Nippon Steel Corp. detected cyanide in waterways near its East Nippon Works in Kimitsu, Chiba Prefecture, 59 times over five years, according to a report the steelmaking giant submitted to local governments in the prefecture.
The company did not disclose the information at the time, and in some cases, the company denied the substance had been detected.
Going forward, Nippon Steel said it will release data on water quality to the public and take other measures to prevent a recurrence of the problem.
Nippon Steel submitted the report on Friday to the Chiba prefectural government and the Kimitsu municipal government, as well as the local governments of neighboring municipalities Kisarazu and Futtsu.
"We sincerely apologize for betraying the trust of the local community and public administration. We will do our utmost to prevent a recurrence," East Nippon Works Director Junichi Tani said at a press conference on the day.
In June, red-colored water was observed in local waterways and in the Koitogawa river near the plant. Following the reports, Nippon Steel conducted daily checks on the water flowing out of its drains and detected cyanide 12 times from June to July.
The company reported those cases to the prefectural government.
Cyanide is highly toxic. Under the Water Pollution Prevention Law, the standard for "no detection" is less than 0.1 milligram per liter.
If cyanide is detected in water quality checks conducted once every three months, plant operators and others are required under the law to record the results and take preventive measures.
Following an internal investigation, Nippon Steel revealed cases when the cyanide levels in local waterways were higher than the levels reported to authorities.
It also revealed that the plant conducted additional checks on subsequent days when cyanide had been detected, recording the levels of the day the measurements fell within the standards.
The company reexamined past records and found that cyanide was detected 59 times between 2017 and this year in statutory and voluntary investigations.
Cyanide was also detected in a drainage ditch inside the plant, but Nippon Steel failed to report it to the prefecture and the three cities even though it is obliged to do so under an agreement between the company and the local governments.
"This wasn't a cover-up," Tani said. "We mistakenly thought we were not required to submit reports when [cyanide levels] exceeded the standards, as long as [the substance] was detected in voluntary investigations."
Nippon Steel has identified the causes of the problems, including damage to a tank at the site, and has taken measures to prevent further leaks.
No colored water or hazardous substances have been detected in local waterways since the problems were identified.
Nippon Steel said it is considering taking disciplinary action against those involved and conducting unannounced environmental inspections at the plant.
The prefectural government and the three municipalities are also considering handing out administrative punishments.
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