Acid Rock Drainage - Mining and the Environment: From Ore to Metal - page 689

Similarly, rain falling on a pile of oxidizing waste rock or tailings ini ltrates and dissolves
salts as it percolates by gravity l ow through the pile. Particularly in arid and seasonally
dry climates, ARD often occurs as a series of pulses, each the result of a specii c rainfall
event. The highest acidities and highest concentrations of salts tend to result from the i rst
rainfall following a prolonged dry period.

Drainage
The most common drainage pathways by which oxidation products are transported to the
receiving environments are:
●

Dewatering of surface and underground mines to remove groundwater which com-
monly includes ARD derived from contact with the ore body or its surrounds. Water
pumped or drained from operating mines may be used as process water, discharged to
evaporation ponds (a common practice in arid Australia), or discharged to the envi-
ronment. In this context it is also important to recognize that mine dewatering may
expose acid generating rock to oxidation. Groundwater rebound after mining ceases
may l ush oxidation products into the drainage system (e.g. into the pit lake);

●

Rainfall runoff into surface mines is handled similarly to groundwater, with which it
may be mixed;

●

Seepage from waste rock dumps or ore stockpiles, which commonly emerges from the
toe at the base of the dump but may also emerge from internal percolation barriers;

●

Seepage from the base of tailings storages, and

●

Overl ow from tailings storages or evaporation ponds.

Capillary rise represents another pathway by which oxidation products may reach the surface.
This mechanism commonly occurs on the surface of tailings storages, and may also occur on
waste rock dumps where i ne-grained soils occur at the surface.

Effects of ARD

Much of the acidity resulting from sulphide oxidation associated with mining activities may,
by design or natural circumstances, remain isolated from the biosphere, in which case it may
be of no concern. Examples are found in many underground mines and some surface mines
where the acidic solutions are trapped at depth with no drainage path to the surface. In other
cases, however, such solutions may be in contact with aquifers which do eventually discharge
to the surface through seeps or springs, or through water supply bores. In these cases, the
acidic solutions may not emerge for many years following the end of mining. As in natural
systems, neutralization and dilution may remove much or all of the acidity in the meantime.

Effects on Operations
Sulphide oxidation, with or without subsequent leaching and drainage, may adversely
affect mining operations in numerous ways, such as:
●

Corrosion of concrete foundations, culverts, metallic pipes, and walkways;

●

In some open pit situations, such as the North Davao Copper Mine in the Philippines,
sulphur dioxide fumes may be generated from the pit walls, which in the absence of
ventilating winds, may settle in lower parts of the pit, presenting a hazard to the oper-
ations workforce. A similar situation has occurred at the Mt Newman Iron Ore Mine
in Western Australia where pyrite nodules which are widespread in the black shale
interburden, oxidize rapidly upon exposure;

Sulphur dioxide fumes may
be generated from the pit
walls, which in the absence of
ventilating winds, may settle in
lower parts of the pit.