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Acid Mine Drainage
What Is Acid Mine Drainage?
by Frederic Beaudry
In a nutshell, acid mine drainage is a form of water
pollution that happens when rain, runoff, or streams come in contact with
rock that is rich in sulfur.
As a result, the water becomes very acidic and damages downstream
aquatic ecosystems.
In some regions, it is the most common form of stream and river
pollution.
Sulfur-bearing rock, especially one type of mineral called pyrite, is
routinely fractured or crushed during coal or metal mining operations, and
accumulated in piles of mine tailings.
Pyrite contains iron sulfide which, when in contact with water,
dissociates into sulfuric acid and iron.
The sulfuric acid dramatically lowers the pH, and the iron
can precipitate and form an orange or red deposit of iron oxide that smothers
the bottom of the stream.
Other harmful elements like lead, copper, arsenic, or mercury may also
be stripped from the rocks by the acidic water, further contaminating the
stream.
Where Does Acid Mine Drainage Happen?
It mostly occurs where mining is done to extract coal or metals from
sulfur-bearing rocks.
Silver, gold, copper, zinc, and lead are commonly found in association
with metal sulfates, so their extraction can cause acid mine drainage.
Rainwater or streams become acidified after they run through the
mine’s tailings.
In hilly terrain, older coal mines were sometimes built so that gravity
would drain out water from inside the mine.
Long after those mines are closed, acid mine drainage continues to
come out and contaminate waters downstream.
In the coal mining regions of the eastern United States, over 4,000
miles of stream have been impacted by acid mine drainage.
These streams are mostly located in Pennsylvania, West Virginia, and
Ohio. In the western U.S., on Forest Service land alone there are over 5,000
miles of affected streams.
In some circumstances, sulfur-bearing rock can be exposed to water in
non-mining operations. For example, when construction equipment cuts a path
through bedrock to build a road, pyrite can be broken up and exposed to air and
water.
Many geologists thus prefer the term acid rock drainage,
since mining is not always involved.
Environmental Effects
· Drinking water becomes contaminated. Groundwater can be affected,
impacting local water wells.
· Waters with a very low pH can support only severely reduced
animal and plant diversity. Fish species are some of the first to disappear. In
the most acidic streams, only some specialized bacteria survive.
· Because of how corrosive it is, acidic stream water damages infrastructure
such as culverts, bridges, and stormwater pipes.
· Any recreational potential (e.g., fishing, swimming) and scenic value
for streams or rivers affected by acid mine drainage are greatly reduced.
Solutions
· Passive treatment of acidic streams can be conducted by routing the
water into a purpose-built wetland designed to buffer the low pH.
Yet, these systems require complex engineering, regular maintenance,
and are applicable only when certain conditions are present.
· Active treatment options include isolating or treating the waste rock
to avoid contact of water with sulfates.
Once water has been contaminated, options include pushing it through a
permeable reactive barrier that neutralizes the acid or routing it through a
specialized wastewater treatment plant.
Frederic Beaudry
Introduction
Associate professor of environmental
science at Alfred University in New York
Ph.D. in wildlife ecology from the University
of Maine
Experience
Dr. Frederic Beaudry is a former writer for
ThoughtCo who contributed articles on pollution, global warming, and climate
science for three years. He is an associate professor of environmental science
at Alfred University in
New York. Prior to teaching, he worked as a wildlife biologist, focusing on the
ecology and conservation of birds and turtles. Beaudry has authored several
scientific papers on land use and conservation and has conducted research
examining land use changes and their effects on bird and amphibian communities.
Education
Beaudry has a B.S. in biology
from Université du Québec à Rimouski and an M.A. in natural resources
from Humboldt State University. He earned a Ph.D. in wildlife ecology at
the University of Maine. Beaudry completed postdoctoral research at the University
of Wisconsin-Madison.
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