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1 Summary and Synthesis Over the past decade or so, the phenomenon of acid rain, or more properly, acid deposition, has evolved in the United States from a scientific curiosity to an issue of considerable public concern and controversy. The issues raised by its possible adverse effects are not confined to specific localized areas, but are regional, national, and even international in scope. A wide variety of effects have been attributed to acid deposition, its gaseous precursors, and certain products of their chemical reactions including ozone. Possible environmental con- sequences include adverse effects on human health, acidi- fication of surface waters with subsequent decreases in fish populations, the acidification of soils, reduced forest productivity, erosion and corrosion of engineering materials, degradation of cultural resources, and impaired visibility over much of the United States and Canada. To evaluate these possibilities, scientific hypotheses have been formulated linking postulated or observed effects to acid deposition and/or its precursors. For example, acidification of lakes is thought to be the result of the deposition of acidifying substances, either directly as deposition to the water surface or indirectly by interaction with soils in the watershed to enhance transport of hydrogen and aluminum ions to surface waters Fish are adversely affected by acidification and the increased concentrations of aluminum that frequently accompany it. Alternatively, other hypotheses involving both natural acidification processes and/or other factors related to human activity have been proposed to explain the same effects. For example, land use practices such as timber harvesting, agriculture, and residential development are known to affect surface water chemistry and in specific circumstances might be more important for 1

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2 water quality than acid deposition. Similarly, changes in fish populations may be influenced by changes in stocking policies, the introduction of competing fish species, commercial or sport fishing, and pollution from pesticides or other commercial chemicals. It appears that an alternative explanation based on other human activities or natural phenomena can be or has been proposed for every proposed link between the depo- sition of airborne chemicals and an adverse environmental effect. In many instances several alternatives are plausible. This study was organized to investigate spatial pat- - trends in acid deposition and its terns and temporal gaseous~precursors in eastern North America and patterns and trends in environmental parameters that might result from acid deposition. The Committee on Monitoring and Assessment of Trends in Acid Deposition was asked not only to review previous efforts in this regard, but also to extend the analyses if there were approriate data. To meet these requirements we had to perform new analyses and make extensive checks on the quality of original data. We assumed that if a mechanism existed linking acid deposition to an environmental effect, it should be possible to demonstrate that acid deposition is associated spatially and/or temporally with the effect. If such an association cannot be established, then either a cause- and-effect relationship does not exist or our understand- ing of the mechanisms and rate-governing factors is not adequate. Despite imperfect knowledge of the relation- ships between emissions and deposition and rates of responses of ecosystems, careful evaluation of data on phenomena that are linked to acid deposition by plausible mechanisms should provide additional insight into the relationships among emissions, deposition, and effects. Some of the questions asked about the spatial relations of acid deposition and related phenomena were the follow- ing: Is the deposition of acidic sulfates and nitrates highest in areas where densities of emissions of sulfur and nitrogen oxides are highest? Are patterns of ecosystem changes attributed to acid deposition also . found in regions of low deposition? With regard to temporal associations, we asked: How has the chemical composition of precipitation changed with time? How acidic was precipitation or dry deposition 30 years ago? 50 years ago? 100 years ago? Does the timing of postulated changes in aquatic and terrestrial ecosystems coincide with changes in patterns of sulfur and nitrogen oxide emissions?

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3 Simultaneous examination of multiple patterns of spatial distributions and temporal trends provides a more robust test for the existence of linkages between emis- sions, deposition, and environmental effects than infer- ences based on data for pairs of phenomena. In conducting the study, it quickly became apparent that answers to questions about acid deposition have proved elusive because historical data from which to judge trends and hypothesize environmental responses are scarce. The earliest records of the direct measurement of levels of acidity, sulfate, and nitrate in deposition in North America date from as early as 1910 (McIntyre and Young 1923, Harper 1942, Hidy et al. 1984), one year after the invention of the pH scale for measurements of the acidity of aqueous solutions. Although they are informative, data obtained before 1950 are sketchy and of limited value because they pertain only to a few locations over short periods of time and in many cases are not reliable. Only since the late 1970s have extensive deposition monitoring networks been established to gather quality- assured data in a systematic way. In some cases however, longer and more extensive records do exist for systems thought to be affected by acid deposition or its airborne precursors. These records include long-term data on visibility, chemical composition of waters in lakes and streams, chemical and biological composition of lake sediments, fish popula- tions, Growth patterns in trees as evidenced in ring widths, distributions of lichens, erosion of tombstones, and chemical composition of glacial ice cores, ground- water, and soils. Unfortunately, investigations in a number of these areas conducted before the early 1970s were not designed to study acid deposition per se, and hence the original records usually do not include all the information required for a definitive intepretation of temporal changes. For example, the interpretation of earlier lake and stream chemical data is difficult because the records frequently fail to include adequate documentation of the sampling procedures and chemical analytical methods employed. Also often missing are quantitative descriptions of the variability in the data that may have been introduced by analogous changes in climate or weather or by human activities such as changes in land use patterns. On the other hand, the establishment of spatial rela- tionships among current values for emissions, deposition, and environmental responses appears to be more straight-

