Determining the origin, circulation path and residence time of geothermal groundwater using multiple isotopic techniques in the Heyuan Fault Zone of Southern China

Medium–low temperature geothermal resources are abundant in Southern China, but their heat source and link to fault zones is poorly understood. Consequently, geothermal energy is only used at small scale. In order to broaden the footprint of geothermal energy use in Southern China, it is first necessary to understand and track the geothermal groundwater circulation pattern associated with fault zones. The Heyuan Fault Zone serves as a typical medium–low temperature geothermal system of South China. Here we show that the geothermal groundwater circulation pattern can be traced by using stable hydrogen and oxygen isotopes, helium and neon isotopes, carbon-13 and carbon-14 as well as hydrochemical parameters. The results show that: in the Heyuan Fault Zone, the main hydrochemical type of the hot springs is HCO3-Na + K, while the shallow cold groundwater is mainly enriched in HCO3-Ca. The results of the helium isotope and neon isotope indicate that the geothermal groundwater of this area is derived from the crust, thus excluding the possibility of extremely deep paths of groundwater tapping into mantle helium sources. Furthermore, the characteristics of the stable hydrogen and oxygen isotopes imply that geothermal groundwater is of local meteoric origin, and the recharge area located at the hilly area of the hanging wall of the Heyuan main fault, with the recharge elevations ranging from about 440 m to 670 m. The hot spring geothermometer shows that the highest reservoir temperature is about 157 °C, and the deepest circulation depth is about 6500 m. Carbon-14 isotope age dating suggests that the geothermal groundwater ages are mostly from 9.9 kyr BP to 12.3 kyr BP. According to the geological structural characteristics of the study area, the main upward channel for geothermal groundwater is the Heyuan main fault. Generally, the hot springs in this area are mixed with shallow cold groundwater and surface water, which raises the ratio of the Ca2+ in water and dilutes the heavy hydrogen and oxygen isotopes.