2/7/2025
WT Staff
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February 10, 2025 950 am EST updated 354 pm EST Feb 12, 2025
What is Acid Mine Drainage anyway?
Edited for clarity and length, updated for further clarity
Metals Leaching-Acid Rock Drainage (ML-ARD) is a serious issue. Pretty well all the things we do in our daily lives involve products derived from mining ML-ARD generating rock. So it's not an issue of whether or not we are mining, we are going to continue. It's a societal responsibility to make sure mining is done properly, so it does not have devastating effects on the environment.
Iron floc acidic mine drainage into Kississing Lake, MB
An interview with Dr. Bill Price - Natural Resources Canada Mine Drainage and Reclamation Expert
WT: Thanks for doing this Bill. I understand acid mine drainage is a complicated subject. Can you tell us, what is acid mine drainage, anyway?
Bill Price: We prefer to call it Metal Leaching - Acid Rock Drainage (ML-AMD), emphasizing that acidic drainage is a subset of the metal leaching issue.
Acid drainage is the biggest concern, though not the only concern. Treatment plants for neutral drainage can be more expensive than for acid drainage, and there are lots of instances of neutral drainage. Basically, acid rock drainage is one type of drainage quality. We refer to all of it as ML-ARD. Just understand, elevated concentrations of contaminants can be found in mine drainage at all pH levels.
(Drainage here refers to the surface water, precipitation, snow melt and groundwater that flows in, around and through a mine site and downstream through the watershed and aquifer.)
You started this off by saying it's complicated. The definition is a complication. Some people will say, "I don't have acid drainage, therefore I don't have any problems." To that, we say, "If you have acidic drainage, you are sure to have problems. If you have neutral or basic drainage, you may or may not have problems, depending on the concentration of contaminants."
The biggest source of contaminants and acidity are the sulphide minerals. These are found in deposits that mining wants to target - the coal, uranium, gold, precious metals - all those mines usually have rock types with elevated sulphides. Oxidation of the sulfides is what leads to the release of contaminants in the water draining through the site. The sulphides include combinations of sulphide with iron, or with arsenic, with lead and all the different metals. Iron is usually not an issue in itself, it is the other trace contaminants released through oxidation that become an issue.
WT: You say the metal leaching is separate from the acid, basic or neutral drainage aspect. So, it is a site-specific issue, is that fair to say?
Price: Yes. Most mines, be they precious metals, or base metals like copper-zinc, uranium, diamonds or coal mines have these elevated sulphides present. So, you can say this issue is common to most forms of mining. What I was saying about metal leaching, its not the pH that causes the environmental impacts, it's the trace elements, contaminants in elevated concentrations. For example, if I drank my annual beer consumption all in five minutes, it would kill me. Many of these elements are actually nutrients, so it's not whether they are toxic or not. Concentration is key, and also the sensitivity of species in the receiving environment. For example, humans can stand quite high levels of molybdenum with no ill effects, but infants cannot. Ruminants cannot. Similarly, we have copper pipes in our homes, but fish cannot handle much copper. The potential damage is specific to the (concentration of contaminants in the) drainage, and to the receiving environment.
Even when properly managed, the ML-ARD producing rock creates substantial liability and environmental risk for mines. Mitigation strategies are not risk-free, this environmental protection requires long term monitoring and maintenance. Most mine sites, you might think "mining occurs, mining finished, no more problems". That's not true. Mining requires ongoing, usually perpetual forms of mitigation, such as covers or drainage treatment.
There are a variety of mitigation strategies. One of them is to flood the waste rock and tailings material. If you flood the mine by building a dam, you have to maintain that dam forever. A dam doesn't maintain itself. If your mitigation strategy is to put in something simple like a diversion ditch, to divert the water away, making sure most of the water never gets contaminated, you have to maintain that ditch forever. Ditches don't maintain themselves.
If you are going to put a cover over the waste rock and tailings, such as a piece of plastic or dense soil like they might place over a landfill, again you need to maintain it. This is not a walk-away scenario.
Environmental risk mitigation is a critical part of any mine plan. When a new mine proposal comes in, part of that proposal is the closure plan. When I say close here, I am not saying walk-away. There are strategies that maybe allow walk-away but it is relatively rare that you could come up with those strategies, you probably cannot handle all the problems on a mine site to be able to walk away.
