A hydrogeochemical and isotopic approach for assessing the origin, recharge and mixing process of geothermal water in the Yinchuan Basin

The hydrogeochemistry of geothermal fluids is fundamental to reveal the genesis, recharge mechanism and circulation pattern of geothermal water. However, the origin and hydrogeochemical process of geothermal water in deep aquifer system still remain unclear. In this study, 17 water samples were analyzed to study the origin, recharge and mixing process of geothermal water in the Yinchuan basin by using a hydrogeochemical and isotopic approach. The results showed that the concentrations of major ions (SO₄²⁻, Na⁺, Cl⁻, TDS, Ca²⁺, K⁺, Mg²⁺ and NH₄⁺) and trace elements (Li, F⁻, Br⁻, I⁻, Sr and Mn) in geothermal water are significantly greater than those in shallow water, hot spring and cold spring in the study area. The hydrochemical type of geothermal water is dominated by Cl·SO₄-Na, which is mainly influenced by dissolution of halides, chlorides and sulfates under strong fluid-rock interactions. The isotope analysis demonstrated that the atmospheric precipitation in the Helan Mountain area is the major recharge source of geothermal water, the recharge elevation is 1118 m–1133 m, and the deep geothermal water is formed by a mixing process of ancient precipitation and modern precipitation. The silica-enthalpy mixing model suggested that the reservoir temperature of deep geothermal fluid is between 110 °C and 175 °C, and the mixing ratio of cold water is about 54 % to 92 %. The present study sheds some light on the genesis, recharge mechanism and hydrogeochemical evolution of geothermal water in deep aquifers, which are vital for sustainable exploitation and utilization of geothermal resources.