Acid Mine Drainage (hard rock mines)/Acid Rock Drainage Working Group

Snake River, CO

The Snake River in Colorado. The water is at a pH between 4 and 5. The reddish coloring is due to iron oxide precipitates and the whitish coloring is due to aluminum oxide precipitates. Photo by: Garrett Rue.

Acid mine drainage (AMD) is one of the major water quality issues in Colorado and at locations throughout the western U.S. USGS and INSTAAR researchers are collaborating on a variety of AMD projects as well as investigations of naturally-occurring acid rock drainage (ARD).

CWEST Participants: Kate Campbell, Blaine McCleskey, Diane McKnight, Kirk Nordstrom, Rob Runkel, Joe Ryan, Andrew Todd


Metals

Pennsylvania Mine Summit County, CO

Structures of the former Pennsylvania Mine in Summit County, Colorado. Photo by: Garrett Rue

Metals are vital for modern civilization. They are in our cars and airplanes, our computers and cell phones, our houses and office buildings, our electrical wiring, cooking pots, and more. They also form an essential part of the history of the western U.S. and Alaska. The California gold rush, Colorado’s Pike’s Peak gold rush, and Alaska’s Klondike gold rush led to the migration of hundreds of thousands of miners to the region in the last half of the 19th century. The General Mining Act of 1872 likewise contributed to the exploration and settlement of the west by giving individuals exclusive rights for the mining of hardrock minerals such as gold, silver, lead, copper, and zinc on federal lands (USGAO 2011).

 

Map of Known Abandoned Mine Lands

Map of Known Abandoned Mine Lands (AML) on Bureau of Land Management (BLM) lands only. From BLM AML Inventory website. Downloaded on March 21, 2014.

However, once ores were depleted, mines were often abandoned, leaving a legacy of ghost towns, deteriorating mine structures, and in some cases, environmental contamination. A 2011 Government Accounting Office report estimates the number of abandoned mines on federal lands to be at least 161,000 (USGAO 2011). This figure does not include abandoned mines on private lands. At least 33,000 of the mines on federal lands are causing some form of environmental degradation including acid mine drainage (AMD) that is contaminating surface and groundwaters (USGAO 2011).

 

What are acid mine and acid rock drainage?

Pyrite

Pyrite (also known as fool's gold). Photo: Wikimedia Commons, Jon Zander.

Acid mine drainage (AMD) occurs primarily when the mineral ore pyrite (FeS2) is exposed to water and oxygen from air, producing soluble iron and sulfuric acid. One reason that pyrite is important in AMD generation is that it forms under a variety of conditions and is widespread in different rock types. It may thus be unearthed during the mining processes for various metals. Another reason that pyrite is important is because iron, unlike many other metals, has a variety of oxidation states. Microorganisms can take advantage of this fact and grow by oxidizing the Fe2+ in pyrite to Fe3+. The Fe3+ form of iron thus generated can, in turn, act as an oxidant and react with pyrite and water to produce even more sulfuric acid.

Acid Drainage

Acid drainage with a pH of -0.7. Although water with negative pH may occur, it is rare to find it accumulating in amounts that you would be able to collect in a cup as shown above.

Sulfuric acid produces extremely low pH values in waters receiving AMD. At times pHs are on par with lemon juice or vinegar (~pH 2) and battery acid (~pH 0). In some cases pH values are even negative (Nordstrom 2011)! As pH decreases, metals in other mineral ores become more readily dissolved. The resulting acid mine drainage may thus not only contain high levels of sulfate and iron but other dissolved metals as well. Microbes, as noted above, play an important role in acid drainage generation because they greatly increase the rate and degree to which such acid drainage generation processes occur (Nordstrom 2011).

The production of drainage with low pH and high dissolved metal concentrations can also occur naturally (acid rock drainage or ARD) through the weathering of pyrite and other mineral ores. However, the often large-scale land disturbances introduced by mining and mineral processing activities leads to the greater exposure of minerals to both air and water, intensifying acid drainage generation.

Acid drainage pathways in the water system

Tailings Piles Keystone, CO

Tailings piles near Keystone, Colorado. Adit, Richmond Mine, Iron Mountain, California. Tailings piles are the waste soils that remain once mineral ores have been processed. Photo by: Sabre Duren.

Adit, Richmond Mine, CA

Adit, Richmond Mine, Iron Mountain, California. Adits are horizontal or nearly horizontal shafts built into hill or mountain sides enabling mining operations to drain mines and access underground mineral ores.

Acid drainage can enter ground and surface waters through various watershed pathways. For example when it rains or when snow melts, water flowing through pyrite and other mineral ores in tailings piles or exiting out of adits can become overland acid drainage directly entering streams. It can also infiltrate into groundwater systems. Groundwater containing AMD or ARD discharging into streams provides a more continuous source of acidity and metals.

 

Impacts of acid mine and acid rock drainage

Green filamentous algae

Green filamentous algae grows in an AMD stream with a pH of 2.5.

Low pH values and high metal concentrations is harmful to fish and other aquatic life, and affects the ability of water to be used for drinking and for recreational purposes such as fishing. It also causes corrosion on infrastructure like bridges. When AMD or ARD water is used for snow-making, sometimes issues arise if the acid drainage is exported to a non-AMD/ARD stream. Alternatively if non-AMD/ARD water is diverted for snowmaking, this could potentially exacerbate metals levels in an AMD/ARD stream if the snowmaking water would have acted to dilute the instream metals concentrations. 

There is some life though that not only survives but thrives in the extreme environments of AMD/ARD streams including organisms such as bacteria, yeast, archea, fungi, and algae.

Collaborative research

USGS and INSTAAR scientists are collaborating on a variety of research topics including studying the implications of climate change for AMD/ARD streams and remediation efforts, characterizing AMD sources and working with stakeholders to develop remediation strategies for a Colorado river basin, and studying AMD geochemistry and microbiology that could help us understand processes on the planet Mars. For more information on some of this research, please check out the links below.

References

Nordstrom, D. K. (2011). Mine Waters: Acidic to Circumneutral. Elements7(6), 393-398.

U.S. Government Accountability Office [USGAO] (2011) Abandoned Mines: Information on the Number of Hardrock Mines, Cost of Cleanup, and Value of Financial Assurances, Testimony Before the Subcommittee on Energy and Mineral Resources, Committee on Natural Resources, House of Representatives.

Conducting a Tracer Experiment in Peru Creek, Colorado

Learn what a tracer experiment is and how such experiments can provide us with information about metal transport in acid mine drainage streams.