Enbridge aims to reverse a section of its Line 9 pipeline between Sarnia and Westover in Ontario. The company said the pipeline is delivering 240,000 barrels of oil per day of crude oil in a westward direction.
Gillian McEachern, campaign director for Environmental Defense, said Canadians need "real answers" from Enbridge given its safety record.
"Enbridge's Line 9 oil pipeline proposal is all risk and no reward for Ontario," she said in a statement. "Our drinking water and farmland should not be put at risk of a potential tar sands oil spill just to help big oil get even bigger."
A spill in 2010 from an Enbridge pipeline in southern Michigan was the costliest onshore incident in the United States. That pipeline carried so-called tar sands, considered by some to be a corrosive form of crude oil that lingers longer in the environment.
Enbridge has defended its record, saying safety comes first in its pipeline plans.
McEachern said the people of Ontario are calling for "a full review of the risks of this project."