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OCR for page 131
11
lakes, Forests, and
Acid Deposition
Even though the British scientist Angus Smith coined the
term "acid rain" over a century ago, only in the last few decacles
have scientists recognized that widespread acidity in precipita-
tion causes damage far from its source. Over large stretches
of the world, acid deposition has damaged life in lakes and
streams and corroded building materials and accelerated the
aging of structures. In addition, it has become a key suspect
in the declining health of some species of forest trees in North
America and Europe.
Acid deposition results when pollutants, particularly oxides
of nitrogen and sulfur, are emitted from smokestacks, smelters,
and automobile exhausts into the atmosphere. These oxides
are converted, through a series of chemical reactions with other
substances in the atmosphere, to acids that fall back to the
earth's surface dissolved in rain, snow, or fog, or as gases or dry
particles.
The political tensions surrounding acid deposition arise
largely because the effects of pollutants produced in one re-
gion can be felt in another. Lakes in far-upstate New York are
131
OCR for page 132
132
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
acidified, in part, by emissions from the smokestacks of mid-
western power plants. Acids that rain into Scandinavia originate
in central Europe or the United Kingdom, and about 50 percent
of the acid deposition falling in eastern Canada comes from the
United States.
Acid deposition has been detected recently in other indus-
triaTized areas, including western North America, China, Japan,
the Soviet Union, and South America. In some areas of Africa
that are not heavily industrialized, high levels of nitric oxide
and other gases implicated in acid deposition have been mea-
sured. Fires set by farmers to clear the forests and Savannah are
a possible source.
The main cause of acid deposition in the industrialized
world is the sulfur oxide emitted to the atmosphere when coal
is burned as fuel or when high-sulfide ores are used in smelters.
The amount of sulfur in coal varies from deposit to deposit.
The higher the sulfur content, the greater is the contribution to
acid deposition once the coal is used as fuel. Not all coal is
high in sulfur. Coal from the Midwestern United States is high
in sulfur about 3 percent by weight. Coals from Appalachia
contain from ~ to 3 percent sulfur. Western coal has relatively
little sulfur, less than ~ percent. China, which is embarking on
ambitious plans for industrial development, will build factories
and power plants drawing on that country's large reserves of
high sulfur coal. Coal that contains high concentrations of sulfur
is also burned elsewhere in the world, such as in some Eastern
European countries.
Nitrogen oxides are the second major source of acidifying
compounds. Nitrogen oxides are emitted as a by-product when
fossil fuels like gasoline, oil, and natural gas are burned. The
amount emitted depends on a variety of factors, particularly
the temperature of combustion. A large fraction of the nitrogen
oxides responsible for acid deposition is emitted from auto-
mobiles and other vehicles. Stationary sources such as power
plants also contribute significant amounts of nitrogen oxides to
the atmosphere.
Scientists and the public in Europe became increasingly
OCR for page 133
LAKES, FORESTS, AND ACID DEPOSITION
133
aware in the 1960s and 1970s that the amount of dissolved acids
in precipitation depends on the direction, timing, and speed of
air flowing over Europe and England. In North America and the
rest of the world, concern took longer to set in. Throughout the
early 1970s, research in North America on air pollution was mo-
tivated more by interest in the potential effects of atmospheric
pollutants on human health than by concerns over the effects
on ecosystems in water or on the land. Gradually it became
clear that changes in the chemical composition of precipitation
were having-or had the potential to have significant effects
on ecosystems. Since then, scientists have conducted millions
of measurements and produced thousands of publications to
understand the causes and consequences of acid deposition.
THE ACIDIFICATION PROCESS
Nearly all of the acidification causing environmental dam-
age comes from sulfur dioxide (SO2) and nitrogen oxides (NO=)
released as gases when fossil fuels, particularly coal, are burned.
In the atmosphere, these oxides are transformed through a se-
ries of chemical reactions into sulfuric and nitric acids (H2SO4
and HNO3) and then into sulfate and nitrate. The rates of trans-
formation are governed by environmental conditions such as
sunlight, temperature, humidity, clouds, and the presence of
various other chemicals. The acids dissolve in water droplets
and eventually rain back to the earth.
The processes that govern how quickly the acids or acidi-
fying substances are deposited on the earth, and how far they
travel in the atmosphere, are complex and depend on meteo-
rological events, the characteristics of the earth's surface, and
the particular form of the pollutant. Typically, the pollutants
remain In the atmosphere for periods ranging from hours to
weeks, in which dine they may travel distances from a few to
over a thousand kilometers.
