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Prevention of Secondary Pollution Caused by Dredging Bottom Sediment Containing Mercury in Minamata Bay,

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In Minamata Bay, sediment containing 25 ppm mercury about 1,500,000 m3 has accumulated on the bottom about 2,110,000 M2, and the fish and shellfish in Minamata Bay are now polluted by mercury. Since October 1977, we have been dredging to provide a safe environment to ensure the inhabitants health. We have restored Minamata Bay by dredging a part about 1,500,000 m2 and using the other part about 600,000 m2 as a disposal area. During dredging, we have guarded against secondary pollution. We have made sure that water quality did not deteriorate due to stirring of the bottom sediment or by soakage of mercury from the disposal area and that marine products were not further polluted. We have established several stations along the border of the working area and measured the concentration of mercury in seawater collected at these stations. The equipment for the analysis is located onsite so that we can get the results of the analysis immediately. If the analysis shows concentration over the allowed level, we stop or slow down the work at once and find the cause of the pollution. However, since the analysis for mercury is slow, we use the estimation method. This method measures the turbidity in seawater collected from stations in the work area for use as indicators of mercury we then estimated the diffusion of mercury. In the first disposal area dredging was carried out from June 1981 to March 1984, the estimation method was used and satisfactory results were obtained. In the second disposal area dredging began in October 1982, we are also using the estimation method for the diffusion of mercury by dredging.

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  • Water Pollution and Control
  • Geography

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