Abstract: The seismic observation wells in Baodi area, Tianjin, have been the precursor sensitive wells in the Beijing-Tianjin-Hebei region, but their hydrogeochemical characteristics is less studied. In this paper, three seismic observation wells on the north and south sides of the Baodi Fracture were selected to analyze the ionic component concentrations, hydrogen and oxygen isotope compositions, silica and carbon isotope contents, and to carry out hydrogeochemical characteristics, and the results show that: ① The hydrochemical types of Baodi Xin well and the surrounding cold-water wells in the northern part of the Baodi Fracture were dominated by Na-SO₄-HCO₃ types, and the hydrochemical types of Wang 3 well, Wang 4 well and the surrounding geothermal wells in the southern part of the Baodi Fracture were dominated by Na-HCO₃ types. Wang 3 and Wang 4 wells in the southern part of Baodi Fracture and the surrounding geothermal wells are dominated by Na-HCO₃ type of water chemistry. The source of groundwater is atmospheric precipitation, and the recharge area is the northern Yanshan Mountains. The cold water and geothermal water on both sides of the Baodi Fracture are located in the immature water zone, indicating that both of them are mixed by shallow cold water in the upward transportation process. The depth of geothermal water circulation on the south side of the fracture is significantly higher than that of the cold water on the north side, and the Cl^- of geothermal water has a deep origin. The inorganic carbon isotopes of the cold water and the geothermal water show zoning characteristics. The carbonate minerals in the cold water on the north side of the fracture are not saturated, and the carbon source of the geothermal water on the south side of the fracture is the leaching of carbonate minerals from the recharge zone and the dissolution of the carbonate aquifer. ④ Although the recharge zones of cold water and geothermal water on both sides of the Baodi Fracture are the same in the northern Yanshan Mountains, different types of groundwater on both sides of the fracture are formed due to the differences in hydrogeological conditions, circulation paths, and circulation depths at the locations of the observation wells, as well as the controlling effect of the Baodi Fracture on the hydrological and geochemical evolution of the groundwater on both sides of the fracture. The results of this paper enrich the geochemical background field data in the Beijing-Tianjin-Hebei region, and provide a scientific basis for exploring the geochemical characteristics of regional subsurface fluids.

Key words: Hydrogeochemistry, Ionic fractions, Thermal storage temperature scale, Carbon isotopes, Genesis models