When I started reading about mining and its environmental impacts I soon realized that I would have a number of different topics to pick from, but I had to narrow it to one so today I’m going to be talking about acid mine drainage, or AMD. AMD is a byproduct of the mining process (usually from abandoned mines) and is polluting groundwater and waterways world wide and at the same time turning them orange. It occurs when mineral deposits containing sulfides, the most common being pyrite (fools gold), are uncovered in the mining process. There’s a lot of chemistry that goes into the production of AMD, but for our purposes I’ll try and simplify it. Basically what you need to know is that once the dirt is dug out of the ground it goes through a process to separate any metals from the waste (dirt). Once the metals are separated they are dumped back into a pit and often mix with water that is around the mining site. When something like pyrite mixes with the air or the water a reaction occurs and forms sulfuric acid which gets into underground water supplies and/or nearby rivers or streams polluting all of the water for miles around.
The pollution comes in a couple of forms. One is that the acid lowers the pH of the water which can make the water inhospitable to many life forms. Some of the most affected are immature fish and insects, however as the pH level drops it will affect more and more organisms. Another way it can harm an ecosystem is by adding a participate into the water system that can block out the sun making plant life unable to grow. Plants are another crucial part of any ecosystem, and without plants the balance of the ecosystem is compromised. With increased levels of AMD rivers and waterways will be wiped clean of all of its plant and animal life, and a red sludgy waterway will be left. The acidity of the water is also not good for humans. Water with a very low pH level can lead to copper, zinc, cadmium, and lead being dissolved in drinking water which can lead to some forms of cancer. Also, high levels of total dissolved solids (definition: www.bit.ly/HwuOPr) can lead to problems with the cardiovascular system as well as toxemia which effects pregnant women. Some other problems that might be less important but should still be mentioned include bad tasting water, rust colored stains on clothes, and the rusting of plumbing pipes and equipment.
In the United States the EPA is well aware of AMD and included it in the 1972 Clean Water Act, but by that time the effects were already widespread so people started looking at treatment options. There are a number of ways to treat high AMD chemically but several factors need to be known before the specific treatment can be picked. These include the flow rate, the flow and quality of the waterway that will be affected, if electrical power is available, the distance between where chemicals are added and the settling pond, the settling pond’s volume, and the amount of heavy metals. For those who don’t know a settling pond is a lined hole in the ground that the water is pumped into and then the solids settle due to gravity. Once you have all of that info there are six main chemicals that are used to treat AMD; limestone, hydrated lime, pebble quicklime, soda ash, caustic soda, 20% and 50% liquid caustic, and ammonia. The chemical that is chosen is based on the factors listed above as well as economic factors. If the correct chemical is picked and the quantity added is right you should have taken care of the AWD problem. There are a number of other biological and chemical treatments that can be done, but to be honest they’re a bit over my head and if I try and explain them I’ll probably just confuse everyone. If you’d like some more information here’s a document that goes more into it: www.hvrd.me/Ii5fQU.
That was how you can treat AWD chemically, but there are other options as well. One of the most simple treatments involve putting all of the toxic material in a lined hole and then covering it, and by keeping air and water out of the material the AMD is eliminated. Something else you can do to biologically treat it is add acid-generating and acid-consuming materials to the toxic material which should leave behind a non-toxic, non-acidic mixture that will be safe. A scaled down traditional wastewater treatment plant is another option. This is a good option when you have a mining site with frequently changing water characteristics because an engineered system can usually be calibrated to give you the required discharge independent of what’s in the water that’s being treated. Water treatment plants are also beneficial if you don’t have a lot of space, and because there is an abundance of professionals that know how these systems work. Another option is called Permeable Reactive Barriers (PRBs). PRBs treat groundwater using a subsurface reactive section that can be placed either in the natural path of the groundwater or barriers can be installed to direct the groundwater to the reactive section. The reactive section is usually made up of compost material that has sulfate reducing bacteria in it that treats the water as it moves through the section.
Another option is called Anoxic Limestone Drains (ALDs). This is basically a channel that is filled with crushed limestone and then drains into a settling pond. The limestone pulls all of the acidity out of the water, and then the heavy metals are separated from the water in the settling pond. Along the same path are Successive Alkalinity Producing Systems, and they use this general design:
The open water keeps the water below from being infiltrated with oxygen. The water below is being cleaned with the organic mulch layer and then with the limestone to get rid of the acid. Then the water is discharged into a settling pond to separate out the metals. This is a fairly simple and very efficient way of cleaning the water before discharging it. There are a number of other options that are in use but most of the others are similar to this and use a basic system of filtering the water through mulch or plants or limestone in some way so I’m not going to get into them in this post.
Throughout the world where you find mining you find acid mine drainage. In the U.S. the mining industry’s wastewater is monitored and they are held to strict standards, but in other parts of the world there is little to no monitoring or restrictions on the mining industry and so they dump whatever and wherever they like. Often this leads to protests and fighting between concerned citizens and the mining companies. Sometimes the people win and the mining company either pays a fine or has their license suspended. Other times nothing is done and the pollution continues. In the U.S. we’re now in the cleanup stage of our relationship with AMD, and through education of governments and their people hopefully developing countries will soon start doing a better job monitoring and punishing the huge mining companies that up till now take advantage of the lax regulations.