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4 forward because information about factors that might bias the analyses is generally more readily available. To determine which types of phenomena to study and which data were appropriate and feasible to include in the study, we developed certain criteria, the most important of which were the following: 1. A documented or postulated relationship, either direct or indirect, to acid deposition. 2. Availability of published data or original data with sufficient documentation to permit peer review. 3. Availability of data representative of broad geographical regions and/or temporal data with unambiguous dating. Consequently, we selected the following for inclusion in the study: 1. Emissions of sulfur and nitrogen oxides (Chapter 2). The major contributors to atmospheric deposition of sulfur and nitrogen compounds in North America are anthropogenic emissions of sulfur and nitrogen oxides. Estimates of emissions have been compiled in this report based on data on the production and use of fossil fuels, estimates of their sulfur content, and emission factors for nitrogen oxides released during combustion. 2. Precipitation chemistry (Chapter 5). Quality- assured data on precipitation chemistry for a broad region in eastern North America have been available since about 1978. These data permit spatial analyses of the chemistry of wet deposition in the region, but the time period of this record is not sufficiently long to estab- lish statistically significant temporal trends. Time series of longer duration, dating from the early to the middle 1960s, are available at a few sites, however, and we have performed trend analyses on some of these data. 3. Atmospheric sulfates and visibility (Chapter 4). A direct effect of sulfur dioxide emissions is the production of atmospheric sulfate aerosols that reduce visibility. Historical and spatial data on atmospheric sulfate and visibility are available from a number of stations. 4. Surface water chemistry (Chapter 7). One of the most studied effects of the deposition of acidic chemical species is the acidification of surface waters. Many data are available for analysis. We selected what we judged to be key data sets and included in our analysis _

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5 those that were amenable to rigorous assessment of their reliability. In all cases checks for internal consistency of the original historical data had to be Der formed befor e data were incorporated into the analyses. , ~ 5. Sediment chemistry and abundance of diatom tax a - (Chapter 9). Changes in watershed and lake chemistry are recorded in lake sediments, which provide historical data of the longest time series. Dating and chemical analysis of successive intervals of sediment cores provide chrono- logical information on changes in water chemistry. Assemblages of diatoms and chrysophytes in sediments can be analyzed to reconstruct historical lake water acidity. 6. Fish populations (Chapter 8). Fish populations are hypothesized to decline in acidified lakes. Some records are available relating fluctuations in popula- tions of selected fish species to historical records of lake and stream chemistry. 7. Tree rings (Chapter 6). The decline of forest trees is perhaps the most controversial phenomenon that some researchers have attributed to acid deposition. Some tree ring data are available for red spruce populations in the higher elevation forests of the northern Appalachian Mountains. It became apparent during the evaluation processes that we would not be able to rely exclusively on the published literature if we were to meet our goal of examining spatial patterns or temporal trends of multiple phenomena. In some cases extensive evaluations of ~ _ _, _ spatial distributions or temporal trends had not been performed for even a single phenomenon and thus our evaluation depended, at least in part, on our own analysis of unpublished data. Owing to limitations in available data, the effects of oxidants and other air pollutants are not considered in this report. The committee's findings and conclusions are listed below. In subsequent sections of this chapter we describe the rationale employed in drawing these conclusions. First we present our methodology for assessing the likelihood of a cause-and-effect rela- tionship based on the criteria of mechanism and consistency of data. We then briefly discuss the specific mechanisms that may link acid deposition to related phenomena and some factors that complicate our analysis. We follow this section with a detailed analysis of the degree of consistency in spatial patterns and temporal trends among acid deposition, emissions, and .

