THE PROMISES built into a ballyhooed new proposal for a $500 million propane export terminal on the Columbia River are substantial:

Millions in annual property taxes for Portland, Multnomah County, and Portland Public Schools! Hundreds of construction jobs and dozens of permanent postings! Tens of millions more dollars every year in local trade!

The Port of Portland, which hopes to place the propane terminal near Kelley Point Park on the east end of Terminal 6, even issued a statement last month calling the facility, pitched by Canadian firm Pembina, "one of the largest single private capital investments in the city's history."

That lure was enough to win a warm embrace from Mayor Charlie Hales, even though some might argue a propane terminal runs counter to the spirit of the city's 2009 Climate Action Plan, which urges working toward reduced carbon emissions in a bid to stanch the tide of global warming.

And yet, the Mercury has learned, the fate of those big promises—indeed, the fate of the propane terminal itself—is already in doubt. Ironically, thanks to something quite small.

The project, as currently envisioned, runs afoul of the city's zoning code—specifically, the city's rules for safeguarding sensitive wildlife along the Columbia. And unless Portland City Council is willing to slightly tweak those rules, at a hearing as soon as next spring, then the project would be impossible to build.

"It's a fairly minor change," insists Jackie Dingfelder, the policy director for Hales who's been tracking this issue. "But regardless, there will be a fully public process."

Dingfelder's correct. The technical change in the zoning code really would be minor—just a few sentences, officials say. But the consequences of that change might not be.

The issue, which came up after Pembina announced its intentions, involves a thin strip of riverfront protected by some of the city's strongest conservation rules. Pembina has talked about running a pipe across the beach, sending propane from two inland storage tanks out to a floating dock already on the river. But the code covering that beach bans the transportation of hazardous materials, like propane, unless it's done by rail or motor vehicle.

Planning and code enforcement officials have been weighing what seems like a fairly clean revision: adding pipes and pipelines to that short list of exemptions. Except that's not as easy as it seems. That kind of change in the zoning code, without limits, could apply to land with similar protections elsewhere in the city, farther up the Columbia and around places like Balch and Johnson Creeks.

"This is a code change that applies to more than this project," says Tom Armstrong, a supervising planner in the Portland Bureau of Planning and Sustainability.

To hedge against that, planning officials say they're looking at an exemption that focuses solely on shipping terminals. But even that might bring some wrinkles.

Any change allowing hazmat pipelines for Pembina, Armstrong says, would likely apply to the rest of the Port of Portland's Terminal 6—removing one more hurdle if the Port ever decided to seek a terminal for something more controversial than propane: liquefied natural gas. The Port already has that capability, in the zoning code, along the Willamette River.

"There is no intent to open the door for prospective future uses at Terminal 6 beyond the containers, autos, and bulk that are already handled there," says Josh Thomas, a spokesman for the Port. "That kind of addition is not something we are pursuing or contemplating. It is more about consistency with various other existing Portland Harbor activities/uses."

Another idea initially discussed, sources say, would have exempted the Pembina site from the environmental rules along the Columbia altogether. While that might have avoided unintended consequences at other spots in Portland, Pembina also would've been sprung from provisions requiring it to mitigate or pay for any environmental disturbance its piping causes.

Pembina officials didn't return messages seeking comment.

But Thomas continually stressed the "embryonic" nature of the Pembina proposal—making clear that the Canadian company had planned all along to do some "due diligence" before breaking ground.

Thomas did say, however, that the environmental issue came up "more recently" in discussions about the terminal.

Details about what Pembina might do with a pipeline, the storage tanks, and the floating dock have so far come from informal conversations with city and Port officials. Some of those details were also reported by the Oregonian, which ran an interview with Pembina's CEO last month.

The company's working on details at the same time as the city's pushing through its code change. Pembina has said it hopes to have the terminal open in 2018.

"I don't have a site plan," says Rebecca Esau of the Portland Bureau of Development Services. "We haven't seen anything."

Armstrong, the supervising planner, says a proposed code change could be out for public review by December, followed by a Planning and Sustainability Commission hearing in January. That would put a city council discussion on the change in March or April, he says.

Those hearings could open a window for environmentalists and critics of fossil fuel shipping to weigh in. Representatives of one advocacy group, Columbia Riverkeeper, declined to comment when asked by the Mercury if they'd seize on such an opportunity.

"The more [Pembina] can define this project," Armstrong says, "it will help people get comfortable with what the implications of this change might be. The onus is kind of on them to help put this change in context."