[Chemical Evolution and Formation Mechanism of Groundwater in Hetao Irrigation Area]

Groundwater resources are important sources of water in the arid region of northwestern China, but their overexploitation and utilization has led to a series of ecological and environmental problems. Exploring the characteristics and mechanism of groundwater chemical evolution is important for the rational use of groundwater resources. The characteristics of groundwater chemical evolution were studied in the Yongji Irrigation Area of Hetao Irrigation District and the formation mechanism of the chemical compounds in groundwater were investigated using cluster analysis, factor analysis, and other statistical methods. The influence degree of different factors was calculated. The results showed that the major cations in groundwater in the study area were Na⁺ and K⁺, and the major anions were Cl^- and HCO₃^-. Moreover, Na⁺, K⁺, and Cl^- showed high spatial variability and were the main factors contributing to groundwater salinization. The major chemical compounds in the groundwater in the study area were Cl-Na, HCO₃ ·Cl ·SO₄-Na, and HCO₃-Na. Based on the cluster analysis results, the groundwater was divided into four categories (A₁, A₂, B₁, and B₂), of which A₁ was highly mineralized by Cl-Na type water, while A₂, B₁, and B₂ were mainly HCO₃ ·Cl ·SO₄-Na and HCO₃-Na type water. Principal component analysis results suggest that groundwater chemistry was mainly affected by salinization, carbonate karstification, and human activities with the influence degrees of 45.976%, 23.853% and 16.678%, respectively. Evaporation, salt rock dissolution, and cation exchange were important sources of Na⁺ and Cl^- accumulation in the irrigation area. Agricultural irrigation (leaching of soil salts) and drought (intense transpiration) were the key drivers of groundwater salinization in the irrigation area.