| Summary: | Thermal water from the hot springs around Bugok, South Korea, has the highest discharge temperature (78 °C), and the source of that heat is of primary interest. The key <sup>3</sup>He/<sup>4</sup>He ratio runs along a single air-mixing line between the mantle and the crust, with the latter accounting for 97.0–97.3%. This suggests that the thermal source is radioactive decay in granodiorite, rock that intruded beneath the Cetaceous era sedimentary rock. Thermal water containing Na–HCO<sub>3</sub> (SO<sub>4</sub>) evolved geochemically from stream water and groundwater containing Ca–HCO<sub>3</sub>. With respect to δ<sup>34</sup>S, there are two types of thermal water: low temperature with low δ<sup>34</sup>S (−3.00~+1.00‰), and high temperature with high δ<sup>34</sup>S (+4.60~+15.0‰), which is enriched by the kinetic fractionation of H<sub>2</sub>S. The thermal water samples, except for a few, reached partial chemical equilibrium. The thermal reservoir temperatures were estimated as in the range of 90–126 °C by the K–Mg geothermometer of Giggenbach and the thermodynamic equilibrium of quartz and muscovite. This study suggests a conceptual model for the formation of geothermal water, including the thermal reservoir in the Bugok area.
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