Acid mine drainage
Acid mine drainage, acid and metalliferous drainage (AMD), or acid rock drainage (ARD) is the
outflow of acidic water from metal mines or coal mines.
Acid rock drainage occurs naturally within some environments as part of the rock weathering
process but is exacerbated by large-scale earth disturbances characteristic of mining and other
large construction activities, usually within rocks containing an abundance of sulfide minerals.
Areas where the earth has been disturbed (e.g. construction sites, subdivisions, and
transportation corridors) may create acid rock drainage. In many localities, the liquid that drains
from coal stocks, coal handling facilities, coal washeries, and coal waste tips can be highly
acidic, and in such cases it is treated as acid rock drainage. This liquid often contains toxic
metals, such as copper or iron. These, combined with reduced pH, have a detrimental impact on
the streams aquatic environments.
Acid drainage problem is associated with the mining of certain minerals that cause long term
harm to waterways and biodiversity. Effluents from these mining industries usually contain high
quantities of toxic substances such as cyanides and heavy metals which have harmful effects on
ecology and the health of living beings. Acid mine drainage (AMD) is acidic water having pH of
less than 5, and also containing iron, sulfate and other metals which are formed under natural
conditions when hardrock mines containing pyrite are exposed to the atmosphere or oxidizing
environments. In other words, AMD can also be stated as outflow of the acidic water from mining
sites. Naturally occurring microbes can also increase AMD production by increasing the
breakdown of sulfide minerals.
Apart from low pH, the effluents of AMD also have high specific conductivity, high concentration
of aluminum and manganese. As the treatment of AMD is expensive and inadequate, it is often
left untreated. Pyrite is one of the most important sulfides found in the waste rock of mines.
When exposed to water and oxygen, it can react to form sulfuric acid (H2SO4). The following
oxidation and reduction reactions express the breakdown of pyrite that leads to AMD.
Sources of AMD
There are mainly two sources of AMD. They are: Primary sources and Secondary sources. Primary
sources : Mine rock dumps Tailings impoundment Underground and open pit mine
workings Pumped/ natural discharged underground water Diffused seeps from replaced over
burden in rehabilitated areas Construction rock used in roads, dams etc.
Secondary sources : Treatment of sludge pounds Rock cuts Concentrated load out Stock piles
Concentrated spills along roads Emergency ponds.
Factors determining the rate of acid generation
pH
Temperature
Oxygen content of the gas phase, if saturation is less than 100%
Oxygen concentration in the water phase
Degree of saturation with water
Chemical activity of Fe3+
Surface area of exposed metal sulfide
Chemical activation energy required to initiate acid generation
Bacterial activity
Other factors include waste rock dump permeability. Dumps with high permeability have high
oxygen access, which contributes to the higher chemical reaction rate.
Effects of AMD
The major effects of AMD are seen with respect to the aquatic resources. Not much direct impact
is seen on humans as long as they are not consuming the water resources affected by deposits of
AMD. Many river systems and former mine sites are totally inhospitable to aquatic life with the
exception of "extremophile" bacteria. Additionally, heightened acidity reduces the ability of
streams to buffer against further chemical changes. The clumps reduce the amount of light that
can penetrate the water, affecting photosynthesis and visibility for animal life. Furthermore, when
the precipitate settles, it blankets the stream bed, smothering the bottom-dwellers and their food
resources.
Acid mine drainage is produced by
natural oxidation of
sulde containing minerals like Iron-
