Acid Rain and Related Programs
2009 Environmental Results
The Acid Rain Program (ARP), established under Ti-
tle IV of the 1990 Clean Air Act (CAA) Amendments,
requires major emission reductions of sulfur diox-
ide (SC>2) and nitrogen oxides (NOX), the primary precur-
sors of acid rain, from the electric power industry. The SC>2
program sets a permanent cap on the total amount of SC>2
that may be emitted by electric generating units (EGUs) in
the contiguous United States. The program is phased in,
with the final 2010 S02 cap set at 8.95 million tons, a level
of about one-half of the emissions from the power sector in
1980. NOX reductions under the ARP are achieved through
a program that applies to a subset of coal-fired EGUs and is
closer to a traditional, rate-based regulatory system.
The emission reductions achieved under the ARP have led
to important environmental and public health benefits.
These include improvements in air quality with significant
benefits to human health; reductions in acid deposition;
the beginnings of recovery from acidification in fresh water
lakes and streams; improvements in visibility; and reduced
risk to forests, materials, and structures. Table 1 on the fol-
lowing page shows the regional changes in key air quality
and atmospheric deposition measurements linked to the
ARP's SC>2 and NOX emission reductions.
During 2010, EPA is releasing a series of reports summa-
rizing progress under the ARP. This third report compares
changes in emissions to changes in air quality, acid depo-
sition, and surface water chemistry. For more information
on the ARP, please visit .
Air Quality
Sulfur Dioxide
Data collected from monitoring networks show that the
decline in SC>2 emissions from the power industry has im-
proved air quality. Based on data from EPA's latest air emis-
sion trends report, the national composite average of SC>2
annual mean ambient concentrations decreased 76 per-
cent between 1980 and 2009, as shown in Figure 1 (based
on state, local, and EPA monitoring sites located primarily
United States
Environmental Protection
Agency
At a Glance: ARP Results in 2009
Air Quality: Between 1989-1991 and 2007-2009, aver-
age ambient sulfate concentrations have decreased by 44
percent in the Mid-Atlantic, 47 percent in the Midwest, 49
percent in the Northeast, and 41 percent in the Southeast.
Acid Deposition: Between the 1989-1991 and 2007-
2009 observation periods, regional decreases in wet
deposition of sulfate across the Eastern United States
averaged 43 percent.
Surface Water Chemistry: Levels of Acid Neutralizing
Capacity (ANC), the ability of a water body to neutralize
acid deposition, have increased significantly from 1990 to
2008 in lake and stream long-term monitoring sites in the
Adirondack Mountains and the Northern Appalachian Pla-
teau. These increasing ANC levels indicate trends toward
recovery from acidification.
in urban areas). The largest single-year reduction (20 per-
cent) occurred in the first year of the ARP, between 1994
and 1995. The second largest single-year reduction (16
percent) occurred most recently between 2008 and 2009.
These trends are consistent with the regional ambient air
quality trends observed in the Clean Air Status and Trends
Network (CASTNET).
