Workers dragged yellow booms Tuesday across different parts of the Chaudiere River in an attempt to contain the gunky crude that continues to creep downstream with the speedy current.
A rainbow sheen now covers the snaking Chaudiere, which flows northward from Lac-Megantic and eventually spills into the St. Lawrence River.
The shoreline is enveloped by the smell of used motor oil and features greasy rocks that create countless mini-pools of the brown substance.
Environment Minister Yves-Francois Blanchet said the chances were "very slim" that the oil might spill into the St. Lawrence, the backbone of the province's water supply.
He predicted a lingering, but not devastating, effect.
"We are never going to get to a stage where it’ll be as if this never happened," Blanchet told a Quebec City news conference.
"There are always going to be a traces of this event — but the river won't be irreversibly contaminated in terms of the well being of its important ecosystems."
There are a variety of advisories for communities in the area to either boil their water or reduce their consumption.
While more than half of the Lac-Megantic's displaced inhabitants headed home Tuesday, they were asked to continue to boil water for at least five minutes before drinking it, and to reduce their overall water consumption to a bare minimum.
The boil-water advisory — ongoing since 10 p.m. Saturday — was issued by municipal authorities as a precautionary measure, despite no evidence yet of contamination in the area's potable water.
Blanchet said the news could have been far more grim.
"If the municipalities and environmental teams were not as efficient as they have been in establishing booms, we would probably be giving you much worse news today."
He said that the light oil would float, which made it easier to scoop up. But if not handled quickly, it would sink eventually — which is what Blanchet said he wants to avoid.
Environment Canada said it was still working closely with its provincial counterpart in testing the waters of both the Chaudiere River and Lake Megantic.
The Chaudiere, a major source of the town of Lac-Megantic's potable water, has been hit hard by the disaster. When a rogue train came barrelling off track last Saturday and exploded in the downtown core, the river nearby received an estimated 100,000 litres of oil.
A water surface of 10 km has reportedly been glazed with the oily film.
There are also still traces of fire in the pipes that evacuate rainfall, as well as in the town's sewage system.
The banks of Lake Megantic and the Chaudiere River are both contaminated, as is water flowing out of them.
The town's sewage system is also contaminated.
In the meantime, Lake Megantic has been tapping into an alternate source to obtain its potable water.
But the town razed by the fire hasn't been the only one affected by contaminated water — the repercussions have also trickled downstream, affecting several communities nearby.
Like the town of Lac-Megantic, the neighbouring communities of Levis, St-Georges and Ste-Marie all relied on the Chaudiere River as a source of potable water.
Those towns are, until further notice, being asked to reduce water use because reserves of clean water are running low.
As for the larger cities in the province, Quebec City's mayor said it would take a major disaster for the oil to get into the St. Lawrence and affect the reservoir that sustains 110,000 of his area's residents, with 60,000 cubic metres per day.
"It would take extraordinary events — like a tsunami," Regis Labeaume said.
"But if that were to happen, we are prepared."
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