Recent seismic monitoring mainly by the Hi-net (High sensitivity seismograph network in Japan) for the last decade has been revealing the 3D structure of velocity anomaly in the crust and mantle due to heterogeneous presence of deep-seated fluids and fluid-related deep low frequency earthquakes in subduction zones of Japan arc. Petrological water circulation models combined with geophysical subduction models quantitatively show the water budget in the solid earth. The recent findings infer the importance of deep hydrothermal fluid on the occurrence of inland earthquakes. As the models are built with the result obtained by monitoring, experimental techniques or simulations, implications from evidence-based geochemical and geological studies are expected for the proof of water circulation models. Hence, we examined chemical features of deep groundwaters in SW Japan arc, and showed spatial distribution of deep-seated fluid mixed into groundwater. We found that the deep-seated fluid, whose isotopic composition is similar to magmatic, has the high Li/Cl ratio (>0.001 in wt. ratio),and concluded that Li/Cl is a good indicator for detecting the slab-related deep-seated fluid in groundwaters. Spatial distribution of Li/Cl reveals that slab-related deep-seated fluid upwells along the faults and tectonic lines, and at close to Quaternary volcanoes in SW Japan arc. In most cases, upwelling places are found close to the areas where deep low frequency (DLF) earthquakes are occurring, implying that deep-seated fluid causes DLF events.

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To identify of metamorphic dehydrated fluid as source fluid of hot spring water, we conducted chemical and isotopic analyses of water and accompanied gas samples collected from hot-spring wells along the Median Tectonic Line (MTL) in the forearc region of the southwestern part of Japan. As a result, we found the hot spring waters having anomalous δD and δ¹⁸O compositions as compared with modern seawater and shallow groundwater in Wakayama and Shikoku regions. Judging from data in relative B–Li–Cl composition and He isotopic systematics, the source fluid of the hot springs in Shikoku could be identified to be one of diagenetic fluids. On the other hand, the source fluid of the hot springs of Wakayama had different B–Li–Cl composition and higher ³He/⁴He ratio in comparison with diagenetic dehydrated fluids and then the fluid was thought to be originated from metamorphic dehydrated fluid as well as Oita plain. There was another striking contrast between the source fluid of Wakayama and Oita and that of Shikoku and Miyazaki; accompanied gases by the former were rich in CO₂, whereas those with the latter were rich in CH₄, and CO₂ in the accompanied gases of Wakayama and Oita is mostly derived from marine carbonate like volcanic gases in subduction zones. Moreover, the Li–B–Cl compositions of them showed transitive values between the relative composition of diagenetic fluids and those of volcanic thermal waters. Consequently, the source fluid of hot springs in Wakayama and Oita was likely to be dehydrated metamorphic fluids released from the subducting Philippine-Sea plate.

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Groundwater dating methods employed in projects to develop deep geological repositories for radioactive wastes, or to research technologies and methods that may be used when developing a repository, have been reviewed. The reviewed projects are being, or have been, undertaken in Japan, Finland, Sweden, Belgium, the UK, Germany, France, Switzerland, Canada and the U.S.A.. A wide range of actual and potential repository host rock types and hydrogeological settings have been investigated. The most commonly used dating methods are based on physical hydrogeology and measurements of δ¹⁸O/¹⁶O, ³H, ¹⁴C, ³⁶Cl and ⁴He in groundwater or pore water. Only two of the reviewed projects have used noble gas data to estimate recharge temperatures, and hence deduce the timing of recharge. A single project used ¹²⁹I. Analyses of U-series isotopes have generally not been used successfully, owing to the complex radiological and redox processes that influence U migration, which complicates the interpretation of such U-series data. None of the reviewed projects have used methods based on Tritium (³H)–Helium-3 (³He),Chlorofluorocarbons (CFCs)or ⁸⁵Kr. These methods have not been needed because they indicate the presence of water recharged within the last ca. 50 – ca. 60 years, which could be identified readily at all the sites by the presence of ³H. In the case of ⁸¹Kr, sampling and analytical difficulties have probably prevented its use. Recently, improved analytical techniques enable smaller samples to be analyzed, but ⁸¹Kr gives similar information to ³⁶Cl, which can be investigated more easily. Whatever the combinations of methods that have been used in a particular program, their results have invariably been interpreted in combination with one another and in the context of other information that is available for the investigated site. That is, a particular groundwater dating method is never applied in isolation.

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