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6 the environmental changes often attributed to acid deposition. F INDINGS AND CONCLUS IONS When trends and patterns are found that establish temporal and spatial consistency in cases for which plausible mechanisms link acid deposition to other phenomena, cause-and-effect relationships can be postulated with some degree of confidence. Previous attempts to evaluate temporal trends have been limited because of large uncertainties inherent in historical data bases. Our premise was that through careful selection of a number of types of data and a number of quality-assured data bases, a more robust analysis for consistency and associations among the data might be possible. We believe that the results of our analyses, presented in later sections of this chapter and in the following chapters of this report, demonstrate the validity of this approach, and that we can formulate the following major findings and conclusions: 1. Through statistical analysis of regional spatial patterns, we find a strong association among the following five parameters: (a) emission densities of sulfur dioxide (SO2), tb) concentrations of sulfate aerosol, (c) ranges of visibility, (d) sulfate concen- trations in wet precipitation, and (e) sulfate fluxes in U.S. Geological Survey Bench-Mark streams. From this result and because of the existence of plausible mechanisms linking the phenomena, we conclude that in eastern North America a causal relationship exists between anthropogenic sources of emissions of SO2 and the presence of sulfate aerosol, reduced visibility, and wet deposition of sulfate. Our analysis also indicates that for Bench-Mark streams in watersheds showing no evidence of dominating internal sources of sulfate there is a cause-and-effect relationship between SO2 emissions and stream sulfate fluxes. Magnitudes of sulfur emissions and deposition of sulfur oxides are highest in a region spanning the midwestern and northeastern United States. 2. Based on data on fossil fuel production and consumption, we conclude that acid precursors, par- ticularly S02, have been emitted in substantial quantities in the atmosphere over eastern North America

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since the early l900s. In particular, SO2 emissions in the northeastern quadrant of the United States have fluctuated near current amounts since the 1920s. These conclusions are supported by limited data on long-term trends in visibility and the presence in lake sediments of chemicals emitted during combustion of coal and other fossil fuels. 3. Substantial differences in temporal trends in SO2 emissions among regions of the United States have emerged since about 1970. Before 1970, temporal trends in SO2 emissions in the various regions were congruent, although the amounts of emissions were of different magnitudes. From data on SO2 emissions, reduction in visibility, and sulfate in Bench-Mark streams since about 1970, we conclude that the southeastern United States has experienced the greatest rates of increase in parameters related to acid deposition. The midwestern United States has experienced rates of increase somewhat lower than the Southeast. In the northeastern United States the trend has been one of modest decreases. 4. The record of the chemistry of Bench-Mark streams suggests that changes in stream sulfate flux determine changes in stream water alkalinity and base cation concentrations in drainage basins that have acid soils and low-alkalinity waters. Increases in stream sulfate flux are associated with decreases in alkalinity and/or increases in amounts of base cation in surface waters. The change in alkalinity per unit change in sulfate depends on site-specific characteristics. Changes in sulfate observed in Bench-Mark streams are consistent with changes in SO2 emissions on a regional basis. Analysis of a sulfur mass balance for 626 lakes in the northeastern United States and southeastern Canada demonstrates that the sulfate output from lakes in general is proportional to sulfate inputs in wet deposition. The ratio of output to input decreases with distance from major source regions, suggesting that dry deposition is an important contributor to sulfate flux inputs, especially near major source regions. 5. Data on alkalinity of some lakes in New York, New Hampshire, and Wisconsin suggest that changes in alkalinity greater in magnitude than about 100 peq/L can occur over time periods of about 50 years. Changes of this magnitude are too large to be caused by acid deposition alone and may result from other human activities or natural causes. We have not attempted to identify the exact nature of the causes of these large changes.

