Where Does Acid Mine Drainage/Acid and Metalliferous Drainage Come From?

Acid and metalliferous drainage (AMD) also known as Acid Mine Drainage (AMD), acid rock drainage (ARD), and Metal Leaching/Acid Rock Drainage (ML/ARD) can pose a significant liability for mine operators. AMD/ARD, when not properly mitigated, can cause the release of acidic and metalliferous water into the surrounding environment and waterways. AMD/ARD can impact the health of those living in communities within the mine’s zone of influence and can have adverse effects on surrounding ecosystems and wildlife.

The potential liability surrounding AMD/ARD for mine operators is significant. It can have a direct impact on asset value if there isn’t a proper plan to mitigate AMD/ARD risk. Ensuring AMD/ARD management plans are incorporated into Life of Mine plans early in the asset lifecycle is one of the best ways to mitigate risk and demonstrate responsible mining practices to stakeholders.

AMD/ARD: The Basics

Understanding what AMD/ARD is and identifying where it comes from provides the framework for identifying strategies to manage it.

AMD/ARD is the outflow of acidic or metalliferous water into the environment. At a mine site, mined rock is brought to the surface. It can be stored in mine rock stockpiles or in tailings storage facilities after undergoing the process of separating the ore from the mine rock. Mined rock stored at these facilities can contain sulfide minerals such as pyrite or sphalerite, and these sulfides commonly host many trace metals such as copper, lead, and zinc in their mineral structure.

How Is AMD/ARD Produced?

The largest factors affecting the production of AMD/ARD are precipitation (water) and oxygen. When exposed sulfides in the mined rock interact with oxygen and water, sulfide oxidation occurs and causes acid generation and metal mobilization. Water that comes into contact with the mined rock can transport the sulfide oxidation products to nearby receptors such as rivers, lakes, or groundwater.

Why does AMD/ARD need to be managed?

Contamination by AMD/ARD is costly to remediate and can take many years to correct. When AMD/ARD is released into the environment from tailings storage facilities or mine rock stockpiles via seepage or surface runoff, the effect on ecosystems can include:

Disrupted growth of plant life
Inability for plant life to reproduce
Bioaccumulation in microorganisms and higher order organisms
Decreased water quality

Additionally, AMD/ARD released from tailings storage facilities or mine rock stockpiles can contaminate water surrounding mine sites that can enter nearby freshwater sources.

In addition to the potential environmental impacts of AMD/ARD, it can also create challenges for the infrastructure of a mine site. The acidic properties of AMD/ARD can accelerate the breakdown of metal and concrete structures.

Okane Archives: Creek bed tinted orange due to high amounts of iron in the water as a result of AMD/ARD

AMD/ARD Management Strategies- Okane’s Approach

Manage Air Contact

Oxygen is one of two main catalysts that cause the production of AMD/ARD. As such, one of the critical components in an AMD/ARD management plan is limiting air contact. This can be done through the application of cover systems or through constructing mine rock stockpiles in a way that minimizes particle size segregation to decrease the number and length of pathways air can enter. By reducing the interaction with oxygen, we can limit the potential for oxidation of AMD/ARD generating material.

Manage Water Contact

The other main reactant for AMD/ARD generation is water. Water can enter tailings storage facilities and mine rock stockpiles through precipitation or, in cold weather regions, from seasonal snow melt. To limit water from contacting AMD/ARD producing material, Okane designs landforms that support the re-direction of surface water runoff, and cover systems that minimize water contact with tailings or mined rock.

Okane has 25 years’ experience in developing both conceptual and detailed AMD/ARD management plans. Our team of experienced engineers and geoscientists take a systematic approach to developing AMD/ARD management plans by focusing on the site-specific material characteristic and climate conditions.

By identifying, managing, and monitoring AMD/ARD, Okane can help you significantly decrease the liability related AMD/ARD at your site and ensure that management strategies are long-term, sustainable, and adaptable to site specific conditions.

References

INAP. (2022). Acid and Metalliferous Drainage. https://www.inap.com.au/acid-drainage/#amdlinks

Pearce, J. Pearce, S. Scott, P. Weber, P. (2016). Acid Metalliferous Drainage Contaminant Load Prediction for Operational or Legacy Mines at Closure. Okane Consultants. https://www.okc-sk.com/nz/wp-content/uploads/2016/04/Pearce-et-al-2016-Acid-and-metalliferous-drainage-contaminant-load-prediction.pdf