Heavy Canadian crude oil shipped through northern Minnesota pipelines poses a special risk when spilled into waterways because it soon turns into a thick, hard-to-recover residue that doesn't degrade and whose toxic effects are poorly understood, the National Academies of Sciences said in a research report Tuesday.

The report for the U.S. Transportation Department recommended regulatory changes to speed cleanups of spilled heavy crude oil, to require disclosure of what oil pipelines carry and to conduct more research of human and ecological risks and other knowledge gaps.

The findings are important to Minnesota, Wisconsin and Michigan, where pipelines carry diluted bitumen from Alberta to Midwest oil refineries. The oil, called bitumen, must be diluted with lighter ­hydrocarbons to flow through pipelines. The diluted bitumen is called dilbit.

The National Academies report said that when dilbit spills, the light hydrocarbons usually evaporate. The "relatively dense and viscous" material left behind tends to sink to the bottom of rivers and lakes and adhere to shoreline and wetland plants, the report said. Unlike lighter crude oils, which biodegrade, "the recalcitrant nature of bitumen" means that aquatic organisms are exposed to its toxic effects for longer periods, the report said.

"There is an opportunity to act quickly within the first several days," Diane McKnight, a University of Colorado Boulder environmental researcher who chaired the research panel, said in an interview. "We're optimistic that these practical and pragmatic recommendations could influence how these spills are handled."

Environmentalists opposed to crude oil pipelines said the report shows why no new projects should be built.

"There are definitely unique risks to Minnesota waters, and we are not ready to face those risks," said Paul Blackburn, a Twin Cities environmental lawyer who has fought pipeline projects in the state.

The report, "Spills of Diluted Bitumen from Pipelines: A Comparative Study of Environmental Fate, Effects, and Response," said that 250 million barrels of diluted bitumen is imported to the United States from Canada each year. Blackburn estimated that two thirds of the heavy crude flows through northern Minnesota pipelines to Superior, Wis., where Enbridge Energy has a terminal on Lake Superior.

"It's a dangerous substance," added Jim Murphy, senior counsel for the National Wildlife Federation. "It puts resources at risk in a way that regular oil doesn't, and it is much more carbon polluting than regular oil. We really just shouldn't be subjecting wildlife and communities to this risk. Certainly, expansion and building of new pipelines is a mistake."

The same heavy Canadian oil was to be carried in the Keystone XL pipeline that President Obama rejected in November.

The report comes nine days before the Minnesota Public Utilities Commission is scheduled to make key procedural decisions on Calgary-based Enbridge Energy's plans to replace and expand a 1960s-era pipeline from Canada through Minnesota to carry diluted bitumen and other crude. Enbridge also wants to build a separate crude oil line on the same northern Minnesota corridor to ship North Dakota crude oil. Both projects face key decisions about the scope of planned environmental studies.

"Our priority is preventing an incident from ever occurring," Enbridge spokeswoman Lorraine Little said of the academies' report. "We have robust emergency response plans in place. That is why we focus our efforts on providing the training, equipment and personnel to be able to act quickly in coordination with local, state and — if warranted — federal responders in order to protect people, property and the environment."

Enbridge's 2010 pipeline rupture in Marshall, Mich., that spilled 843,000 gallons of dilbit into a tributary of the Kalamazoo River was mentioned prominently in the academies' report. Enbridge has spent more than $1 billion to clean up the spill.

In a statement, the U.S. Department of Transportation Pipeline and Hazardous Materials Safety Administration said it will publish a bulletin to the pipeline industry on the dilbit report, suggest voluntary improvements to spill response plans and take other steps, including hosting a public meeting next year to consider revising federal pipeline regulations.

David Shaffer • 612-673-7090

Twitter: @ShafferStrib