Acid Rock Drainage - Mining and the Environment: From Ore to Metal - page 690

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Corrosion, leading to failure of metal rock bolts, resulting in slope or roof collapse. In an
underground nickel mine at Kambalda, Western Australia, oxidation of pyrrhotite in a warm,
humid environment, caused corrosive failure of rock bolts within two months of installation.

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Physical disintegration of rock due to crystallization of sulphates in micro-cracks or
inter-granular pores. This process has the potential to cause damage to structures such
as retaining walls or rock-i ll dams constructed of rock containing sulphides.

Ecological Effects
The most serious effects of ARD occur in freshwater aquatic ecosystems. While ARD
may kill most terrestrial organisms in its path, the effects do not extend beyond the con-
tact zone, which is generally coni ned to one or more l ow paths ( Figure 17.2 ) rather than
spread over broad areas. In rivers, streams, and lakes, however, large areas can be affected.
In general, any increase in acidity of surface water is likely to adversely affect the
resident aquatic biota. The extent to which ARD causes ecological damage depends on
numerous factors, including:
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The most serious effects of
ARD occur in freshwater aquatic
ecosystems.

The quantity, degree of acidity, and concentrations of heavy metals in ARD solutions;

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The degree to which the ARD can be coni ned close to the source;

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The assimilative capacity of the receiving environment, which includes attributes such as
the diluting, adsorbing, neutralizing, and buffering capacities of receiving soil or waters;

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The sensitivity of key organisms in the receiving environment, which may rel ect
exposure and adaptation of these organisms to acid environments during their recent
evolutionary history, and

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Interaction with other substances such as cyanide.

Typically, where ARD occurs, there are a range of adverse impacts which are most acute
close to the discharge point(s), and diminish with distance downstream. In general, where
ARD results in pH values below 5, signii cant effects can be expected, including both
reduced diversity and reduced populations of aquatic plants and aquatic invertebrates,
particularly insect larvae. Fish, in such circumstances may be stunted and breeding success

In general, where ARD results
in pH values below 5, signifi cant
effects can be expected.

FIGURE 17.2
Vegetation Absent from Path of
Overland Flow of ARD