Acid Rock Drainage and other Doomsday Tales

During the last two weeks I’ve attended a course at the University of Witwatersrand on Environmental Engineering Topics.
It was run by the School of Mining Engineering (Wits have “schools,” not departments, in what I imagine is an attempt to emphasise teaching over bureaucracy) and so focussed on environmental issues with respect to mines. A very useful course which covered all aspects of mining, and how mines tend to break the pristine conditions of the world’s environments and generally annoy the people who live in them.
There were recurring themes of road blockades, pipeline sabotage, and civil unrest. A wonderful recipe for edge-of-your-seat learning.

Then, towards the end of the second week, an external presenter spoke to us about Acid Rock Drainage (ARD). Shortly afterwards I was overcome by a deep sense of the futility of my actions in attempting to get mines to manage the environmental impacts they create, or have the potential to create.

This is in the United States, but some South African rivers look equally bad.

Acid Rock Drainage (or Acid Mine Drainage) is a nasty bastard. The people over at Save the Wild UP appear to share my general horror at the situation, and are trying to make a noise about it.
They also have something more than horror. They have pictures like the one above.

Simplistically, material that is sulphide-rich (typically pyrite) gets exposed to water and oxygen when it gets pulled up from underground. This tends to happen on gold mine rock dumps (on the Witwatersrand which is a pyritic ore-body) and with coal mines (since there is plenty of sulphur associated with coal).
The water and oxygen causes oxidation, liberating hydrogen and iron ions and ultimately leading to the generation of acid. Secondary reactions keep it ticking along in a cheerful, self-sustaining manner. The reaction never reaches equilibrium and will continue until all sulphide-rich material has been oxidised. The time this will take is best measured on the geological scale.
The good news is that the reaction can be stopped. Take away the oxygen or the water.
If you have lived on Earth, you should easily see why that won’t work. To isolate huge mines or rock dumps from things as ubiquitous as water and oxygen isn’t going to be practical.

The real issue isn’t the acid. The real issue is that metals are very fond of dissolving in acid. Once they dissolve, they are very happy to travel along with water into rivers and reservoirs.  Water is fairly important, so chances are that living beings are going to consume it, even if it is laced with all manner of dissolved metals.
If the living creature cannot process the metal biologically, it leads to bioaccumulation of the metal (or some slightly processed, but not entirely metabolised compound of the metal).
Animals eat other animals, which further concentrates the bioaccumulated metal compounds. It should be obvious that eventually, people will fit into this food-chain at some point. I mention people fitting into this, not because I think people are more important than animals, but because most people think they are. So when I say bioaccumulation ultimately leads to the onset of cancer those people, who don’t really think it significant if animals get cancer, will perk up and listen.

Global Warming. Bah! ARD is what we should really be concerned about.