[A-HW24] 同位体水文学2024

[AHW24-05] Identifying a Mixing Process of Hidden Geothermal Fluids with Shallow Groundwater in the Mt. Endut Field, West Java, Indonesia

*Mochamad Iqbal^1,2、Koki Kashiwaya¹、Yohei Tada¹、Katsuaki Koike¹ (1.Kyoto University、2.Institut Teknologi Sumatera)

キーワード：hidden geothermal system, geochemistry, stable isotope, mixing

This research explores hidden geothermal systems (HGS), focusing on Mt. Endut in Banten, Indonesia, approximately 100 km southwest of Jakarta. Mt. Endut features two geothermal systems: the medium enthalpy Cikawah system in the west and the Endut HGS in the east, the latter lacking surface manifestations. This study aims to examine the impact of geothermal fluids on nearby shallow groundwater within the Endut HGS. It involves the analysis of 23 water samples from various sources in the Endut area, including river, well, spring, and rainwater, along with hot spring samples from the Cikawah system. The research scrutinizes major ions and stable isotopes δ¹⁸O and δD to understand the characteristics of the Endut HGS. According to a Piper plot analysis, the water samples are grouped into different categories: hot springs as Na-Cl type, rainwater as Na-HCO₃, rivers as Ca-HCO₃, and groundwater varying between Na-Cl and Ca-HCO₃. The hot spring samples, identified as chloride water type, are believed to emerge from the Cikawah system's geothermal reservoir. Solute geothermometer estimates suggest the reservoir's temperature is about 160±5°C at depths between 517-595 m. Isotopic analysis of δ¹⁸O and δD reveals most water samples originated from meteoric sources, with distinct slope values for each type: rivers (4.97), groundwater (5.76), and hot springs (2.15). Variations in groundwater slope provide initial indications of geothermal fluid influence. Consequently, the shallow groundwater is affected by geothermal fluids from the Endut HGS, as evidenced by the integration of major ion, isotope, and conservative element (Cl, Li, B) data.