MARQUETTE - The results of more testing by Rio Tinto at the company's Eagle Mine site affirm prior tests showing trace amounts of uranium found recently in water samples are being removed by the mine's water treatment plant.
Earlier this month, officials said testing confirmed the presence of uranium in water samples from within aggregate layers at the bottom of the mine's Temporary Development Rock Storage Area that exceeded the federal maximum concentration level for safe drinking water.
The finding did not violate any state or federal regulatory permits at the mine, but Rio Tinto and the Superior Watershed Partnership - which conducts the independent Community Environmental Monitoring of Rio Tinto's mining activities - wanted to make sure the element was being contained, learn more about its potential sources and report the information to the public.
Later tests on water entering and leaving the water treatment plant - by Rio Tinto and the partnership - showed the uranium was being successfully contained and was not being released into the ground or back into mining processes.
Rio Tinto had also received results of tests on solids retained by the plant showing the uranium was being contained.
The latest tests were performed April 8. Those results were reported this week.
Rio Tinto spokesman Dan Blondeau said the water treatment plant creates two solid waste products, one in the filter press and the second is crystallizer waste. Solids removed from the plant are disposed of in a municipal landfill.
Tests on the filter press waste showed 1.9 milligrams per kilogram of uranium concentration, which compares favorably to the company's previous solid waste test which registered 2.5 milligrams per kilogram. Tests on the crystallizer waste did not detect any uranium, duplicating the result from Rio Tinto's previous testing.
"Once more, this indicates that the system is working as designed," Blondeau said. "The uranium is being contained and treated and poses no risk to the environment or the community."
The mine's Temporary Development Rock Storage Area is designed to be an environmentally secure feature which holds waste rock from mining tunnel excavation until it is later put back underground to fill voids where ore was removed.
The bottom of the storage area has two multi-layered lining systems: a primary contact water sump and a lower secondary lining, called the leak detection sump.
Last month, Underwriters Laboratories in Indiana determined a water sample taken from the leak sump in February by partnership staff was found to contain 72.6 parts per billion of uranium. Partnership workers were test sampling water quality in the leak sump to compare with previous test results produced by Rio Tinto.
The U.S. Environmental Protection Agency - which has been regulating uranium in community drinking water supplies to reduce the risk of kidney disease and cancer since 1993 - set a maximum concentration level for uranium under the Safe Drinking Water Act at 30 parts per billion, with higher levels considered unsafe.
Rio Tinto's rock storage area and water treatment plant are not governed by the Safe Drinking Water Act, but by the company's mining and groundwater discharge permits. Mining company officials said they were confident the EPA-recommended processes in the mine's water treatment plant would remove the uranium and no threat was posed to the environment.
John Pepin can be reached at 906-228-2500, ext. 206.