Experriments of cation leaching (Na⁺, K⁺, Ca²⁺, and Mg²⁺) from rocks to CO₂ saturated water were carried out to clarify the relation between water quality and surrounding rocks. Rock samples use were: 15 granite, 5 rhyolite, 15 andesite, 6 granodiorite, 13 basalt, 6 tuff, 3 mudstone, 6 sandstone, 19 feldspar, and 7 mica samples. On the trilinear diagram, results of granite, rhylite, and basalt agreed with water quality in respective rock areas. The rocks, therefore, affect the quality of water nearby. Alkali feldsper in granite and rhyolite, and plagioclase in olivine basalt readily dissolved in the CO₂ saturated water. The quality of ground and stream water depends on the ratio of cations leached into water from those feldspars.

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Kashio brines in the deep mountain area in the central Honshu have been noted for the high salinity, the non-meteoric δ¹⁸O and δ²H values of water, and its close relation to the Median Tectonic Line, the largest fault in Japan. Similar brines discharge in other localities along the fault. The Cl^- content of the brine seasonally varied from 670 to 60 meq/l in harmony with the variations of δ¹⁸O from -2.4 to -8.7‰ and δ²H from -52 to -64‰. The stable isotopic and chemical data indicate that these variations are caused by dilution of ascending deep endmembers with two types of meteoric circulation. The first is a local meteoric circulation directly supplied from precipitation to the drainage basin of Kashio, having δ¹⁸O of -10.5 to -13‰ and δ²H of -72 to -86‰. The second is a circulation whose isotopic values are greater (δ¹⁸O: -9 to -10‰ and δ²H: -60 to -70‰) than the local meteoric water. The enrichment of heavy isotopes in the latter circulation may have resulted from ultra or ion-filtration of deep-circulating water during its passage through sedimentary rocks. A heavy precipitation in the drainage area increased after a few months the proportion of the first meteoric component and decreased that of the second.

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Up to 100 ppm of As was found in siliceous sinters from Nakabusa Hot Springs, despite a low As concentration (0.16 mg/l) in sinter-depositing thermal waters. In the sinters, As is strongly correlated with Ca, but not with Al, which is considered as being capable of adsorbing As. Evidently As-adsorbing sites on Al were blocked by Na and Ca ions. Excess Ca in the sinters may be present as CaCO₃, as the waters are supersaturated with aragonite. CaCO₃ behaves as an effective collector of finely dispersed As precipitates, thus leading to the formation of As-enriched sinters. This conclusion is supported by the fact that Ca and As were dissolved simultaneously by mild acid leaching of powdered sinter samples.

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