Acid Mine Drainage, the Unseen Enemy
by Walter D. Lawhorn

Walter is a Biology major at Valdosta State University. Visit his homepage here.

A quiet mountain stream flows through a beautiful mountain valley full of life and vigor. But unknown to the naked eye is the fact that this stream is polluted by a force that originates from nature itself, and not from some manmade toxic waste or radioactive material. It is a process induced by mining. It can be years after the closing of a mine before a change is noticeable. By the time someone realizes what is happening to the streams, rivers, and lakes, it may be too late. This blight that is traveling through the water ways and infecting the aquifer is known as acid mine drainage (AMD.). The above picture illustrates the effects of acid mine drainage in a stream This occurrence results from the continuos quest for wealth that mankind tries to quench by scoring the earth in order to obtain precious minerals and /or metals. The main culprits for this action are metal sulfides such as iron disulfide, also known as pyrite or fools gold.1 Acid mine drainage can be found almost everywhere in the United States, and it does not require a special process to become toxic to the waterways. The lethal effect results from the reaction of pyrite to oxygen and water cause a lowering of pH due to the formation of acid. Also, there is a precipitant that blocks out the sunlight, restricting plant growth and virtually upsetting the balance of many waterways. The effects are sometimes devastating to the plants and animals in the environment surrounding the mine. Presently, many companies are developing options to help solve the acid mine drainage problem. Some solutions are more expensive than others, but all require time. Acid mine drainage is an indirect problem due to the uncovering of pyrite which reacts and pollutes the environment with an acidity problem difficult to solve.

Acid mine drainage occurs when mineral deposits that contain sulfides are uncovered during a mining process such as strip mining, cavern mining, or sedimentary mining. This type of activity can occur when mining numerous amounts of minerals and metals including coal, copper, gold, silver, zinc, lead, and uranium.2 Many companies are scoring the earth in search of precious metals. One area includes the open strip mining located in Starke, Florida. Companies like Dupont buy the land and begin a number of steps to refine heavy metals including titanium, aluminum, iron, bearing minerals and silicates. This process requires a number of steps in which the land is stripped of all topsoil, dug out, and mined through a dredging process. During the final stages, the metals are separated from waste materials in a mill. These tailings are then dumped back into the open pit where they were mined. Illustration three summarizes the heavy metal mining sequence in a simplified flow chart.3

During the mining process, a sulfide containing compound can be uncovered. One of the most common examples is when pyrite (FeS2) is disturbed during the mining process. Once exposed to the oxygen in the air and the water surrounding the mine, the pyrite reacts in a series of steps ending with the formation of sulfuric acid.1 This sulfuric acid seeps into the underground waterways and pollutes the area surrounding the mine for many miles. The reaction of pyrite with oxygen and water is really quite simple and is as follows:

Step one: The pyrite oxidizes upon contact with air and water.

1) Fe+2 + 1/4 O2 + H+ --> Fe+3 +1/2 H2O

Step two: Iron oxidizes to ferric iron

2) FeS2 + 7/2 O2 + H2O --> 2SO4-2 + Fe+2 + 2H+

Step three: Precipitation occurs with ferric iron to ferric hydroxide.

3) Fe+3 + 3H2O --> Fe(OH)3 + 3H+

Step four: All combined to show a full formation of sulfuric acid

4) FeS2 + 15/4 O2 + 7/2 H2O --> 2H2SO4 + Fe(OH)3 4

The acidity is caused when the hydrogen (H+) ions are released into the water in one of the above steps. This reaction can sometimes occur many years after a mine has shut down. Once in the waterways, the sulfuric acid lowers the pH of the streams, rivers, etc. virtually killing anything that cannot handle the stress of the stronger acidity levels. Sometimes, a stream's natural buffering system, due to carbonates, may hide the fact that the pH is being lowered and something is wrong. The acid in the water neutralizes the carbonate and bicarbonate ions resulting in the formation of carbonic acid (HCO), which weakens the natural buffering action of the water.2 When the buffering system is unable to react anymore, usually at a pH of 4.2, the water way suddenly becomes acidic and it is too late to reverse the damage.2 Also, another hazardous reaction resulting directly from the lowering of the pH comes from the (Fe(OH)3) precipitating out in the water. It affects streams by blocking sunlight and covering the stream bed with a thick red blanket. Acid mine drainage becomes hazardous to the environment, thus disrupting the cycle of nature.

