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The Walkerton Tragedy is a series of events that accompanied the contamination of the water supply of Walkerton, Ontario by E. coli bacteria in May 2000.

The water supply for the town of Walkerton was operated by the Walkerton Public Utilities Commission, with Stan Koebel as supervisor. The water supply became contaminated with the highly dangerous O157:H7 strain of E. coli bacteria, likely from farm runoff. Starting May 15, 2000, many of the residents of the town of about 5,000 began to experience bloody diarrhea and other symptoms of E. coli infection. For days the Walkerton Public Utilities Commission insisted that the water supply was safe. On May 21, an escalation in the number of patients with similar symptoms finally spurred the region's Medical Health Office to warn residents not to drink the water.

Seven people died directly from E.coli, another 14 elderly people died of E.coli complications, and at least 2,300 people (more than 40% of the population at the time) became ill.

The Ontario Clean Water Agency was put in charge of the cleanup of Walkerton's water system.

An inquiry, known as the Walkerton Commission led by Ontario Court of Appeal Associate Chief Justice Dennis O'Connor, reported in 2002. The Part 1 report, released in November 2001, estimated that the Walkerton water tragedy cost a minimum of $64.5-155 million CAD and laid much of the blame at the door of the Walkerton Public Utilities Commission.

From the report:

For years, the Walkerton Public Utilities Commission operators engaged in a host of improper operating practices, including failing to use adequate doses of chlorine, failing to monitor chlorine residuals daily, making false entries about residuals in daily operating records, and misstating the locations at which microbiological samples were taken. The operators knew that these practices were unacceptable and contrary to Ministry of Environment guidelines and directives.

The Ontario government was also blamed for not regulating water quality and not enforcing the guidelines that had been in place.

The Part 2 report made many recommendations for improving the quality of water and public health in Ontario. All its recommendations have been accepted by succeeding governments of the province. The recommendations have also influenced provincial policies across Canada.

Key recommendations touched on source water protection, the training and certification of operators, a quality management system for water suppliers, and more competent enforcement. In Ontario, these requirements have been incorporated in new legislation.

The many lessons learned from this incident were described by Kim Vicente in his book The Human Factor.

A 2005 CBC Television docu-drama called Betrayed was based loosely upon the Walkerton tragedy.

A similar outbreak in North Battleford, Saskatchewan in 2001 caused illness in 200 people.

• Contamination: The Poisonous Legacy of Ontario's Environmental Cutbacks. History and context of the tragedy.
• Background article from the CBC
• Water Science and Technology article
• CBC Digital Archives - Death on tap: The poisoning of Walkerton
• The Walkerton Commission Reports
• Song "Waterton" by Melwood Cutlery