鹿児島湾の湾口部に位置する阿多カルデラの西半分は陸地にあり，カルデラ内には池田湖・鰻池などの第四紀更新世以降に形成された火山地形が多く見られる．近隣には指宿温泉をはじめとする複数の温泉や噴気地帯も多く分布する．写真は，大隅半島西側の伏目地区の砂浜で，干潮時にだけ沸騰した湯の噴出が観察できる温泉である．すぐ横の崖には約6300年前に池田湖を火口とする噴火時に噴出・堆積した池田火砕流堆積物が露出する（川辺・坂口，2005）．この地区内にある山川地熱発電所の開発に関係して，この地域の熱水系の研究が多方面から行われ，2000 mの掘削深度から330°Cを超える超高温熱水が発見されている（例えば，吉村・伊藤，1994）．その始原熱水は海水がこのような高温で反応して生まれるが，上昇過程で陸域からの地下水に希釈され，またさまざまな鉱物の沈殿によって，湧出する熱水の水質は変化していると説明されている（例えば，Okada et al., 2000)．周辺には市営の温泉施設・砂蒸し風呂温泉のほかに，かつて温泉水を蒸発させて製塩していた塩田跡などもある．背後に見える独立峰は日本百名山の一つ開聞岳（標高942 m）である．この地域は火山と地熱を体感できる地質学・地球化学の第一級の研究フィールドである．

（写真・説明：益田晴恵）

 The history of hot spring studies and research in Japan is reviewed mainly from the viewpoint of Earth science. Studies on the geochemistry of hot spring waters started with geochemical analyses of hot spring water in the 1830s. The relationships between the issuing mechanisms of hot springs and volcanism, active faults, and earthquakes have been studied since around 1910. These studies reveal sources and recycling paths of waters and dissolved materials in the Earth's crust and the upper mantle in relation to plate tectonics and subsequent volcanism. Microbiological studies on living organisms, such as algae, bacteria, and archaea, living in hot springs, began in the late 1880s, and have contributed to documenting the origins and evolution of life. Combined with mineralogical studies, minor and trace element analyses of hot spring water have contributed to an understanding of the formation mechanisms of ore deposits. Studies on hot spring waters triggered an understanding of the essential role of water in the Earth and its evolution.

 The relationships between eruptive activities and changes in groundwater level, temperature, and chemical composition of thermal water at Toyako Onsen are discussed in relation to the 1977-1978 and 2000 eruptions of Usu volcano, Hokkaido, Japan. The temperature and chemical composition of thermal water observed since the 1977-1978 eruption are found to have changed after the eruptions. The groundwater levels in the hot spring wells, observed before the 2000 eruption, are found to have changed approximately six months before the eruption. Changes of groundwater level, which reflect pore water pressure, are generally faster than those of the temperature and chemical composition of thermal water attributed to fluid flow. However, thermal water flow during the 2000 eruption of Usu volcano was probably triggered as fast as propagation. Because the thermal water at Toyako Onsen is affected by the eruptive activities of Usu volcano, understanding the relationship between volcanic activity of Usu volcano and changes in temperature, chemical composition and groundwater levels of the thermal water is useful for inferring volcanic eruption and activity of Usu volcano.

 Geothermal fluid heated by a magma heat source is available as a geothermal resource when the fluid or vapor is obtained continuously from the subsurface and is introduced into power generation facilities. Exploration of geothermal resources using a combination of various techniques is an essential process to successful developing geothermal energy, which has become attractive as a renewable and sustainable resource. Geochemical exploration provides direct information on the properties and behaviors of geothermal fluid, based on chemical analysis of hot spring waters, fumaroles collected at geothermal areas, and high-temperature fluids obtained from the heads of drilled wells. Chemical properties of those fluids reflect histories of various chemical processes during fluid circulation within a subsurface geothermal system where groundwater evolves into a high temperature fluid. Compositions of major cations (Na, K, Ca, Mg) and SiO₂ reflect the course of fluid-mineral interactions, and function as a geothermometer when compositions are controlled by chemical equilibria within the reservoir. Meanwhile, compositions of major anions (Cl, SO₄, HCO₃) and B reflect contribution from various sources, such as seawater, magmatic volatiles, and crustal fluid. Gas species (CO₂, H₂S, H₂, N₂, CH₄, Ar, He) are also derived from various sources such as the atmosphere, organic matter decomposition, and a magma heat source. These species provide information on the history of fluid circulation, because they are unaffected by fluid-mineral interactions in spite of some exceptions. Hydrogen and oxygen isotopes of H₂O mainly provide an index of the origin of a fluid, such as meteoric water, seawater, and magmatic volatile, as well as evidence for fluid-mineral interactions and boiling processes. Isotopic compositions of light elements such as carbon and sulfur of gas species are useful for discriminating contributions from magmatic volatiles and from microbiological activities. Recent technical advances in mass spectrometry have enabled the isotopic composition of several elements to be obtained. These newly proposed geochemical indices would provide information that is difficult to extract using conventional geochemical tools. Developments in new geochemical extrapolation techniques are expected in the light of the present situation where expansion in the use of geothermal resources is demanded.

