Acid mine drainage (AMD) is a type of water pollution that occurs when rocks containing sulfide minerals are exposed to air and water.
This process causes the release of sulfuric acid, which in turn can lead to contamination of nearby bodies of water.
AMD is a major environmental problem associated with mining operations, and it can have serious consequences for both human health and the environment.
In this article, we’ll discuss what acid mine drainage is, its effects, and how it can be prevented or managed.
Here are the top facts about Acid Mine Drainage that you must know.
1. What Causes Acid Mine Drainage?
The primary cause of acid mine drainage is the oxidation of sulfide minerals found in rocks such as pyrite (FeS2).
When these minerals come into contact with oxygen and water, they react to form sulfuric acid (H2SO4).
This acid dissolves heavy metals leading to the creation of water that is high in sulfuric acid and dissolved iron.
The iron, as it separates from the water, can form striking deposits of red, orange, or yellow sediments in streams.
This process is accelerated by bacteria present in the environment, which convert the sulfide minerals into sulfuric acid more quickly than would occur naturally.
So why does it occur?
There are several different sources of acid mine drainage, including acid runoff from abandoned mines and natural deposits of sulfide minerals.
2. Acid Mine Drainage Can Heavily Affect the Environment
Acid mine drainage is a devastating consequence of mining activities that can have severe environmental impacts.
It occurs when the iron sulfides found in rocks and soil are exposed to air and water.
The reaction between the iron sulfides, air, and water causes a chemical process that creates sulfuric acid, a highly corrosive substance.
This acid has the power to deteriorate rocks and soil, which can result in the release of toxic metals into the water.
The release of toxic metals into the water system can have far-reaching effects, as the metals can travel downstream and contaminate other water sources, potentially affecting the health of plants, animals, and even humans.
Pollutants created by acid mine drainage, such as heavy metals, are not biodegradable and will build up in organisms at every level of the food chain.
The release of sulfuric acid into the water can also lead to a drop in pH levels, making the water highly acidic.
This acidic water can be harmful to aquatic life and can also make it difficult for plants to grow and make areas uninhabitable for many species that require neutral pH levels to survive.
This, in turn, can cause a ripple effect on the ecosystem, disrupting the food chain and causing harm to other species.
Acid mine drainage is a serious problem that must be addressed, as it has far-reaching and long-lasting effects on the environment.
It is essential that steps are taken to prevent this problem, through proper management and disposal of waste products, and the use of protective measures to prevent water and air from coming into contact with iron sulfides.
As mentioned earlier, certain bacteria can also intensify the rate and extent of acid mine drainage, further exacerbating its negative impact on the environment.
This makes it imperative that measures are taken to manage and prevent mine drainage, as it can lead to long-lasting damage to water sources and ecosystems.
3. Mine Drainage pH Can Reach as Low as 2.5-4
The accuracy of water sample analysis in the study of mine drainage is of utmost importance.
This is because the chemical composition of water samples can rapidly change if they are taken away from the mine environment.
As a result, most mine drainage measurements are conducted in the field.
The initial step of measurement usually involves determining the acidity of the sample, which is reported in terms of pH.
A pH of 7 is considered neutral, while a value below 7 is classified as acidic, and a value above 7 is considered basic.
The lower the pH, the more corrosive the water, making it more effective in eroding mining slag, rocks, and other materials.
This acidic water can then transport the contaminated materials to nearby rivers, eventually depositing them downstream.
Some mine drainage samples have been found to have a pH as low as 2.5-4, making it imperative to take measures to prevent and manage mine drainage to avoid further harm to the environment.
4. Acid Mine Drainage Can Cause Ocean Acidification
The impacts of acid mine drainage are not limited to the land and freshwater sources; it can also have a significant impact on the ocean.
When acidic water flows into the sea, it can have devastating effects on the underwater ecosystem.
Ocean acidification is a phenomenon where the ocean absorbs increasing amounts of carbon dioxide (CO2) from the atmosphere, leading to a decrease in the pH level of seawater.
This increase in CO2 makes the ocean more acidic, altering the chemical composition of seawater.
