Billionaire businessmen Charles and David Koch are channeling money into an Arizona-based organization that’s fighting a plan that would include a permanent ban on uranium mining around the Grand Canyon.

A proposal to declare the area around the Grand Canyon a national monument – Greater Grand Canyon Heritage National Monument – calls for protecting 1.7m acres of land from uranium mining. A number of environmental groups and native tribes as well as the vast majority of Arizonans support the plan.

Still, a handful of Arizona Republicans and a major not-for-profit group are trying to block it. Much of the group’s efforts apparently are being funded by the Koch brothers, according to Greg Zimmerman of the Center for Western Priorities.

Zimmerman told the Phoenix New Times he became suspicious about the Koch brothers’ involvement with the anti-Grand Canyon national monument movement after reading a recent report co-authored by the Arizona Chamber Foundation and another Arizona-based group called the Prosper Foundation. The report referred to the plan as a “monumental mistake”.

Interested in learning more about the Prosper Foundation, Zimmerman looked through its 990 tax forms, which not-for-profit groups must file with the Internal Revenue Service. He found that between 2013 and 2014, the foundation received more than $1.5m – or 83% of its total budget – from a political-advocacy organization called American Encore.

Sean Noble, a political consultant who has deep ties to the Koch brothers, leads American Encore. The organization is formerly known as the Center to Protect Patient Rights. It changed its name in 2014.

Zimmerman discovered that a donor from the Koch brothers’ funding network was funneling money into the Prosper Foundation, so the group could continue its on-the-ground campaign to block the Grand Canyon national monument plan.

We know that these anti-public land efforts have a lot of money behind them Greg Zimmerman

“I wish I could say I was surprised by this, but honestly I wasn’t,” Zimmerman said.

“We know that these anti-public land efforts have a lot of money behind them,” he continued. “It’s not surprising to learn that the Koch brothers and other wealthy, ultra-conservative industrialists are funding these efforts to roll back conservation measures across the American west.”

Messages to Prosper Foundation and American Encore were not immediately returned on Wednesday.

Zimmerman’s discovery comes a few months after the release of a poll by the Grand Canyon Trust that found 80% of Arizona voters are in favor of the Grand Canyon national monument plan.

Despite the wide support, a number of Arizona Republicans, including governor Doug Ducey and US senators John McCain and Jeff Flake, oppose the plan. Paul Gosar, a Republican congressman from Arizona and a vocal opponent, stated the plan would “stifle development, kill jobs and erode the extensive cooperation and success that federal and state agencies in Arizona have achieved to date”.

Zimmerman said he supports the plan because it would protect “sacred” public lands from possible contamination caused by uranium mining.

“The Grand Canyon and the lands around the Grand Canyon are sacred – I think all Americans recognize that,” he said. “It’s truly an incredible landscape, and the threat of uranium mining around the Grand Canyon poses a risk.”