For mining to be sustained, it needs to occur in a proactive, well informed, environmentally sound manner. This is to say, not every mine proposal should go ahead. There are sites where we might decide mining is NOT the best use of this land. As a whole, we do need the products, so unless we are going back to the stone age, we will continue mining. The public needs to realize, we are all responsible for ensuring environmentally sound mining.
WT: New mine proposals include a closure plan, so this presumes we know the metal leaching and drainage quality before the mine permit is granted?
Price: We have to be proactive, we have to manage to prevent failure. You don't let the mess occur then try to mop it up afterwards. Here is what I call "the stitch in time saves nine" approach: if we were to advise someone about crossing the road, we wouldn't say, "Go ahead, step out in the road, we will handle whatever comes." The possible negative effects of stepping out in the road and being hit by a car is a problem you probably cannot fix. Similarly with the ML/ARD from mining, the cost (of failure) is so great, the environmental impacts are so great, risk mitigation has to be proactive.
Environmental impacts are devastating when things go wrong. You might have heard about the Mount Polley dam failure in British Columbia? When the plan fails, the cost is phenomenal.
By the way, thank you very much for your contribution for dealing with these kind of issues. You as a taxpayer are paying for the Faro mine liability.
The company went broke and the Canadian taxpayer is on the hook for approximately four billion dollars. Think of how many elementary schools or hospitals that would pay for! Also the Giant mine, similarly.
WT: How do the mines deal with pollution prevention? What are the risk mitigation and closure plans that you see?
Price: There are various mitigation strategies, all of them have been done better over time. Flooding, I mentioned that previously. It prevents the sulphides from oxidizing in the first place. We put a cover on. We build diversion ditches to help prevent water from coming in contact with waste materials. We can collect and treat drainage after it has drained through the mine materials. Those are the big ones. Then there are strategies where you decrease ML/ARD by how and where you place the waste materials or how you modify the material compositions. Modification in material placement and material composition are where some of our recent big gains in mitigation have occurred. An example of this is removing sulphide from tailings.
WT: Describe these mine waste materials for us, you have mentioned tailings, waste rock, what are these?
Price: When a mine excavates material, they can do so with an open pit or by tunneling underground. The excavated material fits into one of three categories: the first is topsoil you may save for later reclamation. The other two are products of excavated bedrock - waste rock and tailings, which are of concern for ML-ARD. Some of the bedrock material doesn't have enough coal or copper in it to warrant extraction. This is the second category, waste rock, which being in the way must be removed, it may be placed in piles and sometimes backfilled into pits or underground workings. The material with sufficient economic commodity-of-value such as coal or copper ore goes on for processing, which usually involves grinding the bedrock down. Crushers and grinders take the bedrock down to sand and silt. After the economic component is extracted, the remaining small particles are a third category, the tailings.
Back to environmental risk mitigation strategies:
- Flood the waste rock, tailings and mine workings
- Cover the waste material
- Divert upstream drainage (around the mine site materials)
- Treatment of the drainage (coming from the mine)
- modify material placement and material composition such as tailings by removing sulphide
A mine could remove some of the sulphide minerals from the tailings during processing for the target mineral. Maybe instead of 10 million tonnes of poor drainage quality tailings, you leave behind 2 million tonnes, allowing you to do something easier with the long-term management and mitigation.
Waste rock has large particles, so you can't remove sulphides. You could put the waste rock back inside the mine, back-filling which may restrict water or air entry, or the mine may flood, as the flood mitigation strategy. A modification strategy is to add an amendment like lime that raises the pH, or iron sulphate that reduces the solubility of contaminants. With tailings, it might be possible to add the amendment during processing.
Everything I have said so far are generalities. There are exceptions, conditions or materials that don't necessarily fit in these general descriptions. For example, while sulphides are generally not very soluble unless they oxidize, some arsenic and antimony sulphides are quite soluble and can have high rates of trace element dissolution.
WT: Just to make sure I am understanding, I have a periodic table, what is a sulphide?
Sulphide is a form of mineral. Minerals are solids, organized and formed from the elements of the periodic table, with a defined chemical, elemental composition, crystal structure and manner in which the different elements, such as sulphur and iron, are organized. A sulphide is a type of mineral containing the elements sulphur and metals and/or metalloids, such as copper and arsenic. Sulphur is one of those elements that can change its oxidation state. Sulphide minerals contain sulphur in the most reduced form. When sulphur is exposed to oxygen in the atmosphere or in water, it oxidizes. First it can oxidize to intermediary sulphur species, but these are usually transitory, and eventually oxidize to sulphate-sulphurs.
