PREDICTED RESTORATION OF THE SURROUNDING MARINE-ENVIRONMENT AFTER AN ARTIFICIAL MERCURY DECONTAMINATION AT MINAMATA BAY, JAPAN - ECONOMIC VALUES FOR NATURAL AND ARTIFICIAL PROCESSES

More than 35 years ago, an accident surfaced because of human misery at Minamata Bay, Japan. By 1960, more than 100 people suffered (mortality rate was over 20%) by eating fish containing methylmercury. By 1987, 1742 people were identified with this Minamata disease. The methylmercury along with inorganic mercury was released from a nearby chemical factory into the Bay, contaminating not only fish but the entire ecosystem including the bottom sediments. The amount of mercury deposited in the sediments was considered to be 150 tons. The Japanese Government initiated, in 1984, a decontamination project vacuuming the contaminated sediments into a sealed area creating a reclaimed land mass of 582,000 m2. The movement of the mercury deposited in the Bay outward to Yatsushiro Sea, was observed during the last 16 years by sampling and analyzing the surface sediments of Yatsushiro Sea. Since 1975, the deposited mercury moved continuously outside from the Bay at an annual rate of 3.7 tons. The initiation of the artificial decontamination (over $500 million in US funds), however, changed the pattern of mercury movement and drastically reduced its contents with a half-life of 1.83 years. Since 1985, the amounts of mercury in the surface sediments of Yatsushiro Sea have been decreasing dramatically with a half-life of 9.5 years. If this present trend of decrease continues, it becomes possible to predict that the Sea would be completely restored by 2011 AD. Another interesting observation was a historical storm in 1982, which cleaned-up the surface sediments of Yatsushiro Sea with a half-life of 2.76 years. If the clean-up effect of the natural processes in Yatsushiro Sea was compared to the artificial one, its economic value was calculated to be $96 million (US funds). Any decontamination attempt for polluted sites should consider a combination of the natural and artificial decontamination processes for economic reasons.