Hydrochemical zonation and geochemical processes of geothermal water in Beijing

Geothermal resource constitutes a vital clean energy in Beijing, playing a pivotal role in advancing the city's 'dual carbon' target. However, the hindrance to the widespread utilization of geothermal resources in Beijing lies in the idleness of over 400 geothermal wells, primarily due to an insufficient understanding of geothermal geological conditions. To address this, we integrated geochemical data from 149 geothermal wells across the city with geological and seismic data, and explored the processes and controlling factors influencing the hydrochemical zonation within the karst thermal reservoir. Our findings are as follows: 1) The geothermal water exhibits distinct zonation in terms of water types and ion content, with high SO₄^2- levels (≥100 mg/L, up to 1420 mg/L), high F^- levels (≥7 mg/L, up to 30 mg/L), and high Cl^- levels (≥300 mg/L, peaking at 3444 mg/L) distributed in strips, areas, and spots, respectively. 2) The dissolution of sulfate-bearing rocks and fluorite is a crucial factor controlling the high concentrations of SO₄^2- and F^- in geothermal water. The NY-TX fault, which serves as the boundary of the geothermal field, and the active fault indentified through seismic activities within the geothermal field, are significant factors influencing the zonation. 3) Geothermal water in Beijing is a mixture of atmospheric precipitation and formation water. From a planar perspective, three subsystems of geothermal water circulation can be identified in the kasrt reservoir. GF8-2 can be regarded as an independent geothermal water system, characterized by Cl^- and TDS concentrations that reach 3444 mg/L and 6286.7 mg/L, respectively, suggesting minimal influence from atmospheric precipitation. This insight has significant implications for guiding the large-scale utilization and management of geothermal resources in the capital city.