Hydrochemical Characteristics and Water–Rock Interaction of Typical Geothermal Reservoirs in Northern China: A Case Study from Tianjin Geothermal Field

Tianjin, nestled on the North China Plain, possesses abundant geothermal resources with tremendous potential for development and utilization. This study employs hydrogeochemical and isotopic analysis techniques to thoroughly explore the geochemical characteristics and circulation patterns of geothermal fluids in Tianjin, shedding light on the mechanisms underlying the formation and evolution of deep geothermal fluids. The findings reveal that atmospheric precipitation serves as the primary recharge source for the region’s geothermal fluids, with the calculated recharge heights coinciding with those of the Jixian mountainous area. This precipitation infiltrates through permeable layers and the deep, large faults surrounding the southern plain, entering relatively enclosed or semi-enclosed geothermal reservoirs. As they circulate, the geothermal fluids undergo intricate interactions with the surrounding rocks, including processes such as leaching, adsorption, carbonate reprecipitation, cation exchange, and decarbonation. The fluids circulate at depths ranging from 1.6 to 3.5 km, with temperatures spanning from 67 to 133 °C. Along the flow path, the anionic composition of the geothermal fluids shifts from HCO3− dominance in the north to a coexistence of Cl− and SO42−, ultimately dominated by Cl− in the south, accompanied by an increase in total dissolved solids (TDS). The results indicate that Tianjin geothermal fluids are mainly recharged by meteoric water and evolve along their flow paths through dissolution of evaporitic and carbonate minerals, cation exchange, and carbonate precipitation. Hydrochemical and Sr-isotope differences suggest generally limited vertical connectivity among the studied reservoirs, although local hydraulic interaction may occur near conductive faults. These results provide constraints on the hydrogeochemical evolution and management of geothermal resources in the Tianjin sedimentary basin.