Figure 1: National S02 Air Quality, 1980-2009
0.04
0.02
E
<
National Ambient Air
Quality Standard
90% of sites have
concentrations below this line
Average Concentration
10% of sites have
concentrations below this line
Source: EPA, 2010
October 2010
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Acid Rain and Related Programs: 2009 Environmental Results
Table 1: Regional Changes in Air Quality and Deposition of Sulfur and Nitrogen Compounds, 1989-1991 versus 2007-2009, from
Rural Monitoring Networks
Measurement Region Average, 1989-1 991 Average, 2007-2009 PercentChange Number of Sites
Ambient S02 Concentra-
tion (ug/m3)
Ambient Sulfate Concen-
tration (ug/m3)
Wet Sulfate Deposition
(kg-S/ha)
Dry Sulfur Deposition
(kg-S/ha)
Total Sulfur Deposition
(kg-S/ha)
Total Ambient Nitrate
Concentration (Nitrate +
Nitric Acid) (ug/m3)
Wet Inorganic Nitrogen
Deposition (kg-N/ha)
Dry Inorganic Nitrogen
Deposition (kg-N/ha)
Total Inorganic Nitrogen
Deposition (kg-N/ha)
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
13
11
5.5
5.1
6.3
5.8
3.5
5.4
9.2
7.1
7.5
6.1
6.7
6.5
2.9
1.2
16
15
9.8
8
3.3
4.6
1.8
2.2
6.2
5.8
5.6
4.4
2.5
2.5
1.4
0.9
8.7
9
6.5
5.9
5
4.1
1.7
2.2
3.5
3.1
1.8
3.2
5.3
4
4.3
3.5
2.9
2.8
1
0.7
8
7
4.7
4.6
2
3.2
1
1.5
4.5
4.9
4.1
3.4
1.5
1.8
0.6
0.8
6
6.9
4.4
4.8
-62
-63
-69
-57
-44
-47
-49
-41
-42
-44
-43
-43
-57
-57
-66
-42
-50
-53
-52
-43
-39
-30
-44
-32
-27
-16
-27
-23
-40
-28
-57
-11
-31
-23
-32
-19
12
10
3
8
12
10
3
8
11
27
17
23
11
10
3
2
11
10
3
2
12
10
3
8
11
27
17
23
11
10
3
2
11
10
3
2
Notes:
Averages are the arithmetic mean of all sites in a region that were present and met the completeness criteria in both averaging periods. Thus,
average concentrations for 1989-1991 may differ from past reports.
Total deposition is estimated from raw measurement data, not rounded, and may not equal the sum of dry and wet deposition.
Percent change and values in bold indicates that differences were statistically significant at the 95 percent confidence level. Changes that are
not statistically significant may be unduly influenced by measurements at only a few locations or large variability in measurements.
Source: EPA, 2010
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Acid Rain and Related Programs: 2009 Environmental Results
Figure 2: Annual Mean Ambient S02 Concentration
1989-1991
2007-2009
Figure 3: Annual Mean Ambient Sulfate Concentration
1989-1991
2007-2009
Notes:
For maps depicting these trends for the entire continental United States, visit .
Dots on all maps represent monitoring sites. Lack of shading for southern Florida indicates lack of monitoring coverage in the 1989-1991
period.
Source: CASTNET, 2010
Dramatic regional improvements in SC>2 and ambient sul-
fate concentrations were observed following implementa-
tion of Phase I of the ARP during the late 1990s at CAST-
NET sites throughout the eastern United States, and these
improvements continue today. Analyses of regional moni-
toring data from CASTNET show the geographic pattern
of SC>2 and airborne sulfate in the eastern United States.
Three-year mean annual concentrations of SC>2 and sulfate
from CASTNET long-term monitoring sites are compared
from 1989 to 1991 and 2007 to 2009 in both tabular form
and graphically in maps (see Table 1 and Figures 2 and 3).
The maps in Figure 2 show that the average annual ambi-
ent concentrations of SC>2 from 1989 to 1991 were highest
in western Pennsylvania and along the Ohio River Valley.
The maps indicate a significant decline in those concentra-
tions in nearly all affected areas after implementation of
the ARP and other programs.
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Acid Rain and Related Programs: 2009 Environmental Results
Figure 4: Annual Mean Ambient Total Nitrate Concentration
1989-1991
Total NO3
(|jg/m3)
2007-2009
Notes:
For maps depicting these trends for the entire continental United
States, visit .
Dots on all maps represent monitoring sites. Lack of shading for
southern Florida indicates lack of monitoring coverage in the
1989-1991 period.
Source: CASTNET, 2010
Like SC>2 concentrations, the highest average annual am-
bient sulfate concentrations from 1989 to 1991 were ob-
served in western Pennsylvania and along the Ohio River
Valley. Most of the eastern United States experienced an-
nual ambient sulfate concentrations greater than 5 micro-
grams per cubic meter (ug/m3).
Ambient sulfate concentrations have also decreased since
the program was implemented, with average concentra-
tions decreasing from 41 to 49 percent in regions of the
East (see Table 1). Both the magnitude and spatial extent
of the highest concentrations have dramatically declined,
with the largest decreases observed along the Ohio River
Valley (see Figure 3).
