[A-HW30] Isotope Hydrology 2025

[AHW30-06] Developing a Hydrothermal Index based on geochemical properties of groundwater to Reveal Hidden Geothermal Signatures

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

Keywords:hidden geothermal system, geochemistry, hydrothermal index, trace element

Hidden geothermal systems, which lack surface manifestations such as hot springs or fumaroles, remain difficult to detect yet hold considerable potential for low–carbon energy development. This study proposes a geochemical approach to identifying these concealed resources through a trace element analysis of groundwater, with developing a Hydrothermal Index (HI) that integrates normalized concentrations of elements commonly enriched in geothermal fluids (e.g., Cl, Li, B, As, Cs, and Rb). Groundwater samples were collected from multiple locations in a region suspected of harboring a hidden geothermal system and analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Our results reveal several sites with elevated HI values, i.e., significantly above the background, suggesting the ascent of deep geothermal fluids and mixing with shallow groundwater. Some of these elements (notably As) also indicate concerns about environmental quality, underscoring the dual importance of resource exploration and water quality monitoring. Spatial mapping of the HI highlights potential zones of the geothermal system and guides further investigation into reservoir properties and drilling targets. By demonstrating the effectiveness of trace element signatures and the HI, this work offers a practical method for uncovering hidden geothermal systems. Such identification is critical not only for expanding renewable energy portfolios but also for assessing possible environmental risks associated with fluid mixing in shallow aquifers. Ultimately, the approach outlined here can support sustainable geothermal development and contribute to global efforts to reduce carbon emissions.
This study was supported by JSPS KAKENHI (Grant Nos. 23K26602 and 21KK0090) and Joint Research Grant for the Environmental Isotope Study of Research Institute for Humanity and Nature.