Acid Mine Drainage
Acid mine drainage in a watershed can be a consequence of mining coal or mineral deposits. A significant amount of scientific research has been conducted to determine the chemical reactions that create acidity and lead to the precipitation of dissolved metals, but despite improvements in both prediction and prevention methods, acid mine drainage problems persist. The acidity of coal-mine drainage is caused primarily by the oxidation of the mineral pyrite (FeS2), which is found in coal, coal overburden, and mine waste piles. The rate of pyrite oxidation depends on the following: reactive surface area of the pyrite, the oxygen concentration and pH of the water, the forms of pyrite, and the presence of Fe-oxidizing bacteria (Thiobacillus ferroxidans).
Many passive and active treatment systems have been developed to treat coal-mine drainage in order to raise the pH of the water and to control the precipitation of dissolved metals. However, predicting and preventing acid mine drainage from occurring is preferable to having to perform remedial treatment once the problem has occurred. Both static and kinetic chemical tests have been developed to aid in predicting potentially acidic drainages. Acid-base accounting of the rock strata that is disturbed (overburden) is commonly determined. One test measure resulting from these chemical analyses is the neutralization potential (NP), which can be combined with other parameters such as maxiumum potential acidity to provide a predictive guide as to whether acidic drainage will be created.