Acid Mine Drainage
Evans, Thomas 1 ; Reese, Michael 2
1 Monarch High School
2 Monarch High School
Acid Mine Drainage (AMD) occurs when surface and groundwater pass through heavy metal or coal mines. More often than not, mines that produce acid mine drainage are abandoned mines that have not been properly maintained. Massive tailing piles (pulverized rock) are left behind, exposing pyrite (contained in the rocks) to oxygen and water in the atmosphere. The combination of these three components leads to the following reactions:
2 FeS2 + 7 02 + 2 H2O -> 2 Fe2+ + 4 SO4 + 4 H+ (Equation 1) 4 Fe 2+ + O2 + 4 H+ -> 4 Fe3+ + 2 H2O (Equation 2)
4 Fe3+ + 12 H2O -> 4 Fe(OH)3 + 12 H+ (Equation 3)
The production of hydronium ions is the key aspect. Hydronium ions lower the pH of contaminated water, resulting in the precipitation of heavy metals such as iron hydroxide (which is responsible for the orange color in streams). Iron hydroxide is hazardous to aquatic life in the stream, as it forms silt that blocks sunlight and covers the food of microbes. Microbial community decline travels up the food chain and affects the entire aquatic ecosystem. In addition acid mine drainage contaminates drinking water.
Acidic stream conditions result in a series of chemical changes in streams. High pH levels increase geochemical weathering of rocks in the stream bed, which contain calcium carbonate. Calcium carbonate acts as a buffer, increasing stream alkalinity (ability to buffer). However, increased alkalinity does not always outweigh the effects of high stream acidity. By overcoming stream buffering capacity, acid mine drainage severely limits the streams ability to adapt to natural conditions.
The data we collected on Little James and James Creek corresponds to the classic result of acid mine drainage. Comparing data points from before and after passage through the mine, which is part of the Golden Age Mining District after the mine there is a lower pH and an elevated number of cations due to geochemical weathering. Alkalinity (buffering capacity) is lowered but increases downstream.
There are various prospects for remediation. One utilizes microbes. Certain bacteria reduce AMD compounds and ameliorate the effects on the environment. If AMD streams can be colonized by these bacteria, AMD will have little effect on the stream. Another tactic is known as liming. Sodium carbonate acts as a base and neutralizes the excess hydronium ions. But biological remediation can only exist if the microbes survive the low pH, and adding mass amounts of base is not cost effective. So far there are no long term solutions to the effects of Acid Mine Drainage.
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