The CN derailment east of Prince George, B.C., last week spilled 40 tonnes of petroleum coke in and along a creek in the Fraser River watershed, according to the provincial environment ministry.

A ministry official said solid and powder forms of the substance spilled from two open rail cars that tipped along a 20-metre stretch of Hay Creek in the community of Giscome, about 44 kilometres northeast of Prince George.

Petroleum coke is a byproduct of the oil sands refining process that burns hotter than coal.

In a written statement, CN said the product is "non-hazardous."

The derailed cars were also carrying liquid petroleum gas and methanol.

Dale Bull, a senior environmental emergency response officer with the ministry, says there's no evidence the powder has travelled any distance downstream the creek, which is part of the watershed that feeds the Fraser River.

"Our biggest environmental concern is going to be with that small chunk of Hay Creek that's been impacted," he told CBC News.

"[Fine powder] was piled up on the shoreline and into the water," he added.

CN cleanup crews are working to remove petroleum coke from a northern creek, after the derailment on March 5. (Mike Kubilius/Contributed )

Challenging cleanup

The fact that much of the coke is in powdered form means removing it from the creek will be challenging, Bull said.

"You need to be very careful getting the powder out [of the creek] so that the powder doesn't go back in," he said.

Bull says it will be up to CN Rail to mitigate any environmental damage that might occur.

CN says cleanup crews have been using skirted booms and silt curtains to physically remove the substance from the water and creek banks.

Next, it will use excavators to clean up nearby soil and snow and move the recovered petroleum coke out of the area.

The company has also been doing water testing.

The Transportation Safety Board has concluded its on-site investigation, but the cause of the derailment isn't yet known.

CN Rail cars carrying petroleum coke, liquid petroleum gas and methanol derailed in a rural area northeast of Prince George on March 5. (Submitted by Mike Kubilius)

Evacuated school still closed

Twenty-eight cars went off the track during the derailment March 5.

Initially, CN reported there had been no leaks. The following day, the railway company said a "small amount of petroleum coke... has spilled into an adjacent creek."

The derailment forced the evacuation of a rural elementary school just 200 metres from the rail line. Six days after the derailment, the school remains closed as the cleanup continues.

VIA Rail has cancelled passenger service along the line serving Prince Rupert, Prince George and Jasper, Alta., due to the derailment.