Hydrochemical and isotopic characteristics of selected geothermal groundwater in Korea to understand groundwater circulation.

Abstract:

The Korean peninsula has non-volcanic thermal springs with heat sources assumed to be the radioactive decays. The representative geology including geothermal waters are Mesozoic granite and Age-unknown metamorphic rock in Korea. This study investigated geochemical characteristics of the 4 thermal water (Suanbo, Yuseong, Deoksan, Seokmodo) according to the geology which are located in the middle and the south part in Korea. The Suanbo, Yuseong, and Deoksan thermal waters show Na-HCO₃ type and the Seokmodo thermal water on the coast Na-Ca-Cl type implying the sea-water intrusion. The LMWL (Local Meteoric Water Line) of the surface water in the sites shows δD = 5.86δ¹⁸O - 8.31 (R² = 0.99) relation, and also thermal water and groundwater plots are distributed on the LMWL. This implies that the precipitation rarely undergoes isotope fractionation during the circulation in the aquifer. The average δ¹³C value of the Suanbo, Deoksan and Seokmodo thermal waters have -11.93±0.31 ‰, -17.40±0.26 ‰, and -15.73±0.68 ‰, respectively, and these values maintained constantly during the research period. But the surface waters show seasonal variations which could be resulted from activities of microbe and hydrophyte. The estimated age using δ¹⁴C_DIC value of the Suanbo, Yuseong, and Deoksan thermal waters suggested 5580±30, 1980±30, 3240±30 BP (Before Present), respectively. The ¹⁴C_DIC age dating represent that the deeper well depth, the older thermal water. This suggests that the period of the groundwater circulation between surface water and thermal water has been achieved very slowly during 2000 and 5600 years. This data indicates that the present thermal waters were originated from meteoric water, and recharged in the past at least 2000 years ago.