The Natural Resources Defense Council said it was vindicated by peer-reviewed studies showing heavier grades of crude oil should be treated with caution.

A 180-page study from the National Academies of Sciences, Engineering, and Medicine finds diluted bitumen, a heavier grade of oil found in North America, has similar properties to other forms of oil immediately after a spill. Weathering processes, however, mean bitumen doesn't evaporate, as is the case with lighter oils, but forms a residue.

"For this reason, spills of diluted bitumen post particular challenges when they reach waters bodies," the report reads. "In some cases, the residues can submerge or sink to the bottom of the water body."

That property can make cleanup operations difficult. Anthony Swift, director of Canadian programs for the NRDC, said the journal is "skewering" talking points by energy companies behind the delivery of the heavier oil.

"The tar sands industry has continued to insist that diluted bitumen creates no deeper environmental threat as they push for unsustainable growth," he said in a statement.

Line 6b, carrying heavy oil through a broader network operated by Canadian company Enbridge, ruptured near Marshall, Mich., in 2010, dumping more than 25,000 barrels of oil into the surrounding community. More than 30 miles of the region's Kalamazoo River were soiled by the spill, making it one of the worst incidents of its kind.

In a settlement with the state of Michigan, Enbridge is required to monitor the environmental impacts of the spill and spend as much as $75 million in restoration and remediation work in and around the Kalamazoo River.

Some of the environmental concerns surrounding bitumen were offered by the White House in its reasoning behind a rejection of the planned Keystone XL oil pipeline from Alberta, Canada. A series of pipelines are planned across North America that could carry heavier oils, projects the NRDC's Swift characterized as "risky."