Geochemical characteristics and formation mechanisms of the seawater-recharged geothermal systems in Yantian of Fujian, China

Seawater-recharged geothermal systems are characterized by abundant recharge, low temperature and salinization. Ascertaining the circulation recharge conditions and genetic mechanisms of geothermal water in coastal areas of southeast China is of important significance in the rational exploration, utilization and protection of geothermal resources in these areas. In this study, 14 samples of geothermal water, groundwater and seawater are collected from the Yantian geothermal field near Guanqiao Town in Fujian Province, China and the hydrochemical and isotopic characteristics are analyzed. The circulation recharge of the geothermal water and the genetic mechanisms of the geothermal resources in the geothermal field are revealed using geothermometers. The results show that hydrochemical type of the geothermal water in the Yantian geothermal field is of Cl—Na type, which is similar to that of the seawater. Total dissolved solids of geothermal water samples H01 and H02 are 2 610 mg/L and 3 090 mg/L, respectively. By contrast, the groundwater in the geothermal field is dominated by the HCO₃—Na type, and the groundwater samples have TDS of less than 400 mg/L. Moreover, the geothermal water is rich in Br⁻, which is not detected in the groundwater. These results indicate that modern seawater or ancient seawater in marine sedimentary layers is a recharge source of the geothermal water. As shown by the results of H01 and H02 calculated using the Cl⁻ mixing model, geothermal water samples H01 and H02 have seawater mixing ratios of 9.13% and 10.76%, respectively, and H01 is mixed with more groundwater after being exposed to Quaternary sediments. The comprehensive analyses show that the geothermal water in the Yantian geothermal field is primarily recharged by seawater and its chemical composition is significantly affected by seawater mixing. Furthermore, the comprehensive analyses also suggest that the deep geothermal water is mixed with groundwater or seawater twice or more times as it rises upward, thus forming shallow geothermal reservoirs with a temperature of 89 °C to 121 °C, as estimated by using the SiO₂ geothermometer and the multi-mineral equilibrium method.