Although concern about acid deposition has focused on wet
forms (hence the term acid rain), the acids can also be deposited
"dry." Less is known about this process, mainly because it is
OCR for page 134
134
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
very difficult to measure, but evidence indicates that when the
dry material comes in contact with moisture, it causes the same
environmental effects as acids in rain, snow, or fog. In addition,
sulfate in the atmosphere that takes the form of fine particles
scatters light and reduces visibility.
The degree of acidification is measured by the pH value on a
scale of O to 14, which depends on the concentration of positively
.
charged hydrogen ions in solution. A neutral solution such as
distilled water has a pH of 7.
Acidic solutions have higher
concentrations of hydrogen ions, indicated by lower pH values.
The lower the pH value, the higher the acidity.
Most natural waters, including rain and snow, are somewhat
acidic because of naturally occurring chemicals. In remote areas
untouched by anthropogenic sulfur and nitrogen, rain with pH
~ ,
values of 5.2 to 5.4 is common, and values of less than 5 have
been recorded in extreme cases. But the organic acids and small
amounts of naturally occurring nitric and sulfuric acids respon-
sible for this acidity do not have the same consequences for life
in lakes and streams and on the land as do the mineral acids
from acid deposition.
Scientists have determined that in soils receiving doses of
acid deposition, the excess hydrogen ions displace other ele-
ments, including nutrients such as potassium, magnesium, and
calcium, and retard plant growth. High concentrations of hy-
drogen also release aluminum, a metal naturally occurring in
soil. Once released, the aluminum can be toxic to plants by
interfering with the ability of roots to absorb water and nutri-
ents. Acidity in water can cause lakes and waterways to lose
their abilities to support species that thrived under less-acid
conditions.
How ecological systems respond to the addition of acidic
material depends on the rate at which it is deposited and on a
region's geologic composition. Some of the rainwater entering
a lake or stream falls directly on the water's surface, but most
falls on the soil surrounding the water body and travels over the
surface and through the soil. Minerals in the soil can react with
the hydrogen ion to neutralize the acidity of the water while it
OCR for page 135
LAKES, FORESTS, AND ACID DEPOSITION
135
is en route to the lake or stream. This buffering capability of
the soil can slow down or prevent the acidification of lakes and
streams, but only where the particular type of soil contains the
right types of minerals. Sandy soils or soils that are thin and
highly weathered generally are already acidic, and so they have
only limited abilities to neutralize the effects of acids deposited
from the atmosphere. Soils capable of retaining large amounts of
sulfate and nitrate, or soils rich in elements such as calcium that
can neutralize the acid, can forestall the input of acidic water to
lakes and streams, at least until the buffering capacity is used up.
In the lake or stream itself acid-neutralizing chemical reactions
and minerals in the water and sediment can further counteract
the acidity, but with heavy inputs and with time some of this
capability can also be exhausted.
Because of the makeup of the soils, extensive regions of
Norm America and northern Europe are sensitive to acid depo-
sition. Researchers are finding that more and larger areas are
more vulnerable than they believed 10 years ago. In the United
States, these areas occur in the Northeast, as well as in M~n-
nesota, Wisconsin, upper Michigan, parts of the Southeast, and
many mountainous areas of the West. Surveys reveal that in
Canada, half of the 700,000 lakes in the six eastern provinces are
extremely sensitive to acid deposition. So are large stretches of
Canada's western anct northwestern provinces, as well as much
of northern Europe and vast expanses of Asia, Africa, and South
America.
LAKES
As researchers try to understand how acidification affects
ecosystems, they are hindered by the lack of comprehensive
biological surveys of large acid-sensitive areas, including North
America. As David W. SchindIer, an ecologist at the University
of Alberta in Edmonton, Canada, explains, "As a result of never
knowing what we had, we cannot know what we have lost."
Studies of the effects of acidification have been based largely
on observations of lakes where little is known of their state
OCR for page 136
136
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
before air pollution became a significant influence on them. As
a result, in many of the lakes known to have been affected, only
obvious changes such as the disappearance of large game fish
have been recorded. The surveys indicate that the sensitivity
of lakes to acid deposition varies from place to place. Even
neighboring lakes can respond differently.