As a result, the ocean’s ability to absorb and maintain essential minerals and nutrients necessary for marine life is reduced, leading to changes in the abundance and distribution of different species in the ocean.
Ocean acidification can impact all sorts of marine life including crabs, coral reefs, fish, seaweed, and mollusks.
One of the major impacts of AMD is coral mortality, which occurs when acidic water reduces the coral’s ability to produce calcium carbonate, a critical component of their structure.
This can lead to the death of large areas of coral, reducing the overall health and diversity of the coral reefs.
In addition to coral, bottom-dwelling organisms in the ocean are also affected by acid mine drainage.
The acidic water can affect the delicate balance of the underwater environment, making it difficult for these organisms to thrive.
This can have a significantly negative chain reaction on the underwater ecosystem.
The impact of acid mine drainage on the ocean highlights the need for measures to prevent and manage mine drainage, as its effects can be far-reaching and long-lasting, impacting not only the land but also the oceans and the creatures that call them home.
5. Acid Mine Drainage Causes Damage to Infrastructure and Landscapes
The corrosive nature of acidic mine drainage is not limited to drinking water; it can also cause severe damage to infrastructure.
The highly acidic water can corrode bridges, underground pipes, and other important structures, leading to costly repairs and replacements.
6. Acid Mine Drainage Effects on Humans Is Concerning
When it comes to the health effects of acid mine drainage (AMD), there are two main concerns:
the solubilization of metals such as aluminum, arsenic, cadmium, copper, lead, nickel, and zinc, and;
the direct exposure to sulfuric acid.
The government states that while there is no direct risk to public health from AMD, the solubilization of these metals can make it easier for them to enter streams and rivers and contaminate drinking water.
Regarding sulfuric acid, it is extremely corrosive, and direct exposure can cause severe irritation and burns to the skin, lung damage, and possible blindness.
If swallowed, it can cause burns to the mouth, throat, esophagus, and stomach.
Long-term exposure to low levels of sulfuric acid mist can cause dermatitis, erosion of teeth, and other health problems.
However, there is limited information available on the effects of low-level exposure and its synergistic effects with other chemicals.
Unfortunately, there is no medical test to determine whether someone has been exposed to sulfuric acid.
While increased acidity in saliva can be measured through tests, it will not tell which type of acid has been inhaled.
The good news is that sulfuric acid is unlikely to accumulate in the body, so any short-term exposure should not cause major health concerns.
7. Constant Sulfuric Acid Exposure Increases the Risk of Cancer
The National Cancer Institute and multiple studies on the matter conclude that there is sufficient evidence linking workplace exposure to strong inorganic acid mists containing sulfuric acid to cancer.
Basically, long-term exposure may also increase an individual’s risk of developing certain types of cancers.
8. Acid Mine Drainage is Heavily Ignored
A recent study of mines in the United States revealed that many mines are failing to accurately predict or prevent acid mine drainage (AMD).
The study found that 89% of the mines that developed AMD either underestimated or ignored the potential for AMD in their environmental impact statements.
This lack of attention to AMD can have significant consequences for the surrounding environment and communities.
Additionally, the study found that 64% of the mines in the study had failures in the mitigation measures they claimed would prevent or limit AMD.
These failures were due to factors such as improper design, maintenance, and management of mitigation measures, leading to greater AMD impacts than predicted.
The study also found that the water quality impacts from the mines were closer to the potential impacts (pre-mitigation) rather than the predicted impacts (post-mitigation) in the environmental impact statements.
This means that the actual impact of AMD was closer to what was expected before mitigation measures were put in place, rather than after they were implemented.
The results of this study highlight the importance of accurate prediction and effective mitigation measures to prevent or limit the negative impacts of AMD on the environment and communities.
It is critical that mines accurately assess and address the potential for AMD to ensure that their operations do not have negative consequences on the environment and communities in the long term.
