DEVELOPMENT OF ECONOMICAL DESIGN AND PREDICTION CONCEPT SOFT FOR DECONTAMINATION PROJECTS OF ENVIRONMENTAL CONTAMINANTS.

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During the last 25 years, surface sediments have been collected from 24 fixed sampling stations to a depth of 4.0cm in the Yatsusiro Sea. Researchers have found that the mercury content of the samples was not a cause for concern and that the concentrations outside the bay were below the acutely toxic level. At the same time the investigators spotted a trend in mercury behavior in the sea of 633km^2. Between 1975 and 1984 mercury concentrations gradually increased, with sampling stations close to Minamata Bay showing the greatest rise, from 0.62ppm in 1975, to 10.3ppm in 1984. At another, more remote station, concentrations rose slightly. The pattern was interrupted in 1982, an exceptional year when 400 mm of rain fell in three hours, a 200-year event. Millions of tons of clean soils were washed from surrounding mountain slopes to cover the sediments in the Yatsushiro Sea. Three years after this storm, the government initiated a decontamination project. Mercury concentrations at one sampling station near the bay dropped from 10.5 to 1.56ppm from 1985 to 1990 and decreased further to a background level of 0.3ppm in 1997. The Japanese government eventually took responsible action against mercury pollution in the bay, but it was too slow to act. The decontamination project costing US$500 million and the first such effort in the world, will not only clean up the bay and protect the people around the Yatsushiro Sea for many years to come.