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

The water of the Longzici Spring catchment is the source of industrial, agricultural, and urban domestic water in Linfen City, Shanxi Province. It is of great significance to identify the sources and influencing factors of karst water resources for their development and utilization, and the protection of the ecological environment. Taking the karst groundwater in the Longzici Spring Area as the research object, systematic sample collection and isotope analysis were carried out. Hydrochemistry methods (Durov diagram, ion ratio system, Gibbs diagram, hydrogen and oxygen isotopes, and sulfur isotopes) were used to analyze the hydrochemical characteristics of groundwater. The hydrogeochemical and environmental isotopic characteristics of Longzici Spring water are revealed. The hydrochemical field of karst groundwater in the Longzici Spring domain has obvious zonation, from the deep buried detention area to the supply area to the runoff and discharge area. The hydrochemical types of karst groundwater detention area are SO₄-Ca and SO₄-Ca·Mg, with total dissolved solids (TDS) of 960.5-2340 mg·L^-1; the supply area is HCO₃-Ca·Mg and HCO₃·SO₄-Ca·Mg, with TDS of 340.5-562 mg·L^-1; and the runoff and discharge areas are SO₄·HCO₃-Ca·Mg·Na, SO₄·HCO₃-Ca, and SO₄·HCO₃-Ca·Mg, with TDS of 494.0-1168.5 mg·L^-1. The karst groundwater is mainly SO₄·HCO₃-Ca and SO₄·HCO₃-Ca·Mg type water. The content range of sulfate ions is 61.6-1503 mg·L^-1, with an average of 481.4 mg·L^-1. A total of 70.3% of the karst groundwater SO₄^2- samples exceed the standard. The sulfate ions in karst groundwater mainly come from the dissolution of gypsum in the Ordovician aquifer and the oxidation of pyrite in a coal aquifer. The ratio of sulfate ions in the Longzici Spring catchment that come from pyrite oxidation in water is 20.2%. According to the analysis of hydrogen, oxygen and sulfur isotopes, 70% of karst groundwater is polluted by the acid water of coal mines to different degrees. Using the software PHREEQC, it is concluded that the hydrochemical characteristics of the study area are mainly affected by dissolution and dedolomitization, rock salt, and gypsum.