Origin and evolution of two contrasting thermal groundwaters(CO2-rich and alkaline) in the Jungwon area, South Korea: Hydrochemical and isotopic evidence

Abstract

In the Jungwon area, South Korea, two contrasting types of deep thermal groundwater (around 20-33 degrees C) occur together in granite. Compared to shallow groundwater and surface water, thermal groundwaters have significantly lower delta O-18 and delta D values (>1 parts per thousand lower in delta O-18) and negligible tritium content (mostly <2 TU), suggesting a relatively high age of these waters (at least pre-thermonuclear period) and relatively long subsurface circulation. However, the hydrochemical evolution yielded two distinct water types. CO2-rich water (P-CO2,=0.1 to 2 atm) is characterized by lower pH (5.7-6.4) and higher TDS content (up to 3300 mg/L), whereas alkaline water (P-CO2=10(-4.1)-10(-4.6) atm) has higher pH (9.1-9.5) and lower TDS (<254 mg/L). Carbon isotope data indicate that the CO2-rich water is influenced by a local supply of deep CO2 (potentially, magmatic), which enhanced dissolution of silicate minerals in surrounding rocks and resulted in elevated concentrations of Ca2+, Na+, Mg2+, K+, HCO3- and silica under lower pH conditions. In contrast, the evolution of the alkaline water was characterized by a lesser degree of water-rock (granite) interaction under the negligible inflow Of CO2. The application of chemical thermometers indicates that the alkaline water represents partially equilibrated waters coming from a geothermal reservoir with a temperature of about 40 degrees C, while the immature characteristics of the CO2-rich water resulted from the input Of CO2 in Na-HCO3 waters and subsequent rock leaching. (C) 2008 Elsevier B.V. All rights reserved.