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[SVC33-P04] Concentration of dissolved arsenic and its fluctuation past five decades in Manza hot springs, Gunma, Japan
キーワード：火山、草津白根山、ヒ素
There are many volcanic hot springs in and around Kusatsu-Shirane volcano, an active volcano in the northwestern Gunma prefecture, Japan. Arsenic is a ubiquitous element in volcanic areas, and its behavior in volcanic hot springs is considered to reflect hydrothermal activities. The fluctuations in arsenic concentration for a few decades of Kusatsu hot springs located at the eastern foot of Kusatsu-Shirane volcano have already been reported. Bandaiko hot spring, a spring with a high temperature and discharge rate in Kusatsu hot springs, shows a significant fluctuation in arsenic concentration. The potential source of arsenic in Bandaiko hot spring is arsenopyrite considering the relationship between arsenic and iron concentrations1).
This study focused on the three major hot springs, Manza-Yubatake, Okumanza, and Manza-Karabuki, in Manza hot springs located at the western foot of Kusatsu-Shirane volcano. We constructed the time series data on the concentrations of arsenic and major dissolved chemical components from 1967 to 2021, then discussed the relationship among those concentrations.
The fluctuation behavior in arsenic of three major hot springs in Manza hot springs are similar, and the concentrations show a downward trend overall.
In the case of Manza-Yubatake and Manza-Karabuki, the concentrations of some dissolved componentss, especially chloride and sodium, seem to fluctuate in response to the volcanic activity of the Kusatsu-Shirane volcano. In addition, the concentrations of arsenic have fluctuated similar to that of chloride and sodium, suggesting that NaCl-type hydrothermal fluids have supplied arsenic.
The concentration of arsenic in Manza-Karabuki is almost a hundred times higher than that in the two hot springs, Manza-Yubatake and Okumanza. In the case of Manza-Karabuki, the concentrations of arsenic and iron are near equimolar and observed a positive correlation in their fluctuations. Considering these relationships found in Manza-Karabuki, it can be assumed that arsenic is supplied by the dissolution of arsenopyrite (FeAsS), as same as the case of Bandaiko in Kusatsu hot springs. Contrary to this, a negative correlation was found between the concentrations of arsenic and iron in Manza-Yubatake and Okumanza. This relationship may be explained that the concentration of arsenic in the two hot spring is controlled by the solubility of arsenic-bearing iron sulfide, since the two hot springs contain relatively large amount of hydrogen sulfide.
1) Yoshikazu Kikawada, Satoshi Kawai, Takao Oi(2006) Long-term changes in the concentration of dissoived arsenic and its present supply in the Kusatsu hot springs, Gunma, Japan, Chikyukagaku (Geochemistry) 40,125-136
This study focused on the three major hot springs, Manza-Yubatake, Okumanza, and Manza-Karabuki, in Manza hot springs located at the western foot of Kusatsu-Shirane volcano. We constructed the time series data on the concentrations of arsenic and major dissolved chemical components from 1967 to 2021, then discussed the relationship among those concentrations.
The fluctuation behavior in arsenic of three major hot springs in Manza hot springs are similar, and the concentrations show a downward trend overall.
In the case of Manza-Yubatake and Manza-Karabuki, the concentrations of some dissolved componentss, especially chloride and sodium, seem to fluctuate in response to the volcanic activity of the Kusatsu-Shirane volcano. In addition, the concentrations of arsenic have fluctuated similar to that of chloride and sodium, suggesting that NaCl-type hydrothermal fluids have supplied arsenic.
The concentration of arsenic in Manza-Karabuki is almost a hundred times higher than that in the two hot springs, Manza-Yubatake and Okumanza. In the case of Manza-Karabuki, the concentrations of arsenic and iron are near equimolar and observed a positive correlation in their fluctuations. Considering these relationships found in Manza-Karabuki, it can be assumed that arsenic is supplied by the dissolution of arsenopyrite (FeAsS), as same as the case of Bandaiko in Kusatsu hot springs. Contrary to this, a negative correlation was found between the concentrations of arsenic and iron in Manza-Yubatake and Okumanza. This relationship may be explained that the concentration of arsenic in the two hot spring is controlled by the solubility of arsenic-bearing iron sulfide, since the two hot springs contain relatively large amount of hydrogen sulfide.
1) Yoshikazu Kikawada, Satoshi Kawai, Takao Oi(2006) Long-term changes in the concentration of dissoived arsenic and its present supply in the Kusatsu hot springs, Gunma, Japan, Chikyukagaku (Geochemistry) 40,125-136