Beyond addressing risks for oxidation of tailings, in some climates, use of water covers can be more cost-effective for mitigating ML-ARD risk when compared to dry covers, such as a terrestrial cover system (MEND, 1997). Dry cover techniques can be relatively costly due to complex earthworks and soil processing activities (Cacciuttolo & Tabra, 2015), although water covers may require upgrading of dam infrastructure, such as a dam raise, and/or buttressing.

However, as the use of water covers also means the TSF becomes a water storage facility, it becomes crucial to ensure the TSF and its embankments can withstand extreme weather events like droughts, floods or seismic events (Bjelkevik, 2005; Price, 2001). Furthermore, water covers are subject to wind exposure, which can create waves and currents that could increase oxygen levels in the water column and cause particle resuspension, leading to surface tailings oxidation (Bjelkevik, 2005).

Water covers must be managed and maintained in perpetuity. As a result, water covers require an understanding of, and a commitment to long-term inspections, monitoring, and maintenance to ensure stability and prevent physical TSF dam failures (Szymanski & Davies, 2004).

Aligning with the Global Industry Standard on Tailings Management (GISTM)

Promoting and implementing responsible mining practices towards achieving the ultimate goal of preventing harm to the environment and people requires mining owners and operators to prioritize alignment with the Global Industry Standard on Tailings Management (GISTM).

All GISTM principles are aimed at ensuring safe TSF management. We are highlighting below the key principles to consider for ensuring the safe closure of a TSF with a water cover.

• Risk management and mitigation: GISTM Principles 2, 3
  • Developing and maintaining an integrated knowledge base detailing the social, environmental, local economic, and technical aspects of the TSF to reduce risks, and to inform its closure.
• Design and construction: GISTM Principles 4, 5, 6
  • Design for TSFs should integrate the knowledge base, including climate and site conditions, the consequence of failure classification, and the feasibility of the closure scenario.
• Monitoring and maintenance: GISTM Principles 7, 10
  • Designs should incorporate an engineering monitoring system to verify design assumptions and monitor potential failure modes throughout the life of the structure.

Since TSFs rely on construction of dams, water covers require long-term attention and maintenance. Hence, water covers have a higher portfolio risk than dry covers due to their unlimited exposure period for dam failure (Bjelkevik, 2005). Therefore, a water cover closure design for TSFs needs to consider and integrate all aspects from GISTM principles 2, 3, 4, 5 and 6.

Water cover depth over the tailings surface needs to be deep enough to ensure the reactive tailings remain submerged and disturbance of the tailings materials due to waves is minimized during dry seasons and drought conditions(Price & Aziz, 2012). Climate change uncertainty and major storm events must also be considered when designing the depth of a water cover, dam height, and spillway. Surface erosion, flood overtopping, and tailings liquefaction (Brett, 2009) are some of the potential failure modes with water covers.

By following best practices as outlined in GISTM principles 2, 3, 7, and 10, these potential failure modes can be identified through risk assessments and largely addressed in the design. An adaptive management approach including monitoring and instrumentation can also help manage these risks through the lifecycle. For example:

• Erosion:
  • Incorporation of additional rip rap protection along the tailing dams protects the tailings from erosion by wave action (Price & Aziz, 2012).
• Flood overtopping:
  • Right-sizing a TSF contributing catchment area is critical. While a large contributing area will provide adequate inflow capacity to maintain the desired water balance, a small contributing area will minimize peak inflows and reduce the risk of overtopping the TSF dams.
• Tailings liquefaction:
  • Strategic placement of mine rock as part of the TSF design to create buttresses can help with improving embankment stability and reduce the potential of initiating failure due to tailings liquefaction (An Bord Pleanála EIAR, 2024).

Okane’s Approach

Under suitable conditions, water covers are generally effective at controlling ML-ARD, especially when applied during operations to manage potential sulfide oxidation from reactive tailings. However, as the closure plan progresses, site conditions change, and long-term management costs are considered, the suitability of a water cover may diminish. Therefore, it is essential to plan well in advance for scenarios where a transition to a dry cover, such as a terrestrial cover system, is needed to reduce risk related to long-term stability and climate change.

At Okane, our integrated mine planning and mine closure planning teams ensure this transition is considered while prioritizing a cost-effective and safe approach. We believe that incorporating this transition into the initial design of the TSF will better support long-term TSF management and closure.

We collaborate closely with our clients to assess long-term risks and costs of water covers, evaluating them against the risks and costs of terrestrial covers to develop tailored solutions aligned with their risk tolerance. Our comprehensive support ensures our clients understand long-term water and climate change risks, infrastructure design, geotechnical stability, geochemistry, and water quality monitoring.

Okane guides clients through the requirements for their tailings infrastructure during operations and closure. We assist with initial strategic project decisions to ensure full compliance with industry regulations, guidelines, and best practices, including GISTM, the International Commission on Large Dams (ICOLD), the Australian National Committee on Large Dams (ANCOLD) and the Canadian Dam Association (CDA).

For more information about our solutions and how we can help, contact us at info@okaneconsultants.com.