THE CAUSE, EFFECTS AND TREATMENT OF ACID ROCK DRAINAGE
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 low pH has a lot
of bad effects on the "bicarbonate buffering system."(Kelly 1988) At low pH solutions
carbonate and bicarbonate are changed over to carbonic acid and then on to water and
carbon dioxide. Because of this water looses its ability to buffer the pH of the water
and plants in and around the water that use the bicarbonate in the process of
photosynthesis. Another effect of low pH is the increase in the rate of the
decomposition of clay minerals and carbonates, releasing toxic metals such as aluminum
and silica. Ironically however, Aluminum silicates can aid in the "buffering" of pH.
HEAVY METALS
The presence of high concentrations of heavy metals from acid mine drainage is just as
much a threat to the environment as acidity is. When sulfide is oxidized, heavy metal
ions are released into the water. "The key concept in this case is the specialization
of the metal distinguishes between 'filterable' and 'particulate' fraction of a
metal.(Kelly 1988)" Filterable means that particles can be trapped by a filter. The
particulate fraction of the metal includes solid minerals, crystals, and metals that set
up into organisms.
The presence of heavy metals in the aquatic environment can have a serious effect on the
plants and animals in an ecosystem. Plants uptake the metals and because plants are at
the bottom of the food chain, these metals are passed on to animals. The animals become
contaminated with the metals through eating and drinking. There are actually some types
of algae that actually thrive in harsh metal environments because they are not affected
by the toxicity and therefore they have no competition. These types of species are
blue-green algae: Plectonema, and green algae: Mougeoutia, Stigeoclonium, and Holmidium
rivular (Kelly 1988). These species are the exception because there are "very few
aquatic plants known to be naturally tolerant to heavy metals.(Kelly 1988)"
LAWS AND REGULATIONS
Recently, many laws and regulations have been passed to help treat and control the acid
mine drainage. The EPA has helped establish new limits and regulations such as no net
acidity of drainage (pH between 6-9), average total iron content of discharge must be
less than 3 mg/L, and the average total manganese content less than 2 mg/L. Processes
used now to prevent acid discharge are proper filtering equipment and drainage ponds that
contain acid rock indefinitely. The most common methods of treating acid mine drainage
are through chemical and biological processes.(Klepper 1989)
The Appalachian Clean Stream Initiative was established by the Office of Surface Mining
(OSM) and is trying to clean up acid drainage by combining the efforts of citizen groups,
corporations and government agencies. President of the OSM, Robert Uram said, "Private
organizations both grassroots and national have joined, in addition to government
programs at the federal, state, and local levels."
"The most effective way to control acid generation is to prevent its initiation.(Siwik
1989)" The biggest part of the reclamation and restoration is to research into the use
of peat/wetland treatment for heavy metal removal from acid mine drainage.(Siwik 1989)
According to the EPA standards, many of the mines will have to be designed and operated
to meet the standard of "zero discharge" from the mines.
CHEMICAL TREATMENT
Chemical treatment is the most common method used to eliminate acid drainage from
abandoned underground mines. There is three major working parts that do just this;
complexion, oxidation, and reduction"(Kelly 1989) Neutralization of acid water with lime
is a common practice. Chemicals commonly used in neutralization techniques are lime and
sodium bicarbonate or "costic soda." Other examples of substances that have been found
to reduce acid mine drainage are bactericides including antibiotics, detergents, heavy
metals and food preservatives. Antibiotics and heavy metals are to costly and to
dangerous to the surrounding aquatic life. Alconex, an inexpensive detergent, and sodium
laurel sulfate both are found to reduce acid in mine drainage.
BIOLOGICAL TREATMENT
Some choose to use biological treatment to treat acid mine drainage and these ways can
include: Biodegration of a chemical into basic oxidation products such as carbon dioxide,
water, and nitrogen. To me, a very interesting way of treating acid mine drainage
successfully and also high metal removal. The reason for this is that the plants that
are in the wetland are anaerobic and therefore the rates of decomposition and
mineralization of organic matter from the plants of the wetlands is slowed, and organic
matter tends to accumulate on the surface of sediments. Wetland, therefore can gather
and transform organic material and nutrients.(Bastian 1993) Natural and constructed
wetland have been used to treat wastewater. The first one that was ever constructed was
in 1982. There are over 200 systems in Appalachia alone.(Bastian 1993)
Even though this is safer for the ecosystem it is found that at most sites, chemical
treatment is still necessary to meet efficient standards, but the costs of chemical
treatment is greatly reduced with the initial biological treatment. Most operators find
that the costs of the construction of the wetlands are made up within one year due to the
money saved on chemicals.
CONCLUSION
In conclusion, acid rock drainage is a big problem all throughout the world due to alot
of industrialization and mining. This is not only a serious problem around the world, it
touches home here, especially here in Appalachia, but it seems to be under or getting
under control with all the new regulations and standards the EPA is setting. Low pH and
a high acidity level is harmful to us our wildlife and our plants. With the help of more
education and more research it will not have to be a problem for our future.
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