Acid Rain And North America
In the past century, one of the greatest threats to North America's
aquatic
ecosystem has been the widespread acidification of hundreds of
thousands of
waterways. Acid rain has effected plant and animal life within
aquatic
ecosystems, as well as microbiologic activity by affecting the rates
of
decomposition and the accumulation of organic matter. What causes this
poisonous
rain, and what can be done to improve North America's water quality
and prevent
future catastrophes? To answer these questions, we must first
examine the cause
and formation of acid rain, as well as understand ways to
decrease or prevent
its formation. Formation of acid rain. Acid deposition,
more commonly known as
acid rain, occurs when emissions of sulfur dioxide
(SO2) and nitrogen oxides (NOx)
react in the atmosphere with water, oxygen,
and oxidants to form acidic
compounds. This mixture forms a mild solution of
sulfuric and nitric acid which
then falls to the earth in either wet (rain,
snow, sleet or fog) or dry (gas and
particles) form. Approximately one-half
of the atmosphere's acidity falls back
to earth through dry deposition in the
form of particles and gases, and are then
spread hundreds of miles by winds
where they settle on surfaces of buildings,
cars, homes, and trees. When acid
rain falls, the dry deposited gases and
particles are sometimes washed from
buildings, trees and other surfaces making
the runoff water combine with the
acid rain more acidic than the falling acid
rain alone. This new combination
is referred to as acid deposition. The runoff
water is then transported by
strong prevailing winds and public sewer systems
into lakes and streams.
Although some natural sources such as volcanic
eruptions, fire and lightening
contribute to the emissions of sulfur dioxide and
nitrogen oxides in the
atmosphere, more than 90% is the result of human activies
such as coal
burning, smelting of metals such as zinc, nickel and copper, and
the burning
of oil, coal and gas in power plants and automobiles. When does rain
become
acidic? Scientists determine whether rain or lake water is acidic
by
measuring its pH (the measure of acidity or alkalinity of a solution on a
scale
of 0 to 14). A value of 7 is considered neutral, whereas values less
than 7 are
acidic and values over 7 are alkaline or basic. A change of one
unit on the pH
scale represents a factor of ten in acidity; for example, a
solution with a pH
of five is ten times as acid as one with a pH of six
(Somerville, 1996, p.174).
Normal or clean rainfall--without
pollutants--is slighty acidic due to carbon
dioxide, a natural gas in the air
that dissolves in water to form weak carbonic
acid. But rain, snow, or other
moisture is not called "acid rain"
until it has a pH value below 5.6 (Gay,
1992, p.44). Rainfall in eastern North
America is often acidic with a pH
of 4 to 5. Why is North America greatly at
risk? Acid rain is more common in
the Eastern U.S. and Canada than in the
Western U.S. because emissions
rise high into the atmosphere and are carried by
prevailing winds from the
west, falling out with precipitation in the east. Some
areas in the U.S.
where acid rain is most common include the New York
Adirondacks,
mid-Appalachian highlands, and the upper Midwest. Canada shows an
even
greater threat with half of its acid deposition caused by a large amount
of
metal smelting industries in Ontario and the other half attributed to
pollution
from combustion in U.S. factories in Ohio, Indiana, Pennsylvania,
Illinois,
Missouri, West Virginia, and Tennessee. Most lakes have a pH
between 6 and 8;
however, some are naturally acidic even without the effects
of acid rain. Lakes
and streams become acidic (pH value goes down) when the
water itself and its
surrounding soil cannot buffer, or shield, the acid rain
enough to balance its
pH level. In areas such as the northeastern United
States and parts of Canada
where soil buffering is poor, many lakes now have
a pH value of less than 5. One
of the most acidic lakes reported is Little
Echo Pond in Franklin, New York,
which has a pH of only 4.2. In New York's
Adirondack region, acid deposition has
affected hundreds of lakes and
thousands of miles of headwater streams, while
300,000 lakes in eastern
Canada are now vulnerable to acid deposition. How does
Acid Rain effect
Aquatic Ecosystems? As lakes and streams become more acidic,
the amount of
fish, aquatic plants and animals that live in these waters
decrease. Although
some plants and animals can survive acidic waters, others are
acid-sensitive
and will die as the pH declines. Plants and animals living within
an
ecosystem are highly interdependent. If acid rain causes the loss
of
acid-sensitive plants and animals, organisms at all trophic levels within
the
food chain may be affected which then causes a disruption to the
entire
ecosystem. In New York's Adirondack region, the diversity of life in
these
acidic waters has been greatly reduced. Fish population have
disappeared and
loons and otters have moved to other lakes where they can
find food (Simonin,
1998, p4). In Canada, over 14,000 lakes have been
acidified to the point where
they have lost significant amounts of fish. The
chart below shows that not all
fish, shellfish or their foot insects can
tolerate the same amount of acid. The
shaded bars represent the highest
degree of pH balance that animal can tolerate
within an acidic lake before it
becomes extinct from that lake. For example,
frogs seem to be the toughest
survivor by being able to tolerate a pH up to 4.0,
whereas clams and snails
are the weakest only being able to tolerate a pH of 6.0
before it will become
extinct. (*Source: United States Environmental Protection
Agency;
www.epa.gov): Animals pH 6.5 pH 6.0 pH 5.5 pH 5.0 PH 4.5 pH 4.0
Trout
Bass Perch Frogs Salamanders Clams Crayfish Snails Mayfly There are
two patterns
that contribute to the disappearance of fish from acidic bodies
of water. The
first pattern is known as "acid shock", which is a sudden drop
in pH.
