HYDROCHEMICAL AND ISOTOPIC CHARACTERIZATION OF THE SEDIMENTARY GEOTHERMAL FLUIDS IN SOUTHWESTERN TAIWAN

The Guanziling and Chunlun hot spring areas, located near the Chiayi-Tainan alluvial plain in southwestern Taiwan, are exemplary sedimentary rock geothermal systems with significant potential to become Taiwan's first geothermal power generation site in the sedimentary region. The hydrochemical and isotopic characteristics of fluids from the geothermal fields were determined to elucidate the potential thermal reservoir, temperature, as well as the pathways and circulation of these fluids. Hence, seasonal sampling (wet and dry seasons) of thermal fluids, river water, groundwater, and rain water was conducted. Our data demonstrate that thermal fluids exhibit high levels of Na⁺, Cl^-, and HCO₃^- while low concentrations of SO₄^2-, which stands out as a notable distinction compared to groundwater and river water. The metal/Na molar ratios and δD-δ¹⁸O values of the fluids suggest a seasonal variation in water discharge, fluid flow pathways, and the degrees of water-rock interactions within the system. The ⁸⁷Sr/⁸⁶Sr ratios of fluids and the drill sediments cores indicate the potential thermal reservoirs. Based on the δD-δ¹⁸O, ⁸⁷Sr/⁸⁶Sr ratios, and chemical geothermometers, we argue that these two geothermal systems possess contrasting thermal fluid flow paths and circulations. Specifically, the Chunlun system exhibits a predominantly shallow circulation, while the Guanziling system indicates a deeper circulation. Those findings were supported by the drilling core mineralogical observations. By identifying these geochemical and mineralogical features, we can discern the ideal target site for geothermal energy development in the sedimentary region of Taiwan.