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

Acid mine drainage (AMD) represents the most widespread and pervasive mining-related impact in the three
basins studied. Almost all of the mineralised targets for mining consist of sulphide ore bodies or are
contaminated with sulphides, especially iron sulphide (“pyrites”). Whenever these ores are exposed to air and
moisture, the sulphide minerals present start to oxidize via a well-known set of autocatalytic reactions to give
high concentrations of total dissolved salts (particularly sulphates), low pH values and high concentrations of
dissolved metal ions (especially iron). The resulting solution is toxic to most forms of aquatic life and can lead to
dramatic changes in ecosystem functioning as well as changes in the structure and chemical composition of
soils. The process of sulphide oxidation is very difficult to stop once it has started and the resulting AMD can
persist for centuries, as proven in some English lead and tin mines.
Specific areas of concern around AMD in the three river basins studied are:
• The prevalence of AMD in the high-sulphur coalfields located within the Olifants basin in South Africa.
These problems of AMD are accentuated both by the number of collieries and by their size, as well as the
long time periods over which they have operated (decades), and the relatively abundant supplies of water
that are available in this area. Water quality changes attributable to AMD from this area can be discerned
(principally as changes in the sulphate to chloride ratio in water samples) for up to two hundred kilometres
downstream in the Olifants River.
• The large-scale AMD problems associated with mines on the Zambian Copperbelt, where numerous, very
large copper mines have operated for decades. Many of the Zambian copper mines appear to have
operated for prolonged periods without due attention being paid to the correct siting and operation of tailings
dams, and proper treatment of effluent before discharge to the Kafue River. Fortunately, the carbonate-rich
local lithology provides some safety in the form of a large buffering capacity that can effectively neutralize
much of the AMD produced. However, this situation is not sustainable indefinitely and concerted effort will
be needed to redesign, and perhaps even relocate, some of the tailings dams, whilst improving general
housekeeping practices and effluent treatment process efficiencies.
• At a smaller scale, gold and base metal mines that exploit these commodities in the different Greenstone
Formations in the three basins studied here also experience problems due to AMD. In these cases,
however, the presence of arsenopyrite in the gangue rock provides an added problem in the form of arsenic
liberation during sulphide oxidation. This arsenic is highly toxic and can have dramatic negative effects on
aquatic ecosystems, coupled to adverse impacts on human health if water users drink contaminated water
downstream of these operations.
• The extent of AMD impacts and perceptions of their severity appears to be closely related to the tendency
for the acidic solution to move away from its source and contaminate water resources progressively further
away from its origin. In sites where the AMD remains close to its source, the severity of the problem is
perceived to be less. This contrasts with situations where the influence of AMD can be seen several tens of
kilometres away from its source; here the problem is perceived to be far worse, even if the total quantities of
acid involved are similar. This so-called “mobility” of AMD, is the result of increased quantities of water
being available to “move” the AMD away from its starting point. The water responsible for “moving” the acid
from its source can be derived either from high local rainfalls, or from mining operations that use large
volumes of process water that must be discarded after use.
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