Comparative study on genesis mechanism of high arsenic groundwater in typical alluvial plain of the Upper and lower Yellow River, China.

Groundwater with naturally high‑arsenic (As) concentrations is a pervasive issue across several major plains (basins) traversed by the Yellow River in China. The genesis of this high-As groundwater and its interrelationships among different plains (basins) have consistently been focal and challenging topics for domestic and international experts. The study focuses on the Hetao Basin in the upper Yellow River and the North Henan Plain in its lower reaches. Selecting typical profiles within the study area, a comparative analysis is performed based on the hydrochemical and isotopic signatures of these profiles. This analysis elucidates the distribution patterns, formation environments, and hydrogeochemical evolution models of high-As groundwater under the influence of different sedimentary environments. The results demonstrate that the shallow groundwater in the upper Yellow River is progressively salinizing, and conversely, the lower reaches are exhibiting a gradual alkalization. Redox conditions, the degree of evaporative concentration, and the intensity of groundwater recharge can influence the distribution disparities of high-As groundwater in the upper and lower reaches of the Yellow River. High-As groundwater in the Hetao Basin of the upper Yellow River is predominantly affected by the strength of groundwater reduction. Conversely, in the North Henan Plain of the lower Yellow River, the chemical conditions of the groundwater are more intricate, with As enrichment being influenced by a confluence of factors such as redox conditions, evaporative concentration, and the intensity of groundwater replenishment.