Minamata Bay's 1.5 Million Cubic Metres of Mercury-Tainted Sediment Sealed in 58-Hectare Landfill

September 13, 2026 at 08:39 AM1 min

This digest was compiled by AI from multiple sources — links to the originals are below.

Kumamoto Prefecture dredged and sealed about 1.5 million cubic metres of mercury-contaminated sediment from Minamata Bay between 1974 and 1990. The project created a 58-hectare reclaimed landfill capped with mountain soil at a total cost of 48 billion yen. Chisso Corporation, the company responsible for the pollution, bore 30.5 billion yen of that cost.

Key Facts

Kumamoto Prefecture sealed about 1.5 million cubic metres of Minamata Bay sediment with total mercury above 25 parts per million between 1974 and 1990.
The dredging and landfill project produced a 58-hectare reclaimed area at a total cost of 48 billion yen.
Chisso Corporation, the company responsible for the pollution, paid 30.5 billion yen of the project cost.
An estimated 70 to 150 tons of mercury were discharged and deposited in Minamata Bay over decades of production.
The removal standard for the project was set at 25 parts per million total mercury in sediment.

The Dredging Project

Kumamoto Prefecture dredged and walled in roughly 1,500,000 cubic meters of bottom sludge carrying total mercury above 25 parts per million between 1974 and 1990. The project produced a 58-hectare landfill at a total cost of 48 billion yen, of which Chisso bore 30.5 billion yen. The treated area came to roughly 2,092,000 square meters in total, with about 1,510,000 cubic meters of sludge disposed of.

Pollution History

Chisso's acetaldehyde process caused Minamata disease, with a perfect circulation system installed in 1966 and acetaldehyde production ceasing in 1968. Factory effluent containing methylmercury flowing into Minamata Bay was brought under regulation in 1969, and the Water Pollution Control Law followed in 1970. ICETT estimates the quantity of mercury discharged and deposited in the bay at 70 to 150 tons, a range rather than a measured total.