What is acid mine drainage?
Well, mining means that rock is being removed to uncover valuable minerals like gold, copper, iron or coal. It is safe to say that most of this rock has not been exposed to oxygen for a long time, if ever, and therefore have been in a state of anaerobic conditions. The process of exposing minerals like pyrite to oxygen is called oxidation. When it erodes from the process of weathering, the sulfur content reacts with the oxygen and nearby water supply. We can see that the water is polluted because it turns a reddish-brown color, which comes from the high levels of iron that has also been oxidized.
We can also tell if the water is acidic using a pH meter to measure the current levels. Natural water is a pH of 7 and pure water is 5, but the lower you get to 0, the more acidic you get. This causes a huge problem on the environment because acidic water is bad for plants and animals. If you reach a pH level of 3-4, many fish cannot reproduce or even die and many plants absorb the acidic water and do not survive for very long. Much of the acid mine drainage issues we see today come from coal mines.
You may be picturing a black and white picture from a textbook with men carrying chisels and picks into a hole in the mountain. Well, those days are quite last century. In fact, last year I got to see a few mines with both my GeoChemistry and Applied Hydrogeology classes when we took a field trip up north to see the Soudan Underground Mine State Park as well as the Mesabi Range Taconite Mine in Hibbing, MN (both shown below with the Mesabi Range photos on bottom).
Solutions include treatment and storage. Sometimes the tailings are returned to the mine pit after it has served its purpose. Other times the tailings are dumped in lakes or behind dams with water dumped on top to slow down the process of sulfuric acid formation. Treatment is the better ones choice of course, but take a long time and lots of space. There are quite a few methods of treatment, but I am more familiar with the passive treatment of acid mine drainage. A few examples include wetlands, anoxic limestone drains (ALDs), open limestone channels, etc. ALDs are simply trenches in the bottom usually lined with clay or synthetic material to keep the toxic water from entering the soil and sinking to the groundwater (aquifers). The key is to keep oxygen out of the equation so the trench is covered. The goal of this technique is to run acid mine water through these trenches filled with a layer of limestone to raise the overall alkalinity and pH back to safer levels before possibly running it through a wetland of grasses and cattails to adsorb the heavy metals therefore taking them out of the water.
Well, mining means that rock is being removed to uncover valuable minerals like gold, copper, iron or coal. It is safe to say that most of this rock has not been exposed to oxygen for a long time, if ever, and therefore have been in a state of anaerobic conditions. The process of exposing minerals like pyrite to oxygen is called oxidation. When it erodes from the process of weathering, the sulfur content reacts with the oxygen and nearby water supply. We can see that the water is polluted because it turns a reddish-brown color, which comes from the high levels of iron that has also been oxidized.
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| http://en.wikipedia.org/wiki/File:Rio_tinto_river_CarolStoker_NASA_Ames_Research_Center.jpg |
Have you ever seen what a mine looks like?
You may be picturing a black and white picture from a textbook with men carrying chisels and picks into a hole in the mountain. Well, those days are quite last century. In fact, last year I got to see a few mines with both my GeoChemistry and Applied Hydrogeology classes when we took a field trip up north to see the Soudan Underground Mine State Park as well as the Mesabi Range Taconite Mine in Hibbing, MN (both shown below with the Mesabi Range photos on bottom).
The differences between the two are quite significant so I'll mention just a few. The Soudan Mine is not in operation and is currently a State Park near Lake Vermilion. It is Minnesota's oldest iron ore mine and there were issues of it's safety (as do all mining operations). It started out as an open pit mine, but they started drilling down due to stricter safety precautions. The main attraction for most is the deep mine pit that you are able to explore on tours provided there as well as various buildings that were used for crushing, drilling, etc.
The Hibbing Taconite Mine in the Mesabi Range is a huge open pit mine. They mine iron as well, but it is called taconite because it is considered a "low-grade" iron ore. The trucks you see in the photo are actually quite large.
A better example of what an open mine and the equipment look like can be found in the short video below as well as this link to a copper mine: Kennecott Copper Mine.
So, mining creates acidic water pollution due to pyrite dissolution and sulfur oxidation, but there are more environmental concerns that arise from mining including the waste and mountaintop removal. The unused material waste that are left over from excavating are called "tailings". Mine tailings are separated from the valuable materials with dangerous chemicals such as cyanide. Not only that, but the rocks themselves may contain toxic chemicals such as arsenic and mercury. Both create a problem when the time comes to dispose of the tailings since they are extremely toxic.
Solutions
Solutions include treatment and storage. Sometimes the tailings are returned to the mine pit after it has served its purpose. Other times the tailings are dumped in lakes or behind dams with water dumped on top to slow down the process of sulfuric acid formation. Treatment is the better ones choice of course, but take a long time and lots of space. There are quite a few methods of treatment, but I am more familiar with the passive treatment of acid mine drainage. A few examples include wetlands, anoxic limestone drains (ALDs), open limestone channels, etc. ALDs are simply trenches in the bottom usually lined with clay or synthetic material to keep the toxic water from entering the soil and sinking to the groundwater (aquifers). The key is to keep oxygen out of the equation so the trench is covered. The goal of this technique is to run acid mine water through these trenches filled with a layer of limestone to raise the overall alkalinity and pH back to safer levels before possibly running it through a wetland of grasses and cattails to adsorb the heavy metals therefore taking them out of the water.
Here is an example of some aerobic limestone treatment systems I saw on the same field trip:
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| As you can see, it is nothing flashy, really it is just a slow process of running the water through these aerobic wetlands with limestone and cattails. |
Mountaintop Removal
Mountaintop mining is something altogether different as it isn't quite digging a canyon, but leveling off the mountains. I don't know if you are much of a hiker, climber, skier, and whatnot, but I love to do all three and appreciate mountains for their aesthetic quality as well. Living in Minnesota my whole life has definitely limited my experiences of real mountains but there are bluffs to ski and while I was in Taiwan in the summer of 2009, I was able to hike up quite a few mountains. The fact that this is being done with no consideration for the surrounding communities should shock you. There are water and air quality dangers that come from exploding rocks.
The following image was taken from the Mountain Justice website:
You can see from this that trees are removed and the mountains are leveled. Many companies have been sued because of their lack of communication with the surrounding towns and undeniable choice to be ignorant of their responsibility to restore the area. Total restoration would be impossible, but some have consented to at least replanting what's left of the mountain. What is usually planted is nothing compared to the glorious forests they once were because the chemicals used and the seeds they plant belong to types of grasses that are either an alien species or prevents other plants from growing.
Due to the loss of trees, soil erosion is much more common as well as flooding since the soil is not held together by roots and the water flows through the smoother surface of the mountain. Mine waste that enters streams destroy habitats and wildlife.
Black Diamonds: Mountaintop Removal and the Fight for Coalfield Justice is a documentary I watched on campus during the Geoscience Department's Earth Talk Series. It goes into more detail about the coal mines in the Appalachian mountains and the various environmental hazards created. Most of my early knowledge of the subject came from watching this documentary. It takes place in West Virginia and the dangers this mining process has on human welfare is shown by this perfect example of a community asking an unwilling government to enforce the law and thousands of lawsuits filed against the mining company being swept under the rug as the company buys out the town. It is a scary thought when the system does not work for the people.
For more information about mountaintop removal check out these sites:
Mountaintop Removal 101
Citizens Fight Back
Earth Justice
I Love Mountains
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