HAYASHI Sachio University of Miyazaki, Faculty of Engineering, Professor, 工学部, 教授 (90148916)
SEZAKI Mitsuhiro University of Miyazaki, Faculty of Engineering, Associate professor, 工学部, 助教授 (80136803)
YOKOTA Hiroshi University of Miyazaki, Faculty of Engineering, Professor, 工学部, 教授 (90037888)
The arsenic pollution in groundwater is a serious problem in the global scale. It is said that the arsenic pollution in groundwater in Ganges basin is largest, and the arsenic seems to have eluted from the stratum to the groundwater. We carried out various investigations for elucidation of arsenic elution mechanism in groundwater of the region mainly in Samta village in Bangladesh where the arsenic pollution had been confirmed, since 1997. In this study, we examined the mechanism of arsenic elution to the groundwater based on arsenic speciation.
High concentration of arsenic is found in the south part of the village, and its concentration tended to lower toward the north. 68 -100 % of the arsenic existed as arsenate (As^<5+>) in the north. But in the south, 98 -100 % of the arsenic existed as arsenite (As^<3+>) From the groundwater analysis, there were a correlation between arsenic concentration and ferrous ion (Fe^<2+>) hydrogencarbonate ion and distribution of the conductivity. From t
he geological survey, there was no upper muddy layer in the north, and they thickened toward south. The groundwater flowed from the north to the south in the village. From the analysis of the boring core, the arsenic was mainly included in this upper muddy layer.
The results on the arsenic elution to groundwater in Samta are summarized as follows. The environmental condition of the groundwater in the northern part of the village is in the oxidation state, where the arsenic concentration is low. There is no upper muddy layer where arsenic is contained. The iron exists in the form of trivalent. And in the groundwater, ferric ion becomes iron oxyhydroxide, in which the arsenic is trapped, and they have precipitated in the groundwater. As the groundwater flows from the north to the south in the village, the upper muddy layer thickened, and the groundwater becomes a reduced state. In the reduced state, iron oxyhydroxide changes to iron hydroxide. Then the arsenic is released into the groundwater and arsenic concentration becomes higher in the groundwater. Less