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 (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.

The purpose of this study is to delineate the main contributions controlling the chemical composition of groundwater and spring water samples at Jeju Island, South Korea. Groundwater and spring water can be grouped as three chemical types: Fresh water (Ca, Na-HCO₃), sea water mixing (Na-Cl, SO₄) and contamination (Ca-NO₃, Cl). By factor analysis, three factors can explain about 87% of chemical variances: Factor 1 (Na⁺ and Cl⁻), factor 2 (NO₃⁻), and factors 3 (HCO₃⁻), which are regarded as indicative ions of sea water mixing, contamination and mineral-water interaction, respectively. Sea water intrusion in the southeastern area of Jeju Island is due to downward distribution of Seogwipo Formation, which plays a role as an aquiclude preventing sea water intrusion in other areas, under sea level at the southeastern coastal area. Nitrate contamination is likely to be affected by pollution sources, i.e., agricultural activities, horse farms and municipal sewage. Thermodynamic saturation index indicates that most of waters are undersaturated with respect to major minerals, which may result from insufficient reaction time within a permeable aquifer. The unsaturation state corresponds to low load scores of Ca, Mg, Na, K, and SiO₂ in factor 3, which means a low contribution of mineral-water interaction.