Geochemical evolution of geothermal waters in the Pearl River Delta region, South China: Insights from water chemistry and isotope geochemistry

The Pearl River Delta (PRD) region in South China is abundant of geothermal water resources whereas geochemical evolution of the geothermal water is still poorly understood. In this study, geothermal water is sampled to investigate the provenance of CO2 and sulphate, residence time, recharge elevation, circulation depth, and water-interaction processes by analyzing chemistry of major ions and isotope geochemistry. The range of delta 13C and content of bicarbonate reveal multiple sources of soil CO2, carbonate rocks, and small amounts of mantle CO2. The delta 34S ratio and sulphate concentration are attributed to evaporites dissolution, input of atmospheric sulphur, seawater intrusion, and oxidation of pyrite. The residence time of up to 21.01 ka for geothermal water likely suggests the earliest precipitation recharge during the Late Pleistocene. Mountains or/and hills at an altitude of 300-760 m in north PRD region are currently the major recharge areas for the geothermal water which can circulate at the depths of up to 3000-4800 m along regional structures. The isotopic and hydrochemical analysis confirms that extensive water interaction with silicate rock, carbonate rock and evaporite, evaporation-crystallization process, and cation exchange are the key mechanisms that control the geochemical evolution of geothermal water in the PRD geothermal system.