Preliminary Analysis Of Hydrogeochemical Characteristics And Origin Of Geothermal Water In Hexigten Global Geopark?North Eastern China

The Hexigten Hot Water Development Zone is located in the northeast of Hexigten Banner,Inner Mongolia Autonomous Region.It is famous for the hot springs,which are the second largest class in the country.The total area of the Development Zone is 6 km²,and the built area is about 3.8 km².Up to now,4 hot water wells have been successfully drilled in the zone,the depth of the wells is 60 to 150 meters,the temperature of the geothermal water is 43-66.5°C,and the annual production is about160000 tons.With the development of hot spring tourism in the development zone,the geothermal water has an accelerating trend of decline in water temperature,water level and water quality.It is urgent to find out the circulation conditions and causes of geothermal system in the development zone and to evaluate geothermal water resources for the protection of hot water resources.Through field investigation and water sampling tests,the data of chemical composition and isotopic concentrations of D,¹⁸O,³⁴S,⁸⁷Sr/⁸⁶Sr,³H and ¹⁴C in 7 water sampling sites?4 hot water wells,2 diving wells and 1 surface water?were obtained.By using the geochemistry method and FLOWPC,PHREEQC,SPSS and other analytical software,the hydrochemical and isotopic characteristics of groundwater in the development area are discussed,the reservoir temperature of geothermal field is estimated,and the origin of the middle and low temperature geothermal system in the area is preliminarily analyzed.According to the research results,the groundwater in the phreatic aquifer of the development zone are mostly HCO₃-Ca type or HCO₃-Na,Ca type.The water temperature is 16-20°C,and the water flow through the surrounding rock lithology as gravel pebbles,sand pebbles and granite near the surface.The ³H age of unconfined groundwater is about 76a,and the update rate is 1.32%/a.The deep geothermally confined water differs according to the location of the fault zone.The groundwater in the main fault zone is HCO₃,Cl.-Na type,and the groundwater in the secondary water-conducting fault zone is mostly HCO₃,SO₄,Cl-Na type.The temperature of geothermal water is 43-66.5°C,and the water flow through the surrounding rock is fractured granite which is affected by hydrothermal alteration.Deep hot water ¹⁴C calculated age4ka-30ka,renewal rate 0.03-0.25‰/a.There is a strong mixing between normal temperature water and geothermal water in the development area.The ratio of normal temperature phreatic water mixing is between 0.35 and 0.72.The upper limit of mixed water ³H age is between 74a and>81a,and the renewal rate is 1.23-1.35%/a.The formation process of hot water can be interpreted as:the deep circulation of meteoric water seeps through deep water-conducting faults such as 09-F2,09-F3,etc.In the course of runoff,it continuously absorbs the heat of the deep geothermal fluid in the earth's crust and forms the middle-low temperature geothermal water,which eventually occurs in the closed reduction environment of the fractured granite intrusive thermal reservoir.It is maintained under a blanket of clayey sand and its hydrothermal alteration.Based on the above analysis results,it is concluded that the genesis of the geothermal field is mainly affected by the fault structure and is a typical fault-controlled mid-low temperature geothermal system.