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

2. The dissolution of feldspars and clay minerals becomes enhanced in acidic solution, and this results in the
release of aluminium into the water (Kelly, 1988).
3. Beryllium, cadmium, copper, cobalt, chromium, mercury, manganese, nickel, lead, vanadium and zinc
become more soluble in acidic solution (Dallas & Day, 1993; Edmunds & Smedley, 1996; Aström, 1998).
4. The formation of a sulphate-rich solution leads to the increasing solubility of arsenic, cobalt, iron,
magnesium, nickel and uranium (Brink et al., 1990).
When acidic mine waters on the surface evaporate, they leave behind metal-bearing salts, such as chalcanthite,
a complex hydrated copper sulphate. When the next pulse of water passes through the area, these salts redissolve,
releasing acid and metals into solution, leading to sharp seasonal peaks in metal content in water
systems (King, 1995).
Associated with acid mine drainage is the precipitation of iron (III) hydroxide and oxyhydroxide complexes,
which give the water an orange colour. These are referred to frequently as "ochres" or "yellow boy". The
hydroxides and sulphides are precipitated at moderately low pH (5-6), but once the pH drops below 4.3 they are
soluble. This often means that iron (III) stays in solution near the source of contamination, but precipitates
downstream, where the pH has been neutralized somewhat, and has risen above 4.3. These hydroxides form
flocs, which can block light penetration, clog the bottom of the river and have abrasive affects on biota.
Dissolved metals may re-precipitate when the iron (III) hydroxide flocs form at slightly raised pH (Kelly, 1988).
The accumulation of nickel, manganese and aluminium in these precipitates has been demonstrated (Rose &
Ghazi, 1998). Iron (III) flocs also precipitate in old, flooded mines, since once mining operations cease, no fresh
pyrite is exposed, and the pH gradually rises (Mining Journal, 1996). Some of the iron may also be precipitated
as iron phosphate, removing this nutrient from the water system (Kelly, 1998).
The release of sulphates and other sulphide breakdown products leads to increased suspended solids and
dissolved solids, and thus to salinization. This is enhanced by the dissolving effect of acidic waters on country
rock.
Acid mine drainage also leads to a reduction in dissolved oxygen concentration in the water system (Dallas &
Day, 1993).
Organisms such as dragonflies, damselflies, mayflies, bivalves and gastropods are often destroyed in water
systems subjected to acid mine drainage. Precipitation of iron hydroxides on aquatic plant leaves inhibits
photosynthesis (Dallas & Day, 1993).
2.4.2 Release of metals
Another major problem associated with mining operations is the release of metals into rivers and lakes.
Speciation refers to the proportion of the metal in different forms, such as ions, complex molecules or ion pairs
and combinations, colloids and precipitates. The impact of metals on aquatic life within a water system is
dependant on the specific complex that the metal is in, the ligands it is associated with, and the thermodynamic
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