The Nova Scotia provincial government recently announced it plans on spending $47.9 million (Cdn.) to clean up two former gold mines in the province. The two mines – Goldenville, near Sherbrooke on the Eastern Shore, and Montague Gold Mines, in Dartmouth – are deemed to be the most contaminated of dozens of abandoned sites in Nova Scotia.
The two sites were mined extensively from the 1860s to the early 1940s. Back then, environmental regulations were non-existent, or, at best, inadequate. Miners used liquid mercury to extract gold from crushed rock, and the mine tailings were disposed in nearby waterways. Arsenic, which occurs naturally in rock, was also released as part of the mining process.
Analysis of samples from the two abandoned mines site reveal that levels up to 200,000 mg/kg at the Goldenville mine and 41,000 mg/kg at the Montague mine. The Nova Scotia Environment Department’s human health soil quality guideline is 31 mg/kg.
With respect to inorganic mercury, samples from the two mine sites were found to be at levels reaching 48 mg/kg at Goldenville and 8.4 mg/kg at Montague. The Canadian Council of Ministers of the Environment’s human health and ecological soil quality guidelines for inorganic mercury is 6.6mg/kg.
The remediation plans involve excavating the tailings with the greatest contamination to a depth of two metres and placing them in a lined containment cell. The cells will than be capped so water cannot enter them and clean backfill will be added on top.
At Montague, two containment cells will each be 95 metres by 95 metres and five metres high, made with a berm, an impermeable liner, a leachate collection system and an impermeable cover system. At Goldenville, the same structures will be built, but one will be 180 metres by 180 metres and the other will by 135 metres by 135 metres.
The two sites will also require a water treatment system as well as a wall to prevent contaminated water from leaving the excavation zone.
In other areas with lower levels of contamination, a protective, low-permeability cover will be placed on top of the tailings to prevent precipitation from getting into the contaminated soils. That barrier will then be covered with soil and vegetation.