Pit lakes are often perceived as liabilities when planning for mine closure. However, when closure planning is integrated into mine operations, opportunities can be realized to leverage pit lakes as waste containment and treatment. Consider the following potential advantages of using your pit lake for waste treatment.
Prevention of Accidental Release
When mine wastes are confined within the pit, they do not rely on elevated impoundment structure. Pit lakes can also provide general water treatment includes flow equalization and sediment trapping. To achieve this goal, the water layer needs to be sufficiently deep to prevent wave erosion and resuspension of fine particles. Pit lakes are also a location to discharge water treatment effluent and by-products such as lime sludge. This is an economically beneficial practice because no extra storage facility needs to be constructed (Morgenstern et al. 2015, Verburg et al. 2009).
Preventing Acid Generation
The water cover above waste rock can effectively restrict exposure of mine waste to atmospheric oxygen and thus limit the formation of acid and metalliferous drainage (AMD). A water cover is considered one of the best practices for permanent storage of saturating potentially acid generating (PAG) materials.
A stratified pit lake can provide some opportunities for mining companies to treat the water: Algae growing in the surface layer of pit lake can provide a surface adsorption site for metals. When algae die in the winter, those absorbed metals along with algae bodies will sink to the bottom layer of the lake and store there permanently.
Management of Selenium and Nitrates
Saturated Rock Fills (SRFs) occur when pits or voids at mine operations are backfilled with mine rock and the backfilled rock is allowed to saturate or flood with water. In saturated environments, it is not uncommon for low oxygen conditions to develop. These low oxygens, or suboxic, conditions can create conditions suitable for the geochemical reduction of some oxidized species within mine impacted water. The geochemically reduced forms are typically much less mobile and/or pose little to no water quality risk. For example, nitrate can be reduced to form inert nitrogen gas, or selenate (oxidized form of selenium) can be reduced to form selenite, which easily adsorbs, or ‘sticks’, to minerals and is immobilized from transport to receiving environments. The high hydraulic conductivity of backfilled mine rock, coupled with sufficiently low oxygen, or geochemically reducing, conditions can result in powerful opportunity for low cost and effective water quality management. Mine impacted water can be diverted to SRFs and given sufficient residence time and geochemical conditions within the SRF resulting in effective treatment of water quality issues.
Management of Organics and Sulphate
Some solid mining wastes are not acid generating but contain degradable organic contaminants (e.g. oil sands tailings). If oxygen is available, the organic contaminants existing in solid tailings can be slowly degraded below the water cap. In this situation, the water column of a pit lake needs to be fully mixed to create an aerobic condition in the lake bottom. If pit lakes become stratified for a while, the bottom portion of the lakes and sediments are likely to become anaerobic and produce hydrogen sulphide, carbon dioxide and methane (Schultze and Boehre, 2009). Some tailings ponds from oil sands operations produce significant quantities of methane. In some oil sands operations, oil sands tailings have also been treated with gypsum (CaSO4·2H2O) or aluminum sulphate (e.g., alum: Al2(SO4)3) to accelerate the consolidation. If these sulphate-rich tails are stored under a stratified water column, sulphate reduction may occur in the presence of organic carbon source, which is rich in oil sands tailings. When water column turnover occurs under an extreme weather event, the sudden release of these gas may damage the biologic system that had evolved in the pit lake.
The hydrogeology of the pit is often the most critical factor for determining the sustainability of a pit lake for waste disposal. The main purpose of hydrogeologic control is to minimize the outflow seepage which may transport the contaminants to the downstream aquafer. The ideal pit for mine waste disposal has a minimal groundwater gradient across the pit so that contaminant transportation is minimized.
Okane ensures the geologic and hydrogeologic condition of the project site are thoroughly investigated. Some of the factors we evaluate include the permeability of rock around the pit, gradient of the groundwater table, pit lake water balance, hydrology of the downgradient receiving water body, presence of any faults or fractures which increase the hydraulic transportation.
Okane supports mine sites in the realization of opportunities associate with pit lake waste management. By integrating active closure planning into the strategic mine planning phases, your pit could go from being a liability to being your best asset at closure.
Morgenstern NR, Vick SG, Van Zyl D. 2015. Independent expert engineering investigation and review panel report on Mount Polley tailings storage facility breach. Available at: https://www.mountpolleyreviewpanel.ca/
Schultze, B and M. Boehrer, (2009), Induced meromixis, in Castendyk, D.N., and Eary, L.E.,eds., Mine Pit Lakes: Characteristics, Predictive Modelling, and Sustainability: Society for Mining, Metallurgy, and Exploration, Inc., Littleton, Colorado, p. 239-246.