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When rock surfaces are exposed
to air and rain, a reaction can occur with the elements in the rock
which results in a change in the characteristics of the water that
drains off. If the rock contains sulphides, a natural oxidation process
can acidify the water. This is known as acid drainage (also acid rock
drainage (ARD) or acid mine drainage (AMD)). As the water becomes
more acidic, its capacity to leach out other elements from the rock,
such as metals, increases. The resulting drainage can become very
acidic and contain a number of harmful constituents. In some cases,
elements from the rock can leach out into contact water without acidification
and result in water contamination – this is known as metal leaching
(ML). In either case, polluted water drains away from the exposed
rock and can have significant impacts on surrounding water bodies
(rivers, lakes, coastal areas, groundwater) and the wildlife or people
who come in contact with these sources.
Although this is a natural process, mining activities can trigger
this phenomenon by exposing large surface areas of rock to water
and oxygen. Rock is exposed on the walls of open pits and underground
structures - but the most significant newly exposed rock surfaces
are in the fragmented pieces of waste rock that are removed from
the ground and placed in dumps.
As there is presently little consistency
in the acid drainage terminology used by practitioners, INAP has
developed a draft glossary of terms (derived in large part from
that developed by William A. Price, Kevin Morin and Nora Hutt for
the British Columbia Ministry of Energy and Mines). The glossary
was produced to assist companies, regulators and members of the
public who are interested in carrying out or reviewing ML/AD work.
These can be viewed below.
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The International Network for Acid Prevention
