PINCHI LAKE MERCURY - Terrestrial Areas
From 2000-2004 site stability was addressed through resloping, armoring and capping of the waste dumps, and backfilling and capping former lagoons that had mine residues present.
Terrestrial Ecological Risk Assessment (TERA)
From 2004-2009 a TERA was undertaken through literature review, field investigations including habitat surveys, food chain modeling, and the establishment of reference values for methyl mercury toxicity. Forty wildlife species were considered in the TERA. This approach was untaken to determine whether contamination was associated with environmental effects, and evaluate the benefits of remediating different areas of the mine site. The purpose of the TERA was to inform the Final Closure Plan and identify high and low risk areas.
Metal analysis was conducted on soil, vegetation, invertebrates, and small mammals. Through the TERA contaminants of potential concern that were identified include: arsenic, antimony, cobalt, copper, molybdenum, nickel, selenium, inorganic mercury, and methyl mercury. Soil at the modern mill site and the old mill were found to have levels of mercury that exceeded the Contaminated Sites Regulation. Mercury concentrations decreased in the soil with increased distance from the mill sites and at 10 km from the mill sites mercury was at ‘background’ conditions.
Dose-response relationships in combination with sampling results were used to determine the probability and magnitude of effects from contaminants of potential concern on wildlife at PLM. A post-closure risk reduction analysis was conducted using this information. The main risk pathways for mercury were identified as the ingestion of flying or ground insects by small mammals and birds. In remediating the historic mill site, the modern mill site, and the tailings impoundment, the greatest risks from mercury would be reduced to acceptable levels. It was also concluded that leaving the reforested areas from the historic disturbance in the 1940s was favorable to disturbing the area to address potential contaminants of concern.
Due to the water content and fine material in the tailings, it was necessary to cap the tailings during the winter, to facilitate heavy machinery operation without sinking. Decommissioning the 22 ha tailings area involved the construction of a spillway to ensure that the tailings area dried out and geotechnical stability improved. Keeping the tailings dry was important to eliminate the methylating aquatic environment, which could promote the uptake of harmful methyl mercury by aquatic organisms. Leaving the dry tailings uncapped posed a risk to browsing and burrowing animals and was therefore capped with 0.5 m of till. The previously ponded area of the tailings where the methyl mercury concentrations were higher was capped with 1 m of till. In total, 150,000 m3 of till was used to cap the tailings area.
Chemically stable iron sulphate sludge from the Sullivan Mine was also spread over the previously ponded area to help with mercury sequestration. Mercury uptake by plants was not a concern, as plants do not uptake mercury through their root systems. The tailings area was re-vegetated with a mix of grasses, trees and shrubs. Reclamation was completed in 2012, but revegetation efforts have already shown rapid results. Reclamation trials were also undertaken on the tailings beach subsequent to the end of the modern operations; test plots were developed and the beach has naturally revegetated.
The emergency spills lagoon and the sewage lagoon were reclaimed in 2004. Both lagoons were capped with 3 to 4 m of glacial till over the lagoon sediments. As the reclamation was being completed on the emergency spills lagoon it failed and slid into the near shore area of Pinchi Lake. The 16,000 m3 spill released some contaminated sediments into the lake; however, the till covered much of the contaminated sediments. Mercury was assessed in the contaminated sediments and it was found that the concentration had been reduced, due to mixing with the till. In 2005, the disturbed area was stabilized with geotextile and rip rap to protect it from erosive wave action from the lake. The remainder of the lagoon was graded and revegetated with grasses, which was successful.
The configuration of the upper pit (main pit) is at the top of a low mountain. The pit has several benches. During the historical operations mining was underground in this area. A 1 m till cap was placed on the filled upper pit and was planted with agronomic grasses and legumes using island and corridor patterns to encourage re-vegetation of native shrub and tree species. Some natural revegetation also took hold in this area. The pit walls were capped with 0.3 to 0.5 m till in areas where natural revegetation was not occurring.
The configuration of the lower pit (west zone pit) is as a side cut into a steeply sloping hill. The pit has very steep highwalls, and was therefore made difficult to access by the public to limit safety issues. The lower part of the west zone pit was used as an on site landfill for demolition debris and waste rock that was not contaminated. The pit was filled with approximately 2000 tons of demolition debris and 114,000 m3 of waste rock, and subsequently capped with 1 m of clean till.
The mill site was also capped with 1 m of clean till in the area near the processing facilities and 0.5 m in the remaining areas. The area was seeded 80% with agronomic grasses and legumes, and then fertilized. Revegetation with native trees will be completed on the remaining 20%, with islands and corridors. In consultation with First Nations upland willow, rose, cottonwood, birch, and buffaloberry were selected to promoted moose and whitetail deer habitat.
A 3.2 ha gravel quarry was developed in 1999. Reclamation was undertaken by regrading the quarry walls and capping with 0.3 m till in 60-70% of the area with agronomic grasses and legumes with fertilizer, and 0.6 m till in 30-40% of the area with native trees and shrubs. Tree and shrub species included Douglas-fir, upland willow, rose, aspen, cottonwood, birch, and buffaloberry.
Till areas used to source cover material were developed more recently as reclamation requirements became clear. These borrow areas were resloped and revegetated. Salvaged topsoil was applied prior to revegetation.
The final remediation efforts were focused on the modern mill site and tailings impoundment, while the old mill site and adjacent forested lands were left to continue on the current trajectory of recovery
Human Health Risk Assessment (HHRA)
A HHRA was also conducted and found no unacceptable risks to people expected to be on site (the caretaker and occasional visitors). The HHRA for off site receptors found risks through mercury in soil, though they were deemed acceptable. Acceptable risks were also found for wild game and plant consumption. However, consuming lake trout and burbot routinely from Pinchi Lake could result in unacceptable risks to human health.
Decommissioning activities at PLM included:
Part of the decommissioning efforts included dismantling the mercury-contaminated ore processing facilities. The modern mill building was not maintained or used after production stopped in 1975. Mercury sulphide dust was present throughout the mill, as was asbestos that was used as insulation, and PCBs used in transformers. As well, the top 0.5 m of soil near the mill was contaminated with mercury.
Following the demolition of site infrastructure, approximately 47,000 kg of mercury-contaminated material was shipped to the Clean Harbour facility in Quebec for safe disposal. Liquid mercury was recovered from this process and shipped to the US for roasting and creation into a marketable product, while the rest of the material was disposed of in the Clean Harbour landfill in Ontario. Demolition of the facilities was complete by early 2011.
Mercury contaminated soils were excavated, and areas of concern were capped with clean till. The total till requirement for capping material was estimated at 300,000 cubic metres for the whole mine site.
Waste Rock Dumps
The waste rock dumps from the historical operations were revegetated and herbaceous growth is currently self-sustaining. These dumps are considered to be reclaimed. The waste rock dumps from the modern operations are partially reclaimed through natural colonization. Other areas were revegetated with alfalfa following the cessation of operations in the 1970s. This alfalfa dominated the disturbance area, outcompeting other vegetation with the exception of a few trees and shrubs.
The waste rock dump at the shore of Pinchi Lake was recontoured and capped with 1 m of dense low-permeability till in 2003. The purpose was to reduce surface water infiltration and leaching, promote runoff, and limit losses of mercury to the atmosphere. This option was chosen in favour of relocating the waste to another dump location due to the potential of a large mercury release to Pinchi Lake during the disturbance of the dump and relocation.