Nitrogen Oxides
Although the ARP has met its NOX emission reduction tar-
gets, emissions from other sources (such as motor vehicles
and agriculture) contribute to ambient nitrate concentra-
tions in many areas. Ambient nitrate levels can also be af-
fected by emissions transported via air currents over wide
regions.
Total NO3
(|jg/m3)
-0.0
From 2007 to 2009, reductions in NOX emissions during the
ozone season from power plants under the NOX SIP Call and
Clean Air Interstate Rule (CAIR) have continued to result in
significant region-specific improvements in ambient total ni-
trate (NOs- plus HNOs) concentrations. For instance, annual
mean ambient total nitrate concentrations for 2007 to 2009 in
the Mid-Atlantic region were 39 percent less than the annual
mean concentration in 1989 to 1991 (see Table 1 and Figure 4).
While these improvements might be partly attributed to added
NOX controls installed for compliance with the NOX SIP Call and
CAIR, the findings at this time are not conclusive.
Acid Deposition
National Atmospheric Deposition Program/National Deposi-
tion Trends Network (NADP/NTN) monitoring data show sig-
nificant improvements in the primary acid deposition indica-
tors. For example, wet sulfate deposition (sulfate that falls to
the earth through rain, snow, and fog) has decreased since the
implementation of the ARP in much of the Ohio River Valley
and northeastern United States. Some of the greatest reduc-
tions have occurred in the mid-Appalachian region, including
Maryland, New York, West Virginia, Virginia, and most of Penn-
sylvania. Other less dramatic reductions have been observed
across much of New England, portions of the southern Appa-
lachian Mountains, and some areas of the Midwest. Between
the 1989 to 1991 and 2007 to 2009 observation periods, aver-
age decreases in wet deposition of sulfate averaged more than
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Acid Rain and Related Programs: 2009 Environmental Results
Figure 5: Annual Mean Wet Sulfate Deposition
1989-1991
Source: NADP, 2010
Figure 6: Annual Mean Wet Inorganic Nitrogen Deposition
1989-1991
Source: NADP, 2010
43 percent for the eastern United States (see Table 1 and
Figure 5). Along with wet sulfate deposition, wet sulfate
concentrations have also decreased by similar percentag-
es. A strong correlation between large-scale SC>2 emission
reductions and large reductions in sulfate concentrations
in precipitation has been noted in the Northeast, one of the
areas most affected by acid deposition. The reduction in
total sulfur deposition (wet plus dry) has been even more
dramatic than that of wet deposition in the Mid-Atlantic
and Midwest, with reductions of 50 and 53 percent, respec-
tively (see Table 1). Because continuous data records are
available from only a few sites in the Northeast and South-
east, it is unclear if the observed reductions in total deposi-
tion are representative for those regions.
A principal reason for reduced sulfate deposition in the
Northeast is a reduction in the long-range transport of sul-
fate from emission sources located in the Ohio River Val-
ley. The reductions in sulfate documented in the Northeast,
2007-2009
2007-2009
particularly across New England and portions of New York,
were also affected by SC>2 emission reductions in eastern
Canada. NADP data indicate that similar reductions in pre-
cipitation acidity, expressed as hydrogen ion (H+) concen-
trations, occurred concurrently with sulfate reductions,
with reductions of 30 to 40 percent over much of the East.
Reductions in nitrogen deposition recorded since the early
1990s have been less pronounced than those for sulfur. As
noted earlier, emission trends from source categories oth-
er than ARP sources significantly affect air concentrations
and deposition of nitrogen. Inorganic nitrogen in wet depo-
sition decreased commensurately in the Mid-Atlantic and
Northeast (see Figure 6). Decreases in dry and total inor-
ganic nitrogen deposition at CASTNET sites have generally
been greater than that of wet deposition, with a 31 and 23
percent decrease in total nitrogen deposition for the Mid-
Atlantic and Midwest, respectively (see Table 1).
