Geochemical and Isotopic Techniques Constraints on the Origin, Evolution, and Residence Time of Low-enthalpy Geothermal Water in Western Wugongshan, SE China

Geothermal resources are increasingly gaining attention as a competitive, clean energy source to address the energy crisis and mitigate climate change. The Wugongshan area, situated in the southeast coast geothermal belt of China, is a typical geothermal anomaly and contains abundant medium- and low-temperature geothermal resources. This study employed hydrogeochemical and isotopic techniques to explore the cyclic evolution of geothermal water in the western Wugongshan region, encompassing the recharge origin, water-rock interaction mechanisms, and residence time. The results show that the geothermal water in the western region of Wugongshan is weakly alkaline, with low enthalpy and mineralization levels. The hydrochemistry of geothermal waters is dominated by Na-HCO3 and Na-SO4, while the hydrochemistry types of cold springs are all Na-HCO3. The hydrochemistry types of surface waters and rain waters are Na-HCO3 or Ca-HCO3. The delta D and delta O-18 values reveal that the geothermal waters are recharged by atmospheric precipitation at an altitude between 550.0 and 1218.6 m. Molar ratios of major solutes and isotopic compositions of Sr-87/Sr-86 underscore the significant role of silicate weathering, dissolution, and cation exchange in controlling geothermal water chemistry. Additionally, geothermal waters experienced varying degrees of mixing with cold water during their ascent. The delta C-13 values suggest that the primary sources of carbon in the geothermal waters were biogenic and organic. The delta S-34 value suggests that the sulfates in geothermal water originate from sulfide minerals in the surrounding rock. Age dating using H-3 and C-14 isotopes suggests that geothermal waters have a residence time exceeding 1 kaBP and undergo a long-distance cycling process.