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

The acidification of the water has immediate
deleterious effects on aquatic ecosystems. A direct
effect is the conversion below pH 4.2 of all carbonate
and bicarbonate into carbonic acid, which dissociates
into carbon dioxide and water. This destroys the
bicarbonate buffer system in the water, which acts as
a control on acidity. Secondly, since many
photosynthetic organisms use bicarbonate as their
inorganic carbon source, their ability to
photosynthesise is limited or destroyed altogether as
bicarbonate decomposes and becomes less available
(Kelly, 1988). Thirdly, decomposition (and hence
nutrient cycling) will be reduced and eventually cease,
in water bodies severely affected by acid inflow
(Dallas & Day, 1993). Fourthly, acid waters kill some
organisms, by destroying ionic balances, or damaging
cell components or carbonate exoskeletons (Kelly,
1988).
However, the most important effects of acidification of water are the changes that result in the speciation of
metals. The solubility, mobility and bio-availability of metals are generally increased by acidification of water.
This is probably the result of the combination of washout processes in rocks and soils in the catchment,
enhancing the mobility of metals, and the active remobilisation of metals from aquatic sediments (Förstner &
Salomons, 1991). Specifically:
1. At very low pH levels, aluminium is present as the toxic species Al 3+ . Al 3+ 2-
is complexed by SO4 anions and
since these are a major component of water subject to acid mine drainage, the affect on aluminium may be
reduced somewhat by this process (Driscoll & Schecher, 1990).
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