the Yellow River’s hydrochemistry is primarily controlled by a combination of human activities and rainfall. Using Gibbs diagram analysis, it is identified that rock weathering is the main source of ionic constituents, while agricultural fertilization, industrial emissions, and domestic wastewater discharge have significant impacts on the hydrochemical constituents. Compared to other rivers worldwide, the concentration of hydrochemical constituents in the lower reaches of the Yellow River is relatively high, especially nitrate and sulfate, which is closely related to the geological characteristics of the Yellow River basin and intense human activities in the middle and lower reaches. Principal component analysis reveals that the main controlling factors for hydrochemical constituents during the dry season in the lower reaches of the Yellow River are rock weathering dissolution and industrial activities, followed by domestic wastewater; during the flood season, the main controlling factors are rock weathering dissolution and industrial activities, followed by agricultural activities and domestic wastewater. The research findings provide theoretical support for water resource management and water quality protection in the lower reaches of the Yellow River.

Introduction: The Zhengzhou section of the lower Yellow River is an ecologically vulnerable region, where the groundwater chemistry has experienced significant variations due to the combined effects of natural factors and human activities.Methods: In this study, we employed methods such as the Piper trilinear diagram, Gibbs plot, ion ratio coefficient method, and multivariate statistical techniques to explore the major controlling factors influencing groundwater chemical evolution.Results: The results indicate that 46% of shallow groundwater in the intensive conversion zone between surface water and groundwater is of the HCO₃—Na·Ca·Mg water type. Influenced by lateral infiltration from the Yellow River, groundwater on both sides of the river evolves into Cl·SO₄·HCO₃ and Cl·HCO₃ types. Groundwater near the riverbanks and ponds, directly replenished by Yellow River water or by leakage, is more significantly impacted by human activities compared to other regions. Factor analysis indicates that the chemical evolution of regional shallow groundwater is primarily controlled by factors such as mineral dissolution, human activities, and Yellow River water recharge. More pronounced water-rock interactions in unconfined aquifers, leads to an increase in the concentrations of conventional components like TDS and Mg²⁺.Discussion: Due to the long-term unidirectional transformation of surface water into groundwater, SO₄^2- and Na⁺ in the groundwater are strongly influenced by surface water. NO₃⁻ is mainly affected by human activities, including domestic and agricultural activities. The concentrations of Fe and Mn are primarily influenced by the high background values of aquifer sediments and, to a lesser extent, by the significant enhancement of the aquifer’s reducing conditions, leading to higher concentrations of Fe and Mn in groundwater. The findings of this study provide theoretical and data-based support for the scientific formulation of groundwater protection and rational development measures.

Shallow groundwater in the lower Yellow River alluvial plain is influenced by river seepage and multiple human activities, yet its hydrochemical evolution mechanisms remain unexplored. To bridge this gap, our study integrates hydrochemical analysis, ion ratio plots, and multivariate statistical techniques to reveal the hydrochemical characteristics, controlling factors, and anthropogenic influence mechanisms on groundwater chemistry in the Henan section of the lower Yellow River during the wet season. The results indicate that groundwater chemistry in the lower Yellow River is strongly influenced by rock weathering and human activities. When compared with the Class III limits of GB/T 14848-2017, 42.6%, 25.7%, 20.8%, 17.8%, and 17.8% of the samples exceed the limits for TH, TDS, Na⁺, Cl⁻, and SO₄²⁻, respectively. The dominant hydrochemical facies is HCO₃-Na·Ca, accounting for 28.7% of all samples, and rock weathering constitutes the fundamental control on groundwater hydrochemistry. Within this framework, silicate weathering serves as the principal natural source for dissolved ions, while domestic sewage, industrial effluents and agricultural non-point pollution represent anthropogenic contributors to groundwater chemical composition. Principal component analysis results indicate that a mixed factor jointly influenced by evaporite–silicate weathering, evaporation–crystallization processes, and sewage discharge (44.1% contribution) is the main driving mechanism of groundwater hydrochemical variation in the wet season, while carbonate weathering (19.0%) and agricultural fertilization (13.4%) exert a comparatively minor driving influence. This study provides an in-depth understanding of the evolutionary mechanisms of groundwater chemical constituents in the lower Yellow River, offering a scientific basis for groundwater protection in this region and similar intensely human-disturbed areas.