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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.
OCR for page 40
40
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OCR for page 42
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,
OCR for page 43
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.
OCR for page 45
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.
OCR for page 46
46
REFERENCES
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National Research Council. 1983. Acid Deposition:
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Representative terms from entire chapter:
surface waters
