Atlas of Contaminated Sites

Notable case, Minamata, Kumamoto Prefecture, Japan

Minamata Bay

水俣湾

The bay into which the Chisso plant discharged, for decades, wastewater containing methylmercury, a by-product of acetaldehyde synthesis. The neurological disease caused by eating the contaminated fish is named after the town, as is the international convention on mercury.

Status
Contained
Area
58 ha on land, 209 ha in water1
Activity
1932–1968
People affected
2,265 (2001)2
Media
sediment, surface water, living organisms
Kind of site
chemical works, port
  1. Area. 58 hectares of reclaimed land built from the dredged sludge; the works covered about 2,090,000 m² of the bay (Kumamoto Prefecture information board, transcribed in the 2010 CACP report).
  2. People affected. Officially certified patients on the Yatsushiro Sea coast at the end of March 2001 (Ministry of the Environment). According to Wikipedia more than 10,000 people had received compensation from Chisso by the same date.

Contaminants

Metals

Status

Contained

Mercury-bearing sludge was dredged and sealed inside a 58-hectare reclaimed area between 1977 and 1990; the mercury was not removed from the sludge, which remains under the fill. The nets that fenced off the bay were taken away after the safety declaration of 29 July 1997.

Story

From 1932 to 1968 the Chisso plant in Minamata made acetaldehyde and acetic acid using a mercury catalyst. The methylmercury formed in the process went out to sea with the wastewater, through the Hyakken outlet and for a time into the Minamata River. An estimated 70 to 150 tonnes of mercury ended up in the bay; near the outlet the contaminated sludge was up to four metres thick.

The first patient was reported in 1956. In November of that year Kumamoto University identified the cause as heavy-metal poisoning transmitted through fish and shellfish; the government formally acknowledged the link with the plant only in 1968. Bay sediments held 29–713 ppm mercury (dry weight) in 1963 and 14–586 ppm in 1971.

In 1975 the bay was fenced with nets to hold the contaminated fish in. From 1977 Kumamoto Prefecture dredged the sludge and placed it in a confined reclamation area by the outlet, covered with synthetic sheets and soil: 1,510,000 m³ of sludge, 58 hectares of new land, fourteen years of work and about 48.5 billion yen. Work ended in 1990; the reclaimed land now holds a park with the memorial to the victims.

Timeline

  1. 1932The Chisso plant starts making acetaldehyde.
  2. 19561 May: the first cases are officially reported. In November Kumamoto University points to heavy-metal poisoning through fish.
  3. 1965A second outbreak is reported in the Agano River basin, Niigata Prefecture.
  4. 1968Acetaldehyde production ends; on 26 September the government acknowledges the cause.
  5. 1975The bay is fenced off with fishing nets.
  6. 1977Kumamoto Prefecture starts dredging the sludge.
  7. 1990Dredging and reclamation are completed.
  8. 199729 July: the bay is declared safe and the nets are removed.
  9. 201029 March: settlement between 2,123 uncertified victims, the state, the prefecture and Chisso.

Health

Minamata disease is methylmercury poisoning of the nervous system, contracted by eating fish and shellfish from the bay; it also affected children exposed in the womb. By the end of March 2001, 2,265 patients had been officially certified on the Yatsushiro Sea coast (Japanese Ministry of the Environment). Hair of affected residents held up to 705 ppm mercury.

Remediation

  1. 1975containment netsFishing nets across the bay to stop contaminated fish from leaving; removed in 1997.
  2. 1977dredging and confined disposalDredging of 1,510,000 m³ of sludge with cutterless suction dredgers and disposal in a 58-hectare confined reclamation area, covered with synthetic sheeting, lightweight volcanic ash and soil; completed in 1990 at a cost of about 48.5 billion yen.

From the literature

Species that acted on one of this site's contaminants in at least one study. The list is automatic and takes no account of this site's soil, concentrations or climate.

Plants

Fungi

Sources

  1. Minamata Disease: The History and Measures, chapter 2, Ministry of the Environment, Japan, 2002
  2. Kumamoto and Minamata Visit, technical report (transcribes the Kumamoto Prefecture information boards at the Hyakken outlet and on the reclaimed land), Citizens Against Chemicals Pollution / Zero Mercury Working Group, 2010
  3. Fujiki M., Tajima S. The pollution of Minamata Bay by mercury. Water Science and Technology (abstract record), US EPA HERO, 1992
  4. Yoshinaga K. Mercury-Contaminated Sludge Treatment by Dredging in Minamata Bay. ASTM STP 1293, ASTM International, 1995
  5. Minamata disease, Wikipedia, 2026
  6. Minamata, Kumamoto, Wikipedia, 2026

Draft compiled from public sources, not yet reviewed.