No cleanup will be required at three abandoned uranium mines near Edgemont after the U.S. Environmental Protection Agency was unable to document a release of hazardous substances.

The EPA announced its decision Tuesday. It is based on water and sediment sampling conducted in September 2015 by contractor Weston Solutions Inc.

The EPA deemed the sampling necessary after a 2014 preliminary assessment. The assessment was requested by the nonprofit Institute of Range and the American Mustang, owner of the Black Hills Wild Horse Sanctuary.

Susan Watt, executive director of the sanctuary, was concerned about potentially hazardous runoff from the mines entering creeks and flowing into the Cheyenne River. The Journal was unable to reach Watt for comment Tuesday afternoon after the EPA released its decision.

The abandoned mines are 13 miles northwest of Edgemont, on the southwestern edge of the Black Hills. The Black Hills Wild Horse Sanctuary is about 20 miles east of Edgemont.

The EPA’s contractor conducted sampling upstream and downstream from the abandoned mines. But the EPA failed to gain landowner permission to access the actual mine sites, known as the Darrow, Triangle and Freezeout mines, which collectively cover more than two square miles on private land.

So, for data about the mine sites, the EPA relied on sampling conducted in 2007 and 2008 by Powertech Inc., now known as Azarga Uranium, which wants to conduct further uranium mining near Edgemont via the in situ method.

That earlier sampling detected radionuclides in water-filled pits on the mine sites. The biggest of those pits, at the Triangle mine site, measures about 400 by 150 yards on the surface and has an estimated depth of 90 feet.

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The mine sites also include large soil and mine-waste piles. The Powertech sampling of that soil and waste detected concentrations of the radium isotope Ra-226 and the thorium isotope Th-230 that exceeded cancer risk screening benchmarks. Additionally, some samples exceeded the Ra-226 concentration standards of the Uranium Mill Tailings Radiation Control Act.

But the EPA contractor’s September 2015 sampling of sediment and water downstream from the mines did not detect concentrations of hazardous substances in excess of three times the natural or “background” levels. Therefore, the EPA could not document any occurrence of a “release,” such as runoff from the mine pits.

In other words, the mine sites may contain hazardous substances, but those substances do not appear to be escaping in amounts that would cause serious human health or ecological effects.

“If the private property owners of the mine source areas are concerned about exceedances of screening levels for these contaminants, they could consult with EPA to understand potential risks and how to manage them,” said Dania Zinner, an EPA site assessment manager, in a written response to Journal questions.

Uranium was discovered near Edgemont in 1951, and the area was mined extensively until the 1970s to help supply the U.S. government with material for nuclear weapons. Weak regulation allowed companies to abandon mines and mills without cleaning them up or disposing of radioactive or other hazardous materials.

Contact Seth Tupper at seth.tupper@rapidcityjournal.com

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Enterprise reporter for the Rapid City Journal.