Abstract
In Lake Inawashiro, pH has been affected by inflow of acid thermal water and acid mine drainage at Mt. Adatara, and has been rising rapidly from 5 to 6.9 since 1995. Negative impacts due to pH variation have become a concern. In this study, the future trend of the sulfate ion concentration in acid waters at Mt. Adatara was estimated using non-linear regression analysis to study preventative measures. The δ34S value of sulfates and the seasonal variation of the sulfate ion concentration in Iwo river and Ama stream suggested that acid waters was predominantly generated by disproportionation reaction of SO2 gas, and oxidation of hydrogen sulfide accompanied with the formation of sulfur deposits. The presence or absence of ground water in the liquid phase was an important factor for the generation of acid waters. It was also suggested that reduction in the sulfate ion concentration of acid waters was caused by the increase in temperature and the evaporation of ground water inside Mt. Adatara. In order to validate the hypothesis about reduction in sulfate ion concentration and to estimate overall future trend, non-linear regression analysis was employed for Numajiri thermal water, because its trend of sulfate ion concentration was representative of that of acid waters at Mt. Adatara. From these results, we concluded that sulfate ion concentration would increase again in the future if the temperature inside Mt. Adatara decreases.