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Acid Rain and Related Programs: 2009 Environmental Results
About Long-term Ambient and Deposition Monitoring Networks
To evaluate the impact of emission reductions on the en-
vironment, scientists and policymakers use data collect-
ed from long-term national monitoring networks such
as CASTNET and the NADP/NTN. These complementary,
long-term monitoring networks provide information on
a variety of indicators necessary for tracking temporal
and spatial trends in regional air quality and acid depo-
sition (see Table 2).
CASTNET provides atmospheric data on the dry depo-
sition component of total acid deposition, ground-level
ozone, and other forms of atmospheric pollution. Es-
tablished in 1987, CASTNET now consists of more than
80 sites across the United States. EPA's Office of Air and
Radiation operates 59 of the monitoring stations; the
National Park Service (NFS) funds and operates ap-
proximately 25 stations in cooperation with EPA. Many
CASTNET sites have a continuous 20-year data record,
reflecting EPA's commitment to long-term environmen-
tal monitoring.
NADP/NTN is a nationwide, long-term network track-
ing the chemistry of precipitation. NADP/NTN provides
Table 2: Air Quality and Acid Deposition Measures
concentration and wet deposition data on hydrogen
ion (acidity as pH), sulfate, nitrate, ammonium, chlo-
ride, and base cations. The network is a cooperative
effort involving many groups, including the State Ag-
ricultural Experiment Stations, U.S. Geological Survey
(USGS), U.S. Department of Agriculture (USDA), EPA,
NFS, the National Oceanic and Atmospheric Adminis-
tration (NOAA), and other governmental and private
entities. NADP/NTN has grown from 22 stations at
the end of 1978 to more than 250 sites spanning the
continental United States, Alaska, Puerto Rico, and
the Virgin Islands. Information and data from NADP/
NTN are available at the NADP's website.
NADP is running a pilot study to determine the fea-
sibility of operating a long-term passive ammo-
nia (NHs) monitoring network. The pilot network
(AMoN) has been measuring 2-week samples of am-
bient NHs for over two years at more than 20 sites. It
will be the first nationwide network to measure NHs
routinely. More information on AMoN can be found
on the NADP website.
Chemical Measured in:
Chemical Name Symbol Ambient Air Wet Deposition Why are these measured by the networks?
Sulfur Dioxide
Sulfate Ion
Nitrate Ion
Nitric Acid
Ammonium Ion
Ionic Hydrogen
Calcium
Magnesium
Potassium
Sodium
S02
S042-
N03-
HN03
NH4+
H+
Ca2+
Mg2+
K+
Na+
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Primary precursor of wet and dry deposition; primary precursor of fine particles (PlVts).
Major contributor to wet acid deposition; major component of fine particles in the
Midwest and East; can be transported over large distances; formed from reaction of SQi
in the atmosphere.
Contributor to acid and nitrogen wet deposition; major component of fine particles in
urban areas; formed from reaction of NOx in the atmosphere.
Strong acid and major component of dry nitrogen deposition; formed as a secondary
product from NOX in the atmosphere.
Contributor to wet and dry nitrogen deposition; major component of fine particles;
provides neutralizing role for acidic compounds; formed from ammonia gas in the
atmosphere.
Indicator of acidity in precipitation; formed from the reaction of sulfate and nitrate in
water.
These base cations neutralize acidic compounds in precipitation and the environment;
also play a major role in plant nutrition and soil productivity.
Source: EPA, 2009
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Acid Rain and Related Programs: 2009 Environmental Results
Ambient Mercury Monitoring
In addition to SC>2 and NOX, coal-fired power plants release
mercury into the atmosphere where it can be transported
and deposited locally, regionally, and globally. NADP recent-
ly launched the Atmospheric Mercury Network (AMNet)
for monitoring three atmospheric mercury species: gas-
eous elemental mercury (GEM), gaseous oxidized mercury
(COM), and particulate-bound mercury (PBM2.s). Data sets
generated from this network are used to estimate mer-
cury dry deposition, assess mercury source/receptor re-
lationships, evaluate atmospheric models, and determine
long-term trends. Currently, 20 sites in North America (as
shown in the map in Figure 7) participate in the network,
generating high-resolution, high-quality speciated atmo-
spheric data. In 2010, the University of California - Santa
Cruz was the most recent partner to join AMNet, establish-
ing a new network site in Elkhorn Slough, California.
