Multi-layer cover reduced Ontario acid mine drainage, study finds

liner installation
Crews install a geosythetic clay liner at Kam Kotia Mine in 2008. Photo Credit: Ontario Ministry of Mines

University of Waterloo researchers have found that multi-layer covers placed over oxidized sulfide-rich mine tailings in Northern Ontario reduced the production and release of acid mine drainage over decades.

As old mines are remediated and professionals design covers and select remediation methods to use, Aria Zhang, who studied a method for covering mine tailings as part of her Master’s degree at the University of Waterloo, said findings recently published in Applied Geochemistry can help guide future remediation.

Zhang, who says it is critical to control toxic metals in mine waste from being released into the environment so that it doesn’t threaten the drinking water supply or cause health concerns, notes that once toxins are released they can be difficult to control.

“They can take our results into consideration,” Zhang says in a statement about the new study. “This cover is already applied at many mine sites, but because of this uncertainty about the chemistry there were some concerns. Now, we know better.”

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Under the supervision of University of Waterloo professors David Blowes and Carol Ptacek, as well as hydrogeochemist Jeff Bain, Zhang assessed the effectiveness of a geosynthetic clay liner that also consisted of five layers of soil, sand and gravel placed over mine waste near Timmins, Ontario. The mine had mainly produced zinc and copper, as well as silver and gold. The Kam Kotia mine started operation in 1942 with open pit mining, and its tailings were deposited between 1968 and 1972. The Ontario Ministry of Mines then initiated the rehabilitation with the cover in 2008.

According to the ministry, there were about six million tonnes of unmanaged acid generating tailings covering more than 500 hectares.

Zhang’s work showed that the multi-layer cover resulted in increased pH values and decreases in contaminant concentrations where the cover remained intact. Over decades of subaerial oxidation, sulfides were depleted. They also found that the cover did not destabilize toxic minerals at the site and was preventing more toxins from developing.

Where there were defects in the mine waste cover there was continued sulfide oxidation and indications that some contaminants were released. In particular, sulfate ion, zinc, manganese and arsenic, are likely to remain water quality concerns, the study states.

The study found that the cover reduced the rate of sulfide oxidation at the mine site,  lowering the acidity production and metal leaching over time. Naturally-occurring peat under the tailings also promoted sulfate reduction in the peat and underlying Quaternary sand aquifer, which further attenuated sulfate ion and trace metals.

“In a multi-layer cover system, a layer of low-permeability and highly saturated material with sufficient thickness acts as a barrier to the ingress of both O2 and water. Clay, silt, and geomembranes are the most common low-permeability materials,” the study states. 

Part of the research was performed at the Canadian Light Source, a national research facility of the University of Saskatchewan. The research is part of the TERRE-NET program, which spans six Canadian universities and funds interdisciplinary research on responsible mining practices.

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