Acid Mine Drainage

Acid seep in Nevada - photo Glenn Miller

Acid Mine Drainage refers to the outflow of acidic water from a mining site. In most cases, this acid comes from oxidation of iron sulfide (FeS2, also known as pyrite or "fool's gold"). Acid mine drainage poisons downstream waters, in many cases to the point where nothing other than bacteria can survive. Although mining is by far the largest cause of this type of acid leaching, the process can also occur during non-mining land disturbances such as construction or even naturally in some environments, and is an example of the more general phenomena of acid rock drainage.

Acid mine drainage is a major problem with many hardrock mines, including almost all mines where the metal ore is bound up with sulfur (metal sulfide mines). A significant number of coal mines also suffer from acid mine drainage.

Geology and chemistry

Many desirable metals such as copper, gold, lead, and zinc are often found in ores containing large amounts of iron sulfides. When iron sulfide is exposed to air, or to dissolved oxygen in water, the iron rusts and the sulfur reacts with water to form sulfuric acid (H2SO4).This sulfuric acid makes the water more acidic, sometimes dramatically so. The Iron Mountain Mine in California contains water with a pH as low as -3.6 which is 1000 times more acidic than battery acid. In addition to the direct effects of acidity, acid water causes toxic heavy metals such as arsenic and lead to leach from the surrounding rocks and contaminate the water.  Additionally, the change in acidity reduces the ability of the stream to buffer further chemical changes.

The process of mining allows these acid and heavy metal generating reactions to occur at much higher rates than in nature. Metal mining produces waste rock and mine tailings with a surface area that is vastly greater than the undisturbed rock. For example if you had a cubic meter of sulfide ore buried in the ground, you'd have one square meter exposed on the surface where sulfuric acid could be formed. If you then lifted this out of the ground and broke it into a bunch of 1 cubic centimeter pieces, the surface area will have increased by 600 times. Often in mines the ore is crushed to mud, where each particle can be less than 50 millionths of a meter across, thus increasing the surface area by a factor of 120,000 or more.

Sometimes iron sulfides are also found in conjunction with neutralizing compounds such as carbonates.  The ratio of acid-generating material to neutralizing material is an important component of predicting whether a mine will have acid mine drainage.  For example, metal at the historic Kennecott Copper Mine in Alaska was found in conjunction with large amounts of carbonate that neutralized any acid-forming compounds.

Microbes can also play an important role in acid mine drainage. Once acid mine drainage gets going, there are a number of microbes which thrive in the acidic environment and increase the production of sulfuric acid from the surrounding rock.

Prevention and mitigation

Attempts to limit acid mine drainage fall into two major categories; preventing sulfuric acid from forming, or neutralizing the acid after it forms.

In order to stop the formation of sulfuric acid, the waste rock and tailings from a mine must be prevented from contacting oxygen. Oxygen can come from flowing water or air. Strategies for keeping tailings separate from oxygen include submerging the tailings under still water, sealing them behind a synthetic barrier, or burying them underground.

Large mines typically use water to reduce the rate at which oxygen interacts with the tailings or other potentially acid generating rock since other options are ineffective or too expensive on a large scale. This strategy slows, but doesn't totally prevent, the formation of sulfuric acid. Some waste facilities limit acid formation to such an extent that active water treatment is not required. However, in many cases the water flowing out from a waste facility needs to be treated to neutralize some of the acid before release into the environment.

Often, acid prevention strategies fail. Once acid has been generated, there are a few possibilities for treatment. One form of passive water treatment involves creating of artificial wetlands where natural microbial processes can be encouraged to precipitate out some of the metals released by the acid. However, this solution is impractical for large mines due to the massive area of wetlands that would be required.

Berkeley Pit

Berkeley Pit water treatment facility

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Active water treatment consists of using bases such as hydrated lime, sodium hydroxide, sodium carbonate, or ammonia to reduce the acidity directly. Crushed limestone is one of the most common neutralization solutions in use today, primarily due to its low cost. However, the use of neutralization solutions causes a significant increase in the "total dissolved solids (TDS)" of the water which can have negative impacts on both aquatic life and human health. Another major problem with active water treatment is that the treatment needs to be maintained forever in order to prevent acid mine drainage. For example, the Berkeley Pit in Butte, Montana is a massive open pit mine that is slowly filling with very acidic water. Once the water gets close to the groundwater level, treatment of the pit will need to commence and continue forever.

Another potentially promising option is the use of "bioreactors" which utilize sulfate-reducing bacteria to precipitate out dissolved metals. While small-scale tests at locations such as the Leviathan Mine in California have been promising, the technology has not been expanded to large-scale water purification.

The goal of active water treatment is to produce water flowing from the mining area that doesn't present a risk to downstream ecosystems or human health. However a large study in 2006 found that the over 60% of large hardrock mines had failed to meet downstream water control standards using a variety of measures. In particular, 90% of the mines that predicted "low acid mine" drainage potential had acid mine drainage problems at the time of the study.

Leviathan mine yellowboy in Bryant Creek
at confluence with Mountaineer Creek, Nevada
photo Glenn Miller

Environmental Impacts

The increased acidity caused by acid mine drainage has a range of negative effects depending on the severity of the pH change. Many river systems and former mine sites are totally inhospitable to aquatic life, with the exception of "extremophile" bacteria. A large number of acid mine drainage sites in the US are designated as EPA Superfund sites.  Acid mine drainge is an example of an "externality", a cost that is not borne by either the mining company or the purchaser of the minerals (see our article on "True Cost").

Mining near the Iron Mountain Mine in California began in the 1860's, the mine closed in 1963 and was designated as an EPA Superfund site in 1983. Water passing through the mine site has resulted in periodic fish kills of migrating salmon since at least the 1940's. The site requires expensive active maintenance in order to prevent it from also contaminating the drinking water of nearby communities. Large portions of nearby creeks have been sterilized by the high levels of acidity and toxic metals in the runoff from this area. As with all acid mine drainage sites, the rocks will remain capable of generating sulfuric acid for many many years. Some Roman mining sites in Great Britain are still producing significant acid mine drainage, 2000 years after the completion of mining(1).

Acid mine drainage is not just a historical problem however; a large number of active mines around the world (including in Alaska) are currently facing this problem. This can occur even before a mine is finished being constructed. For example the under-construction Kensington Mine in Southeast Alaska didn't even expect to have acid mine drainage issues because of the presence of neutralizing rock in the ore. However, in 2007 acidic water was discovered near the construction site and mitigation is still ongoing.

In addition to the direct negative effects of increased acidity and the increased release of toxic metals, an additional problem can also be created when the acid reacts with rock that neutralizes it. As the water becomes less acidic, metals and other solids come out of solution. These precipitates, known as "yellow boy," can smother life on the streambed and turn the stream a distinctive orange/red color.

In Alaska

Acid Mine drainage is a problem at a number of active Alaskan metal mines including Red Dog, Greens Creek, and Kensington Mine. The potential impacts of acid mine drainage on Bristol Bay fisheries is the primary source of opposition to the proposed Pebble Mine in southwestern AK and a concern also at the proposed Donlin Creek Mine. One inactive mine in Alaska, the Salt Chuck Mine, is being considered for Superfund status by the EPA as a result of acid mine drainage.

References

1Wildeman, T. R., Brodie. G.A. and Gusek, J.F. 1991 Draft handbook for constructed wetlands receiving acid mine drainage. Ohio 43268, USA: US Environmental Protection Agency.

Date Created: Wed, 5 May 2010 00:07:54 -0800

Last Modified: Sun, 14 Aug 2011 14:06:40 -0800