Refining the origin of thermal water using stable isotope, hydrochemistry and GIS approaches in Jiaoxi, Taiwan

Comprehending the recharge of thermal springs is crucial for preserving the water quality of such springs. However, limited groundwater well data in mountainous areas presents a challenge to determining thermal-spring recharge areas. This study analyzes the isotopes (delta H-2 and delta O-18) and major ions in rainwater, streamwater, mountain groundwater and thermal spring water in Jiaoxi, located near Taiwan's largest city, Taipei. The results of the isotope-elevation relationship and GIS recharge analysis indicate that the thermal spring water originated from a remote mountain at elevations of approximately 1170-1480 m, rather than the upstream watershed. The thermal spring water was divided into two groups based on the carbonate saturation index (SI) and isotope composition: the foothill group, which had a positive SI and lower delta value, and the downstream group, which had a negative SI and higher delta value. This study improved the conceptual model of the thermal flow path by demonstrating that the recharge of thermal water is a transboundary regional flow and that the foothill thermal water does not mix with the meteoric water. The revised model also highlights the role of structural lineament trends (faults, fractures and folds) in the movement of regional flow in fractured mountain blocks.