Much of the understanding about the effects of acid deposi-
tion on lake ecosystems has been obtained from studies in labo-
ratories and artificial acidification of experimental lakes. Lakes
in or near populated areas may be affected by numerous chem-
ical pollutants from a variety of sources including recreational
uses, agriculture, and nearby mining. Consequently, there are
few pristine places where scientists can study the effects of acid-
ity in the absence of these other influences. Recognizing this
need, the Canadian government designated a portion of north-
western Ontario the Experimental Lakes Area. Scientists control
46 lakes and their watersheds, conducting experiments to help
understand what happens when the chemistry of lakes changes.
One of these lakes, "Lake 223," has provided especially valuable
insights.
Lake 223 lies in a region with thin, sandy soils covered by
pristine forests of jack pine and black spruce. It receives lit-
tle acid deposition, and the surrounding watershed has very
poor acid-neutralizing capacity. After 2 years of background
study, researchers began in 1976 to acidify the lake experimen-
tally, adding sulfuric acid incrementally until by 1983 they had
lowered the pH of the lake to 5.0 from its original value of
6.~. In 1985, Schindler and his colleagues reported that the
species living in the lake suffered the effects of acidification ear-
lier than expected and that, even when the pH was relatively
high, changes were extensive. The overall biological productiv-
ity and availability of nutrients were essentially unchanged, but
a handful of species that were food for the healthy population
of trout in the lake were eliminated at pH values as high as
5.~. When the study began, the lake supported a community
of about 220 species. By the time the pH reached 5.0, fewer
than 150 remained. (Even this number is misleading, Schindler
OCR for page 137
LAKES, FORESTS, AND ACID DEPOSITION
137
says, because almost half of that number were acid-resistant or-
ganisms that had come in to replace ones that had been forcer!
out by the acidity.) After ~ years, the trout the top of the
food chain in the lake were no longer reproducing, and their
mortality rates had gone up considerably.
Scientists believe that the number of species in a lake de-
clines continuously with increasing acidity below pH values of
6.5 to 7.0 and that many species that are foraged by fish higher
in the food chain are lost at pH values near 6.0. This disruption
in the food chain means that large predatory fish can starve long
before the direct toxic effects of acidification are evident.
"We are not losing hundreds of thousands of species, we are
losing hundreds," Schindler says. "But in terms of the fraction of
species that make ecosystems function, the relative magnitude
of biological impoverishment in acidifying softwater lakes is
probably just as large as in the tropics."
These findings hold true when researchers estimate the
likely degree of biological impoverishment of lakes in the north-
eastern and Midwestern United States. Such estimates are possi-
ble because, while there are too few data to indicate the number
of species lost, scientists find they can use existing chemical data
to predict the proportion of species lost as a lake moves from
any given pH to a lower one. Schindler and colleagues predict
that more than half of the lakes from the area's most heavily
acidified regions have lost 40 or 50 percent of species such as
molluscs and insects, losses just as important to the ecosystem
(though not as visible) as losses in gamefish. Thorough case
studies of streams, while few, indicate that the life they support
may be even more sensitive to acidification than that In lakes.
While some lakes in Ontario provide an example for what
happens under extreme acidification, others make it possible to
study what happens when acid precipitation lessens. in 1972,
emissions from the Coniston smelter, which contributed massive
amounts of sulfur dioxicle, were reduced by over 60 percent, and
the three smokestacks less than 200 meters tall were replaced by
one 381 meters tall. The combined effect of these changes was
to give the life in the lakes near Sudbury a chance to recover.
OCR for page 138
138
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
(Overall, Canada has reduced its emissions by 50 percent, and
in Ontario by nearly 60 percent, of the 1980 emission level.)
Once additions of the acidic materials tapered off, alkalinity
and pH increased rapidly, but the biology of the lakes was
less resilient. While some species soon recovered, resuming
reproduction when pH levels rose to between 5.4 and 5.6, some,
such as trout, did not. Nor did all species eliminated from the
lakes return; stocking of fish could be required to return the
lakes to states close to their original ones.
FORESTS
Damage to trees from droughts, hurricanes, insects, and dis-
ease is as old as the forests themselves. The transport of pollu-
tants from their inclustrial sources to stands of forests hundreds
of kilometers away subjects these ecosystems to yet another
form of stress.
In the late 1970s, extensive discoloration of needles and
declining rates of growth in forests in northeastern Bavaria in
the Federal Republic of Germany suggested to scientists that
acid deposition, and the ground-level ozone producecl by the
chemical reactions involved in acid deposition, play a major role
in what has come to be known as "forest dieback" or, in German,
waZd:ersterben (forest death). By the early 1980s, 20 to 25 percent
of European forests were classified as moderately or severely
damaged. in the late 1980s, the pace of deterioration decreased.
