Aside from the use of a toxic chemical such as cyanide in gold mining being potentially dangerous to the environment, another danger is the waste rock and tailings that are created (e.g. Hoffert, 1947; Ketelhodt, 2009). These two byproducts are primarily hazardous to the hydrosphere, with implications extending down to the biosphere and anthroposphere.
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| Fig. 6 - AMD damage to the Rio Tinto River, Spain - Carol Stoker |
The dangers present here is that of acid mine drainage (AMD). This is essentially the outflow of acidic water from mines and dumps. Rocks that contain gold are quite likely to also contain other reactive minerals, most commonly pyrite; 'fools gold', a common iron sulfide mineral. This iron sulfide mineral is not wanted during gold mining, so is either dumped as waste rock or dumped as tailings post-gold extraction (Hoffert, 1947). The problem here lies in pyrites reaction with water. It breaks the iron sulfide down into an acid:
This is a chemical reaction that occurs naturally, however, when humans dig a mine, they allow water to access the surface of these rocks faster, and expose more surface area. This speeds the reaction up, which is a problem. The sulfuric acid gets into the water, making any streams nearby turn orange-red due to the rust (see fig. 6), and most likely have a low pH of around 2-3. This is a major environmental hazard to the hydrosphere, which is contaminated, allowing the acid to reach the anthroposphere and biosphere (Sulfuric acid is extremely dangerous to humans, animals and plants).
Another effect is the creation of yellow boy. This yellow-orange solid is an iron(III) hydroxide (more commonly known as ferric acid) precipitate, which discolours the water, smothers plants and animals, and disrupts the natural stream ecosystem. Below (fig. 7) is a picture from the Animas River in Colorado, a waterway heavily stricken by large amounts metals and sulfates, decreasing pH and producing precipitates such as iron(III) hydroxide (Shemel & Cox, 2005).
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Treatment
Currently, there are three general ways of treating and preventing acid mine drainage; engineered structures, active treatments, and passive treatments (e.g. Johnson and Hallberg, 2005; Gray, 1997; Sheoran and Sheoran, 2006).
Engineered Structures (fig. 8): This involves physically changing something regarding the mines. Firstly, we can deal with water management. We can inundate the mine with water. By flooding the mine and sealing it with clay and soil, you are creating an anoxic zone, where the sulfides cannot react (increasing H2O but removing O2). Another way is to completely drain all the water and prevent water from entering (increasing O2 but removing H2O). Secondly, a more engineered structure is to build an impoundment - dig a large hole or use the existing mine, line the hole with cement and plastic, dump all the tailings in, then seal up with more cement. This method is quite often used, however, despite it appearing to be a long-term solution, it is not uncommon to see these break (as can be seen in the picture above, where the tailing pond at the Gold King Mine was accidentally damaged, releasing its contents into the Animas River).
Active Treatment (fig. 9): This method is quite easy; react the acid with a base to neutralise it. Large amounts of limestone (calcium carbonate - CaCO3) or slaked lime (calcium hydroxide - Ca(OH)2) are
thrown into the river. For every one mole of the alkaline carbonate/hydroxide present, two moles of the acid will neutralise, eventually bringing the pH up to a safe level.
Passive Treatment (fig. 10): Probably the best method would be to build a wetlands/swamp over and around the mine. The sulfides in the rock will be in an anoxic zone, unable to react, and will essentially stay in its natural form, allowing the sulfide to stay in full circle. Another method is to build a limestone drain. The acid in the water passing through the drain will passively react with the carbonate and neutralise. This is a near long-term treatment, however, the limestone walls will need sub-annual pressure washing to remove rust flocculation. Either way, this 'passive' treatment is more a combination of active and engineering, but with less time needed to attend and regularly treat it.
Regardless of which method is used, something needs to be done. The corporation of Rio Tinto have recognised their mistakes after their river became highly contaminated, and are seeking to fix this. As of 2015, they have properly disposed of or stored 1,740 million tonnes of waste rock and tailings (Rio Tinto Annual Report), instead of dumping it at the site which is what caused the damage in the first place. This is a positive step forward, and hopefully other companies will try to follow.
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| Fig. 8 - Example of concrete impoundment containing waste. Self-illustrated. |
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| Fig. 9 - Addition of limestone neutralises acidic water. Self-illustrated |
thrown into the river. For every one mole of the alkaline carbonate/hydroxide present, two moles of the acid will neutralise, eventually bringing the pH up to a safe level.
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| Fig. 10 - A limestone drain built in to the ground passively neutralises flowing water. Self-illustrated. |
Passive Treatment (fig. 10): Probably the best method would be to build a wetlands/swamp over and around the mine. The sulfides in the rock will be in an anoxic zone, unable to react, and will essentially stay in its natural form, allowing the sulfide to stay in full circle. Another method is to build a limestone drain. The acid in the water passing through the drain will passively react with the carbonate and neutralise. This is a near long-term treatment, however, the limestone walls will need sub-annual pressure washing to remove rust flocculation. Either way, this 'passive' treatment is more a combination of active and engineering, but with less time needed to attend and regularly treat it.
Regardless of which method is used, something needs to be done. The corporation of Rio Tinto have recognised their mistakes after their river became highly contaminated, and are seeking to fix this. As of 2015, they have properly disposed of or stored 1,740 million tonnes of waste rock and tailings (Rio Tinto Annual Report), instead of dumping it at the site which is what caused the damage in the first place. This is a positive step forward, and hopefully other companies will try to follow.
