Since my post last week, I thought I had better clarify what Acid Mine Drainage (AMD) is. I dug through the files and came up with an essay from my days back in BAMR. I hope this clarifies things.
What is Acid Mine Drainage? The answer is pretty simple and pretty complex. The simple answer is Acid Mine Drainage (AMD) is any water that comes from a mine with a pH less than 7.
Now the complex answer. Let’s start with the water cycle. Water falls as rain or snow from the clouds and settles on the Earth. If it doesn’t hit a body of water it either becomes surface water or groundwater. Surface water runs off and is the basis for flow in many freestone streams. Think of the streams that feed Lake Erie in Pennsylvania. After a rain or snow melt event they go right up. Then, just as quick they go back down. These streams are very dependent on surface water. Water that doesn’t immediately flow into streams becomes groundwater. Sometimes it has a short flow path measured in hours to go from one place to another, and sometimes it has a long flow path taking eons to go from point A to point B. Groundwater is the base flow of our lovely little brook trout streams, the limestoners and all rivers and other bodies of water. When the groundwater drops, down goes the stream. Groundwater and surface water is often intercepted by man for uses such as drinking, manufacturing, agriculture and making beer and things of that ilk. Eventually though it is evaporated or transpired by organisms and off to the atmosphere it goes to eventually fall on another part of this blue orb we call home.
Now we start getting technical. When coal was formed back before there was television, video games and the WWE it was laid down as a layer of carbon bearing materials which were plants….not animals. Over these deposits of ferns, mosses and the lichen (sorry just a little play on words and a poor one at that), minerals of various kinds were deposited. Coarse grain mineral deposits became sandstone, fine grained deposits became shale. Depending on the environment these minerals were dropped in determined the quality of the strata over the compressed plants that would eventually become coal. Some areas that became inundated by the ocean were covered by deposits of limestone. Some river bottoms had large quantities of sand and silt that was mostly leached of soluble minerals. Brackish environments full of mud received mineral deposits near where freshwater met the oceans like the Chesapeake Bay. These deposits were often laid down in an environment quite different from the oxygen rich environment we live in today. The sandstone and shale piled up quite rapidly (geologically speaking) and their intense downward pressure caused the plant matter beneath to become coal.
Okay so after a few million years or so humans found that this black stone cropping out of the cliff burned. Image the first person to discover that and report back to the tribe. They probably stoned him for being a witch. But it was too late. The genie had been let from the bottle and the race for industrialization was on. Fast forward now to the early 18th century. America was a new and growing land. Resources were plentiful and appeared to the 18th century citizen as limitless. Lo and behold after the Europeans got established in Pennsylvania they discovered coal. Bituminous coal was found in western Pennsylvania and Anthracite was found in the northeastern corner of the state. As the country prospered this fuel source was exploited and the population grew causing greater demand. Since the coal was laid in layers called seams in the Bituminous fields and veins in the Anthracite fields and the Earth had shifted and it wasn’t exactly level to begin with there was a slope to the coal that came to be called the dip.
Groundwater, remember what that is, was often associated with the coal seams as aquifers (underground water storage reservoirs) because the underclay (the primordial soil on which the plants that formed coal grew) was relatively impervious (water was unable to readily pass through it thus forming what is called an aquatard). Well the early miners weren’t that dumb. They found if they started on the up dip side of the coal seam the hole they were working in quickly filled with water and either they or their mule would drown. Since mules were hard to come by they decided that if they started on the down dip side and mine up, the water would run out of the mine and away from them thus saving them from the exasperating experience of having to perform artificial respiration on a mule who neither cared nor wanted a grizzled old tobacco chewing miner breathing up his nostrils.
In the Anthracite region things were a bit different. Anthracite coal is actually metamorphosed and the veins were folded and compressed. At the bottom of the fold was the greatest concentration of coal. Of course this is also where the greatest collection of water is. To solve this problem they dug tunnels through the mountain to drain the water away.
Now, remember the overlying rock. There are mineral deposits in them called sulfides. The most common one associated with coal is pyrite or ‘fools gold’. Fools gold has a nasty habit. When it is exposed to air and water it can form sulfuric acid. The fools gold in the overburden is already exposed to water in the form of groundwater but there is no oxygen deep in the ground. Here come the miners and what do they do. Well, it’s sort of like opening a door on a smoldering fire in the wood stove. Poof! So now you have a mine generating sulfuric acid. That is a bad thing insofar as fish are concerned.
