The City of Lethbridge is investigating the source of a yellow, oily substance that was found in the Oldman River but is reassuring the public that the city's drinking water is safe.

Authorities first discovered the substance, that entered the Oldman River through the storm water system, on Tuesday.

Crews have already contained the spill and are actively removing the material from the water.

According to preliminary tests, the substance appears to be cooking oil.

Leanne Lammertsen, water and wastewater operations manager for the City of Lethbridge, says between 3,500 and 4,500 litres of oil spilled into the water.

"The problem is that material could be anywhere in the storm system between the source and the river and we're just containing it at the river and it takes time."

She says the oil is easy to contain with booms because it's sitting on the surface of the water.

"We are using a [vacuum] truck to remove the substance from the surface of the water."

Lammertsen says in addition to cleaning up the mess, they are working to determine the exact source of the oil.

"We've got crews going through the catchment area for this particular outfall that are just looking into the source."

The City of Lethbridge insists the city's water is still safe to drink and residents don't need to take any further steps.

Officials remind residents that all materials released into street drains ends up in the Oldman River completely untreated.

Residents should take care about the pollutants they could inadvertently add to the storm water system such as chemicals from washing vehicles and equipment, debris and liquids from the street or pouring waste directly into the storm drains.

Examples of harmful materials include:

  • cooking oils and grease
  • industrial waste
  • pesticides, herbicides or fertilizers
  • trash, soil, leaves, and grass clippings
  • gasoline, motor oil, transmission fluid and antifreeze
  • solvents and paints
  • water from pools and hot tubs
  • soaps or detergents (including biodegradable products)
  • cement/concrete waste
  • sawdust and construction materials
  • any substance that gives off an odour

The city says only clean water or naturally occurring storm water should enter the drainage system.

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The City of Lethbridge is also working very closely with Alberta Environment in regards to this incident.

(With files from CTV Lethbridge's Terry Vogt)