草津温泉主要源泉における溶存ヒ素濃度経年変化とヒ素負荷量の見積

抄録

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.