OVERVIEW OF THE IMPACT OF MINING ON THE ... - IIED pubs

Clearly, any large mining operation is likely to have a proportionately greater impact on the environment than a
smaller mining operation located in the same area. However, it is important to note that most large mining
operations also tend to employ more effective and efficient environmental management programmes than their
smaller counterparts. This feature tends to reduce the relative size and intensity of impacts associated with
larger operations. Indeed, many small mining operations very often cannot afford to implement effective
environmental management programmes and their impacts on the environment are sometimes
disproportionately large. This is clearly evident in the case of small-scale and artisan mining operations, where
little or no attention is directed towards minimizing impacts on the surrounding environment.
As a consequence, it is often extremely difficult to assign specific proportions of environmental impact to
different scales of mining operations where these occur on close proximity to one another. The only way to
separate the individual impacts in such a situation is through the application of a carefully structured monitoring
programme that examines both water quantity and water quality aspects.
2.4.1 Acid mine drainage
The breakdown of pyrite and other sulphides by water or air releases acid, sulphate and metals into the
environment. This is termed acid mine drainage (also referred to as acid rock drainage). The main reactions
involved (Loos et al., 1990) are:
2FeS2 + 2H2O + 7O2 → 2FeSO4 + 2H2SO4
4FeSO4 + O2 + 2H2SO4 → 2Fe2(SO4)3 + 2H2O
Fe2(SO4)3 + 6H2O → 2Fe(OH)3 + 3H2SO4
4Fe 2+ + O2 + 4H + → 4Fe 3+ + 2H2O (bacterial catalysis)
FeS2 + 14Fe 3+ + 8H2O → 15Fe 2+ 2- +
+ 2SO4 + 16H
The system is autocatalytic: the earlier reactions catalyse the later ones. This makes it difficult to stop the
reaction series once it has started (Mathesen et al., 1997).
It can be seen from these reactions that pyrite can remain in reduced form until exposed to air or water. Certain
chemolithotrophic bacteria, notably Thiobacillus ferrooxidans, use pyrite as an energy source, catalysing pyritic
decomposition. These bacteria have been found in water draining from various mine waste deposits, and in soil
in the seepage zone below the same deposits (Loos et al., 1990). They are also often found in streams near
mine dumps, where they aggregate in filamentous “streamers” of bacteria, coated in fibrillar polymers, which
they secrete (Kelly, 1988). The bacteria can function in an anaerobic environment, such as in a flooded mine.
According to comparisons of abiotic laboratory tests with field studies of sites with known Thiobacillus presence
(Kirby & Elder Brady, 1998), the bacteria can increase oxidation rates by five to eight orders of magnitude.
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