9. Acid Mine Drainage Can Be Prevented and Managed
There are several methods available for preventing or managing AMD at mining sites, according to this study on acid mine drainage formation, control, and treatment:
Overburden analysis: Before any large-scale excavation, pre-mining analysis of soil and overburden is required by law to identify the acid-producing or acid-neutralizing status of rock layers.
Overburden analyses determine the potential for AMD to occur and help in preventing it.
Proper handling and placement of overburden: Proper handling and placement of overburden materials can significantly reduce the risk of AMD.
Precise handling of overburden materials, especially in areas prone to acid production, and quick coverage of acid-producing materials in the backfill can help prevent AMD from forming.
Post-mining hydrology: Understanding the post-mining hydrology of a site is crucial in preventing AMD.
Changes in water flow patterns and rates should be expected after mining, and steps should be taken to prevent water from coming into contact with the acid-producing materials.
Chemical treatment: In the case that AMD problems arise during or after mining, chemical treatment may be required.
A simple water treatment system can be implemented to treat the discharge, which may consist of a storage tank, valve, and settling pond to capture precipitated metals.
Passive treatment systems: Passive treatment plants, such as constructed wetlands, anoxic limestone drains, and open limestone channels, can be used to treat AMD without the need for continuous chemical inputs.
These systems take advantage of natural chemical and biological processes to treat contaminated mine waters.
10. Innovative Treatment Method for Acid Mine Drainage Recovers High Concentrations of Rare Earth Elements
Conventional methods for managing acid mine drainage involve trapping the contaminated water in holding ponds or specialized facilities, then using chemical additives to reduce the acidity.
This causes the metals that have dissolved in the water to settle out and solidify.
But in 2020, scientists at Penn State made a breakthrough by creating a new two-step treatment method to extract more of the precious, rare earth elements from acid mine drainage.
The process starts by injecting carbon dioxide into the drainage, which triggers a chemical reaction that results in the formation of solid minerals known as carbonates.
These carbonates bond with the rare earth elements, causing them to settle out of the water at low pH levels.
This new process allowed the scientists to recover a significantly higher amount of rare earth elements and aluminum, with a recovery rate of over 90% for aluminum and 85% for rare earth elements, compared to traditional methods which only recovered 70% of these elements.
11. One of The Biggest Acid Mine Drainage Disasters
The Doñana Disaster was a devastating industrial accident that occurred in Andalusia, Spain.
The Los Frailes mine, owned by Boliden-Apirsa, a subsidiary of Boliden, produced zinc and silver near Aznalcóllar on a yearly basis.
On April 25th, 1998, a holding dam at the mine burst, releasing a massive amount of mine tailings into the nearby rivers.
The tailings were filled with hazardous levels of heavy metals and quickly contaminated the River Agrio and River Guadiamar, which was the main water source for the Doñana National Park, a UNESCO World Heritage Site and a well-known national park in Europe.
The Impact on the Environment
The disaster at the Los Frailes mine near Aznalcóllar in Andalusia, Spain, had devastating consequences on the local ecosystem.
The toxic waste from the mine, containing heavy metals like lead, copper, zinc, and cadmium along with sulfides, was highly acidic and left nothing in its wake.
The toxic aftermath caused a chain of environmental problems in the region, with heavy metals still present in the soil and water and affecting wildlife.
The impact on wildlife was particularly severe, with over 2,000 birds and chicks, eggs, and nests being killed or destroyed and over 25,000 kilos of dead fish collected.
The Doñana National Park, a UNESCO World Heritage Site, was also impacted, with the migration of over 300 species of birds coming to a halt.
Despite efforts to clean up the disaster, the process took three long years and cost an estimated €240 million.
The tragedy serves as a reminder of the devastating impact that industrial accidents can have on the environment and the need for better safety measures to be implemented in mines.
Acid mine drainage is a significant environmental issue that has far-reaching consequences on the ecosystem and human health.
The low pH levels and high concentrations of toxic metals in the drainage cause damage to surrounding rocks and water sources, leading to a chain of serious environmental and economic issues.
It is important for all stakeholders to acknowledge the severity of this problem and work together toward finding effective solutions to minimize the impact of acid mine drainage on the environment and human health.
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