Work to cap contaminated sediment in St. Clair River expected this summer

Dow Canada-led work to cap historical mercury-contaminated St. Clair River sediment could take place this summer, officials say.

The work was originally planned for last fall, but required permits weren't secured in time, according to Environment and Climate Change Canada.

“The Dow contractor, QM Environmental, anticipates going ahead with the work as soon as weather and fisheries considerations allow, likely summer of 2024,” the department's Samuel Lafontaine said by email.

“Although the conservation authority is not involved in the implementation phase of the sediment management work, we are pleased that plans are still underway to implement the engineering and design plan that was completed in 2021,” Mike Moroney, St. Clair River remedial action plan coordinator with the St. Clair Region Conservation Authority, said by email.

Construction in the summer would follow the fish spawning season, he said.

The contamination is believed to have come from Dow Chemical's Sarnia operation, which closed in 2009. A 2014 recommendation to the federal and provincial governments called for removing the sediment by hydraulic dredging at an estimated cost of about $28 million.

But an engineering consultant recommended in 2021 that the polluted sediment be capped, not removed.

Erosion-resistant cover will be placed in the three priority areas in the river near the Canadian shoreline.

Consultants found upstream sediment cleanup between 2001 and 2005, and along the shore of Dow Chemical's former facility, helped cut mercury concentrations. Testing also determined the highest concentrations were buried in sediment.

Instead of hydraulic dredging, the consultant recommended adding “an erosion-resistant cover” of washed, fine gravel to three areas of river bottom with the highest mercury concentrations, Moroney said.

That approach is expected to meet the project's original goals and avoids risks that come with dredging, like “re-suspending” contamination in the river, he has said.

A 2021 report identified three priority areas for the work:

Near the Transalta site (formerly Dow Canada) and Suncor's site;

Near property owned by Enbridge and Shell;

Near Guthrie Park in St. Clair Township.

Dow will fund the work, Environment and Climate Change Canada has said.

The project is key to plans to restore two beneficial use impairments — restrictions on fish and wildlife consumption and degradation of benthos — following the river's 1987 designation as an area of environmental concern under the Canada-u.s. Great Lakes Water Quality Agreement.

Several of the eight impairments identified in 1987 for the St. Clair River area of concern have been removed and work continues to address the rest.

Sarnia Coun. Terry Burrell sits on a joint Canada-u.s. advisory group that is part of long-running efforts to restore the designated impairments so the river can be removed from the list.

Burrell said he's not concerned with the new timeline and is happy Dow is leading the work.

“That's good news to me, that industry is taking on their part, as well as the government,” he said.

“It is probably the last major item that needs to be done,” Burrell said of the capping project.

“They found that once industries stopped contributing pollution into the water, the river to a great extent was able to heal itself,” he said.

Poisonous mercury found in pickerel in 1970 led to the closure of commercial fishing in Lake St. Clair, the St. Clair River and portions of Lake Erie.

The mercury-contaminated sediment is located upstream of the Detroit River and the City of Windsor's intake source for drinking water.

A number of Great Lakes sediment remediation projects have been recently undertaken. Mercury is just one of the toxic legacies of more than a century of heavy industrial activity.

Scientists warned last year that toxic sediment in the Detroit River was an “urgent” concern and that a U.S. funding deadline for such expensive cleanups was looming.