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.

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 ³He/⁴He 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–HCO₃ (SO₄) evolved geochemically from stream water and groundwater containing Ca–HCO₃. With respect to δ³⁴S, there are two types of thermal water: low temperature with low δ³⁴S (−3.00~+1.00‰), and high temperature with high δ³⁴S (+4.60~+15.0‰), which is enriched by the kinetic fractionation of H₂S. 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.