If the coal was lucky enough to have a limestone layer deposited over top of it, the water leaching through the rock (henceforth called overburden) received alkalinity as it made its way to the coal. This was enough alkalinity that it neutralized the acid. If the overburden was sandstone or a brackish shale there would be no neutralization and the acid would flow out of the mine unabated.
Now I want you to think back to your high school chemistry class. Did you ever see sulfuric acid? Was it yellow? Of course it wasn’t yellow! It was clear, and if you had been paying more attention to the lab work and not your lab partner in the mini-skirt (sorry ladies this doesn’t apply to you) you might have remembered this. Well then, just where does the orange color come from?
Well, in the overburden, the coal and the mine floor there are metals. In Pennsylvania they are primarily Iron, Manganese and Aluminum. These metals dissolve in acid. When there are a lot of metals in solution and they are exposed to air, or they mix with water of a high pH they deposit on whatever happens to be handy, like rocks in the stream. Iron is the most common because it comes out of pyrite. Pyrite is made of iron and sulfur. This iron causes the yellow, orange and reds seen in streams across Pennsylvania and has acquired the name of “Yellow Boy”. Manganese forms a black precipitate. Aluminum is a white precipitate. One of the most common sources of aluminum is not from the overburden but from the mine floor where it is leached from the underclay. Elemental sulfur is almost never found as a precipitate.
Iron and Manganese are not usually in and of themselves toxic. Instead they smother the aquatic substrate and the organisms that live there by a depositional effect. Aluminum on the other hand is deadly at low pH. At a pH of 5.5, aluminum in concentrations of 0.5 mg/L will usually kill all fish and most macroinvertebrates. Other metals such as cadmium, chromium, copper, zinc, and so on are also present but usually in much, much smaller amounts.
But it just keeps getting better. There are microbes called “ferrobacters“ that actually enhance the production of AMD. These little critters thrive in AMD and actually speed up the reaction.
Surface mining that came along around the time of World War II exposed huge quantities of pyrite bearing overburden to air and water. Underground mines left huge voids to be exposed to air and water. Out of the drain tunnels drilled into the anthracite mines flowed massive amounts of water. Pennsylvania coal mines began generating and continue to generate this mixture of acid and metals and unless abated will continue on for the next millennium or so.
Two samples of AMD are as follows.
The Oneida #3 Discharge (Anthracite Tunnel Discharge)
Sulfates 45 mg/L
Total Iron .141 mg/L
Manganese .474 mg/L
Aluminum 1.950 mg/L
Acidity 16.4 mg/l
Alkalinity 0 mg/L
Flow 1399 gpm
The Camp Run Discharge (Surface Mine Discharge in Northern
Sulfates 1740 mg/L
Total Iron 10.92 mg/L
Manganese 46.3 mg/L
Aluminum 249 mg/L
Acidity 2768 mg/L
Alkalinity 0 mg/L
Flow 5 gpm
Both samples are AMD. Both samples are capable of clearing most normal aquatic life for miles downstream. So to answer the complex question of what is AMD? The generally accepted answer is water with a pH less than 5, Sulfates greater than an undisturbed background sample or 50 mg/L, metals elevated beyond undisturbed background samples and acidity greater than alkalinity. This begs a few other questions. Is all drainage from mines acidic? No. Do all mines leach metals? Probably in some form or another. Are certain areas better to mine in than others? Yes. Can trout survive in mine drainage? Yes provided the metal content is low and the pH is not severely depressed. Does AMD only come from coal mines? No. Other types of mines develop it as well, particularly clay mines and in the western states metal mines. Even road cuts that expose coal seams can generate AMD. These and other questions are the subject of thousands of technical articles, books and reports.
There is a lot of new and interesting science being developed to treat this catastrophic problem. The Bureau of Abandoned Mine Reclamation estimates that over 7,000 miles of streams do not meet the clean streams standards of Pennsylvania because of mining. But things are better now than they were even just a few years ago. Pointing fingers and saying who is to blame is pointless. It was the energy from coal, Pennsylvania coal that provided the materials to keep us from speaking German or Japanese. Coal is the fuel that is probably providing the electricity to your computer so that you can read this. It has been a good energy source and will continue to be for the foreseeable future. New technology will help it to be mined cleaner and burned cleaner. New technology will also allow for us to find ways to treat or eliminate the pernicious problem of AMD.