The Range of Environmental Concerns - Mining and the Environment: From Ore to Metal - page 529

waste management hierarchy; i.e. reduce, reuse, recycle, and remove (disposal) ( Case 13.5 ).
The main goal of the waste management hierarchy is to provide an order of preference
for the selection of appropriate waste management techniques. This order is based on the
apparent effectiveness of each technique in conserving resources and protecting the envi-
ronment against pollution.
Landi ll disposal represents the most common means for disposal of solid wastes generated
from a mining project (other than waste rock and tailings). In the past, these landi lls were
badly designed and poorly managed, often resulting in groundwater pollution and sometimes
leaving an eyesore; they were a cheap method of waste removal/disposal. As environmental
legislation has evolved, so has the design and management of landi lls. Today's landi lls at mine
sites are commonly engineered to include impermeable liners and leachate collection systems to
avoid groundwater contamination. Long-term monitoring is adopted to demonstrate that the
landi ll continues to perform over time as designed. As landi lls are no longer cheap options for
waste removal there is a strong incentive to shift waste management to recycling or reuse.

As in the extractive industry
as a whole, the mining industry
increasingly aims to implement
the 'Four R' waste management
hierarchy; i.e. reduce, reuse,
recycle, and remove (disposal).

13.3 MINE EFFLUENTS, ACID ROCK DRAINAGE AND
WATER BALANCE

Degradation of water systems, including rivers, lakes, and marine coastal waters, and
their aquatic ecosystems, is one of the most signii cant environmental impacts of mining.
Sources of water pollution vary from discharge of process efl uents containing remnants of
process chemicals, to acid rock drainage (ARD) from the mine or from mine waste dumps.
The nature of wastewaters from mineral processing varies widely. For example, highly
acidic wastes may be produced from acid leach processes as for lateritic nickel or uranium
extraction; caustic soda used to dissolve alumina from bauxite, produces highly alkaline
residues, while most precious metals are extracted using sodium cyanide solutions and the
wastes may contain potentially toxic levels of cyanide.
Rock containing sulphides oxidizes on contact with the air, producing sulphuric acid.
ARD may originate from rainfall dissolving acidic salts from excavated mine surfaces,
or as leachate emerging from oxidizing waste rock dumps or tailings disposal facilities.
Leachates commonly dissolve metals from materials which they contact. In this way, metal
may migrate to the groundwater or to the surrounding surface environment.
Mine efl uents can also result from seepage through and below impoundment walls, perco-
lation to the subsoil and groundwater, or overl ow. A discussion of these and other common
efl uent streams at mine sites follows, some of them illustrated conceptually in Figure 13.6 .

The nature of wastewaters from
mineral processing varies widely.

Common Effl uent Streams

Process efl uent can be dei ned as water that has come into contact with process materials
or results from the production or use of any raw material, intermediate product, i nished
product, by-product, waste product or wastewater. It also includes blow-down water,
efl uent that results from plant cleaning or maintenance operations, and any efl uent that
comes into contact with cooling water or storm water ( www.ec.gc.ca ). Process efl uent can
be discharged from a mine site to the environment as one or a combination of the fol-
lowing: (1) decanted efl uent from an engineered impoundment area (e.g. tailings pond);
(2) decanted efl uent from a polishing pond or clarii cation pond typically situated down-
stream of a tailings pond; and (3) treated efl uent from an efl uent treatment plant.