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8 6. Analysis of diatom and chrysophyte stratigraphy for sediments in 10 low-alkalinity Adirondack Mountain lakes studied indicates that 6 of them became increasingly acidic between 1930 and 1970. Because the trend in acidification is consistent with both the presence of other substances in sediments that indicate fossil fuel combustion and current lake acidification models, and because the observed acidification cannot be explained by known disturbances of the watersheds or by other natural processes, acid deposition is the most probable causal agent. These findings are supported by fish population data for 9 of the 10 lakes in the Adirondacks for which concurrent data exist. Diatom data from lakes in New England indicate slight or no decrease in pH. Data for southeastern Canada are insufficient to examine trends in acidification. 7. Based on comparisons of historical data on alka- linity and pH recorded in the 1920s, 1930s, and 1940s with recent data for several hundred lakes in Wisconsin, New Hampshire, and New York, we find that many lakes have decreased in pH and alkalinity and many have increased in pH and alkalinity. On average, lakes sampled in Wisconsin have increased in alkalinity and pH. The New Hampshire lakes on average show no overall change in alkalinity and a small increase in pH. Interpretation of changes in the New York lakes is sensitive to assumptions about the application of calorimetric techniques in the historical survey and the selection of recent data bases. Depending on the assumptions, New York lakes on average either experienced no changes in alkalinity and pH or have decreased in alkalinity and pH. In the judgment of the committee, the weight of the evidence indicates that the atmospheric deposition of sulfate has caused some lakes in the Adirondack Mountains to decrease in alkalinity. We base this conclusion on three types of evidence: (a) Sulfate concentrations in wet-only deposition in the region of the Adirondack Mountains and sulfate concentra- tions in Adirondack lakes are relatively high in comparison with those in other areas in the northeastern United States and southeastern Canada. We have demon- strated that increasing sulfate in surface waters is associated with decreasing alkalinity in low-alkalinity surface waters. (See Conclusion 4.) (b) Diatom-inferred pH and other supporting evidence provide a strong indication of acidification from acid deposition in low-alkalinity lakes. (See Conclusion 6.) (c) We calculated alkalinity changes in New York lakes four

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9 different ways to account for different assumptions. Three of the results indicate, on average, a decrease in alkalinity (median values of -28, -44, and -69 peq/L), and one result shows no overall change (median value of +1 peq/L). Because of ambiguities regarding the assumptions employed in the historical New York survey, we cannot currently determine which of these results is most accurate, and hence we cannot quantify the number of New York lakes that have been affected by acid deposition. 8. Emissions of oxides of nitrogen (NOX) are estimated to have increased steadily since the early 1900s, with an accelerated rate of increase in the Southeast since about 1950. Reliable data do not exist to determine historical trends of nitrate concentrations in the atmosphere, precipitation, or surface waters. 9. Although high-quality data to assess trends in fish populations as a function of surface water acidity are sparse, the data that are available indicate that fish populations decline concurrently with acidification. The strongest evidence in support of this finding comes from some Adirondack Mountain lakes. These data demon- strate declines in acid-sensitive fish species populations over the past 20 to 40 years in lakes thought to have been acidified over the same time period. (See Conclusion 6.) The number of cases studied is too small to permit any projections of the total number of fish populations that may have been affected by acidification. 10. Geographically widespread reductions in tree ring width and increased mortality of red spruce in high- elevation forests of the eastern United States began in the early 1960s and have continued to the present. The changes occurred about the same time as important climatic anomalies and in areas subject to comparatively high rates of acid deposition. The roles of competition, climatic and biotic stresses, and acid deposition and other pollutants cannot be adequately evaluated with currently available data. METHODS It is important to establish the requirements for inferring that a relationship between data sets implies causality. Two variables can be considered to be associated if their values are paired in some related way across a population, and they are unassociated if a

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10 special pairing does not exist. To establish that an association exists, it is necessary only to show that the variables do not appear to be paired at random. Thus, as we will see in Chapter 5, sulfate and nitrate in wet deposition are associated across eastern North America; regions with high wet sulfate deposition also tend to have high wet nitrate deposition, and vice versa. Association between variables is necessary but not sufficient to infer the existence of a causal relation- ship. Mosteller and Tukey (1977) list three criteria-- consistency, responsiveness, and mechanism--at least two of which are usually needed to support causation. Con- sistency implies that (all other things being equal) the relationship between the variables is consistent across populations in direction, or perhaps even in amount. If a relationship between the variables holds in each data set, then the relationship is consistent. Responsiveness involves experimentation. If we can manipulate a system by changing one variable, does the other variable also change appropriately? Mechanism means a step-by-step path from the "cause" to the "effect," with the ability to establish linkage at each step. Observation of a correlation between two variables can establish consistency, but it cannot establish either responsiveness or the mechanism of possible causation. Thus, correlation is not adequate to prove a cause-and- effect relationship. Continuing the earlier example, sulfate and nitrate in wet deposition have a clear, con- sistent relationship, but neither experiment nor mechanism implicates changes in one as causing changes in the other. In fact, we know that they both arise from a common source, the high-temperature combustion of fossil fuels (National Research Council 1983). In this report, we use the criteria of mechanism and consistency for suggesting cause-and-effect relationships. Some controlled studies of responsiveness are discussed, but field experiments on most of the variables are gen- erally not considered practicable. In the next section of this chapter, we describe a number of conceptual linkages among the various types of data. These linkages are in fact mechanisms, stepping from one variable, e.g., emissions of SC>, to another variable, e.g., visibility, with causation natural at each step. Ideas about mech- anisms then motivated analyses to determine whether consistent relationships exist among the variables over space and time.

