Hydrochemical and Isotopic Evidence of Origin of Thermal Karst Water at Taiyuan, Northern China

Thermal karst groundwaters with temperatures ranging from 32.8 to 62.5 ℃ were found at Taiyuan (太原) City, the capital of Shanxi (山西) Province. To identify the origin of the thermal groundwater, the following tracers were used in this study: δD, δ¹⁸O, ⁴He, ³He/⁴He, and major chemical constituents in water. Hydrochemical and isotopic data indicate that the thermal groundwaters in the basin area are a mixture of thermal waters from the West Mountain and those from the East Mountain. Furthermore, the ⁴He and ⁴He_exc concentrations of the thermal groundwater samples are usually lower than those of the cold groundwater samples, and there is an evidently negative correlation between the temperature and the ⁴He concentration in thermal groundwaters from the West Mountain and the basin, which means that with the increase in temperature, the He concentration increases in the vapor phase and decreases in the aqueous phase. In the plot of ³He/⁴He vs. ⁴He/²⁰Ne of all water samples: air, crust, and mantle, all thermal groundwater samples are distributed near the line between the point of air and that of crust, suggesting that atmospheric and crustal helium is the main source for that in thermal groundwaters. In other words, there are no mantle-derived fluids mixed in the thermal groundwaters.