Figure 7: Ambient Mercury Monitoring Locations
The AMNet Database
The AMNet database has received extensive quality assur-
ance (QA)/quality control (QC) review by a team of data
quality experts from EPA, USGS, NADP, and other institu-
tions. Quality control flags to determine whether datum
records are valid or invalid were developed based on peer-
reviewed criteria. The AMNet QA/QC review of data is a
robust, tiered approach, including: automatic screening
of each raw datum and quality control flagging; additional
rigorous automatic screening, accounting for calibration
data and other checks (e.g., extensive 24-hour trap bias,
etc.) and quality control flagging; and NADP site liaison
manual review of data, where automatically screened data
are reviewed against corresponding monthly site opera-
tor field report notes (e.g., glassware changes, site shelter
power failure, etc.).
Atmospheric Mercury Sites
AMNet Sites
Interested Speciated Sites
Other Atm. Mercury Sites
CAMNET
Ik (Canadian Atmospheric
Mercury Network)
Source: NADP/AMNet, 2010
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Acid Rain and Related Programs: 2009 Environmental Results
At present, more than 56 site years of data have been qual-
ity assured. NADP now offers mercury speciation data
products available through the AMNet website (nadp.sws.
uiuc.edu/amn/), including bi-hourly graphical plots and
data tables for each site.
Beltsville, Maryland Case Study
Beltsville, MD is home to NADP/AMNet (site ID: MD99),
CASTNET (site ID: BEL116), NADP/Mercury Deposition
Network (site ID: MD99), and other monitoring sites where
mercury, meteorology data, and other ancillary data are re-
corded. The data plot shown below represents an impor-
tant example of the value of maintaining complementary,
collocated monitoring network sites. In this example, col-
located AMNet bi-hourly gaseous oxidized mercury (COM)
and CASTNet hourly SC>2 values (Figure 8) from Beltsville
are plotted over the same time period. COM is measured
in picograms per cubic meter (pg/m3). SC>2 is a trace gas
that is often used as a signature of coal-fired power plants.
The plot shows both COM and SC>2 following a very similar
pattern over the same sample time period, which warrants
further analysis to determine if there is a relationship be-
tween the ambient data and emissions. The measurements
collected at AMNet can also be used with source-receptor
modeling and other tools to provide useful information on
source attributions and dry/total deposition estimates for
sensitive ecosystems.
Figure 8: Bi-Hourly Gaseous Oxidized Mercury Concentrations
and Hourly CASTNET S02 Gas Concentrations at the AMNet Site
ID MD99/CASTNET Site ID BEL116
200
n .-J K^v ^ t W^*-i f-* \i MBhtfoJ: n
U ~^^ ii| n ir^ iiT i I \ ^ ir^^ ^^^^^^^^^T~ U
Sep22 Sep23 Sep 24 Sep 25 Sep 26 Sep 27 Sep 28 Sep 29
Source: NADP/AMNet, 2010
Improvements in Surface Water Chemistry
Acid rain resulting from SC>2 and NOX emissions is one of
many large-scale anthropogenic impacts that negatively af-
fect the health of lakes and streams in the United States.
Surface water chemistry provides direct indicators of the
potential effects of acidic deposition on the overall health
of aquatic ecosystems. Long-term surface water monitor-
ing networks provide information on the chemistry of
lakes and streams and on how water bodies are respond-
ing to changes in emissions. Since the implementation of
the ARP, scientists have measured changes in some lakes
and streams in the eastern United States and found signs
of recovery in many, but not all, of those areas (see Figures
9-12).
Two EPA-administered monitoring programs provide in-
formation on the effects of acid rain on aquatic systems:
the Temporally Integrated Monitoring of Ecosystems
(TIME) program and the Long-Term Monitoring (LTM) pro-
gram. These programs were designed to track the effect of
the 1990 CAA Amendments in reducing the acidity of sur-
face waters in four regions: New England, the Adirondack
Mountains, the Northern Appalachian Plateau, and the
Central Appalachians (the Ridge and Valley and Blue Ridge
Provinces). The surface water chemistry trend data in the
four regions monitored by the TIME and LTM programs are
essential for tracking the ecological response to ARP emis-
sion reductions (see Figure 9).
