"To accurately assess the environmental consequences of the Replacement Project, the PSC should have determined whether the project would be the proximate cause of Line 5's continued operation and its alleged attendant harms," Justice Elizabeth Welch wrote for the majority.

The opinion directs the commission to determine whether the replacement project would extend Line 5's operating life and cause additional environmental harm, compare the project and its alternatives on a consistent basis and separately evaluate potential effects on public trust resources, including fishing, navigation and other public uses of the Great Lakes.

The commission had considered greenhouse gas emissions associated with hydrocarbons transported through the full pipeline system but limited its consideration of other environmental risks, including oil spills, to the replacement segment. The court said that approach was inconsistent and did not allow regulators to make a “reasoned choice” among project alternatives.

The decision does not vacate separate permits issued by the Michigan Dept. of Environment, Great Lakes and Energy (EGLE) or the Michigan Dept. of Natural Resources (DNR). Instead, it removes the commission’s approval under Public Act 16, which governs the siting of crude oil and petroleum product pipelines in Michigan.

EGLE announced July 15 that it had reissued a wetlands and Great Lakes submerged-lands permit following a 16-month review and forwarded the authorization to Enbridge for countersignature.

The agency found that the public and private need for the proposed activity outweighed other public-interest criteria but imposed new wetland and cultural-resource mitigation requirements.

The permit prohibits authorized construction activities to proceed until EGLE approves Enbridge's wetland and cultural resource mitigation plans. It also prohibits disturbance of cultural resources unless the Public Service Commission siting permit, the DNR threatened-and-endangered-species permit, an EGLE wastewater-discharge permit and a U.S. Army Corps of Engineers permit have been issued and are not blocked by a court.

The DNR issued its permit July 15 with conditions intended to protect threatened plants and animal habitat. The Corps authorization remains pending, while EGLE says documents for renewal of the project’s National Pollutant Discharge Elimination System permit are still being drafted.

The Supreme Court did not establish a timetable for the commission's renewed review. As of Aug. 5, the commission had not posted a procedural order outlining its next steps.

The commission’s original approval covered Enbridge’s proposal to replace two existing 20-in.-dia pipelines at the Straits with a single 30-in.-dia pipeline inside a concrete-lined tunnel. The tunnel would be bored through bedrock below the lakebed, allowing Enbridge to deactivate the existing pipelines once the replacement enters service.

Enbridge spokesman Ryan Duffy said the company is reviewing the opinion while weighing its legal options.

“The Court’s decision is unfortunate in that it adds an additional delay to the permitting process for the Great Lakes Tunnel, which is designed to protect the Great Lakes and ensure the ongoing safe, reliable and efficient delivery of energy to our region,” Duffy told ENR in an emailed statement.

Great Lakes Tunnel Project in Michigan Secures Needed Permits

“What remains unchanged is our commitment to operating Line 5 safely and responsibly, incorporating enhanced safety measures in the Straits, and protecting Michigan’s natural resources for generations to come,” he added.

In 2024 Enbridge selected Mackinac Straits Partners, a joint venture of Barnard Construction Co. and Civil and Building North America, to lead construction, with Arup serving as lead design engineer. Excavation has not begun.

When the tunnel proposal was unveiled in 2018, Enbridge estimated the project would cost about $500 million. An outside analysis previously cited by ENR estimated the project would cost at least $750 million by 2022.