What is Acid Mine Drainage (AMD)?
AMD is a term used to describe drainage flowing from or caused by
surface mining, deep mining or coal refuse piles that are typically
highly acidic with elevated levels of dissolved metals.
Formation of acid mine drainage
Acid mine drainage (AMD) forms when sulfide minerals in rocks are
exposed to oxidizing conditions in coal
and metal mining, highway construction, and other large-scale excavations.
There are many types of sulfide minerals, but the iron sulfides, pyrite
and maracasite (FeS2), common in coal regions are the predominant
AMD producers. Upon exposure to water and oxygen, pyritic minerals oxidize
to form acidic, iron and sulfate-rich drainage.
The drainage quality emanating either from underground mines or backfills
of surface mines is dependent on the acid-producing (sulfide) and alkaline
(carbonate) minerals contained in the disturbed rock. In general, sulfide-rich
and carbonate-poor materials are expected to produce acidic drainage.
In contrast, alkaline-rich materials, even with significant sulfide
concentrations, often produce alkaline conditions in waterw.
Acidity in AMD is comprised of mineral acidity (iron, aluminum, manganese,
and other metals depending on the specific geologic
setting and metal sulfide) and hydrogen ion acidity. Approximately 20,000
km of streams and rivers in the United States are degraded
by AMD. About 90% of the AMD reaching streams originates in abandoned
surface and deep mines. Since no company or individual
claims responsibility for reclaiming abandoned mine lands (AML), no
treatment of the AMD occurs and continual contamination of
surface and groundwater resources results.
Problems associated with acid mine drainage
- Contamination of drinking water
- Harmful affects on aquatic plants and animals
- Corrosion of infrastructure such as bridges, monuments, and buildings