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A few years ago, the U.S. Environmental Protection Agency (EPA) developed formulas for predicting the amount of radon emitted by radium-laden uranium mill ponds.

When Ute Mountain Ute tribal officials plugged data from the nearby White Mesa Uranium Mill into one EPA formula, the numbers were alarming.

Predicted emissions for radon-222 were up to 50 times the cap set to protect the environment — and downwind communities in San Juan County — from the odorless cancer-causing gas.

Tribal leaders went to the EPA for answers. But EPA air-quality regulators have yet to step in.

So the tribe and environmental groups took their case to the state Wednesday. In a presentation to the Utah Air Quality Board, Uranium Watch program director Sarah Fields blasted what she considers the federal agency's indifference to community concerns.

"You must take action now. There needs to be a discussion and further investigation," Fields told the board.

The tribe and environmental groups, she said, want the state to intervene.

Mill owner Energy Fuels Resources maintains the mill's radon emissions, which are the subject of constant monitoring, are "de minimus" and within acceptable limits.

"The calculations used by the activist groups and the tribe are wrong. And frankly, their results are preposterous," said Curtis Moore, the company's spokesman. "They are applying an EPA formula incorrectly, with incorrect inputs."

The White Mesa mill's five ponds total 145 acres in area and sit only three miles from the White Mesa tribal community.

A radioactive metal, radium in trace amounts appears in the waste stream from uranium milling and decays into radon gas.

The Clean Air Act has set the emission limit for the radioactive gas at 20 pico Curies per square meter per second. Breaching that limit could threaten people's health within a 50-mile radius.

Environmentalists agree their radon emission numbers seem preposterous, but they are what the federal formula predicts would come off the ponds, given the intense concentration of radium they hold.

"If we are right, this is a public health emergency," said Anne Mariah Tapp, energy program director for Grand Canyon Trust, which has teamed with Uranium Watch on the issue. "At a minimum, they need to be proving us wrong."

At Wednesday's meeting, Utah Division of Air Quality director Bryce Bird noted that the EPA is revising rules for radon emissions.

"Our concern would be: What is the exposure? And what can be done to reduce that?" Bird said.

"There is coordination between two federal agencies and two state agencies," he added. "The only thing we can do is use the existing regulations and wait and see what EPA does."

EPA officials could not comment on the groups' calculations and whether they applied the formulas correctly.

"We are carefully evaluating that information as part of an ongoing rulemaking to revise radon-emission requirements for uranium mills," agency spokesman Rich Mylott said. "We will also examine that information in the context of our work with Utah DEQ to ensure compliance and protect public health at the White Mesa facility."

Mylott noted that fence-line monitoring around the mill has detected no exceedances of any standards for various radionuclides.

Energy Fuels' spokesman dismissed the groups' math exercise as part of a longtime campaign against uranium.

"The activist groups, who oppose nuclear energy and uranium mining at all levels, routinely petition the regulators on various matters related to the mill," Moore said. "We expect the regulators to investigate the allegations as they always do, and respond appropriately to any of these groups' concerns."

But the tribe, not the environmental groups, was the first to raise the issue — earlier this year.

In a joint press release with the environmental groups, Ute Mountain Ute tribal chairman Manuel Heart worried about the disconnect between the new emissions calculations and the low levels assumed by regulators.

"The Ute Mountain Ute Tribe is hopeful that it can work with the regulatory agencies and the state of Utah to resolve this issue, but first we need to understand the actual level of radon-222 emissions from Cell 1," Heart said.

Cell 1 is one of the large evaporation ponds where Energy Fuels stores radium- bearing sediments covered in water. The pond has the highest radium concentrations.

For years, the EPA has claimed that radon emission from such impoundments is negligible.

"EPA assumed that liquids on top of solid tailings reduced radon emissions," Fields said.

"We now know that radon emissions from liquid covers and solution impoundments are significant and must be monitored and controlled under [Clean Air Act] hazardous air pollution standards."

Energy Fuels annually reports concentrations of "gross radium alpha" in its cells to the state. In 2014, the concentrations increased as much as tenfold.

To explain the elevated concentrations, Energy Fuels managers told regulators that they had stopped adding fresh water to the ponds. At the same time, drought conditions resulted in less storm runoff entering the ponds and increased evaporation.

Radium concentrations in liquid waste correlate with radon emissions — the more concentrated the radium, the more radon is produced.

"The drastic increase in the calculated emissions between 2013 and 2014 has elevated the Tribe's concerns about the health and safety of the Ute Mountain Ute Tribal members living close to the White Mesa Mill, and the Tribes believes that the EPA should consider taking emergency actions to protect human health and the environment in southeastern Utah," tribal officials wrote in a brief to EPA in February.