AMD and Mining Environments
By: Bruce Golden, Regional Coordinator
Did you ever wonder why there’s such a wide range in the chemical make-up of coal mine drainage discharges? It’s because there’s a great diversity in the mining environments in which mine drainage is formed. This multipart series of AMP will explore, on an introductory level, the connection of mine drainage characteristics with the mining environments that created it. It’s not meant to be rigorous, but to mainly get you thinking about the kinds of things that can happen underground which are influential in the
making of polluted water.
Part 1: Introduction
The formation of polluted mine drainage is actually a natural process that has been greatly accelerated due to mining activities. The weathering of the common mineral pyrite, a.k.a. fool’s gold, (and its cousins) is the heart of the process. By weathering of pyrite, we mean that pyrite is exposed to the forces of air and water. A common place to see evidence of a non-coal related example is a road cut where rocks have been stained orange. That orange staining is likely to be a result of small amounts of pyrite being exposed to and reacting with water and oxygen.
Coal seams and surrounding strata commonly contain small percentages of pyrite which, being tucked away safely underground, has little chance of coming into contact with air and water. However, mining of coal exposes vastly greater quantities of pyrite to water and oxygen than would happen without the mining disturbance. If you examine a piece of freshly mined coal, you’re sometimes able to see fragments of pyrite in it, but not always because the pyrite particles may be too small. Even though you may not easily observe the pyrite directly, you can see evidence of its presence by what happens if the coal is left out in the weather. Eventually orange or yellow staining will become evident and the water that passes over it will have acquired some newly dissolved although undesirable constituents.
The water that becomes contaminated by this process goes by several names, which itself can cause some confusion. And since the topic comes up often enough, I’ll
try to make some sense of it here. Here’s at least a partial list of names:
- Acid Mine Drainage – perhaps the most common name. It’s accurate except in the case where local conditions have turned the water alkaline. Even if it’s alkaline, it may still be referred to as Acid Mine Drainage. Go figure.
- Alkaline Mine Drainage – see above.
- Coal Mine Drainage – accurate if the drainage occurs as a result of coal mining. However, other types of mining also can produce the same sort of stuff. And occasionally coal mine drainage isn’t even polluted. (Too bad there’s not more of that!)
- Abandoned Mine Drainage – accurate only if it’s coal mine drainage that originates from mining that occurred prior to the passage of the federal SMCRA legislation in 1977.
- Acid Rock Drainage - refers to drainage from “hard rock” mining, i.e. mining for minerals other than coal, but can also refer to non-mining related discharges (such as with the I99 Sky Top debacle).
- Contaminated Mine Drainage or Polluted Mine Drainage - about the most general terms covering all mining.
- Sulfur Water – a very colloquial term referring to Coal Mine Drainage. Many people believe the orange color in the water is sulfur. While sulfur is almost assuredly present in the form of colorless, odorless sulfate ions , the orange color is due to rust-like compounds of iron. Perhaps the rotten egg smell of hydrogen sulfide gas (H2S) that sometimes accompanies mine drainage (due to bacterial action with sulfate) may reinforce the idea it’s sulfur.
- I’m guessing there are others. Let me know.
So for the rest of this series, I’ll just refer to the polluted water simply as AMD. The “A”, in our case normally means “Abandoned”, just like the “A” in WPCAMR. In the next installment in this series we’ll delve into the chemistry involved in AMD formation. Following that, the underground conditions water may experience on its journey to becoming polluted will be explored.