Multi-isotope characteristics and residence time of saline geothermal waters in the coastal area of Xiamen bay, southeast China

Study focus: The formation and circulation of saline geothermal water in coastal areas are crucial for the application of geothermal resources; however, its residence time and interaction with seawater are poorly understood. This study measured chemistry and multiple isotopes of the geothermal water samples collected from granite-hosted coastal areas of Xiamen bay, southeast China, aiming to ascertain the age of geothermal water and its evolution in the seawater recharged system. New hydrological insights for the region: Saline geothermal waters of Cl-type are observed along the coast. The enriched water isotopes in these waters and the co-variation of Cl - and δ 18 O indicate the marine origin of salinity. Cation exchange, sulfate reduction and chloritization subsequently modified the geothermal water. The results of the helium and neon isotopes indicate that He of geothermal water of this area is predominantly crustal radiogenic with a slight mantle contribution of 1 % - 6.4 %. Carbon-14 isotope dating shows that the age of fresh geothermal water is estimated to be 571–10082 years in mountain areas and 3500–17000 years in coastal areas, implying a paleo-recharge in the fracture system. Mixing with ancient seawater trapped in the fractures during Holocene and Late Pleistocene locally was identified. We emphasize the necessity of long-term monitoring on geothermal water salinity to identify whether ancient seawater is finite volume of storage or connects to the present seawater, which is significant for the scientific utilization of geothermal resources. • Saline geothermal water resulted from mixing with ancient seawater. • Cation exchange, sulfate reduction and chloritization modified geothermal water. • Helium is primarily crustal radiogenic with a slight mantle contribution. • Residence times of mixed geothermal water were estimated using 14 C isotope. • Holocene seawater was identified to be trapped in a fracture groundwater system.