The mechanism of high-salinity thermal groundwater in Xinzhou geothermal field, South China: Insight from water chemistry and stable isotopes

Abstract

The high-salinity geothermal water in the coastal zone is naturally considered to be formed by seawater intrusion. However, this is not all the case. Water chemistry and stable isotopes of thermal and non-thermal groundwaters are used to investigate the hydrothermal circulation in Xinzhou geothermal field, located near the coast of South China. Thermal groundwaters are uniformly Cl-Na type with higher salinity (TDS > 1700 mg/L) but non-thermal groundwaters regarded as shallow groundwater have various chemical types with lower salinity (TDS < 180 mg/L). The waters are presumably of meteoric origin according to δD and δ18O. Cation geothermometers (Na-K-Mg, Na-K-Ca-Mg, Na-K, K-Mg, Na-K-Ca) and SiO2-chalcedony geothermometers failed to predict the suitable reservoir temperature. According to SiO2-Quartz geothermometer with maximum steam loss and Multicomponent chemical equilibrium, the maximum temperature in the reservoir is estimated to be in the range of 127.1 °C to 154.0 °C at a maximum circulation depth of 4.34 km. The saturation index (SI) simulated by PHREEQCI and Na/1000-K/100-Mg1/2 triangle plot suggests that the chemical components in the thermal groundwater are possibly derived from water-rock interactions at high temperature. As suggested by the ion ratios, these interactions may be dominated by salty rock dissolution which is mostly related to the layer of salty rock formed by evaporation generally existing within 200 m depth in West Guangdong. The thermal groundwater flow in the deep faults at the average horizontal 3.4 × 10−3 m/d and the average vertical 3.0 × 10−3 m/d. The ascending movement of thermal groundwater is speculatively driven by thermodynamic buoyancy in the conceptual circulation model.