What is AMD
Abandoned Mine Drainage (AMD) is water that has become contaminated as a result of passage through a physical environment created by coal mining activities of the past. This contamination can occur in the underground voids created by deep or underground mining or it can occur by water passing through coal mining refuse left on the surface.
The vast majority of pollution of this sort results from old mining operations that have simply been abandoned after the coal was extracted. In some cases, the AMD is from very old mining operations dating from the turn of the 20th century. Prior to 1977, the laws governing coal mining operations were less stringent concerning their environmental impacts. It was a common practice to simply abandon mining operations following the exhaustion of the coal reserve, then declare bankruptcy. This allowed the mining operators to walk away from liabilities, including environmental devastation.
The nature of AMD contamination varies greatly from site to site, as its formation is dependent a variety of factors. AMD often lowers water quality and impairs aquatic life, and is most often characterized by one or more of the four major components:
- Low pH (high acidity), i.e., acid mine drainage
- High metal concentrations (iron is the most common)
- Elevated sulfate levels
- Excessive suspended solids and/or siltation
Low pH (high acidity)
The majority of AMD problems result from surface water contact with the unreclaimed waste rock and other earthen materials or from the seepage or drainage of ground water which has contacted the coal or rock strata remaining in an underground mine. If the water becomes acidic, it is referred to as "acid mine drainage". Acid is a contaminant of primary concern since it can leach toxic concentrations of metals from rocks at mine sites.
Acids in streams are a problem because they can corrode metal pipes and structures, break down concrete, and kill or stunt plants and other aquatic life-forms. Acidic surface waters or runoff can also break down metallic compounds of iron, sulfur, manganese, and aluminum found in nearby rock or earthen waste piles.
Where does the acid come from?
Acid solutions form when surface or ground water comes into contact with acidic material, mostly pyrite, commonly found in mine rocks, earthen refuse piles, or underground mine works and/or auger holes. The iron-sulfide mineral pyrite is often found near subsurface coal seams along with compounds containing manganese, aluminum, and other metals. in the presence of oxygen, ordinary rain water or ground water can react with the sulfur to form sulfuric acid.
Acid concentrations in AMD can reach levels that are more than 10,000 times higher than neutral waters, presenting a powerful leaching agent that can dissolve significant amounts of metal compounds and leach additional acid from rocks and earthen wastes commonly found at most mine sites.
High Metal Concentrations
Layers of rock and earth above the coal removed during mining commonly contain traces of iron, manganese, and aluminum and can also contain other heavy metals. These metals can be dissolved from mining sites through the action of acid runoff, as described above, or can be washed into streams as sediment Many metals, though common, can be toxic to fish and other aquatic organisms when they are present in high dissolved concentrations. Dissolved iron and iron precipitate, for example, can kill the aquatic biota that fish feed on, thus reducing the overall fish population. Iron precipitate can also clog the gill structures of fish which will eventually lead to their death as well. In addition, precipitation of iron in the stream channel can also wipe out the aquatic food chains and adversely affect fish populations.
Elevated Sulfate Levels
As pyrite wastes are chemically broken down, a sulfate compound is produced in runoff waters. Sulfates can bond with water molecules to form sulfuric acid or can attach to calcium atoms to form a gypsum sludge. Elevated sulfate levels are often found in AMD discharges.
Excessive Silt and Suspended Solids
Most people think contaminated AMD results from chemical reactions in streams, but a significant threat to water quality and aquatic organisms comes from eroding soils at abandoned mining sites. Tiny fly nymphs, insect larvae, and other organisms that form the base of aquatic food chains can be wiped out by heavy accumulations of soil and mine waste particles that wash into streams after rain events. Suspended silt particles can clog the gills of fish and smother eggs on the stream bottom. Streams and rivers muddied by silt and other suspended solids also mean higher costs at municipal and industrial water treatment plants and accelerated sedimentation in reservoirs.
This information was largely taken from EPA Region 3 publication A Citizen's Guide to Address Contaminated Coal Mine Drainage, EPA-903-K-97-003.
See EPA's treatment of What is Acid Mine Drainage.
What is AMD? Hedin Environmental