 Achieving carbon neutrality is a measure for mitigating climate change. As a result, development of zero net buildings and houses is accelerating in Japan. In this connection, ground source heat pump (GSHP) systems are being promoted as a potential energy saving cooling and heating technology. The aquifer thermal energy storage (ATES) system is an GSHP system that has been attracting attention for its low initial cost and high operation efficiency compared to other GSHP systems such as closed-loop systems. However, successful implementation strongly depends on the hydrogeological conditions of the host area. ATES systems require an adequate groundwater supply to meet the thermal demand of buildings. To increase ATES adoption in Japan, research to evaluate areas that are appropriate for ATES, including the hydrogeological environment, have been actively carried out in the last decade. Existing literature on the thermal use of groundwater in Japan and studies related to ATES from the 1970s to the present are reviewed and analyzed. Then, the latest findings on the ATES potential map and/or suitability evaluation for Osaka city and Yamagata basin are presented. Both findings imply that geological conditions might impact the suitability of implementing an ATES system. Finally, challenges surrounding development of a technique to assess the applicability and potential of ATES at different locations in Japan are discussed. The safe and effective installation and operation of an ATES system requires an understanding of prevailing hydrogeological conditions.

 Acid river systems and the actual situation of river neutralization at the Kusatsu-Shirane volcano, Gunma Prefecture, and Ebino highlands of the Kirishima volcanoes, Miyazaki Prefecture, are reviewed. In the case of the acid river system at the Kusatsu-Shirane volcano, sources of acid water are volcanic acidic hot springs and acid mine drainage (AMD) from closed sulfur mines. Volcanic hot springs and AMD have different effects on the river environment. Arsenic is predominantly supplied from hot springs, and heavy metals such as lead are predominantly supplied from AMD. The river neutralization system at the Kusatsu-Shirane volcano targets water from Kusatsu hot springs, the region's largest source of acidic components, resulting in a significant water quality improvement in the downstream river. The system at the Kusatsu hot springs directly injects a neutralizer prepared from powdered limestone into the river. The neutralization products formed in river water are finally accumulated in a dam constructed to function as a sedimentation basin. Then, the dredged sediment from the dam is dumped at dedicated disposal sites. In the case of the Ebino Highlands (Iwoyama volcano), the Kirishima volcano complex, an eruption in 2018 caused river water pollution. Phreatic eruptions at the Iwoyama volcano triggered discharges of acidic hydrothermal fluids containing large amounts of arsenic and heavy metals from vents newly opened with the eruptive events, resulting in a deterioration of river water quality and significant damage to local agriculture. Because the discharge of acidic hydrothermal fluids from the vents continued even after eruptive activity ended, Miyazaki Prefecture decided to neutralize the river in the Ebino Highlands. After starting river neutralization, the water quality of downstream rivers improved significantly. Because the amount of treatment required for river neutralization at the Ebino Highlands is not very large, a passive treatment method was adopted to enable low-cost operation.

 Previous single-gene surveys using 16S rRNA gene-based methods have revealed the phylogenetic classification, distribution, and diversity of as-yet uncultivated microbial lineages in diverse environments. In particular, archaeal communities associated with high-temperature terrestrial hot springs and sediments are dominated by novel thermophilic and hyperthermophilic populations whose physiological characters and ecological roles are unknown. Recent advances in metagenomic approaches and computational processing of huge volumes of DNA sequence data have further unveiled the evolutionary importance of uncultivated hot spring archaeal lineages and previously undescribed roles in biogeochemical cycles of carbon, nitrogen, sulfur, and metals. In this review article, the uncultured thermophilic and hyperthermophilic archaeal lineages that have accelerated our understanding of the tree of life are illustrated.