The causes of the damage are not entirely clear. In the United
States, Tong-distance transport of air pollutants is suspected of
contributing to a decline in forest health in California's San
Bernardino National Forest, in the pine forests of the Southeast,
and in the high-elevation forests of the Appalachians.
All trees experience stress in the form of diseases, insects, ex-
tremes in the weather, and competition with other trees for light,
nutrients, and water. When the accumulated stress becomes too
severe, trees become more vulnerable to opportunistic pests or
injury from extremes in the weather. Eventually, the growth rate
OCR for page 139
WAS, FOREST, ~ ~ ~EPOSI~ON
139
~ _
_
_
_ _1 ~
_ _
~ _
~ i Hal
~ _
I
i I
~ _
_ _.
A stand of Douglas ~ on ~te~ce Mortal ~ me Adirondack Mortals, New
^ saw damage Tom a colon of Actors Clung disease harsh watery
Ad acid p~itabon. (Photo taken ~ 1987 by Y. ~ US Forest sliced
OCR for page 140
140
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
for the tree declines, and, if the stress is severe enough, the tree
dies.
At high enough concentrations, air pollutants such as
grounct-leve! ozone and sulfur dioxide can damage a tree's fo-
liage when they come in direct contact with it. Indirectly, acid
deposition could deplete the soil of nutrients essential for the
tree's growth, elevate the levels of toxic metals such as alu-
minum, and alter the normal functioning and growth processes.
Because clouds can contain high concentrations of ozone and
acids, and because soils at high elevations are relatively thin,
the effects on forests are most pronounced at high elevations,
where clouds and fog come in contact with the trees.
it is difficult to distinguish between damage to trees from
natural causes and from air pollutants. Recent analyses, notably
by the U.S. government's National Acid Precipitation Assess-
ment Program, have considered forest damage in North Amer-
ica and attempted to unravel the causes. Red spruce that grows
at high elevations of the northern Appalachians in the eastern
United States has drawn special attention because analyses of
tree rings reveal that in the past quarter century, the growth
rate of the trees has declined substantially and over half of the
mature red spruce trees in the high elevations of the Adirondack
and Green mountains have died. In New York, Vermont, and
New Hampshire, stands of red spruce began to decline between
the late 1950s and mid-1960s. There is much stress from natu-
ral factors at high execrations, but scientists have now focused
their studies on effects of acidic cloud water and ground-level
ozone, which may compound the other stresses to the point
where red spruce is unable to survive. On the basis of current
scientific understanding, an interaction between injury from air
pollutants and mites seems a likely cause of the spruce decline
In the mountain forests of the Northeast.
Recent research has led to new theories about the decline
of Norway spruce in central Europe. A team of forest scientists
led by Ernst-DetIef SchuIze, of West Germany's University of
Bayreuth, conducted a wide-ranging search for causes of the
dieback in the Bavarian spruce forest but found no one agent
OCR for page 141
LAKES, FORESTS, AND ACID DEPOSITION
141
responsible. His studies suggest that atmospheric deposition-
nitrogen compounds in particular- gradually creates nutrient
unbalances in the trees and soils. Eventually, the trees become
deficient in important nutrients such as magnesium and more
vulnerable to opportunistic blights and to extreme weather such
as drought. SchuIze hypothesizes that the visible symptoms of
forest decline apparent since the late 1970s reflect this constelIa-
tion of conditions.
Changes in climate as a result of the greenhouse effect would
add to the stresses that air pollutants place on forests. Over short
intervals such as seasons, of course, weather is a governing fac-
tor. But longer-term changes in temperature and precipitation
would not only influence where and how rapidly acidic ma-
terial is deposited, but would also change the environmental
conditions for the trees.
BUILDINGS AND MONUMENTS
From an economic standpoint, corrosion of building ma-
terials is one of the most serious effects of acid deposition.
According to the EPA, estimates of the annual costs of repairing
or replacing structures damaged by acid deposition exceed $5
billion.
Marble and limestone, which is the second most commonly
used building material in the United States, are particularly
susceptible to acid deposition. The acids attack the calcium
carbonate, the principal constituent of these materials. Lime-
stone monuments like the Acropolis in Athens and the Jefferson
Memorial in Washington, D.C., show signs of damage. Emis-
sions from Mexico's refineries are corroding Mayan artifacts.
