The owners of the planned Pebble Mine in Alaska are firing back against a longtime mining executive who raised eyebrows when he said the project would lose $3 billion over its 20-year lifespan.

Richard Borden’s prediction that the mine would produce only a small amount of low-grade minerals has sparked controversy because he admits he was paid by an environmental group to write his analysis, even though he is widely seen as pro-business after spending 23 years at British mining giant Rio Tinto Plc.

Borden’s prognosis could scare off potential Pebble investors at a time when the company appears to be trying to raise funds. Twice in the last month, the firm has sold large blocks of shares, saying it needed the money to fund an environmental impact study and for “enhanced outreach and engagement with political and regulatory offices.”

Borden also told Bloomberg Environment he thinks his findings, based on publicly available information, constitute sufficient reason for the U.S. Army Corps of Engineers to extend the 90-day comment period now ticking on the agency’s draft environmental impact statement for the mine.

Borden sent his conclusions to the Corps in a March 28 letter.

John Budnik, a Corps spokesman, said the agency couldn’t respond to the letter because its key personnel are attending public meetings on a draft environmental impact statement in Alaska.

Pebble says its mine will generate 318 million tons of copper, 1.8 million ounces of silver, and 362,000 ounces of gold every year.

Pebble: No Objectivity

Borden, former head of environment at Rio Tinto’s copper, diamonds, and coal groups, predicted the mine would produce only about half the metals that were forecast in a 2011 economic assessment report written by Wardrop Engineering Inc., now owned by the consultancy Tetra Tech.

The gold, copper, and molybdenum at the site is also relatively low grade, and Pebble has itself said it’s only mining 10 percent of the total mineral resource at the site, Borden wrote.

“I think they intentionally avoid highlighting this number to the general public, investors, and regulators,” Borden said.

He also said the Pebble Mine is “the most sensitive environmental siting for a mine that I’ve ever seen in my career.”

But Sean Magee, a spokesman at Pebble owner Hunter Dickinson Inc., said the Natural Resources Defense Council—one of the mine’s most vocal opponents—paid Borden to study the project, diluting his findings.

That renders Borden’s findings “considerably less than objective or independent,” Magee told Bloomberg Environment.

Borden also never contacted Pebble to request or verify information about the project, Magee said.

Pebble Says Information Outdated

“From our review, we cannot see any evidence that Mr. Borden has completed the proper financial and technical analysis that is required to establish whether or not a mine at the Pebble Project is feasible,” Borden said. “He has based his conclusions on information he admits as being outdated and has qualified his conclusion as being conceptual only.”

Borden said his conclusions were based only on publicly available documents.

Mike Heatwole, a spokesman for the Pebble Limited Partnership, added that the Corps’ draft environmental impact assessment “speaks clearly about our approach to responsibly managing the environmental issues at Pebble.”

The Corps document finds that that mine won’t harm returning adult salmon to the Nushagak or Kvichak river systems, and won’t cause long-term damage to the health of the Bristol Bay and Cook Inlet fisheries, Heatwole said.

Consultant Says Acting for ‘Good of the Industry’

Borden confirmed to Bloomberg Environment that NRDC paid him for the study. Neither he nor NRDC specified the amount, but Borden said it was half his usual rate as the owner of consultancy Midgard Environmental Services LLC.

“My professional credibility is worth more than a couple of thousand dollars,” Borden said. “And I won’t accept the money from the NRDC if I’m accused of selling out my professional credibility for such a small amount.”

“I’m doing this for the good of the industry,” he added. “We all get tarred with the same brush when something goes wrong, and it’s not fair to the people who are playing by the rules.”

He also said he approached NRDC with the idea of studying Pebble, not the other way around.

Borden said he worries that Pebble can only become profitable if it vastly expands its current mine plan. That would require more environmental permits. Ronald Thiessen, Hunter Dickinson’s chief executive officer, strongly hinted at that possibility in a September 2017 speech to investors at the Denver Gold Forum.

“Regulators look at footprint and impacts, and as we try and grow the economic outcomes, our footprint grows, and so does our impacts,” Thiessen said about Pebble Mine.

In Meetings, Cons Outnumber Pros

The Army Corps’ first five of nine scheduled hearings on its draft EIS wrapped up in late March.

Of those who made comments, 83 opposed the mine and 23 supported it, according to Pebble opponent United Tribes of Bristol Bay.

Shane McCoy, program manager with the Corps’ Alaska district, said March 20 that the agency hadn’t heard any “compelling” reasons to extend the comment period beyond its 90-day duration, which ends May 30.