Kikawada Yoshikazu

草津白根火山地域の温泉水ならびに硫黄鉱山排水が周辺河川の水質に与える影響の見積

Sequential extraction procedure was applied to the volcanic marine sediments collected at Wakamiko Caldera in Kagoshima Bay and Teishi Knoll in Sagami Bay for interpretation of their sedimentary environments. The extraction behaviors of arsenic and antimony in the Wakamiko Caldera sample suggest that those elements, which have been introduced to seawater through volcanic hydrothermal fluids, have been held in sediments in two states;one is in the oxidative form, in which they may be scavenged by iron-manganese hydroxides, and the other is in the reductive form, like sulfides. Contrary to this, arsenic sulfides are not found in the Teishi Knoll sample, although the existence of antimony sulfides is suggested. The existence of arsenic sulfides in the Wakamiko Caldera sample may be ascribed to the relatively high concentration of hydrogen sulfide in the hydrothermal fluids ejected there. The sequential extraction procedure for volcanic marine sediments is thus a useful tool for the chemical characterization of hydrothermal fluids supplied from the seabed, where the sediment samples were collected.

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.