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Imperial's Kearl oil sands operation.

Imperial Oil Ltd.’s IMO-A Kearl oil sands facility is again under the scrutiny of the Alberta Energy Regulator after hundreds of thousands of litres of water from a settling pond poured into the Muskeg River.

The most recent incident at Kearl happened on Nov. 13, following two other incidents early this year.

Settling ponds are a common feature of oil sands projects. They capture surface water runoff to let contaminants settle before the water can be released. In this case, the total suspended solids in the water that escaped reached around 140 milligrams per litre – more than four times the legal limit of 30 mg/l set by the AER.

Approximately 670 cubic metres of water was released into the Muskeg River, according to the regulator, or around 670,000 litres.

This most recent incident will have less environmental impact than the previous two, which led the AER to issue Imperial a non-compliance order and an environmental protection order.

In the first incident, water tainted with dangerous levels of arsenic, dissolved metals and hydrocarbons seeped from tailings ponds for more than a year into muskeg, public lands and waterways that are home to wildlife and fish. Then, in February, a drainage pond – which collects industrial wastewater laced with pollutants from the surface of the site – overflowed, spilling an estimated 5.3 million litres into the environment.

The federal government, local Indigenous communities and the public at large were not informed of the first incident at Kearl until after the February spill. As a result, Imperial executives were called to Ottawa to testify before the House of Commons environment committee, to explain how the company handled the problem.

The release from an oil sands settlement pond isn’t the first this year, either.

In April, almost six million litres of water with more than twice the legal limit of suspended solids escaped from a pond at Suncor Energy Inc.’s Fort Hills oil sands project into the Athabasca River watershed.

Imperial spokeswoman Lisa Schmidt said in an interview Tuesday that the incident at Kearl last week was caused by erosion of a culvert.

The water itself was already treated and ready for release, she said, but soil got into it when the culvert eroded, sending it into a channel that leads to the Muskeg River.

Ms. Schmidt said the company halted pumps and stopped the flow of water as soon as workers noticed. She added that work to fix the culvert is still under way.

The AER said Imperial will conduct additional sampling to ensure that the amount of total suspended solids in the water is reduced before it begins releasing water from the pond.

Imperial has also taken water samples upstream and downstream in the Muskeg River, and at a federal government environmental protection compliance point, which measures water quality and is downstream of the culvert failure.

The AER said it would review the water quality samples to determine any potential effects to fish and fish habitat.

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