Mixing mechanisms control the hydrogeochemical composition in medium-low enthalpy geothermal areas

Abstract

China has abundant medium-low enthalpy geothermal resources, primarily distributed across sedimentary basins, coastal regions, and mountainous fault zones. The Wugongshan area is a typical medium-low enthalpy geothermal field that has attracted considerable attention recently due to its widespread geothermal resources. Previous studies have investigated the hydrochemical evolution and formation mechanisms of hydrothermal systems in the Wugongshan area, and significant mixing processes have been found. However, the effects of these mixing processes on the geothermal reservoir temperature and the hydrochemical evolution in the area have not yet been investigated in detail, which has limited understanding of the hydrothermal system in this area. The integrated multicomponent geothermometry (IMG) method was employed in the present study to estimate the reservoir temperatures, effectively mitigating inaccuracies caused by mixing processes. The results showed that the geothermal fluid in the Wugongshan area comprised 26–82 % initial geothermal fluid and 18–74 % cold water. Ancient meteoric water was the principal source of geothermal fluids, and the isotopic signatures of geothermal fluids and shallow cold water became increasingly similar as more shallow cold water was mixed in. Additionally, the influx of shallow cold water, which carried CO₂ and O₂ from the soil, enhanced the weathering of silicate minerals and the oxidation of sulfide minerals in the research area, altering the concentrations of HCO₃^–, SO₄^2– and trace element concentrations in the geothermal fluid. The results also revealed that the fault zones play a crucial role in facilitating the circulation and upward migration of deep geothermal fluids, and the mixing processes are a key mechanism driving geochemical changes in the geothermal waters.