The data presented here show regional trends in acidifi-
cation from 1990 to 2008 in lakes and streams sampled
through the LTM program (see Figures 10-12). Only sites
that have a complete data record for the time period are
represented. Three indicators of acidity in surface waters
Figure 9: Long-Term Monitoring Program Sites
LTM Sites by Region
New England
o Adirondack Mountains
« Northern Appalachian Plateau
Central Appalachians
(Valley and Ridge and
Blue Ridge Provinces)
Source: EPA, 2010
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Acid Rain and Related Programs: 2009 Environmental Results
Figure 10: Trends in Lake and Stream Water Chemistry at LTM
Sites, 1990-2008 Sulfate Ion Concentration (ueq/L/yr)
Figure 11: Trends in Lake and Stream Water Chemistry at LTM
Sites, 1990-2008 Nitrate Ion Concentration (ueq/L/yr)
1990-2008 Sulfate Ion Concentration
Increasing significant trend
Increasing non-significant trend
O Decreasing non-significant trend
Decreasing significant trend
Source: EPA, 2010
are presented: measured ions of sulfate and nitrate and
acid neutralizing capacity (ANC). These indicators provide
information regarding both sensitivity to surface water
acidification and the level of acidification that has occurred
today and in the past. Trends in these chemical receptors
allow for the determination of whether the conditions of
the water bodies are improving and heading towards re-
covery or if the conditions are degrading. Significant trends
are statistically significant at the 95% confidence interval
(p<0.05). Measurements of sulfate ion concentrations in
surface waters provide important information on the ex-
tent of cation leaching in soils and how sulfate concentra-
tions relate to deposition and to the levels of ambient at-
mospheric sulfur.
Assessments of acidic deposition effects dating from the
1980s to the present have shown sulfate to be the primary
negatively charged ion in most acid-sensitive waters.1 Ni-
trate has the same potential as sulfate to acidify drainage
waters and leach acidic aluminum cations from watershed
soils. In most watersheds, however, nitrogen is a limiting
nutrient for plant growth, and therefore most nitrogen
inputs from deposition are quickly incorporated into bio-
mass as organic nitrogen with little leaching of nitrate into
surface waters.
ANC is an important measure of the sensitivity and the de-
gree of surface water acidification or recovery that occurs
over time. Acidification results in the diminishing ability of
water in the lake or stream to neutralize strong acids that
enter aquatic ecosystems. Water bodies with ANC values
defined as less than or equal to 0 microequivalents2 per li-
1990-2008 Nitrate Ion
Concentration
Increasing significant trend
Increasing non-significant trend
o No change
O Decreasing non-significant trend
« Decreasing significant trend
Source: EPA, 2010
Figure 12: Trends in Lake and Stream Water Chemistry at LTM
Sites, 1990-2008 ANC Levels (ueq/L/yr)
1990-2008 Acid Neutralizing
Capacity (ANC)
O Increasing significant trend
O Increasing non-significant trend
O Decreasing non-significant trend
Decreasing significant trend
Source: EPA, 2010
ter (ueq/L) are of acute concern for acidification. Lakes and
streams having springtime ANC values less than 50 ueq/L
are generally considered of elevated concern for acidifica-
tion. Lakes and streams with ANC higher than 50 ueq/L
are generally considered of moderate to low concern for
acidification. When ANC is low, and especially when it is
negative, stream water pH is also low (less than 6), and
there may be adverse impacts on fish and other animals es-
sential for a healthy aquatic ecosystem. Movement toward
recovery of an aquatic ecosystem is indicated by increasing
trends in ANC and decreasing trends in sulfate and nitrate.
