Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
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
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
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
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
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
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
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.
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
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
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
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
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?
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
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.