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The Magumsan thermal waters of the southeastern Korean Peninsula are pumped out of six deep wells (average depth, 300 m) at temperatures of 30.8−49 °C. The thermal waters are chemically classified into two groups: NaHCO₃ type (<31 °C) and NaCl (HCO₃, SO₄) type (>40 °C), both of which have chemical compositions that are distinct from local groundwater (Ca−HCO₃ type). δ¹⁸O and δD values suggest that the thermal waters originate from meteoric water and they are isotopically fractionated by silicate hydration or H₂S exchange. δ³⁴S values (+7.0 to +15%) of dissolved sulfate in the thermal waters reflect enrichment in ³⁴S through kinetically controlled oxidation of magmatic pyrite in the thermal aquifer and mixing with paleo-seawater. On the ³He/⁴He vs. ⁴He/²⁰Ne diagram, the thermal waters plot along a single air mixing line of dominant crustal He, which indicates that the heat source for the thermal waters is non-volcanogenic thermal energy that is generated from the decay of radioactive elements in crustal rocks. Chalcedony geothermometry and thermodynamic equilibrium calculations using the PHREEQC program indicate a reservoir temperature for the immature thermal waters of 54−86 °C and 55−83 °C, respectively.