Decades after Japan Minamata mercury disaster roughly 1.5 million cubic metres of contaminated seabed sediment remains sealed beneath a 58 hectare reclaimed landfill in Kumamoto Prefecture.
Between 1974 and 1990 the prefecture dredged and contained sediment from Minamata Bay containing more than 25 parts per million of total mercury the removal threshold established for the cleanup. The project cost about 48 billion yen with Chisso, the company responsible for the industrial pollution, contributing about 30.5 billion yen.
The contamination originated from Chisso’s chemical plant, where methylmercury entered wastewater and eventually accumulated in the bay. Although acetaldehyde production ended in 1968 and regulations were introduced soon after, mercury contaminated sediment remained on the bay floor.
The cleanup involved constructing a watertight barrier using steel sheet piles around the reclamation area. Contaminated sediment was then dredged as slurry and pumped into the enclosed area, which was eventually covered with mountain soil. The approach was designed to prevent contaminated mud from being resuspended and re-entering the marine food chain.
According to Japan’s Ministry of the Environment, the overall treatment area covered about 2.09 million square metres. About 1.51 million cubic metres of sludge was disposed of, with the landfill itself covering approximately 582,000 square metres.
The International Center for Environmental Technology Transfer has estimated that between 70 and 150 tonnes of mercury were discharged and deposited in Minamata Bay over the period of industrial contamination. The figure is an estimate rather than a directly measured total.
The cleanup reduced mercury concentrations in the remaining sediment. ICETT reports that residual mercury levels fell to about 4.69 parts per million after the program. However, monitoring of fish continued, with provisional standards of 0.4 ppm for total mercury and 0.3 ppm for methylmercury.
The containment project also did not erase the health consequences of Minamata disease. Japan’s Environment Ministry records continuous exposure to methylmercury at disease-causing levels around Minamata Bay until no later than 1968, years before the dredging operation began. By March 2001, 2,955 people had been officially certified as Minamata disease patients.
The bay’s dividing nets, installed as part of measures to control contaminated fish and sediment, were finally removed in October 1997, seven years after the major dredging and landfill works were completed.
Today, the reclaimed land represents the engineering response to a contamination problem that could not simply be removed from history. The mercury contaminated sediment was isolated rather than eliminated, leaving the long-term integrity of the landfill and its containment measures as an enduring part of Minamata Bay’s environmental legacy.