WT: Oxidation of sulphide-bearing mine waste produces ML-ARD, you say this is an issue for most if not all mines and must be managed forever. How can the public respond, what can be done?
Price: The public needs to be knowledgeable about this subject and ensure government provides the oversight necessary for environmentally sound mining and protection of public resources. Otherwise, ill-informed politicians, facing many other public demands for government resources, won't do much about it. Hypothetically, we might be promoting the need for critical minerals, having largely ignored the issue of ML-ARD for the last twenty years. Suddenly there is a need to accelerate mining, and yet there are large gaps in the knowledge and practises.
Successful ML-ARD management requires that we are well-informed. It is a little like the water in watersheds, when somebody says "the water quality has gone to hell in a handbasket", if you don't have monitoring showing where the contaminants are coming from, where do you begin to respond? When one mitigates a site, you don't just march in there and start acting. You have to understand exactly what is causing the problem, where the problem is occurring and what are the site attributes that will affect your mitigation options.
For example, there is a large abandoned mine, I was asked to advise on mitigation. I figured it was going to take a year and a half to properly assess what is required. I should have said longer. The response was, "We don't want to study the problem, we want to solve it." You can't solve ML-ARD problems without first having a good understanding and studying the site. You need to understand the details, the devil is in the details. You cannot understand the environmental risk of a particular mine if you don't know all the details.
WT: How important is it to contain ML-ARD to prevent contamination downstream, compared to allowing it to discharge?
Price: Source control is often the most effective way of preventing impacts and making sure mitigation costs are manageable. Mitigation strategies each come with their own risk. One of the best forms of source control is putting sulphides underwater. Not enough oxygen gets to the sulphides if they are in flooded sediment, to extensively oxidize them and therefore the geo-chemical risk is greatly reduced. On the other hand, if you have to build a dam for flooding, you have now created geo-technical risk. As I said previously, if you have a dam, you have to maintain it forever, you can't walk away. What are the needs and resulting risks for a dam? One is that you need a spillway big enough to deal with maximum flood. With climate change, the probable maximum flood is increasing all the time.
Dangers include things like ice jams. Beaver can cause a dam to fail by damming up the spillway, causing the water to over-top the dam, causing failure. Or, beavers can build their dam at the base of the dam and flood the foundations, causing failure. Another example of damage caused by beaver is, a site in Ontario had the "one in 100 year" flood event. The tailings impoundment was designed for double that event, however, the floodwater upstream hits a beaver dam, which collapsed. The amount of water released from the beaver dam failure was greater than the design capacity of the spillway. So one needs to be aware of the risks upstream as well as those on the mine site itself.
WT: What are we looking at here in context, how many mines are active and operating compared to closed and/or abandoned mine sites?
Price: All mines will eventually close. Of the major mines that require ML-ARD management, most, perhaps 95% of them are closed. The number of closed mines continually increases, and ML-ARD requirements are continually increasing, it is a progressively greater issue. Every time a new mine opens, it is important to realize this is a site that is eventually going to close.
Example, a mine near Smithers, BC closed in 1994. Cost is three million dollars per year to prevent ML-ARD pollution of the major river running through our region. If things go wrong, ML-ARD may kill the river with salmon in it. Active informed public interest over these decades comes down to one person, the only one with sustained interest.
We as tax payers are responsible for making sure all mines are properly managed. I think of government as "we". This only works if the public is engaged. We regulate, but we also own those projects that have gone broke. When the proponent ceases to fulfill their obligations, the government takes it over. Part of mine regulation is requesting a financial security that will pay for the ML-ARD mitigation liability. Predicting how much that should be is limited by our gaps in knowledge. When a mine goes broke, they tend to be the sites that didn't use the "stitch in time saves nine" principle, hence the four billion dollars clean-up at the Faro mine and Giant mine.
WT: One last question. With more mining coming in the future, we have more ML-ARD to look forward to, requiring investment forever. It seems water is at risk. As water is not a natural resource managed by Natural Resources Canada, I would ask, is water being used to dilute pollution with little oversight?
Price: I wouldn't say that. We have the Fisheries Act to protect fish bearing waters. As a result, water usually has a precedence over land. Mines will sooner decrease moose habitat rather than salmon or trout habitat.
Acid Mine Drainage to the extreme: Sherridon, Manitoba Canada, an interview with Dr. Matt Lindsay, NSERC Industrial Research Chair in Mine Closure Geochemistry
Associate Editor, The Canadian Journal of Mineralogy and Petrology, here.
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