“Why is the water in the Animas River so Orange after the release from the Gold King Mine?”  I had another environmental scientist ask me this question yesterday. I was surprised at the question, but then I thought well of course I know why the water is so orange, I have been working on abandoned mine issues for over twenty years. I’m used to the way mine drainage looks. It mostly looks orange.

But most people generally think water is blue and the sight of a whole river turning orange even puzzled the scientist who asked me this question and he has worked in environmental remediation for over thirty years.

The water from the Gold King Mine is what is called “acid mine drainage.” It is orange in color because it has a lot of iron in it. It turned the Animas River orange because some three million gallons flowed out of the Gold King Mine and went down Cement Creek into the Animas River in a massive spill. The spill occurred after an environmental cleanup company working for the Environmental Protection Agency (EPA) excavated loose material that had collapsed into the mine entrance. From the EPA’s accounts of the incident, it sounds like the water started leaking into the mine tunnel from above it and then just starting flowing out of the mine.  In this case the mine entrance was an adit. Most people envision vertical shafts when they think of mine entryways. An adit is different. Adits are horizontal entryways into a mine that go into the side of a hill or a mountain. You can walk into them, but you might not walk far, because there are often vertical shafts just a few feet into the mine tunnel. Many an unsuspecting person has entered an old adit and fallen right down a shaft.

The iron in the water comes from pyrite and phyrrotite. Most people know pyrite as fool’s gold. It is an iron sulfide. Phyrrotite is another iron sulfide mineral. Most people have never heard of it. It has a funny mineral formula. Pyrite and to an even greater extent phyrrotite have all sorts of inclusions of other elements in them. Elements like arsenic and nickel. Wait a minute you are probably saying, I thought this was a gold mine, not a fool’s gold mine. Yes you are right, but the precious metals in the area are what geologists call sulfide deposits. The gold and other valuable minerals in the San Juan Mountains in Colorado where the Gold King Mine is located were formed as a result of volcanic activity. The whole area is what geologists call a caldera. Calderas are formed when a volcano or volcanic complex literally blows its top; spews all its lava, ash and rock; and then collapses – kind of like Mount St. Helen but on steroids. So iron sulfides as well as all sorts of trace element sulfide minerals are right there with the valuable stuff like gold and silver.

The old mine workings, like tunnel and shafts, expose these sulfide minerals to water and air and they start to weather and oxidize. As a result you get water with a lot of ferrous iron, sulfate (because the sulfur has combined with oxygen in this oxidation reaction), and lots of free hydrogen ions. If you ever had chemistry in school, you will remember that lots of free hydrogen ions means a solution is more acidic. If you didn’t have chemistry in high school, you still probably have heard of pH. The pH is a measure of acidity and when I went to school the scale of measurement was between 0 and 14. Any solution with a 7 was considered neutral. Acid mine drainage usually falls in the range of 2 to 6. However, years ago it was discovered that there is actually negative pH numbers. These are highly acidic solutions and were first discovered by the U.S. Geological Survey while working at the Iron Mountain Mine in California.

So acid mine drainage can be very acidic and can have lots of iron in it. The iron is initially ferrous iron which is soluble in water. What happens next depends on the presence of some highly specialized microorganisms called iron oxidizing microbes with great names like acidithiobacillus ferroxidan. These little pests oxidize ferrous iron to ferric iron which is not very soluble, so it starts to precipitate out. Ferric iron has another little specialty too. It is an even better oxidizer than oxygen. So the ferric iron is precipitating out into these ugly looking orange minerals called jarosite, goethite, ferrihydrite and so on, and it is also causing more oxidation of pyrite and phyrrotite which create more acidity and more precipitation of more ugly orange minerals.

So there you go, that is why the water in the Animas River turned orange. A huge load of iron precipitates in the water were swept into the river. It looks bad but it is not the worst problem. The worst problem is all those other elements that are released in oxidation reactions going on in the mine, like arsenic, nickel, zinc, and copper. Those elements are what EPA is worried about because many are toxic. EPA doesn’t want them in drinking water and there are numerous drinking water intakes on the Animas River. Zinc and copper are especially bad for fish and their presence at certain levels in a waterway can deter fish from even venturing there. If the toxic elements settle into the sediments at the bottom of the river, it can be a long term problem that can affect bugs living down there, which can affect the whole ecosystem. But you’ve also got to remember that there are hundreds of mines in the San Juan Mountains that have been leaking out acid mine drainage for a hundred years or more. EPA and other federal agencies have been working to reduce this type of acid mine drainage entering the Animas River for years.