In southwestern Colorado, air pollutants from several power
plants and a nearby smelter are suspected in the quickening
deterioration of ancient sandstone cities of the Ansazi Indians
Of course, factors other than acid deposition contribute to
aging and deterioration of materials, including sunlight, wind,
and water. But evidence from buildings in areas receiving high
OCR for page 142
442
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
levels of acid deposition indicates that the process is being accel-
erated. In addition to stone, acidic pollutants are also implicated
in widespread damage to paint, wood, fabrics, masonry, con-
crete, and metals, though less is known about these weathering
processes. The evidence is beginning to mount that rusted steel
in bridges and corroded buildings are joining the list of the costs
to society from acid deposition.
CONTROLLING ACID DEPOSITION
While researchers have been exploring the causes and con-
sequences of acid deposition, engineers have been designing
methods to control those emissions responsible for the acidity,
and politicians have been debating the merits of implementing
laws that would put these controls into practice.
Progress has been made over the last 15 years. In the north-
eastern United States, the sulfate content of rain and the concen-
tration of sulfur compounds in the air have decreased, reflecting
pollution control measures mandated by the 1970 Clean Air Act
and efforts of individual states to limit emissions. Regulations
for controlling emissions from automobiles have contributed
substantially to the decline in nitrogen oxide emissions since
their peak in 1978. But there is little doubt that emissions will
have to be reduced much further to reduce the threats of acid
deposition.
Implementing technologies to control emissions or setting
restrictions on the sulfur content of coal raise a host of diffi-
cult and sensitive questions. On the scientific side: How much
should emissions be reduced to protect the environment from
the effects of acid deposition? In what regions should emissions
be reduced to protect sensitive areas far from the source? Are
technologies available to control the emissions? On the politi-
cal side: Would different limitations on emissions for different
regions be politically acceptable? Should controls be retrofitted
to the older plants that predate the Clean Air Act, even though
the remaining economic life of these plants is relatively short?
OCR for page 143
LAKES, FORESTS, AND ACID DEPOSITION
143
Should these older plants be decommissioned? Who should
bear the costs?
Scientists have constructed computer models to help pro-
vide answers to some of these questions. On the basis of emis-
sion levels and forecasts of weather and atmospheric chemistry,
the models predict the amount of acid deposition that would
occur for hypothetical emission levels from different regions. Of
course, the models can only reflect the state of the scientific un-
derstanding about acidification processes. Uncertainties such as
how long the soil can buffer the effects of acid deposition, how
long it would take for lakes to recover from the effects of acict-
ity, and the relation between forest health and acid deposition
need to be resolved before the models can provide more pre-
cise answers. Nevertheless, the models can help policymakers
evaluate and balance issues related to strategies for controlling
acid deposition. Where and by how much should emissions be
reduced? What technologies should be used? Should laws be
implemented now or should action be delayed until the remain-
ing scientific uncertainties are resolved?
Reducing emissions from coal-burning power plants, which
generate more than one half of the nation's electricity, is the
key to controlling deposition of sulfuric acid. Techniques to re-
duce emissions are available for the 410 generating stations built
between 1955 and 1975, when little was done to control emis-
sions. Effective measures include switching to coal with lower
sulfur content, "cleaning" the coal by removing the sulfur be-
fore combustion, and spraying the exhaust with wet limestone
to neutralize the acid (a process known as flue-gas desulfur-
ization). These methods have trade-offs. Fuel switching may
displace coal miners; coal cleaning techniques typically remove
only 10 to 30 percent of the sulfur; anc! conventional flue-gas
desulfurization is expensive and reduces the efficiency of the
plant. Moreover, none of these techniques do anything to re-
duce emissions of nitrogen oxides, also an important contributor
to the buildup of greenhouse gases in the atmosphere.
As these older plants age, they become candidates for re-
placement or refurbishment and provide opportunities for in
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144
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
corporating one of the "clean coal technologies" that recluce
emissions while maintaining, or even increasing, the efficiency
of the plant. For instance, in the system known as atmospheric
fluidized-bed combustion, combustion takes place at a lower
temperature, which reduces the formation of nitrogen oxides.
Meanwhile, limestone is mixect with coal in the combustion
process and efficiently captures sulfur ctioxide. Such techniques
are improving and eventually could yield very large reductions
in the emissions responsible for acid deposition.
Representative terms from entire chapter:
nitrogen oxides