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11 MECHANISMS In this section, we summarize the conceptual mechanisms that could link emissions, atmospheric deposition, and environmental responses; the analyses of trends and spatial patterns are discussed in the following section. The general conceptual relationships are shown schemati- cally in Figure 1.1 and are described further in the following chapters. The figure does not depict all the possible environmental interactions of sulfur and nitrogen oxides or the many possibilities for their ultimate fates. It does, however, indicate relationships among the phenom- ena examined in this report: emissions, visibility, chemistry of precipitation, chemistry of lake and stream waters, fish populations, forests, and chemical and biological stratigraphy of lake sediments. Dry deposition is shown in this general diagram and is discussed in various chapters of this report, but lack of data precluded any detailed quantitative analysis of its temporal trends. ATMOSPHERE dispersion + transformation modulated by climate -2 deposition: wet and dry emissions ~ ambient SO4 Box+ NOx ~ and visibility forests ~ i: :''~''~'~'': __ combustion : ~:~ i:: ~::~: ~~:~ ~~ ~~ -~:~ ~:~ ~~-~;;~;=~_ of fossil fug s : BIOSPHERE ~~ ~ ; watershed) ~~° fish >by : :: :~: :: :~:: : :~ ~ ~ ~~ i::: ~ ~ i: ~ ~~:.~ .~: ~ ~ i: i: ~ ~ ::: ~~:~ ~ :-: ~ :~:: ~ ~ ~ ~ : FIGURE 1.1 Acid deposition: affected ecosystems. diagram of sources and

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37 suggests that the endpoint was bounded in the pH range of 4.19 and 4.04. We compared the 1930s data with two recent New York lake surveys (Pfeiffer and Festa 1980, Colquhoun et al. 1984) and determined that the results depended somewhat on which of the two surveys were included in the calcu- lations. Thus, there are four possible pairings of assumptions as follows: (1) an MO endpoint of 4.04 pH units and comparison of historical data with the 1980 data; (2) an MO endpoint of 4.04 and comparison with the 1984 data; (3) an MO endpoint of 4.19 units and comparison with the 1980 data; and (4) an MO endpoint of 4.19 units and comparison with the 1984 data. Calculations applying assumptions 3 and 4 yield median changes in alkalinity of -69 and -44 peq/L, and median changes in pH of -0.74 and -0.63, respectively. Applying assumptions 1 and 2, the calculations yield median changes in alkalinity of -28 and +1 peq/L, and median changes in pH of -0.14 and -0.12 units, respectively. In each state there are numerous examples of lakes with changes in alkalinity greater in magnitude than 100 peq/L. Since the magnitude of change from acid deposition is estimated at about 100 peq/L or less (Galloway 1984), it is likely that these lakes were affected by factors other than, or in addition to, acid deposition. Relationship of Trends in Diatom-Inferred pH and Fish Populations Analysis of diatoms in sediments of selected lakes offers another indication of long-term acidification. Unlike the lake surveys in Wisconsin, New York, and New Hampshire, the lakes from which diatom data were obtained were selected on the basis of acid sensitivity (i.e., alkalinity less than 200 peq/L) and either little or no disturbance of the watershed or good documentation of disturbance. (See Chapter 9.) Based on paleoecological analysis of the entire history of currently acidic lakes, rates of long-term natural acidification are slow--with decreases of 1 pH unit (from 6.0 to 5.0) occurring over periods of hundreds to thousands of years. In contrast, some lakes in the Adirondack Mountains have apparently experienced decreases in pH on the order of 0.5 to 1.0 pH unit over a 20- to 40-year period in the middle of this century. Diatom data for 31 lakes were evaluated to assess regional trends in lake acidification; data of sufficient

