[Hydrochemical Characteristics and Formation Causes of Ground Karst Water Systems in Gudui Spring Catchment]

As one of the famous karst springs in Shanxi Province, the Gudui spring is the only medium-low temperature hot spring, with a long history of development and a rich cultural accumulation. The karst groundwater in the Gudui spring catchment was taken as the research object. Through systematic sample collection and isotope analysis, hydrochemistry (Durov map, ion ratio, Gibbs map, and hydrogen and oxygen isotope) methods were comprehensively used to analyze groundwater hydrochemistry and groundwater system runoff characteristics. The⁸⁷Sr/⁸⁶Sr value of karst groundwater in the Gudui spring catchment was 0.709 to 0.717, and the Mg/(Mg+Ca) value was 0.27 to 0.74. By analyzing the Sr isotope composition and Mg/(Mg+Ca) and 1/Sr variation characteristics, it was concluded that the karst groundwater in the Gudui spring catchment was a mixture of deep hot water and shallow cold water. The karst water subsystem of Nanliang spring presented the characteristics of carbonate stratum runoff. The karst water subsystem of Fuling Mountain Gaoxian Haitou spring and the deep circulation subsystem of Houma Basin exhibited the runoff characteristics of carbonate rock and igneous rock strata. The karst water subsystem of Taiershan Jiuyuanshan Gudui spring presented the runoff characteristics of carbonate rock and ancient silicoaluminate strata. The δ¹⁸O value in karst groundwater of Guodui spring area ranged from -11.46‰ to -7.81‰, and the average value was -10.08‰. The range of the δD value was -83.7‰ to -60.8‰, and the average value was -73.6‰. This showed that karst groundwater in the spring area was the result of mixing of various types of water. Through comparative analysis of hydrogen and oxygen isotopes of 2014 and 2021 sampling points at the same location, it was concluded that the change in water samples at the Guduiquan resulted from the gradual accumulation of water supplied by Sanquan Reservoir. The change in Sanquan Reservoir was due to the influence of Yellow River diversion. The karst groundwater in the spring area were characterized by large calcium ion, magnesium ion, and sodium ion values; a small potassium ion value; a large sulfate value; and a small chloride value. The hydrochemical types of karst groundwater in Gudui spring catchment could be divided into SO₄-Na, SO₄-Ca, HCO₃-Na, HCO₃-Mg, HCO₃-Ca, and Cl-Na. The hydrochemical types of karst groundwater showed evident hydrochemical composition zoning from HCO₃-Ca·Mg→HCO₃·SO₄-Ca·Mg→SO₄·HCO₃-Na·Ca→SO₄·Cl-Na·Ca. According to the comprehensive analysis of hydrochemical isotope and hydrogeological conditions, the karst water subsystem of Nanliang spring was primarily recharged by rainfall infiltration in the exposed limestone area and river infiltration, and its karst groundwater was recharged by runoff from south to north to the karst water subsystem of Fuling Mountain Gaoxian Haitou spring and the deep circulation subsystem of Houma Basin. The karst water subsystem of Taier Jiuyuan Mountain Gudui spring received rainfall infiltration supplement and upstream runoff supplement from the exposed limestone area. Its karst groundwater flowed from north to south and received the supply of Sanquan Reservoir from Yellow River water in the natural discharge area of Gudui spring.