The fate of two major oil pipelines for carrying crude oil from Canada's tar sands region has been called into question over environmental concerns as judges in Minnesota overturned a key approval for a proposed pipeline and Michigan's attorney general threatened to shut down an aging pipeline under the Great Lakes.

The actions are a further setback for Enbridge, the company behind both pipelines, and for Canadian tar sands oil producers that have struggled in recent years as attempts to build more pipeline capacity failed.

New pipelines for tar sands crude oil have faced fierce opposition from environmental and indigenous rights advocates who fear both the immediate effects of an oil spill and as the climate impact from tar sands oil, a particularly carbon-intensive fuel.

Sign up for InsideClimate News Weekly Our stories. Your inbox. Every weekend.

The Minnesota Court of Appeals on Monday reversed a decision by state regulators who, last June, approved an environmental impact assessment for a larger replacement of the existing Line 3. The project would carry tar sands crude oil across northern Minnesota on its way from Alberta to U.S. refineries.

The judges ruled in favor of environmental and Native American groups who argued that the environmental impact statement of the proposed replacement pipeline did not address the risk of an oil spill into the Lake Superior watershed.

The Latest Pipeline Setbacks for the Tar Sands

The ruling was the latest setback for a series of five pipeline projects designed to bring additional tar sands crude to market that have either been canceled or delayed. The other projects include Energy East and Northern Gateway, both of which were canceled, and Trans Mountain expansion and Keystone XL pipelines, both of which are on hold.

"What you are looking at is an industry that has put forward these five proposals and so far none of them have been completed," Collin Rees of Oil Change International said. "Line 3 I think they actually thought was the one that was almost certainly going to go through, but there has been this incredible resistance. The political terrain has changed, and it's looking much less certain."

Enbridge in March had announced a one-year delay of the Line 3 project due to permitting hold-ups. That decision caused at least one tar sands producer in Alberta to hold off on the completion of a US$200 million expansion of its extraction operations due to limited pipeline capacity.

It remains unclear how this week's ruling will affect the Line 3 project. Enbridge officials said they were "disappointed with the court's decision" and will consult with state regulators about next steps.

Fears an Aging Great Lakes Pipeline Could Leak

The Minnesota ruling came less than a week after Michigan Attorney General Dana Nessel said she would shut down Line 5, another Enbridge pipeline, in a matter of weeks if negotiations between the company and the state's new Democratic governor don't reach a resolution.

Line 5 is an aging oil and gas pipeline that can carry light synthetic crude oil, a partially refined form of tar sands crude, and conventional light crude oil. It crosses the lakebed under the Straits of Mackinac, which connects Lakes Michigan and Huron.

The 66-year-old pipeline had lost chunks of its outer coating and appeared to have been dented by a ship's anchor last year, raising fears of its vulnerability for future leaks that could contaminate Michigan's shorelines. The company proposed building a tunnel to house a replacement pipeline, keeping the old pipeline operating in the interim, and former Republican Gov. Rick Snyder approved an agreement. But Nessel issued a formal opinion in March that a 2018 law related to that plan was unconstitutional. That led to new talks between Gov. Gretchen Whitmer and Enbridge.

Michigan is sensitive to the risks of pipeline ruptures, particularly involving the tar sands: in 2010, more than 1 million gallons of diluted bitumen spilled into the Kalamazoo River from Enbridge's Line 6B in one of the largest inland oil spills in U.S. history. The disaster forced evacuations, shut down miles of the river and took years to clean up.

"I continue to believe that our state is in great peril every single day that Line 5 continues to operate," Nessel said in an interview with the Detroit Free Press last week. "And so I've indicated that I'll give the governor a little bit more time to try to resolve this. But if she's unable to do that, I have independent authority to act and I plan to do so."

Enbridge issued a press release in response , saying it is continuing discussions with Whitmer and that a new pipeline could be in service in 2024. But on Thursday, an Enbridge executive told the Associated Press the company would ask a state court to rule on the legality of the construction agreement it had reached with Snyder shortly before he left office.

The inability to get new pipelines approved and the potential for existing pipelines to be shut down could have a significant impact on financing for future tar sands extraction projects, said Joshua Axelrod, a policy analyst with the Natural Resources Defense Council.

"If they lose a pipeline like this, the delays, they all send a very negative signal to the finance and investment community," Axelrod said. "Already the view of tar sands as a good investment is pretty dim and it just continues to solidify that, that becomes a bigger impediment to future growth. If investors are turning away, they can't grow."

Editor's note: This story was updated to add details from a 2015 Enbridge agreement with the state on the types of oil Line 5 can carry.