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38 quality are available for 11 lakes in the Adirondacks, 10 in New England, 6 in eastern Canada, and a reference set of 4 lakes in the Rocky Mountains. The evaluation suggests that certain poorly buffered eastern lakes have become substantially more acidic during the past 20 to 40 years. The lakes for which evidence is strongest are in the Adirondack Mountains. Of the 11 lakes for which diatom data are available, 6 of the 7 lakes with a current pH at or below about 5.2 show evidence of recent acidification; the seventh is a bog lake. Analyses of chrysophyte scales (mallomonadaceae) agree well with interpretation of the diatom data. None of the 4 lakes with current values of pH above about 5.2 showed strong evidence of a pH decline. Where dating of sediments is available, the most rapid diatom-inferred pH changes (decreases of 0.4 to 1.0 pH units) occurred between 1930 and 1970 beginning in the 1930s to 1950s. Diatom data for the 10 lakes in New England indicate either a slight decrease or no change in pH over the past century. Diatom data for the 6 lakes in eastern Canada indicate no change in pH (4 lakes with current pH greater than 6.0) or a significant decrease in pH that is probably caused by local smelting operations (2 lakes). Rocky Mountain data do not show decreases in pH. Before 1800, several lakes in the Adirondacks and New England had diatom-inferred pH values less than 5.5. These lakes now have a pH of only 0.1 to 0.3 pH units lower, and total aluminum concentrations greater than 100 ug/L. Because of the potential importance of buffering by organic acids, a small decline in pH could be asso- ciated with significant decreases in acid neutralizing capacity. Analysis of the Adirondack data indicates that no other acidifying process except acid deposition has been identified to explain the rapid declines in lake water pH during the past 20 to 40 years. However, watershed disturbances may also play a role, but probably a minor one for the lakes evaluated in this study. Further evidence of acidification trends in lakes of the Adirondacks is provided by comparing measured pH, diatom-inferred pH, sediment chemistry, and fish population data on nine Adirondack Lakes (Table 1.3). They show consistent trends with some exceptions. The lakes with current values of pH of about 5.2 or less have become more acidic in recent times and have lost fish populations, whereas lakes with higher current values of

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39 pH show no obvious trend toward acidification or fish declines. In a number of cases (e.g., Woods Lake, Upper Wallface Pond) the observed change in diatom-inferred pa is small, from about pH 5.2 to about pH 4.8, yet major changes in fish populations have occurred. There are two plausible explanations for this phenomenon. First, fish are sensitive to pH in this range, with survival decreasing abruptly over the pH range 5.2 to 4.7. Many lakes in New York and New England with pH 5.0 to 5.2 currently support fish, whereas lakes in this region with pH below 5.0 rarely support fish (Chapter 8). Second, acidification of surface waters from pH 5.2 to 4.7 may be accompanied by a decrease in dissolved organic carbon (Davis et al. 1985), and an increase in dissolved uncompleted aluminum, which is highly toxic to fish. In summary, we analyzed historical and recent data on pH and alkalinity from three large lake surveys in Wisconsin, New York, and New Hampshire. In all cases, some lakes decreased in alkalinity and pH since the 1930s and some increased. In some lakes the magnitude of the change in alkalinity appears to be too large to be explained solely by acid deposition. The evidence further indicates that on average Wisconsin lakes have increased in pH and alkalinity since about 1930, New Hampshire lakes show no obvious change in alkalinity but may have increased in pH, and New York lakes have shown decreases in alkalinity and pH in three of four possible combinations of assumptions, and show no change if one accepts the fourth assumption. Data on diatom-inferred pH for 11 lakes in the Adirondack Mountains (one of them was a bog lake and was disregarded) indicate that 6 of these lakes have become more acidic over the period from about 1930 to the 1970s. All these lakes have current pH values of about 5.2 or lower. The 4 lakes with current pH values above 5.2 show no trend in pH. For 9 of the lakes, concurrent data exist on measured pH, sediment chemistry, and fish populations. The data are generally consistent and support the findings based on diatom analysis. Data for New England indicate slight or no increase in diatom- inferred pH. There is insufficient information to evaluate trends in eastern Canada. The lakes selected in the diatom studies have low alkalinities (less than 200 peq/L) and little disturbance of their watersheds, and hence may be the most likely to show responses to acid deposition.

