B.C.'s Court of Appeal will decide Friday whether the province can change its environmental laws in a way that could effectively kill the controversial Trans Mountain pipeline expansion project.

The province's top court was asked to weigh in on proposed amendments to B.C.'s Environmental Management Act forcing anyone wanting to transport increased quantities of "heavy oil" to seek provincial permission.

While B.C.'s attorney general argues the government has the right to enact laws aimed at protecting British Columbians from the effects of a catastrophic spill, Ottawa claims the move is a ruse designed with just one target in mind: the proposed twinning of a 1,150 pipeline to move bitumen from Edmonton to the West Coast.

The case revolves around weighty legal and constitutional issues, but it boils down to a few fundamental questions about the role of government in the lives of Canadians — and the balance between local and national interests.

In his opening arguments, Joseph Arvay, counsel for B.C., pointed to the array of lawyers for "pipelines and railways and others in the oil and gas industry" who had lined up to oppose him.

Lawyer Joseph Arvay is representing the attorney general of B.C. in a bid to introduce amendments to provincial legislation regulating the flow of bitumen from Alberta. (Darryl Dyck/Canadian Press)

He claimed their arguments all boiled down to one thing:

"That even if the pipelines or railways they own or operate create a risk of even catastrophic environmental harm because of the substance they carry, the province is nevertheless powerless to enact laws to prevent that risk from materializing," Arvay told the five appeal court judges.

"We say that the province is not required to accept such a fate."

'A Trojan horse'

The appeal court battle is the latest in a series of legal and political hurdles the pipeline expansion has faced since then-owner Kinder Morgan first lodged its plans with the National Energy Board (NEB) in 2013.

Last year, the federal government announced plans to buy the $4.5 billion pipeline from Kinder Morgan shareholders, who expressed dissatisfaction with the slow pace of the approval process.

The Federal Court of Appeal put the project on hold in 2018 after finding that the government had not adequately consulted with First Nations and the NEB had failed to fully consider impacts on marine life.

Ottawa has since moved to rectify those shortcomings.

The proposed twinning of the Trans Mountain pipeline from Alberta to B.C. has faced a series of legal and political hurdles. (Chris Helgren/Reuters)

The reference question was debated over five days in March. It's one of the rare proceedings B.C.'s appeal court has decided to webcast.

Jan Brongers, lawyer for Canada's attorney general, acknowledged the "superficial attraction" of Arvay's argument. But he warned the judges against falling for it.

"Our main concern is that this legislation appears to be a Trojan horse," he said.

"It's one that's designed to appear as constitutionally acceptable local environmental protection measures, but in substance it's an unconstitutional initiative whose only logical reason for being is to limit federally regulated pipelines and railways from moving additional quantities of heavy oil."

The court has been asked to consider three basic questions:

Does B.C. have the authority to enact the proposed amendments?

Would those rules apply to "heavy oil" from another province?

Would existing federal legislation trump the provincial rules in any event?

The federal government argued that it has exclusive jurisdiction over interprovincial pipelines and railways, and that B.C.'s legislation is simply a bid to kill a project the provincial NDP government vigorously opposes.

But Arvay claimed B.C. isn't trying to usurp Ottawa's power — that instead the proposed amendments would work in tandem with federal laws to ensure locals are protected from the risks of projects in their backyards.

"Decisions affecting individuals should as far as reasonably possible be made by the level of government closest to the individuals affected," he told the court.

Strange bedfellows

A host of intervenors have lined up on either side of the debate, with the cities of Vancouver and Burnaby supporting British Columbia along with the Assembly of First Nations.

"Local governments and First Nations governments are on the front lines of emergency response for their communities," the City of Vancouver said in its written arguments.

"It is these communities that are at risk from a heavy oil accident and who will suffer the impacts."

Several individual First Nations have also aligned themselves with the government of Canada, along with a number of oil and gas industry groups and the attorneys general of Alberta and Saskatchewan.

Alberta said the provinces and Ottawa should work together to protect the environment, but B.C. can't take unilateral steps that would result in the obstruction or delay of the project.

Regardless of what the appeal court decides, the ultimate decision on the law is likely to rest with the Supreme Court of Canada.