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> Environmental Assessment Acid Mine Drainage & Salinity Impacts
Acid Rock Drainage (ARD), often also referred
to as Acid Mine Drainage (AMD), typically results from poor management
of waste rock piles at mine sites. Acid drainage, or saline leachates,
occur on most metal producing mines and is also a common contaminant
from coal mines. The effect of ARD on man is generally limited,
but consequences of the ARD impact on aquatic environments can be
disastrous. For example, in Australia, ARD from an abandoned waste
rock pile at a mine was responsible for the loss of aquatic fauna
for some nine kilometres of the Finniss River in the Northern Territory
(1986). In Barberton, South Africa, ARD leachates generated in gold
mines, in the late 1930's, have degraded the local river system
to the point where it has not recovered to this day. Knowledge of the ARD generation potential of a
mine site allows for the implementation of correct environmental
management during the life of a mine and hassle free closure certification.
Acid Generation Acid generation is caused by the oxidation of
sulphide minerals when exposed to air and water. This results in
the production of acidity, sulphate solutions and the release of
metals. Not all sulphide bearing rock is potentially acid producing.
The ability of a particular rock to generate contaminated leachates
is a function of the relative content of acid generating minerals
versus the content of acid consuming minerals. The quantification of the potential of a mine
site to be a "net acid producing mine" is done through
acid-base accounting (ABA) to determine the net neutralisation potential
(NNP) of the geologic materials at a mine site. Stages of ARD Development The development of ARD is generally viewed as
a three stage process: i) Period of primary chemical oxidation of sulphide
minerals. Any carbonate (or equivalent) present in the rock mass
neutralises this small amount of acidity and maintains neutral to
alkaline pH. ii) As acid generation continues, and the carbonate
material is consumed or becomes unavailable, the pH of the water
reduces and the chemical and biological reactions accelerate. iii) The dominating reactions become primarily
biologically catalysed oxidation. The rates of biologically catalysed
oxidation can be from 50 to 1,000,000 times faster then chemical
oxidation. The time scale for each successive stage may vary from a matter of days to a period of hundreds of years, depending on the volume of material and other factors controlling the acid generation.
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