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42 The Record Since 1950 General Trends The decades after World War II are characterized by rapidly changing patterns of fuel consumption and fuel use in eastern North America. Before 1945 coal was the dominant source of fuel and consumption was divided among railroads, residential and commercial heating, oven coke and other industrial processes, and electric utilities. The demand for rail transport was particularly high during the war years of the early 1940s. By the end of the 1950s, however, coal consumption by railroads and by the residential-commercial sector essentially vanished. Overall, coal use declined by about 30 percent from 1945 to 1960. However, coal for electric power more than doubled over this same period, and doubled again for the period from 1960 to 1975. Concurrent with the expansion of coal use for electricity was the construction of new power plants with increasingly higher smokestacks, resulting in more than a doubling in average stack height from the mid-1950s to the mid-1970s (Sloane 1983). The seasonal pattern of consumption also changed during this period. In the early 1950s coal consumption in winter, the peak season, was about 27 percent higher than in summer, the season of lowest consumption. By the mid-1970s both winter and summer were peak periods of comparable consumption. Total coal consumption in the eastern United States is currently comparable to consumption during the peak years of the early 1940s, owing to increased consumption after the decline in the 1950s. However, there has been a con- siderable shift in the regional patterns of consumption over the past two decades; some areas currently consume far greater amounts of coal than they did in the early 1940s and some areas consume far less. (See discussion of regional trends below.) Coal provided about 50 percent of the energy needs of the United States in 1945. Currently, it provides about 20 percent as the total energy consumption has more than doubled over this period. The increasing demand for energy has been met largely by natural gas and oil. Natural gas contains little or no sulfur, and oil, after refinement, is of relatively low sulfur content. Thus, SO2 emissions have not increased in proportion to energy consumption over the past four decades. Nitrogen oxides, however, are formed as a by-product of any high-temperature combustion process in the atmosphere, regardless of the cleanness of the fuel. Consequently,

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43 nitrogen oxide emissions in eastern North America have more than doubled from the period 1945 to 1980, and have become an increasingly important component of acid deposition. The changes in fuel consumption, fuel use, and fuel type that have occurred over the past four decades have undoubtedly affected the geographic distribution and the composition of acid precursors in the atmosphere in subtle or more obvious ways. Environmental effects (e.g., lake acidification) have occurred in sensitive ecosystems over this same time period. However, if acid deposition is a cause, it may be difficult to determine whether the effect is a consequence of relatively recent changes in fuel use or consumption since 1945, or whether it is a result of cumulative exposure to acid deposition over many decades. Regional Trends Estimates of SO2 emissions in eastern North America suggest that the decade of the 1950s was a period of constant or declining emissions in all of the designated Regions A through E. In contrast, during the 1960s SO2 emissions rose sharply in all regions. The 1970s are characterized by strong regional differences in trends of SO2 emissions. There were differences not only in the magnitude of trends but also (for the first time) in their direction. In the northeastern states (Region B) the trend was distinctly downward. In the southeastern states (Region C) the trend was rapidly upward, continuing the trend that began in the 1960s. the midwestern states (Region D) the trend was upward, but not so rapidly as in Region C. Emissions in the north central states (Region E) remain consistently low. The divergence of trends in SO2 emissions along regional lines in the 1970s provides the opportunity of testing whether other types of data also reveal consistent regional differences. We analyzed two different types of data that have continuous records for a number of years and were collected at numerous sites in different regions. One is the record of light extinction (an inverse measure of visibility) at 35 airports from 1949 to 1983 (Chapter 4). The other is the record of sulfate in Bench-Mark streams from 1965 to 1983 (Chapter 7). We have also analyzed the record of pH, sulfate, nitrate, hydrogen ion, and other ions in bulk precipitation at the Hubbard Brook Experimental Forest, New Hampshire, for the period from 1963 to 1979 (Chapter 5), but we do not include

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44 these data in our regional analysis because they represent only one site in one region. However, we note that the observed trend of decreasing sulfate at Hubbard Brook is consistent with our observation of decreasing emissions of SO2 in Region B over this time period. The mechanisms outlined earlier suggest that SO2 emissions, atmospheric sulfate, light extinction, and stream sulfate are related and may exhibit similar temporal trends. We examined this suggestion on a regional basis by testing for associations among the regional trends of these variables by using Friedman's test. For the period l9SO to 1980, only data on SO2 emissions and light extinction are available; the regional rankings for these trends are given in Table 1.4. A ranking of 1 in Region B for the trends of both SC2 emissions and light extinction signifies that this region experienced the lowest rate of increase in each of these parameters over the period from 1950 to 1980. Higher rankings signify greater rates of increase. The p value for Friedman's test in this case is 0.042, signify- ing that if there were in fact no true association between the parameters, then an apparent association to this degree or greater would occur by chance only 4.2 percent of the time. The result is indicative of a strong temporal association between SO2 emissions and light extinction on a regional basis. We applied the same method to test for possible associations among trends in light extinction, sulfate fluxes from Bench-Mark streams, and emissions of SO2 for the period from 1965 to 1980. The regional rankings are shown in Table 1.5. The p value of 0.054 again gives evidence of temporal associations for these data on a regional level. TABLE 1.4 Regional Rankings by Rate of Change in SO2 Emission Densities and Light Extinction for the Period 1950-1980 Region SO2 Emissions Light Extinction - B C D E 4 2 4 2 NOTE: p value of Friedman's test, 0.042.

