An unusual spill of industrial pollution has prompted a two-week cleanup on the Bow River in Calgary.

Last September, members of a group that offers educational boat tours of the Bow River noticed thousands of small white plastic pellets floating on the waterway.

That prompted calls to the city and to Alberta Environment.

Nothing was publicly disclosed at the time. Details of the incident were included in a recent environment, health and safety report to city council.

The city's manager of water quality services, Francois Bouchart, said the source of the spill was traced back to a stormwater outfall and from there, to an industrial plastics company.

"Our investigation determined that these pellets had actually been released from a storage vessel and that the vessel had failed … once, we discovered that, we worked with the party involved in the release to ensure that it was contained."

Rain washed pellets into the river

He said it turned out the pellets were washed into a storm drain during heavy rainstorms last summer.

It's not an every day kind of spill.

"It's actually very unusual. It's not something that we've come across before," said Bouchart.

Francois Bouchart, manager of the city's water quality services, said an investigation tracked the source of the pellet spill to an industrial plastics company. (CBC)

He said typical plastic pollution in the city's waterways includes items like bottles, cups or straws.

Alberta Environment says the white polyethylene pellets, which measure about five millimetres in diameter, were cleaned up and there was no threat to human or animal health.

No charges laid

The department has no plans to lay any charges for the spill.

The cleanup was done by 10 members of Riverwatch, which was paid by the plastics company to do the work.

Bouchart said it's believed about a tonne of the material went into the storm drain and about half a tonne of material has been recovered.

When water levels on the Bow River recede later this fall, he said there will be monitoring done to see if more pellets can be found in the river.

The plastics company has told the city it has put in place measures that will prevent a repeat of the pellets making it to the stormwater system.

Calgarians are being reminded by the city they can help keep waterways healthy by not littering around waterways, near storm drains or anywhere else that could allow items to be washed into the stormwater system.