Minamata’s mercury cap: 1.5 million cubic meters sealed beneath a 58-hectare landfill
From 1974 to 1990 Kumamoto Prefecture sealed about 1.5 million cubic meters of Minamata Bay sediment containing mercury above 25 ppm under a 58-hectare landfill, at a total cost of 48 billion yen (Chisso bore roughly 30.5 billion). The containment used steel sheet piles and pumped dredging to isolate contaminated sediment, and residual mercury in the sludge dropped to about 4.69 ppm. Fish monitoring continued and nets were removed in 1997; the cap stops a exposure pathway but does not erase Minamata’s health history, which remains addressed through ongoing certification and compensation programs.