ORIGIN AND EVOLUTION OF SALINE GEOTHERMAL WATER IN CRYSTALLINE ROCK, BUSAN, SOUTH KOREA

Origin and evolution of saline geothermal in crystalline rock, Busan, South Korea, are evaluated by application of reaction path modeling based on interpretation of geochemical data. According to geochemical analyses, the geothermal waters belong to Na-Cl type and are characterized by much higher electric conductivity (EC) and total dissolved solid (TDS) than other geothermal waters in Korea. The concentration of major ions in geothermal waters shows a weakly positive relationship with temperature except for Mg ion. The content of cations has the order of Na>Mg>Ca>K in seawater, but the order of Na>Ca>K>Mg in geothermal waters, typically enriched in Ca and depleted in Mg compared to seawater. The d18O and dD values indicates that geothermal waters were derived from local meteoric waters, the local meteoric water line for study area is produced as D=7.8d18O+3.4. The measured d34SSO4 values can be considered that geothermal waters were affected by seawater, and the measured values of d13C mean that the carbonate in geothermal water originated from soil CO2 or dissolved inorganic carbon (DIC) in groundwater. Tritium data of geothermal waters show negative correlation with temperature. These results mean that the geothermal waters mixed with shallow groundwater during ascent. The depletion of Mg can be explained by the formation of Mg-chlorite by the reaction between SiO2 and Al2O3 in rock and Mg ion in seawater. The enrichment of Ca also can be explained by the reaction forming albite from calcic plagioclase and seawater Na. In reaction path modeling of geothermal water-groundwater mixing, the major species show overall a positive relationship with temperature except for some ions like Mg. The result of reaction path modeling agrees well with measured and analyzed hydrogeochemical field data. The saline thermal groundwaters in Busan are originally formed by seawater (or mixture of seawater and groundwater) reaction with host rock during circulation at depth, and then this water is cooled and diluted by mixing with local groundwater during ascent. If the geothermal water originated by seawater reaction with host rock, it can be concluded that the seawater reacting with host rock at depth is a paleoseawater intruded at a geological past time when sea level was higher than at present.