That resulted in a list of 46 mines that are close to homes and have high levels of radiation or are close to water sources. Nine of those sites have already been cleaned up, Tenley said.

The EPA has agreements with 30 companies responsible for the mines, and the Justice Department is funding the assessment and cleanup of the additional 16, Tenley said.

Those are just a fraction of the more than 15,000 mines known to have “uranium occurrence” in 14 Western states. About 75 percent of those are on federal and tribal lands, according to the EPA.

Navajo Nation officials did not return calls seeking comment on the issue, but one tribal environment activist called the problem the result of U.S. policy that “legitimized stealing my land … through mining.”

“The United States created this law saying anyone can mine on public land through this patent system, which gave them permission to stake a claim,” said Leona Morgan, the activist. “If they can prove there could be minerals, then they could mine.”

She was referring to the General Mining Act of 1872, which let settlers mine for specific hard minerals, including uranium, on land managed by federal agencies such as the U.S. Forest Service or the Bureau of Land Management.