This paper is the result of mineral exploration engineering.

Objective In order to study the hydrochemical characteristics and evolution of groundwater in the Ejina plain, Inner Mongolia, 87 water quality samples and 69 hydrogen and oxygen isotope samples were collected in August 2020.

Methods Based on the methods of mathematical statistics, ion ratio, and hydrogeochemical simulation, the hydrogeochemical characteristics and the hydrochemical evolution rule of groundwater stored in Quaternary aquifer and Cretaceous aquifer were analyzed.

Results The results showed that: (1) The groundwater was mainly SO₄-Na type. SO₄^2- was the main anion, followed by Cl^-. Na⁺ was the main cation, followed by Ca²⁺ and Mg²⁺, there was little difference between Ca²⁺ and Mg²⁺ concentrations. (2) The concentrations of SO₄^2-, Cl^-, TDS, total hardness, Na+ and Mg²⁺ decreased in the order of Quaternary confined water> Quaternary phreatic water> Cretaceous confined water. (3) Quaternary phreatic ions were mainly controlled by leaching and mixing process, and some areas were significantly affected by evaporation. The ions of Quaternary confined water were mainly controlled by leaching and cation exchange. The ions of the Cretaceous confined water in the northern part of the plain were controlled by leaching and cation exchange. The main ions of groundwater in the study area came from the dissolution of halite, carbonates and gypsum.

Conclusions Along the direction of groundwater flow, the concentration of ion components in Quaternary phreatic showed an increasing trend. In the Quaternary phreatic and confined water in the Ejina plain, the main water- rock interaction processes were dissolution of halite, dolomite and gypsum, precipitation of calcite, and positive cation exchange. In the interlaced zone between the desert and the plain in the southeast of the study area, the dissolution of halite, dolomite and gypsum, the precipitation of calcite, and reverse cation exchange mainly occurred. Halite, dolomite and gypsum dissolution, calcite precipitation, and positive cation exchange mainly occurred in the Cretaceous confined water in the northern plain.