木川田 喜一

Long-term changes in the concentration of arsenic in these 40 years were investigated for four hot springs, Kusatsu-Yubatake, Bandaiko and Kagusa No. 3 and No. 8, located in the Kusatsu hot springs area in the eastern foot of Kusatsu-Shirane volcano, Gunma, Japan. The drastic increase in arsenic content was observed only in Bandaiko between 1985 and 1998. The concentration of arsenic in Bandaiko water showed good positive correlations with those of iron and sulfate ions during that period. In addition, the arsenic content has fluctuated equimolecularly with that of iron through the period. This suggests the major origin of the dissolved arsenic in Bandaiko water is arsenopyrite. The drastic increase in the concentration of arsenic probably have been caused by the accelerating oxidation-dissolution of arsenopyrite present in and around underground hydrothermal reservoir, which started in 1970 since the hot spring issued out, associating with the continuous intrusion of meteoric water containing a considerable amount of dissolved oxygen. The concentration of arsenic in Bandaiko water is now around 10 mg dm^-3, meaning that the annual supply of arsenic from Bandaiko water is about 49 ton y^-1. Bandaiko is by far the largest arsenic supplier in the Kusatsu hot springs area.

We compared two analytical values for sulfur species in water samples of Yugama, a crater lake of the Kusatsu-Shirane volcano, collected between 1966 and 1995. One analytical method was ion chromatograph, by which concentrations of the sulfate ion (SO_4^<2->) were determined. The other was gravimetric analysis preceded by oxidation of the samples, by which total concentrations of sulfur species (T-SO_4^<2->) were determined. The concentration differences between the two were in good accordance with the total concentration of poly-thionates reported by Takano and Watanuki (1990) through the whole period studied. This suggested that the concentration difference between SO_4^<2-> and T-SO_4^<2-> can be used to estimate the concentrations of dissolved poly-thionates. In this way we can grasp very easily the secular change of the total concentration of poly-thionates without any special analytical conditions and/or instruments.

Long-term changes in temperature, pH and major element chemistry during 1969 and 1999 were investigated for the three hot springs, Manza-Yubatake, Okumanza and Manza-Karabuki, in the Manza hot spring area located in the western foot of Kusatsu-Shirane volcano, Gunma, Japan. At Manza-Yubatake and Okumanza, water temperatures have been decreasing for at least these 60 years except occasionally small irregular changes probably accompanied by the change of volcanic activity of Kusatsu-Shirane volcano. The concentrations of dissolved components of Manza-Karabuki water have widely fluctuated obviously related with volcanic activities, while the water temperature has rarely been fluctuated. The waters of the three hot springs show similar ratios among the concentration of the analyzed dissolved components except calcium and sulfate ions, which are possibly controlled by the solubility of gypsum. The similarity of chemical composition and the highest concentration of dissolved species in the Manza-Karabuki waters indicate that Manza-Yubatake and Okumanza waters are formed by the mixing between local meteoric water and the Manza-Karabuki water, which has the closest chemical composition to the primitive hydrothermal fluids related to the volcanic activity in the Manza area.

Geochemical study on Yugama, a crater lake at Kusatsu-Shirane volcano, has been conducted since 1966. Amounts of various cationic species in Yugama water started increasing around 1981, slightly before the phreatic eruptions in 1982-1983, and kept increasing until 1985. In 1986, they turned to decrease and at present restore their former levels before the eruptions. The concentration of sulfate ion showed a secular change similar to those of cationic species, but no such variation was observed for chloride ion even during the 1976 and 1982-1983 eruptions. However, it started increasing in 1989. A high correlation between the concentrations of chloride and hydrogen ions suggests an increasing influx of hydrogen chloride from the deep volcanic systems under the lake. A Cl^--SO₄^2- is an excellent monitor of the variation in volcanic activity at Kusatsu-Shirane volcano; all of the three past activities since 1966, i.e., the high-level subsurface activity in 1968, the eruption in 1976 and the eruptions in 1982-1983, showed a reverse secular change with time in the Cl^--SO₄^2- plot. This could be attributable to the function of Yugama water as a condenser of volatiles released underground.