RECLAMATION AND CLOSURE
> By Gord McKenna and Guy Gilron
FROM THE BOTTOM UP I
A proactive approach of landform design and source control
ncreasingly, two related trends in mine design are becoming apparent. One is mining with the end in mind, or effective landform design (the emerging practice of designing and building truly sustainable mining landscapes from the very outset). The other involves building mine rockpiles that limit the generation and release of potential contaminants within the rockpile (source control) using both old and new technologies, combined with siting and constructing these mining landforms to facilitate the collection and treatment of leachates (leachate control). Together, these trends illustrate a fundamental shift in approach, especially for mines at the permitting stage. At a practical level, it shows the transition from proposing modest measures to control the production and transport of leachate from rockpiles, and then employing adaptive management should conditions worsen, to a proactive approach of limiting water and oxygen fluxes in the design and construction of these landforms that allows for effective capture and treatment of leachates. This strategy better manages the risks by “mining with the end in mind” and adhering to the principles of landform design (see June/July, 2021 issue of CMJ). This article describes how your mine can benefit from these novel approaches. To illustrate this approach, we provide a simplified example drawn from recent efforts to control selenium leaching from metallurgical coal mine rockpiles under neutral pH conditions (this approach is JUNE/JULY 2022
similar for acid rock drainage elsewhere). At many mines, acid rock drainage and metal leaching (ARD/ML), especially from rockpiles, poses a significant problem, often evidenced by bright orange, ironrich acidic seepage waters. Neutral mine drainage with elevated metals and metalloids can also be an issue. At some coal mines, selenium is present in the mined rock at concentrations of about 2 ppm. When the rock is blasted and stacked loosely in piles, water and oxygen enter-
“Doing better” means reducing costs, risks and the need for long term treatment. ing the rockpile cause natural bacteria (the bugs) in the rockpile to oxidize selenium, in turn producing selenate, which is easily leached, reporting to the toe of the rockpile at aqueous concentrations in the range of about 50 ppb and higher. These neutral pH leachate waters flow into local creeks, wetlands, and lakes, potentially leading to bioaccumulation up the food chain, with impacts to fish and bird populations. To manage this situation, many mines employ expensive, long-term seepage and surface water collection facilities, paired with active water treatment plants. Key questions for consideration are how can we “do bet-
ter,” especially for new mines? How can we reduce costs and risks? Can we reduce the need for long-term treatment? To address this issue, landform design can be employed to establish mine rockpiles that restrict contaminant mobility. Initially, a governance team and a multi-disciplinary technical team are established; a design basis memorandum (DBM), which outlines land uses, goals, objectives, and criteria, is clearly set out; and designs are formulated and modelled and subjected to various risk assessments. Pre-planned contingencies are then developed in the case of poor performance, and a monitoring plan is established to identify whether contingencies need to be implemented. The team subsequently monitors the system to ensure that the rockpile is built to design specifications and is performing as intended. A key part of the DBM is to establish acceptable concentrations and leachate loading to the receiving environment. Generally, no single technology or scheme can be relied upon to control leachate. Instead, the use of multiple technologies in a multiple-lines-of-defence approach is used to provide reasonable assurance that the leachates can be effectively managed. This involves incorporating mining and rock handling methods, source control, water management, and mitigation measures (e.g., collection and treatment) into the system from the beginning. CONTINUED ON PAGE 18
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