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ACID RAIN from:
"THE ENCYCLOPEDIA OF THE ENVIRONMENT" by The Rene Dubos
Center for Human Environments.
aR & W Eblen (Editors).
Acid rain is the deposition of airborne acids, not
just in rain, but also in snow, fog, and dry acidic particles. The
primary source of acid rain is burning of coal and oil in electrical
powerplants, industrial boilers, and internal combustion engines.
Fossil fuel burning produces sulfur dioxide (SO2) and oxides of nitrogen
(NOx,). In the atmosphere, SO2, and NOx, react chemically with ozone
and other compounds to form sulfuric and nitric acids - the acids
in acid rain.
Before SO2,
and NOx fall to earth as acids, however, they can be carried by wind
for hundreds of miles. Thus, acid rain may originate in one state,
region, or nation, and fall in another. Roughly half of Canada's acid
rain comes from the U.S., and 80% of Scandinavia's acid rain comes
from elsewhere in Europe. Natural, unpolluted rain can contain small
amounts of organic acids or windblown alkali dust, and thus typically
has a pH ranging from 4.9 to 6.5. However, downwind of major industrial
regions ~ such as the American midwest ~ average rainfall is often
10 -100 times more acidic (pH 4.5 to 4). Individual acid storms can
be over five times more acidic (0.7 pH unit lower) than these long-term
averages.
There is clear evidence that acid rain causes biological damage in
acid-sensitive lakes and streams. Small changes in the balance between
acids and bases in stream and lake water can cause large changes in
pH. Many aquatic organisms lose the ability to absorb and retain sodium
in low-pH waters. Acid waters also often have high concentrations
of dissolved aluminum, which disrupts organisms' salt balances and
interferes with gill function. Often, reproductive failure occurs
before direct damage to adult fish. Less conspicuous organisms, notably
amphibians and aquatic invertebrates, often succumb first to acidification.
Therefore, although disappearance of large sport fish is one of the
most visible effects of acidification, it is not a good early warning
indicator.
Small changes in acid loading, and thus lake acidity, may make lakes
uninhabitable for many species. Even in waters that are not chronically
acidic, organisms may be threatened by short-lived acid pulses that
occur when spring snowmelt or intense rainstorms flush acids through
the watershed faster than they can be neutralized. Other lakes may
be naturally acidic, because they have high concentrations of organic
acids. Organisms living in such waters are adapted to their chemistry
~ in contrast, organisms living in acid-sensitive, but unacidified,
waters may not survive the chemical changes that result from acidification.
Lakes and streams receiving the same acid rain may be too acidic for
fish, or may be habitable, depending on how well their watersheds
neutralize acids. Lakes and streams in watersheds with sedimentary
bedrock will usually not be acidic, because most sedimentary rocks
contain carbonate minerals, which dissolve rapidly to buffer (neutralize)
acids. In contrast. watersheds underlain by igneous and metamorphic
rocks (which contain little or no carbonate minerals) can be moderately
or highly acid-sensitive. The acid sensitivity of these watersheds
is determined by how much buffering capacity is stored in their soils.
This, in turn, depends on how rapidly alkaline chemicals are released
by mineral weathering. Acid rain may damage some lakes, while having
little effect on others in the same region, due to local geological
differences.
A direct link has not been established between acid rain and forest
dieback in the U.S., except for forests on mountainsides that are
frequently bathed in highly acidic clouds and fogs. In some European
forests, acid rain may be contributing to forest dieback by leaching
nutrients such as calcium, magnesium, and potassium from soils. Forest
dieback probably results from a combination of stresses, including
climate, drought, acid rain, ozone, and other air pollutants. A simple,
direct relationship between acid rain and forest health is therefore
difficult to prove.
Acid rain itself probably has little direct effect on human health,
but it may indirectly create health risks for some people. For example,
as water becomes more acidic, it becomes more corrosive; thus acidification
may accelerate leaching of lead from pipes and solder, leading to
higher concentrations of lead in drinking water (although most community
water supplies add alkaline chemicals to drinking water to control
corrosion). Similarly, coal combustion - which leads to acid rain
also releases mercury, and lake acidification converts mercury to
its toxic methyl form. Thus, some people who routinely eat fish from
acidified lakes could be exposed to high levels of methyl mercury.
Furthermore, the same combustion processes that cause acid rain also
generate ozone, airborne paiticulates, and acidic aerosols, all of
which are potentially hazardous when inhaled. Therefore, measures
to control acid rain may benefit human health, even if acid rain itself
does not directly make people sick.
Emissions of SO2 and
NOx, the building blocks of acid rain, can be reduced by switching
to low-sulfur coal and oil supplies, chemically or physically cleaning
sulfur from coal and oil before combustion, mixing powdered limestone
with the fuel during combustion, or using "scrubbers" to
remove SO2 and NOx from flue gases after combustion. NOx emissions
can also be controlled by lowering the combustion temperature or restricting
the supply of oxygen. SO2 and NOx emissions can also be cut through
energy conservation. Energy efficient light bulbs and refrigerators,
for example, reduce electricity demand and thus reduce acid rain,
usually at lower cost than other technical solutions. Where acid rain
cannot be adequately controlled, individual acid lakes can be neutralized
by repeated additions of crushed limestone. This usually permits some
fish to return, but does not restore the lakes' original pre-acidification
chemistry or biology.
Now that scientists
generally agree on the causes and effects of acid rain, the political
debate has shifted from whether a problem exists at all, to whether
it is a problem worth solving and, if so, who should pay the cost.
The environmental costs of acid rain (and thus the benefits expected
from controlling it) are potentially large but highly uncertain, while
the costs of emission control are both substantial and highly certain.
Some people still maintain that we should not control acid rain unless
we are sure that it is economically worthwhile to do so. Others contend
that emission control is cheap insurance against potentially widespread,
insidious, and irreversible environmental damage. This viewpoint has
apparently been persuasive. In the mid 1980s, Canada and seventeen
European countries jointly agreed to reduce their SO2 emissions by
30%, in an ultimately successful attempt to pressure Britain and the
US to make similar reductions. In the U.S., the 1990 Clean Air Act
amendments signaled the end of a decade of political indecision, mandating
40% cuts in emissions from 1980 levels.
Now that commitments
have been made to reduce the emissions that cause acid rain, there
is a growing sense of optimism that the acid rain problem will be
solved. This complacency may be premature. Drastic reductions in acid
deposition can reverse the acidification of lakes, streams, and forest
soils, but there is growing evidence that the proposed reductions
will be too small for many ecosystems to recover. Restoring fish habitat
in some Norwegian streams, for example, may require cutting emissions
by 80% or 90%, far beyond any reductions now planned.
Dealing with acid rain
will require ongoing scientific work and political commitment, long
after its novelty as an environmental issue has faded. Although field
data show that rainfall acidity is now declining in many regions,
it remains - and will remain for the foreseeable future - far higher
than natural levels. If it remains high enough to leach away buffering
capacity from acid-sensitive watersheds faster than geochemical processes
can resupply it, long-term progressive acidification of some lakes,
streams, and soils will continue even as rainfall acidity declines.
There is little indication that acid rain, or its effects, will simply,
go away.
JAMES W. KIRCHNER
For Further Reading: National Acid Precipitation Assessment
Program, Acidic Deposition: State of Science and Technology
(1990); James L. Regens and Robert Rycroft, The Acid Rain
Controversy (1988); David W.Schindler, "Effects of acid
rain on freshwater ecosystems" Science (1988).
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