HUMBOLDT - Rio Tinto officials have reported a mixture of water and non-hazardous clay material flowed into a wetland at the Humboldt Mill Monday during construction of an underground containment wall.
The building of the underground cut off wall is required as a water protection measure at the mill. The wall will prevent water from naturally flowing from a tailings pit into wetlands.
"The pit next to the Humboldt Mill arose from previous iron ore mining. The pit is now filled with water. It contains tailings from earlier mining and that is where tailings from the Humboldt Mill will be stored," said Matt Johnson, Rio Tinto manager for external affairs.
This aerial photograph shows the Humboldt Mill where a mixture of water and clay material flowed into a wetland Monday while workers were constructing an underground containment wall. (Rio Tinto photo)
On Monday, while in the first stage of construction of the wall, a trench was being dug. The trench was filled with bentonite - an inert non-hazardous clay material which will line the trench to act as a sealant to prevent the flow of water into wetlands.
During the digging, an underground channel was discovered and the bentonite slurry ran into the adjacent wetland through the channel.
Johnson said a "relatively small amount" of material reached the wetland and is not expected to have any long-term impacts.
"There's been no harm to the water system," Johnson said.
Rio Tinto said work is under way to clean up the residual bentonite - which is commonly used as a sealant in residential water wells. The slurry was thickened to reduce the flow and the bentonite settled in the wetland.
Michigan Department of Environmental Quality officials were notified and they inspected the wall and the wetlands, along with representatives from the Superior Watershed Partnership, as part of the recently announced independent Community Environmental Monitoring Program.
The cut off wall is being constructed in fill and waste rock material from previous mining operations, which allows water to flow underground from the pit to the wetland.
"There is a further possibility during construction of the cut off wall additional bentonite slurry could flow into the wetland," Johnson said. "Therefore, mitigation measures are being put in place to ensure protection of the wetland and nearby (Escanaba) River system."
Trenching is expected to be completed by the end of the year and the underground wall is expected to be finished by early summer next year. During operations at the mill, the water in the tailings pit will be treated in a water treatment plant before entering the wetlands.
John Pepin can be reached at 906-228-2500, ext. 206.