Acid mine drainage

It can sometimes be surprising to learn that naturally occurring substances on Earth are not always benign or “safe” to humans or our environment. Sulfides are those kind of substances. When left deep within the Earth, unexposed to air, they present no problem.

But when brought to the surface, as done with mineral extraction, sulfide ores can undergo a chemical reaction that can create long-lasting contamination to water and the plants and animals dependent on that water.

When sulfides interact with oxygen (in our air) and water (in rain or snowmelt, for example), they create sulfuric acid – the same caustic substance used in car batteries and one many of us were cautioned about using in our high school chemistry labs. If this acid makes its way into water systems (streams, rivers, lakes, groundwater) in sufficient quantity, it will kill all organisms that cannot tolerate highly acidic waters.

In addition, the acids dissolve harmful metals out of the surrounding rocks, metals that are toxic to fish and other aquatic life, as well as to humans. The term for this pollution is Acid Mine Drainage (AMD). Acid mine drainage can be discharged from a variety of places in the operation of the mine – any place where the ore comes in contact with air and water – such as the open pit walls, the wasterock piles, tailings, and underground tunnels.


Sometimes, if large quantities of iron are leached from the rocks, the water turns a yellow-orange color. Pictures of orange rivers and streams from sulfide mining operations around the country have become visible representations of this pollution. It is important to know, however, that not all acid mine drainage or toxic metal leaching is visible – and rivers, lakes and streams have been contaminated without showing the signature orange color.

Keeping the chemical reaction and the toxic metal leaching from happening is no small task for sulfide mines. The proposed PolyMet project, for example, expects to excavate and pile waste rock on the land – the size of 500 football fields 20 stories tall. To keep Minnesota’s surface and ground waters free from sulfide mining pollution, none of these mountains of sulfide ore can ever be allowed to come into contact with oxygen and water.

This is an extremely difficult, if not impossible, task to accomplish. Indeed, no sulfide mine has ever managed to accomplish it.

AMD has devastated water bodies in many states where this kind of mining has occurred.

“Water contaminated by AMD, often containing elevated concentrations of metals, can be toxic to aquatic organisms, leaving receiving streams devoid of most living creatures.” – Source (PDF)

Research shows fish that experience direct contact through their gills to metals and hydrogen ions experience impaired respiration, both from acute and chronic exposures (source PDF). Indirect exposure to fish can also occur when fish ingest contaminated sediments and food.

The orange colored precipitate, iron hydroxide, that often symbolizes the formation of AMD, may “physically coat the surface of stream sediments and streambeds destroying habitat, diminishing availability of clean gravels used for spawning, and reducing fish food items such as benthic macroinvertebrates.” (source PDF)

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