Notably high salinity of geothermal water in the coastal area in Hainan Island, China, predominantly dominated by tectonic activities

Notably high levels of salinity were detected in the geothermal water of the coastal area on Hainan Island. The underlying mechanism of this phenomenon was identified by analysing the geochemical and isotopic data from geothermal water samples, which is crucial for the development and utilization of these geothermal resources. From the acquired results, it was demonstrated that the geothermal water samples with obviously higher concentrations of Na⁺, Ca²⁺, and Cl⁻ were concentrated in the coastal areas. In addition, the Ca²⁺ concentration in some geothermal water samples was even higher than that of the seawater. On the contrary, a negligible difference between the other components and hydrogen-oxygen isotopic ratios was observed in these geothermal water samples, compared to the other geothermal water samples in Hainan Island. These outcomes contradicted the assumption of seawater infiltration into the geothermal water in the coastal areas. In our study, we revealed that the halite-water interaction could have dominated the notably high salinity of geothermal water in the coastal area, and four stages of the interaction were illustrated. Further study indicated that the enhanced water-rock interactions by the tectonic activities could be responsible for the distinct high concentrations of Na⁺, Ca²⁺, and Cl⁻ in geothermal water at the intersection of faults in the coastal areas, where concentrated earthquakes occurred. The preliminary findings suggested that monitoring Na⁺, Ca²⁺, and Cl⁻ concentrations in geothermal water within the coastal regions is a promising method for regional earthquake prediction.