These pH shocks usually occur in early spring when melting snow
releases acidic
elements accumulated during the winter into a lake or stream
causing a rapid
decrease in pH level, which in turn causes fish to die. A
second pattern is the
gradual decrease in pH level over a prolonged period of
time interfering with
fish reproduction; therefore, causing decrease in fish
population, and a change
in size and age of the population. Other animals are
affected by acidic water as
well. For example, low pH will often stunt the
growth of frogs, toads and
salamanders. Changes in pH level have caused
alterations in the structure of the
aquatic plant life involved in primary
production. Reducing the diversity of the
plant communities in lakes and
streams and disrupting primary production will
most likely reduce the supply
of food; therefore, the energy flow within the
ecosystem will decrease.
Changes in these communities also reduce the supply of
nutrients. These
factors limit the number of organisms that can exist within the
ecosystem
(Brittenbender, B., et. al., p. 4) In addition to affecting the plant
and
animal life, microbiological activity is also reduced affecting the rate
of
decomposition and accumulation of organic matter. Organic matter plays a
central
role in the energy flow of a lake's ecosystem. "The
biochemical
transformations of detrital organic matter by microbial
metabolism are
fundamental to nutrient cycling and energy flux within the
system, and the
trophic relationships within lake ecosystems are almost
entirely dependent on
detrital structure" (Brittenbender, B., et. al., p. 5).
There are two
responsible causes for the slowing rate at which organic matter
decomposes
underwater. First, the disappearance of certain invertebrates such
as snails
that shred organic debris as they feed; and second, a decrease in
the metabolic
rate of decomposition bacteria at a low pH level. Fighting acid
rain. There are
several ways to treat the acid rain problem. The answers
depend heavily upon
local politics and global economics. One solution is to
use low-sulfur coal as
opposed to high-sulfur coal. Unfortunately,
high-sulfur coal is far more
expensive than low-sulfur coal due to the
economics of mining and transporting
it. Another solution is to chemically
treat high-sulfur coal before burning it.
Devices known as scrubbers can
be installed on smokestacks to reduce the amount
of sulfur dioxide being
released into the atmosphere. The pH levels in lakes can
be increased by a
technique called liming. This process involves adding large
quantities of
hydrated lime to the waters in order to increase the alkalinity
and pH. Areas
that have used this method have had some success; however; liming
does not
always work because the lake may be too large and therefore
economically
unfeasible. In other cases, the lake may have a high flush rate, or
poor
buffering, so they quickly become acidified again after liming. Liming
the
acidic soils surrounding the lake so that the lime slowly dissolves over
time to
wash alkalinity into the lake is a more simple answer as well as less
expensive.
Although these solutions decrease sulfur dioxide in the
atmosphere, nitrogen
oxides are still increasing. Reducing nitrogen oxides is
more difficult to treat
because this type of acidic pollution is mainly
caused by automobile exhaust.
Although a reduction in number of
automobiles used is unlikely, regulating the
use of specially designed
catalytic converters could control emissions.
Improvements are being
made. Thanks to environmental regulations and agreements
to control
pollution, lakes and streams in North America are beginning to
recover from
acid rain and life is being restored. In 1995, phase I of the Clean
Air
Act Amendment was launched. Through this Act, over 400 power plants in
the
U.S. were instructed to reduce their sulfur dioxide emissions by 3
million tons.
Power plants are now instructed to reduce their use of
fossil fuels, burn
low-sulfur coal or use scrubbers. In 1991, the United
States and Canada
established the Air Quality Accord that controls the air
pollution that flows
across international boundaries. In this agreement, acid
deposition causing
emissions of sulfur are permanently capped in both
countries (13.3 million tons
for the U.S. and 3.2 million tons for Canada)
and plans were implemented for the
reduction of nitrogen oxides. Phase II of
the Clean Air Act will kick off this
year, mandating even steeper cuts in
sulfur emissions. The National Atmospheric
Deposition Program/National
Trends Network (NADP/NTN) has 191 sites across the
country which measure the
emissions of sulfur dioxide. Establishing more
organizations such as this
will help us understand how and where to combat the
acid rain
problem.
Bibliography
Bittenbender, B., Latendresse, K, Martysz,
I., Mood, P. Acid Deposition and
its Ecological Effects. Retrieved April 24,
2000 from the World Wide
Web:
http://bigmac.civil.mtu.edu/public_html/classes/ce459/projects/t17/r17.html
Gay,
K. (1992, March). Acid Relief? (4p). Cricket, 19 (7). Retrieved
April 24, 2000
from EBSCOhost database (masterfile) on the World Wide Web:
http://www.ebsco.com
Simonin, Howard (1998, April). The Continuing Saga
of Acid Rain (2p). New York
State Convervationist, 52 (5). Retrieved
April 24, 2000 from EBSCOhost database
(masterfile) on the World Wide Web:
http://www.ebsco.com Somerville, Richard C.J.
(1996). The forgiving Air:
Understanding Enviornmental Change. Berkely and Los
Angeles, California:
University of California Press United States Environmental
Protection
Agency. Affects of Acid Rain on Water. Retrieved April 24, 2000 from
the
World Wide Web: http://www.epa.gov/acidrain/student/water.html