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What is acid rock drainage?
Image: G. Plumlee, USGS
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Rocks that contain useful metals
commonly contain sulfide minerals minerals that combine metal with
sulfur.
Image: Travis Hudson
Sulfide minerals in a rock sample
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The most common sulfide mineral is
pyrite - or “Fool’s Gold” - a
combination of iron and sulfur.
Image: R. Busch
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Pyrite occurs widely in rocks
including disseminations through
this entire mountain.
Image: G. Plumlee,
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When pyrite comes in contact with oxygen in
air or water it “oxidizes” or turns into rusty
iron oxides and sulfuric acid. Certain bacteria
make this process go faster.
Oxygen
Pyrite
Bacteria
Sulfuric
Acid
Images: USGS, DeAtley Design
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The sulfuric acid produced by oxidation of
pyrite makes nearby soil and water acidic this is acid rock drainage. Acid rock drainage
can harm habitat.
Image: Travis Hudson
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Acid rock drainage occurs naturally and is an
environmental concern where sulfide minerals
are mined. There are several ways to control
or prevent acid rock drainage at mines.
Image: Travis Hudson
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Controlling Acid Rock Drainage
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Storing waste rock underwater: Storing wastes underwater
reduces the rate of sulphide mineral oxidation by air to almost
zero, effectively stopping ARD formation. Underwater storage is
considered to be the most reliable and effective ARD prevention
strategy.
Flooding and sealing underground mines: The flooding of
underground tunnels is used where it is possible to isolate the
mine by sealing all the entrances. Sealing the mine prevents water
moving in and out, which could result in surface water
or groundwater contamination.
Mixing acid-producing materials with acid-buffering
materials: Combining sulphide wastes with limestone or calcite
can result in ARD neutralization on site.
Controlling Acid Rock Drainage
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Covering waste rock: Installing a cover of clay, plastic, or soil
over piles of waste rock prevents rain and other precipitation
from contributing to ARD formation and transport, and reduces
the amount of oxygen available to react with the sulphide
minerals.
Chemical treatment of sulphide wastes: Organic chemicals
designed to kill sulphide-oxidizing bacteria have been applied to
sulphide wastes in order to slow the rate of ARD. However,
there is concern that some of these chemicals may kill beneficial
microorganisms in the environment, thus becoming pollutants
themselves.See more at: http://www.miningfacts.org/Environment/Whatis-acid-rock-drainage/#sthash.aQbelzxj.dpuf