On June 19, 1998, Notre Development Corporation, based in North Bay, Ontario, received approval from the Environmental Assessment Board of Ontario to develop the Adams Mine near Kirkland Lake as a non-hazardous solid waste landfill. The site is a former open-pit iron ore mine that was owned and operated by Dofasco, a steel manufacturing company based in Hamilton, Ontario.
The landfill provides a unique opportunity to develop a site using existing natural conditions to the best advantage.
The concept of hydraulic containment is applied at sites where a landfill is developed in an excavated pit, like that of the south pit at the Adams Mine. The pit has been excavated to a depth below the surrounding groundwater table. Therefore, groundwater will flow radically inward toward the pit from the surrounding area. Proven civil and mine engineering techniques will be used to pump incoming groundwater and leachate to ensure that the optimum imbalance between the water level in the pit and the surrounding environment is always maintained. This inward movement of groundwater is used to positively contain and prevent any outward movement of leachate.
The landfill design is separated into two distinct phases: a “pumping” phase and a “gravity drainage” phase. The pumping phase encompasses the 20-year landfilling period and an approximately 100-year post-closure period. During this phase, the water level in the pit will be maintained at the base and sidewalls of the pit. The pumping system involves a network of horizontal pipes installed in the drainage blanket at the base of the pit. The pipe network will connect through an adit to two sumps located at the bottom of a 200m deep vertical shaft adjacent to the pit. Leachate generated within the landfill, as well as inflowing groundwater and any rainwater runoff from the pit walls, will flow into the drainage blanket and then downward to the base of the pit where they will mix and be conveyed through the pipe network to the sumps. The mixture will then be pumped to an onsite treatment plant facility.
The pumping rate will vary throughout the pumping phase, reflecting variations in leachate, groundwater and pit all runoff flow rates into the drainage blanket. At the onset of landfilling, the pumping rate is expected to be the highest, at an average annual rate of about 312,000 m3/year (130 IGPM) and will be comprised primarily of runoff from the exposed pit walls. After closure of the landfill, the average annual pumping rate is predicted to be in the range of 85,300 m3/year (36 IGPM) to 110,200 m3/year (46 IGPM) and will be comprised of about 25 per cent leachate and 75 per cent groundwater.
The leachate treatment plant design includes flow equalization, physical /chemical treatment, biological treatment using the PACT process, effluent filtration, and disinfection prior to discharge to a constructed wetland for effluent polishing. Outflow from the wetland system will be directed through an approximately 7km long onsite tailings area. The outflow from the tailings area discharges to the adjacent Misema River and will be required to comply with the Provincial Water Quality Objectives prior to entering the river.
By the end of the proposed pumping phase, it’s estimated that landfill gas generation will be substantially complete and the quality of the leachate/groundwater mixture pumped from the base of the pit will be such that active treatment will no longer be required. At that point, it will no longer be beneficial to continue to pump from the base of the pit, and the second stage of landfill operation, referred to as the gravity drainage phase, will commence. The actual duration of the pumping phase, however, will ultimately be based on the results of monitoring of the leachate/groundwater mixture pumped from the base of the pit.
During the gravity drainage phase, the water level in the pit will be allowed to rise to the level of a perimeter leachate collection pipe system installed along the pit sidewall at an average depth of about 25 m below the perimeter ground surface elevation. This perimeter leachate collection system will be a minimum of 5m below the surrounding groundwater table and therefore there will still be groundwater flow towards the pit. The collected leachate and groundwater will be allowed to drain by gravity to the constructed wetland and tailings area. The predicted average annual outflow rate from the perimeter leachate collection pipe system is approximately 78,000 m3/year (32 IGPM). About 60 per cent of this outflow rate will originate from leachate flow into the drainage layer and the remaining 40 per cent from groundwater inflow.
To demonstrate that hydraulic containment is being maintained, a monitoring program will be carried out for water levels within the landfill and surrounding bedrock as well as for groundwater quality. Bedrock groundwater levels will be monitored at 43 wells (thirteen locations) around the perimeter of the landfill and 32 wells (twelve locations) along the property boundary. In addition, groundwater level monitoring will be carried out for 44 wells at 19 other locations within the property boundary. Groundwater quality will be monitored at 23 wells (five locations) around the perimeter of the landfill and at 32 wells (twelve locations) along the property boundary. The depth of the wells ranges from several metres below ground surface to as much as 380m below ground surface (i.e., 180m below the base of the pit).
Due to the remote location of the site, certain other environmental issues normally associated with landfillssuch as dust, noise, smell, and incoming trafficare not a major consideration.
The south pit has an approved capacity of 20 million tonnes. This extensive capacity, combined with its remote location, is consistent with the trend of new landfill developments in North America. With increased regulatory requirements and new landfill standards in Ontario, the ability of many municipalities or the private sector to develop and operate smaller landfillsthat can only attract small annual tonnagesis not cost effective. The need to meet the standards specified in the Certificate of Approval (i.e., an aggregate drainage liner, leachate treatment plant, long-term closure allowances, and groundwater monitoring) and to also provide customers with economical tipping fees, make large landfills the only feasible disposal alternative in Ontario.
The closure of Toronto’s Keele Valley landfill is expected in 2002. The Adams Mine landfill is poised to ensure that there is an Ontario site that can compete, subject to the necessary tonnages being made available.
Written by Scott Wolfe, general manager of Miller Waste Systems in Markham, Ontario