Chemical forms of arsenic in the modern sediments under microbiologically controlled condition concerned with arsenic polluted groundwater

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The highly arsenic-contaminated groundwaters, causing serious health hazards in the world, monstly present in the modern sediments. In this study, arsenic releasing mechanism is discussed based on the analyses of groundwaters, estuarine waters, and modern sediments of the Osaka basin, and the intertidal sediments from the iriomote island, OkinawaSediments of the intertidal zone of lriomote island record a naturally balanced arsenic-fixation system, where accumulated arsenic compounds are largely fixed into sulfide mineral(s) transformed by iron hydroxide/oxide. This transformation appears to be mediated by microbial reactions in the stagnant aquitard during the initial stage of diagenesis. Significant release of arsenic to groundwater would be unlikely to occur if the transformation rates among arsenic solid phase compounds are stablePhytoplankton and its body accumulates arsenic in the estuarine sediments, suggesting that high concentration of arsenic in the marine clay layers of Osaka Group sediments, from 7 to 20 ppm, is also the results of such a microbial accumulation. After the deposition, most of the arsenic is finally fixed in pyite. In the southem part of Osaka Basin, Senshu area, The heavy suifur isotope ratios (20-27 CDT) of sulfate-sulfur in the groudwater from the spring to fall indicate that the source of the sulfur is the pyite in the sediments. Afterpyrite decomposition supplies the arsenic into the groundwater, arsenic concentrations changes in harmony with dissolution of iron oxyhydroxide in reduced condition produced by bacterial activity. The observation in this well water suggests that the pyrite was he primary source of arsenic in the Osaka Basin, however, arsenic release is dependent on the solubility of iron oxyhydroxide, mainly controlled by the redox condition produced by the bacterial activity in the well