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Acid Rain and Related Programs: 2009 Environmental Results
Table 3: Regional Trends in Sulfate, Nitrate, ANC, and DOC at Long-term Monitoring Sites, 1990-2008
% of Sites with % of Sites with % of Sites with % of Sites with
Improving Sulfate Improving Improving ANC Improving DOC
Region Waterbodies Covered Trend Nitrate Trend Trend Trend
Adirondack Mountains
Catskills/ N.Appalachian Plateau
New England
Central Appalachians
50 lakes in NY
9 streams in NY and PA
26 lakes in ME and VT
66 streams in VA
90%
78%
96%
12%
32%
33%
31%
45%
58%
56%
12%
12%
42% (26 sites)
29% (7 sites)
20% (10 sites)
NA
Notes:
Trends are determined by multivariate Mann-Kendall tests.
DOC was only examined in low-ANC waterbodies (ANC less than
25 neq/L).
DOC is not currently measured in Central Appalachian streams.
Source: EPA, 2010
Dissolved organic carbon (DOC), essentially organic mate-
rial, is derived from many sources, some of which include:
atmospheric deposition, decaying leaf litter, soil organic
matter, aquatic sediments, and aquatic organisms. DOC is
an important part of the acid-base chemistry of most low-
alkalinity freshwater systems. A host of factors control
DOC in surface water including the inputs from acidifying
deposition, discharge, temperature, and nutrient enrich-
ment. Recently, scientists have suggested that increased
concentrations of DOC are likely due to declining sulfate
content from atmospheric deposition, increasing seasonal
temperatures, or a combination of both.3'4 With increasing
loading of acid deposition, soils release lower quantities
of organic acids, thereby causing DOC to decrease in sur-
face water. This means that as surface waters recover from
acidic deposition, DOC concentrations are returning to-
ward pre-industrial levels. On the other hand, as tempera-
tures warm, more organic acids in the soil break down and
are released to surface waters and increase DOC concen-
trations. These increasing DOC concentrations could be a
possible sign of climate change. Another mechanism that
could cause increases in DOC is a soil microbial response
to nitrogen deposition that results in greater export of hu-
mic material to surface waters.5
DOC is an important water chemistry parameter and may
be affected by acidification. Table 3 presents the aggregate
sulfate, nitrate, ANC, and DOC trends represented by the
LTM sites shown in Figures 10-12 for four acid sensitive
regions of the eastern United States, as well as DOC trends
for low ANC (ANC<25 ueq/L) waterbodies.
Acidification of Soils
Soils are also affected by acidic deposition. As acidic
deposition enters the soil, it can cause base cations,
such as calcium (Ca2+), magnesium (Mg+), or potas-
sium (K+), to exchange with H+ and A13+ ions. This
causes base cations to be lost from the soil through
transport to surface water, which may lead to de-
clines in soil pH. As acidic deposition continues, more
base cations exchange, causing a further decline in
base cation adsorption to the soil surface (i.e., per-
cent base saturation). This is important because base
cations are important plant nutrients and their loss is
a potential threat to forest productivity and health.10
Additionally, the immobilization of A13+ by acidic
deposition can also be toxic to plants. While surface
waters have experienced some chemical recovery
(Table 3), soils are still likely acidifying.11 In 2009,
researchers at Syracuse University published results
from a 2001 soil survey for soils in 139 watersheds
across the northeastern United States. Many of these
watersheds had previously been sampled as part of
the EPA's Direct/Delayed Response Project in 1984.
This comparative study showed that over the 17-yr
interval, median base saturation in the Oa-horizon
(organic surface soil layer) exhibited a statistically
significant decrease from 56% in 1984 to 33% in
2001. Soil pH also decreased from 3.05 to 2.95 pH
units over the same time period. These results are
consistent with other research, showing continued
soil acidification for this region.12 Soil acidification
is likely to continue until acidic deposition inputs to
soils decline to the point where soil base cation pools
are sufficient to neutralize them.
10
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Acid Rain and Related Programs: 2009 Environmental Results
The maps and summary results indicate that:
Sulfate concentrations are declining at most6 sites in
the Northeast (New England, Adirondacks, Catskills/
Northern Appalachian Plateau). However, in the Central
Appalachians, sulfate concentrations in some streams
(21%) are increasing. This region has highly weathered
soils that can store large amounts of deposited sulfate.
