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February 10, 2025 950 am EST
What is Acid Mine Drainage anyway?
Edited for clarity and length
ML-ARD is a serious issue. Pretty well all the things we do in our daily lives involve products derived from mining. 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 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.
The public needs to realize, we are all responsible. 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 need the products of mining, so unless we are going back to the stone age, we will continue 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 mining case, 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 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 the $4 billion dollar liability for the Faro mine
. The company went broke and the Canadian taxpayer is on the hook for 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 prevent water and perhaps air 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 around where you place the waste materials or how you might modify the materials. Those are where some of our recent big gains have occurred, 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 save for later reclamation. The other two are of concern for ML-ARD. Some of the excavated material has enough of the target substance, say coal, to warrant processing. The other material doesn't have enough coal in it, so it is moved aside. This is the waste rock to be disposed of under water or underground. The material with the economic commodity of value goes for processing, which usually involves reducing, grinding rock down to extract the valued element. Crushers and grinders take the bedrock and create sand and silt, the economic component is extracted leaving small particles, tailings.
Back to environmental risk mitigation strategies:
- Flood the waste rock and tailings
- Cover the waste material
- Divert upstream drainage (around the mine site)
- treatment of the drainage (coming through the mine site)
- modify the tailings - remove sulphide
A mine could remove some of the sulphide material 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 is large, you can't remove sulphides. You could put the waste rock back inside the mine, back-filling. Maybe there is restricted water entry, or restricted air entry. Or maybe the mine floods, and becomes like the flood mitigation strategy. Another strategy is to add an amendment like lime that raises the pH, or iron sulphate that reduces the solubility of the contaminants. Usually with tailings, you might add the amendment during processing so the end material is not so bad.
Everything I have said so far are generalities. There are exceptions, materials that don't necessarily fit in these categories. For example, sulphides are generally not soluble unless they oxidize.
WT: Just to make sure I am understanding, I have a periodic table, what is a sulphide?
Minerals are organized forms of the elements (of the periodic table) with a defined chemical composition and organization, a method in which those elements are organized. It could be layers of sulphur and layers of oxygen. A sulphide is a type of mineral, associated with the element sulphur. Sulphur is one of those elements that can change its oxidation state. Sulphide is the most reduced form. As sulphur is exposed to oxygen in the atmosphere or in water, it oxidizes it can produce intermediary sulphur species, these are usually transitory, and sulphates.
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 on this subject otherwise politicians won't do anything about it. Just hypothetically, we might be promoting the need for critical minerals, having really 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.
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 is the problem occurring, what are the site attributes that are affecting your mitigation.
For example, there is a mine close to Vancouver, I worked for the provincial government to advise on risk mitigation. I figured it was going to take a year and a half (to properly assess). I should have said longer. Everybody said, "We don't want to study the problem, we want to solve it." You can't solve it without studying it. You need to understand the details, the devil is in the details. It is very difficult for people to understand (the environmental risk of a particular mine) because they 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 come with their own risk. One of the best ways is putting sulphides underwater. Not enough oxygen gets to the sulphides if they are in sediment, to oxidize them. The geo-chemical risk is greatly reduced. If you have to build a dam, you have now created geo-technical risk, as I said previously, you have to maintain it forever, you can't walk away. What are the risks for a dam? You need a spillway, big enough to deal with maximum flood, with climate change the probable maximum flood is increasing all the time.
Then there are things like ice jams. One of my favorites, beaver! 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 dam up the base of the dam and flood the foundations, causing the dam to fail. Another example, a site in Ontario had the "one in 100" 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 four times greater than the design capacity of the spillway. So one needs to be aware, that's an example of the risks all around.
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 management, 95% of them are closed. The amount of closed mines continually increases, it is a progressively greater issue. Every time a new mine opens, realize this is a site that is eventually going to close.
Example, the neighboring mine (Smithers, BC) closed in 1994. Cost is three million dollars per year to prevent pollution of the major river running through our region. If things go wrong, it will kill the river with salmon in it. Here we have one little old lady, 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 (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. I would say water usually has a precedence over land, so people will sooner destroy a huge amount of moose habitat rather than a little bit of salmon or trout habitat. The Fisheries Act protects fish bearing waters.
Related
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.
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