CNRL reported a break in a well at its Wolf Lake project in late October.
Alberta's energy regulator says it later discovered elevated levels of hydrocarbons in the aquifer 60 kilometres northwest of Cold Lake.
The Wolf Lake project was using a method called cyclic steam stimulation. Often described as "huff and puff," it alternates between injecting steam and drawing the softened bitumen to the surface.
The same technology was being used at CNRL's Primrose site during a spill in 2013 where nearly 1 million litres of bitumen leaked into the surrounding area.
The company was forced to stop extraction after the spill, but later applied to resume work at the Primrose site, saying it would modify the steaming method used.
Greenpeace Canada's Keith Stewart says the technology has been a problem since 2009, but government has yet to take any action.
The energy regulator says CNRL must clean up the aquifer.
Stewart, however, says he doubts a clean-up is possible, given how tough the chemicals involved are to filter out.