Geochemical Composition, Source and Geothermometry of Thermal Water in the Bugok Area, South Korea

Abstract

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.