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45 TABLE 1.5 Regional Rankings by Rate of Change of SO2 Emission Densities, Light Extinction, and Stream Sulfate for the Period 1965-1980 Region SO2 Emissions Light Extinction Stream Sulfate B C D E 4 2 2 4 3 2 NOTE: p value of Friedman's test, 0.054. As demonstrated in Chapter 7, changes in the flux of sulfate in soft-water Bench-Mark streams were balanced by changes in alkalinity and base cations. The regional pattern of trends in stream alkalinity for the period 1965 to 1983 was the approximate inverse of that of stream sulfate (see Chapter 7, Figures 7.6 and 7.7) : decreases (or no increases) have occurred at several stations in Region C and Region F while increases (and no decreases) have occurred at stations in Region B. Station-by-station comparisons of alkalinity and sulfate trends, however, do not always show a consistent inverse relationship. Beginning in the 1960s, ring widths of red spruce at high elevations throughout its range in the eastern United States decreased significantly, a change that has persisted to the present. Important regional climatic anomalies occurred when the red spruce decline began and may have been a factor in triggering the response. There is currently no direct evidence linking acid deposition to mortality and decreases in ring width, although this effect has occurred in areas that are receiving relatively large amounts of acidic substances and other types of air pollutants. In summary, based on statistical tests of regional temporal trends, a strong association exists between SC2 emissions and light extinction (30-year records, 1950 to 1980). A similar result is obtained for SO2 emissions, visibility, and sulfate concentrations in Bench-Mark streams (15-year records, 1965 to 1980). Since 1950, the northeastern United States (Region B) nas experienced the smallest rates of change in these parameters. The southeastern United States (Region C) has, in general, experienced the greatest rates of change, and the Midwest (Region D) has experienced rates of change greater than Region B but less than Region C.

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47 National Research Council. 1983. Acid Deposition: Atmospheric Processes in Eastern North America. Washington, D.C.: National Academy Press. National Research Council. 1984. Acid Deposition: Processes of Lake Acidification. Washington, D.C.: National Academy Press. Pfeiffer, M. H., and P. J. Festal 1980. Acidity status of lakes in the Adirondack region of New York in relationship to fish sources. New York Department of Environmental Conservation, Albany, N.Y. FW-P168(10/80). 36 pp. Robinson, E. 1984. Natural emission sources. Pp. 2-1--2-52 in The Acidic Deposition Phenomenon and Its Effects. Critical Assessment Review Papers, Volume I, Atmospheric Sciences. A. P. Altshuller and R. A. Linthurst, eds. Environmental Protection Agency report no. EPA-600/8-83-016BF. Schofield, C. L. 1965. Water quality in relation to survival of brook trout, Salveilnus fontinalis (Mitchell). Trans. Am. Fish. Soc. 94:227-235. Seip, H. M. 1980. Acidification of freshwater--sources and mechanisms. Pp. 358-366 of Ecological Impacts of Acid Precipitation, D. Drablos and A. Tollan, eds. Proceedings of an international congress in Sandefjord, Norway. Sur Nedbors Virking Pa Skog Og Fisk (SNSF) Project, Oslo. Sloan, C. S. 1983. Seasonal acid precipitation and emission trends in the northeastern United States. General Motors Research Laboratories, Warren, Michigan. Research publication no. GMR-4456, ENV$16. Wagner, D. P., D. S. Fanning, J. E. Foss, M. S. Patterson, and P. A. Snow. 1982. Morphological and mineralogical features related to sulfide oxidation under natural and disturbed land surfaces in Maryland. Chapter 7 of Acid Sulfate Weathering. J. A. Kittrick, D. S. Fanning, and L. R. Hossner, eds. SSSA Special publication 10. Soil Society of America, Madison, Wis.