Another way acid mine drainage affects the area is from the deposits of (Fe(OH)3) or other metallic compounds. A reaction occurs that results in a heavy sedimentation that blankets the stream. This "blanket" discolors the stream and covers up vegetation and/or prey on which larger aquatic animals depend. This blanket is caused when the iron hydroxide precipitates out of the water due to a lower pH from acid mine drainage.5 Not only does it cover food, but it also blocks out sunlight important for photosynthesis in plants. When the plants die, the rest of the aquatic cycle is upset and ultimately the stream becomes barren. Illustration three displays a river in which the red precipitate has become evident.

Acid mine drainage affects streams and wildlife in many ways. The rise in acidity levels is sometimes too large a shock on the plants and animals in some regions. In one area along the Tsolum River on Vancouver Island, a copper mine was established in 1964. The area was renowned for its coho, pink, chum, and cutthroat salmon. After two years of operation, the mine was abandoned and pyrite was left exposed which reacted to form a poisonous copper leachate. This lethal toxic substance seeped out into the streams killing 90% of the fish population.4 Now there are virtually no salmon in the river. Several cases have occurred all over Canada where the metal from mines has combined with the acid from pyrite, resulting in the destruction of stream habitats. These problems can be remedied through several types of solutions which can be expensive.

Several solutions have been proposed to reduce acid mine drainage. Some of the buffering agents that have been found affective include: hydrated lime, sodium antibiotics, heavy metals, detergents, and several carbonate compounds. When dumped into the streams, they raise the pH, thus neutralizing the effects of the acid in the water. In one study, it was found the anoxic limestone drains increase the alkalinity by an average of 128-248 mg L -1. The bicarbonate ion in limestone (CaCO3 ) becomes a buffer by reducing the proton acidity caused by the hydrolyzation of metals.6 Limestone seems to be the best solution due to its availability and inexpensive cost. The reaction that occurs in the water is quite simple. The bases used usually dissolve completely in water producing hydroxide ions (OH-). These hydroxide ions in combination with carbonate ions (CO32-) combine with the hydrogen ions (H+), neutralizing them in the water. This reaction produces water and carbonic acid (H2CO3) which is very weak and has hardly any effect on the environment.5 Sometimes even these measures are not enough. The town along Tsolum River puts more than $2 million a year for the removal and clean up of the mines, yet the streams are still barren.4 Dumping limestone and other neutralizing agents into the river does help for a short period, but it to can become expensive over time. Sometimes, because the mine connects directly to the underground aquifer, there is no way of stopping run off from the mine. Virtually, the environment has to pay for the mining in many ways.

It appears that the only way to stop acid mine drainage is to stop new mining completely. This is not likely to occur. Those mines abandoned are still a threat to the environment. When metal sulfides such as pyrite are uncovered and left exposed to the elements of nature, the resulting reactions become more than just "fools gold," it becomes a time bomb of disaster in the water ways in many forms. Companies may end up spending millions to make up for mistakes due to mining. Maybe many mining companies will think twice before trying to extract that "gold" everyone is looking for because they may end up the fool.

Here are a few photographs of what AMD can do to a stream or river. These were taken on the Stoney Creek/Little Conemaugh Rivers in the Cambria County, Pennsylvania.

REFERENCES

1. Baird, Colin (1995) Environmental Chemistry W.H. Freeman and Company, New York. p301-302.

2. http://cotf.edu/ETE/scen/waterq/chemmine.html "Acid Mine Drainage Chemistry"

3. Stouffer, Norman W. "Mining For Heavy Minerals" E. I. Dupont de Nemours & Co., Inc. Starke Florida pp. 10-15.

4. http://www.sunshine.net/miningwatch/AMD.html#anchor610171

5. http://www.ce.vt.edu/enviro/gwprimer/acidmine.htm "Groundwater Pollution Primer".

6. Hedin, Robert S., George R. Watzlaf, and Robert W. Nairn; (1994) "Passive Treatment of Acid Mine Drainage with Limestone" Journal of Environmental Quality, 23, pp. 1338-1344.