The development of a pit lake at Agrium Kapuskasing Phosphate Operations – an integrated geotechnical, geochemical and biological approach

Authors: Munnoch, KG; Renaud, SE; Connors, K Download Paper

DOI https://doi.org/10.36487/ACG_rep/1152_57_Renaud

Cite As:
Munnoch, KG, Renaud, SE & Connors, K 2011, 'The development of a pit lake at Agrium Kapuskasing Phosphate Operations – an integrated geotechnical, geochemical and biological approach', in AB Fourie, M Tibbett & A Beersing (eds), Mine Closure 2011: Proceedings of the Sixth International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 545-554, https://doi.org/10.36487/ACG_rep/1152_57_Renaud



Abstract:
Agrium’s Kapuskasing Phosphate Operations (KPO) is an open pit mining facility located approximately 40 km southwest of the town of Kapuskasing, Ontario. In operation since 1999, three contiguous pits have been developed (North, Centre, and South) with mining activities expected to be complete in 2013. As part of the rehabilitation strategy the open pits will be transformed into a freshwater lake. Agrium KPO has had previous success in constructing and establishing a lake ecosystem. In 2006, in order to further access ore deposits, a small boreal lake was relocated which included the transplantation of fish, benthic invertebrates, plankton and aquatic vegetation. This paper will discuss Agrium KPO’s integrative planning approach based on geotechnical, geochemical, and biological specifications, including lessons learned from the previous lake relocation, to ensure the successful establishment of the pit lake. After closure, the pit will be flooded to approximately 239 masl in the North and South pits while backfill placed in the Centre pit will rise above the projected lake elevation. Geotechnical reports indicate that the safety factor of pit slopes will increase over time due to the stabilising force of the water weight on the slopes. To minimise erosion, rock cover may be required in areas potentially exposed to wave action. An assessment of geochemical loadings has also been completed to model the expected quality of water in the pit once flooded. Results suggest that concentrations of modelled parameters (such as metals) released from the submerged waste rock will be below applicable regulatory acceptability criteria after the lake has filled to capacity. As such, the final flooding of the pit can be done sequentially without in situ treatment of the water within the pit. In terms of biological development of the lake, the ultimate objective is to maintain a viable aquatic ecosystem. The construction of a littoral zone, including enhanced aquatic habitat features, will provide spawning and refuge areas geared towards the successful reproduction of sentinel fish species found in this area. It is predicted that once flooded, the open pit will provide a stable, deep cool water fisheries habitat.

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