1016POLLUTION FROM MINE DRAINAGE
INTRODUCTIONAll forms of mining cause some impact on the aquatic envi-
ronment, just as any other earth disturbance will impact the
local hydrology. Sometimes this impact is very adverse, in
which case there is usually a considerable disruption of the
natural life cycles in the affected water. Other, and less noted
cases may even improve the local waters. Unfortunately, the
adverse impacts greatly outnumber the advantageous cir-
cumstances so that the result of mining is to severely degrade
the aquatic environment.
A generalized characterization of the impact of mining
on water is rather difficult, as almost any specific change in
the chemical qualities of the affected waters may be found
at some specific point. In almost all cases, however, there is
an increase in the total dissolved solids in the mine drain-
age waters. Additionally, the acidity of mine drainage is
increased above normal ground water levels for the area,
and the level of dissolved metal is increased. In some areas,
however, the alkalinity levels are increased by mining. Many
forms of mining also increase the suspended solids content
of water.
Coal mining has received the greatest amount of atten-
tion as the mining which causes water pollution. This is
perhaps deserved as the mining of coal has been a major
operation for many years and more coal has been mined
than any other single mineral. Water pollution from coal
mining was known in medieval England and the mines in
Wales were known to make the creeks run red. This fact
was important to the exploration of the North American
continent, as early explorers deduced the presence of large
deposits of coal from the natural color and character of some
of the streams and creeks. Similar conditions were some-
times noted in relation to other mineral deposits in the US.
As the outcrop materials come into contact with the atmo-
spheric conditions, oxidation and solubilization take place
and the products are transported into the streams. Hence,
the natural production of some mine drainage is a natural
phenomena which has existed almost from the beginning
of time.
Coal mine drainage may vary from waters pure enough
to drink without treatment to waters containing more than
20,000 mg/l acidity with commensurate amounts of iron and
other dissolved solids. Drainage from metal mines may vary
over almost equally wide ranges of acidity but often con-
tain substantial amounts of dissolved heavy metals. In mostrespects the acid drainages from metal mines are similar
to acid coal mine drainages. This similarity is so great that
most of the treatment processes and prevention mechanisms
developed and applicable to coal mine drainage can also be
applied to metal mine drainage.ORIGIN OF ACID MINE DRAINAGEThe earth strata associated with and superjacent to coal and
many other minerals almost always contain the iron sul-
fide mineral pyrite (FeS 2 ). Oxidation of the acidforming
pyritic material associated with mining is necessary for the
formation of mine acids and as oxygen is a necessary part
of the oxidation of these materials, there can be no signifi-
cant amount of oxidation until these are exposed to air. The
process of mining greatly increases the exposure of these
materials to atmospheric oxidation. Oxidation of the sulfide
mineral begins as soon as it is exposed to the air and contin-
ues at a rate characteristic of the geologic and atmospheric
conditions. Usually this oxidation causes spalling of the
mineral substance with a progressive increase in the amount
of surface area available for oxidation. Time then becomes a
significant factor in the amount and rate of formation of acid
mine drainage.
The exact nature of the pyritic mineral which oxidizes
so rapidly and causes the acid drainage from mining has
been sought for many years. In appearance, the mineral
is often grey like marcasite, and its oxidation rate is even
more rapid than that of marcasite. X-ray diffraction stud-
ies of the sulfide material associated with coal, however,
have confirmed by the crystal structure that the material
is pyrite rather than some other crystalline form of iron
sulfide.
The intricate mechanism of oxidation of this pyrite and
the formation of acid drainage has been the subject of many
learned discussions and research efforts extending back over
fifty years or so. The reaction will occur at normal room
temperature and humidity conditions with the release of SO 2
into the atmosphere. Under more humid conditions, the reac-
tion results in all of the sulfur being converted into sulfate.
Typical reactions depicting some of the several paths pos-
sible for the reaction of the iron sulfide, air, water and alkali
materials are listed.
This list is typical rather than all-inclusive to cover all
possible reaction routes.C016_010_r03.indd 1016C016_010_r03.indd 1016 11/18/2005 11:03:03 AM11/18/2005 11:03:03 AM