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INTRODUCTION
Imagine going fishing on a cool Autumn day, the trees are all different
shades of orange, brown and red and the birds are singing their beautiful songs,
but their is a serious problem because when you arrive at the river all plant
and animal life are gone. This is by no means a recent phenomenon. This is due
to the effects of acid rock drainage (ARD). This is a problem that has been
occurring since ancient times, but it was not until the 1800's when fast growing
industrialization and heavy mining that it caught alot of attention.
Acid rock drainage is the term used to describe leachate, seepage, or
drainage that has been affected by the natural oxidation of sulfur minerals
contained in rock which is exposed to air and water. The major components of
ARD formation are reactive sulfide minerals, oxygen, and water. Biological
activity and reactions is what is responsible for the production of ARD. These
reactions make low pH water that has the ability to mobilize heavy metals
contained in geological materials with which it comes in contact. "ARD causes a
devastating impact on the quality of the ground or surface water it discharges
to. (Ellison & Hutchison)"
ACID MINE DRAINAGE
Within the mining process there are several sources that cause ARD. No
matter what activities occur, ARD usually occurs when certain conditions are met.
These conditions are the factors that limit or accelerate the release of ARD.
The initial release of ARD can occur anywhere from a few months to many decades
after the sulfide containing material is disturbed or deposited. ARD has been
associated with mines since mining began. When ARD occurs due to the effects of
mining it is called acid mine drainage, or AMD. The coal mining industry here
in the eastern United States has been associated with a major source of AMD for
decades. When water comes in contact with pyrite in coal and the rock
surrounding it, chemical reactions take place which cause the water to gain
acidity and to pick up iron, manganese and aluminum. Water that comes into
contact with coal has a orange-red yellow and sometimes white color. The metals
stay in the solution beneath the earth due to the lack of oxygen. When the
water comes out of the mine or the borehole it reacts with the oxygen in the air
or some that may be deposited in the stream and deposits the iron, manganese and
aluminum and deposits it on the rocks and the stream bed. Each of the chemicals
in acid mine drainage is toxic to fish and aquatic insects in moderate
concentrations. At real high concentrations all plant life is killed.
"Underground mines that are likely to result in ARD are those where
mining is located above the water table. (Kelly 1988)" Most of the mines are
also located in mountainous terrain. "Underground workings usually result in a
ground water table that has been lowered significantly and permanently. (Kelly
1988)" Mining also helps in the breaking of rock exposing more surface area to
oxidation.
OTHER SOURCES OF ARD
ARD is not necessarily confined to these mining activities. "Any
process, natural or anthropogenic, that exposes sulfide- bearing rock to air and
water will cause it to occur. (Ellison & Hutchison)" There are examples of
natural ARD where springs produce acidic water. These are found near outcrops
of sulfide-bearing rock, but not all exposing sulfide rock will result in ARD
formation. "Acid drainage will not occur if sulfide minerals are nonreactive,
the rock contains sufficient alkaline material to neutralize any acid produced,
or the climate is arid and there is not adequate rainfall infiltration to cause
leakage. (Ellison & Hutchison 1992)"
CHEMISTRY
"The most important factor in determining the extent of the acid mine
drainage is not the pH, but the total acidity. (Ellison &Hutchison 1992)" Total
acidity is a measure of the excess amount of H+ ions over other ions in the
solution. A high acidity is accompanied by a low pH in AMD. This is what
separates AMD from acid rain, which has a low pH and a low acidity. These
differences are due to the sources of acid in different ecosystems.
A buffer, as we learned in class, "is a compound that tends to maintain
the pH of a solution over a narrow range as small amounts of acid or base are
added.(Rhankin 1996)" This is also a substance that can also be either an acid
or a base. A ...
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