[S-GC45] Volatile Cycles in the Deep Earth - from Subduction Zone to Hot Spot

[SGC45-P03] Delineations of groundwater circulations and geothermal water origins using the geochemistry and noble gas of groundwater around the Yangsan fault, Republic of Korea.

*YU JIN LEE¹、CHAN HO JEONG¹、YONG CHEON LEE¹、JUNSIK PARK¹、SONG MIN OU¹ (1. Daejeon Univ.)

キーワード：fault, geochemistry, noble gas, groundwater, geothermal water

The geochemistry and noble gas isotope of groundwater in 10 deep groundwaters and 3 geothermal waters were examined for the characterization of groundwater circulations and the heat source origins of geothermal waters around major fault such as Yangsan fault, southeastern part of the Korean peninsula. The pH values of groundwaters and geothermal waters were in ranges from 5.65 to 8.16 and from 7.0 to 7.98, respectively. In the geothermal waters such as the Haeundae geothermal waters and the Dongrae geothermal waters, the average electrical conductivity was relatively high (3,890 uS/cm) due to the high Na and Cl concentrations. It indicates that the geothermal waters in the hot spring areas are affected by the seawater intrusion. The geochemical types of groundwaters and geothermal waters were Ca-HCO₃ (Ca-SO₄(Cl)) and Na-Cl type, respectively and showed indistinct relationship with the geological characteristics. In the results of noble gas analysis, the most of groundwaters were plotted along the air-crust mixing line on ³He/⁴He vs. ⁴He/²⁰Ne diagram. It implied that the ³He of groundwater except one groundwater sample are dominantly derived from the air and are mixed with the helium originated from the crust. Also, it indicates that the groundwater actively circulates along the faults and the faults cannot play a role of upward pathway of a deep-seated helium gases. The ⁴He concentrations of groundwater were relatively high in the aquifer. It indicated that the groundwaters are sufficiently interacted with the basement rock. The ³He/⁴He ratios of the geothermal waters, plotted on the air-crust mixing line, were in ranged from 0.41010^-6 to 0.81010^-6. It suggested that the He gases in the geothermal waters are mainly originated from the atmosphere and crust, and partly from the mantle sources.