You may have seen this story about the release of an estimated 3 million gallons of toxic water into a river in southwestern Colorado last week. The images were pretty awful, showing the Animas River a sickening yellow-orange color. A USEPA team was working on diverting flow from an abandoned gold mine when they accidently poked a hole in a dam and caused the release. The result was contamination by a classic case of acid mine drainage.

Acid mine drainage is one of the most widespread forms of human contamination in the world. Mining exposes rocks and minerals that have been isolated from the atmosphere for millions of years. Sulfide minerals are common in many of these mined rocks; the most common is pyrite, an iron sulfide mineral commonly known as “Fool’s Gold” (chemical formula FeS₂). When pyrite and other sulfide minerals are exposed to oxygen, they oxidize and release iron and other metals into the water and produce acidity. The distinctive yellow-orange color in the Animas River is due to all the iron released into the water.

These reactions produce a lot of acidity, and pH levels often fall below 2 (there are even cases where the pH drops to negative values!). Once the pH reaches those levels, most metals, which are pretty insoluble at neutral pH values, are dissolved in the water. These can include a host of nasty toxic metals and other elements, including lead, cadmium, copper, and arsenic.

There are tens of thousands of abandoned mines in the U.S., including an estimated 55,000 in the West, and many of them were not properly remediated when they were closed to prevent the buildup and release of acid mine drainage. The USEPA and other agencies are fighting a rearguard action to prevent releases into streams and rivers, but they (obviously) still occur. We’ll be dealing with these abandoned mines for a long long time.