Parks Canada is testing mucky sediment along the Ottawa stretch of the Rideau Canal to decide whether the UNESCO World Heritage Site should be added to an inventory of Canada's most contaminated properties.

At the same time, the Ontario government is investigating whether game fish in the canal have dangerous levels of toxins, and should never be put on a dinner plate.

Both moves this spring follow Parks Canada's decision to temporarily halt repairs along a section of the picturesque canal after workers churned up canal-bed toxins while laying bricks in November.

Historians say parts of the Rideau Canal in Ottawa were once industrial centres, with manufacturers on both banks and steam-powered vessels carrying industrial products. Some toxic waste found its way to the canal bed. (Library and Archives Canada) Tests revealed the presence of heavy metals, such as lead and cadmium, as well as polycyclic aromatic hydrocarbons, which have been linked to various cancers.

"As part of the standard process for reporting contaminated sites, Parks Canada is undertaking further tests to identify the scope and extent of contaminants within the stretch of the Rideau Canal in downtown Ottawa," said spokesperson Darryl Whitehead.

"Once these tests are complete, Parks Canada will take all necessary steps to update the Federal Contaminated Sites Inventory."

For a century, the canal's Ottawa section featured industrial activity on both banks, including a paint factory and regular train traffic, as well as steam-powered vessels hauling industrial goods.

Dark legacy of pollution

Today, the canal is de-industrialized as a major tourist attraction – welcoming skaters in winter, pleasure craft in summer – but with a dark legacy of pollution lurking below.

Canal repairs along a 500-metre section resumed in the winter, but were restricted to work that will not disturb the sediment. In the meantime, Parks Canada has hired consultants to carry out detailed testing to assess the levels of various toxins along several kilometres of the canal bed.

... a very low risk to the public as this type of contamination is generally only a concern if there is dermal contact or ingestion. - Parks Canada on risk from toxins in the sediment bed of the Rideau Canal in downtown Ottawa

"The presence of contaminants in the canal bed, although not unexpected in an urban area, is generally not known to stakeholders and the public," says a Jan. 4, 2017, briefing note for Environment Minister Catherine McKenna, obtained by CBC News under the Access to Information Act.

"A negative reaction and questions about Parks Canada's management of this project are possible."

In a release, Parks Canada said that the contaminated sediment "presents a very low risk to the public as this type of contamination is generally only a concern if there is dermal contact or ingestion."

It added: "During the skateway season, sediment is contained by snow and ice and therefore the risk to human health is even further reduced."

Government 'carefully reviewing' info

The agency acknowledges, however, that fish can ingest sediments. And the Ottawa section of the canal attracts urban fishers looking for carp, muskies and other sports species.

But fishing — even in a federally owned canal — is regulated and monitored by the Ontario government, which now says it is assessing the situation.

Workers drill through ice on Rideau Canada in Ottawa to prepare the skateway. Parks Canada says sediment is contained by snow and ice during skateway season, reducing any risk to human health. (Roger Dubois/CBC)

"The Ministry [of the Environment] is carefully reviewing this new information about sediment contamination in the Ottawa section of the Rideau Canal," said spokesperson Gary Wheeler.

"We are reaching out to Parks Canada for additional details. As soon as we have reviewed this information, we will be able to determine whether further testing is needed in addition to the ongoing monitoring that we conduct."

Parks Canada is preparing the canal and its locks for the annual spring opening on Victoria Day weekend in late May, with some special events to mark the 150th anniversary of Confederation. Pleasure boating traffic is expected to increase this summer because the agency has dropped all lockage fees to celebrate the sesquicentennial.

Raises profile of industrial history

Parks Canada has long known of the historic industrial pollution trapped in the canal bed, but the recent round of repairs — drawing on a $13-million fund for the Ottawa section — appears to have raised the profile of the problem.

The McKenna briefing note says no testing of local contaminants was carried out before the repair work began near Ottawa's Lansdowne Park on Nov. 7, though preliminary tests were conducted at other locales not slated for immediate repairs.

Canadian Museum of Nature scientist André Martel holds an old mussel specimen taken from the Rideau Canal in Ottawa. The species is no longer living in the canal partly because of industrial pollution. (Julie Ireton, CBC Ottawa) Parks Canada says it has finalized a contract for comprehensive contaminant testing to be carried out later this month, after the ice disappears. The section to be sampled stretches from the Hartwells Locks, near Carleton University, to the Ottawa Locks at the base of Parliament Hill, where the canal enters the Ottawa River, about 6.7 kilometres in total.

"The sediments in question are low risk to health but of course we are monitoring them and we'll continue to do so," McKenna said this week. "Parks Canada is going to be working hard making sure that the conditions are safe and taking appropriate measures."

Scientists contacted by CBC News say the industrial waste is likely well buried by sediment, and that even if tests uncover toxins the safest option may be to leave them untouched rather than carry out a cleanup.

The Rideau Canal, which opened in 1832, runs more than 200 kilometres from Kingston, Ont., to Ottawa. Originally a strategic waterway to provide an alternative transportation route in the event of war with the United States, the canal was declared a World Heritage Site by the United Nations Educational, Scientific and Cultural Organization in 2007.

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