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Environmental Impacts of Mining: Acid Mine Drainage
Formation
Why do we need to rehabilitate mine sites and what is causing the
problem? As discussed in the previous section, the soil and rock
(overburden) excavated to expose the materials of interest (i.e.
coal, metal ores, non-metallic ores), in addition to the waste rock
and tailings formed during the processing of valuable minerals,
often contain sulfide minerals such a pyrite (FeS2) that when exposed
to air and water, will oxidize and release large quantities of iron
and sulfate into solution. In addition, H+ ions are liberated during
the oxidation process producing an acidic solution that readily
weathers and releases other trace minerals (i.e. copper and zinc)
into solution. The acidic solution formed, characteristic of high
metals and sulfate and low pH, is generally termed acid mine drainage
(AMD).
Acid generation and drainage affect both surface and groundwater.
The sources of surface water contamination are leachate from mine
openings, seepage and discharges from waste rock, tailings, groundwater
seepage, and surface water runoff from waste rock and tailings piles.
Mined materials such as waste rock or tailings use for construction
or other purposes (i.e. road beds, rock drains, and fill material)
can also produce acid mine drainage.
The environmental impact of AMD can be severe. High concentrations
of metals and acidic conditions can have adverse effects on fish,
aquatic plant communities and humans. Nationwide, over 19,300 km
(12,000 miles) of rivers and streams and over 730 km2 (180,000 acres)
of lakes and reservoirs are adversely affected by contaminated water
draining from abandoned mines. To remedy the problem, industry in
the U.S. spends millions of dollars a day reclaiming mine sites
to prevent the formation of acid mine drainage. However, despite
the magnitude of the problem, the situation is much better than
it was 30 years ago, when the number of stream miles adversely affected
was 50% worse. The improvement can be attributed to the reclamation
of many abandoned operations, and to the regulatory requirements
on mining operations, which now must both prevent AMD generation
and treat their effluent water during and after mining to meet effluent
limits.
For more information on acid mine drainage, see the following journal
references: Cohen
and Gorman (1991) and Robb
(1994). Also, there are some great references on the web. Check
out the following websites to learn more:
Problem | Compliance
| Health & Safety | Sampling
| Analytical | Data
Quality
Site Assessment | Prediction
| Construction | GIS
| Monitoring & Assessment
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