A Brief Explanation of Acid Mine Drainage

I've been reading up on acid mine drainage, the thing that will pose the greatest danger to the environment, should Prosperity go ahead.

The rocks that contain the metals Taseko wants also contain chemicals called sulphides. The  plan is to pile waste rock in the tailings pond... you know, where Teztan Biny or Fish Lake is today. The problem with waste rock is that you are taking a massive chunk of bedrock and breaking it into little pieces and bringing it to the surface. Now, instead of having a little bit of rock exposed you have a massive amount of surface, all of which is exposed to air, water and microbes.

I made a little graph of how surface increases as you break something down into smaller pieces...

To quote the Wikipedia entry:

After being exposed to air and water, oxidation of metal sulfides (often pyrite, which is iron-sulfide) within the surrounding rock and overburden generates acidity. Colonies of bacteria and archaea greatly accelerate the decomposition of metal ions, although the reactions also occur in an abiotic environment. 

These microbes, called extremophiles for their ability to survive in harsh conditions, occur naturally in the rock, but limited water and oxygen supplies usually keep their numbers low. Special extremophiles known as acidophiles especially favor the low pH levels of abandoned mines. In particular, Acidithiobacillus ferrooxidans is a key contributor to pyrite oxidation.

In other words, mine tailings are a Pandora's box full of toxic chemical stew. Like nuclear waste (though not radioactive), they will create this toxic stew for centuries. So when critics complain that the environmental effects of a mine last well past the mine's life, this is why.

Mining companies like Taseko don't really address this issue because it would suggest they aren't actually in complete control of the environment they propose to create. And they aren't...

How can the Toxic Stew leave the tailings pond? Easy, it can:

• leak through the dam holding it back
• the dam can break (see this link for a list of dam breaks List of Tailings Dam Breaks)
• enter the groundwater through cracks in the rock

Mine tailings are forever, and turning Teztan Biny into a tailings pond means perching a toxic stew directly 

above what is for all intents and purposes a pristine watershed critical to a broad spectrum of people, animals and ecosystems.