As long-term sulfate deposition exhausts the soil's ability
to store sulfate, a decreasing proportion of the deposited
sulfate is retained in the soil and an increasing propor-
tion is exported to surface waters. Thus sulfate concen-
trations in surface waters, mainly streams in this region,
are increasing despite reduced sulfate deposition.
Nitrate concentrations are decreasing in some of the sites
in all four regions, but several lakes and streams indicate
flat or slightly increasing nitrate trends. This trend does
not appear to reflect changes in emissions or deposition
in these areas and is likely a result of ecosystem factors.
In 2008, 45% of the Central Appalachian streams had a
decreasing trend in nitrate, compared to 24% in 2007.
This increase in the number of sites with a decreasing
nitrate trend may be due to continued recovery follow-
ing gypsy moth defoliation in the early 1990s. Gypsy
moth defoliation has been shown to increase nitrate ex-
port from affected forests to surface waters by as much
as 50 times.7 While defoliation from gypsy moths may
only occur over several months, impacts on nitrate trans-
port and in-stream concentrations may be seen for many
years.8
ANC, as measured in surface waters, is increasing in
many of the sites in the Adirondack and Catskills/North-
ern Appalachian Plateau regions, which in part can be at-
tributed to declining sulfate deposition. The site trends
also indicate variation within each region. Only 12% of
sites in New England and the Central Appalachians have
improving ANC trends, but overall, only seven sites in all
regions have a significant downward trend in ANC.
DOC is increasing in only about 20% to 42% of the low
ANC lakes and streams of the Adirondack Mountains,
Catskills/Northern Appalachian Plateau, and New Eng-
land. The Adirondack Mountains have the highest per-
centage (42%) of lakes with an increasing DOC trend.
These results suggest that the change of DOC in the LTM
catchments is complex, with the majority of low ANC
waterbodies not changing over the past 18 years. Of the
lakes and streams with increasing DOC, no single envi-
ronmental factor is likely for the cause of the increase.
Declines in sulfate deposition (Figure 5) and warmer
seasonal and annual temperatures may have contributed
to the rise in surface DOC.9
The ANC of northeastern U.S. lakes monitored under the
TIME program was also evaluated for the 1991-1994 and
2006-2008 periods to assess the impacts of ARP imple-
mentation. The analysis in Figure 13 compares average
ANC levels for the northeastern lakes that had data in each
time period. From 1991 to 1994, 7.5 percent of lakes had
three-year mean ANC levels below 0 ueq/L. These lakes are
categorized as "acute concern," in which a near complete
loss of fish populations is expected, and planktonic com-
munities have low diversity and are dominated by acid-
tolerant forms (see Table 4). The percentage of lakes in
this category dropped to 4.3 percent in 2006 to 2008 (see
Figure 13). Additionally, the percentage of elevated con-
cern lakes dropped from 13.8 percent for the 1991-1994
time period to 10.1 percent from 2006-2008, while the
percentage of lakes with in the moderate concern category
increased from 6.4 to 13.5 percent. These results point to a
decrease in acidity, particularly for the subset of lakes with
low ANC.
Figure 13: Northeastern Lakes by ANC Status Category,
1991-1994 versus 2006-2008
70-
60'
50-1
'o
a 40.
_ro
| 30-
OJ
Q_
20-
10-
1991-1994
I 2006-2008
Acute Concern Elevated Concern Moderate Concern Low Concern
(<0^eq/L) (0-50 Meq/L) (50-100 jjeq/L) (>100 ^eq/L)
Notes:
Based on 305 EMAP/TIME monitoring sites.
See Table 4 for descriptions of level of concern categories.
It is important to note that the wide range of ANC values within
these categories makes it likely that substantial improvements
in ANC may occur without changing the categorization of a given
lake.
Source: EPA, 2010
11
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Acid Rain and Related Programs: 2009 Environmental Results
Table 4: Aquatic Ecosystem Status Categories for Northeastern Lakes
Category Label 1 ANC Level 1 Expected Ecological Effects
Acute Concern
Elevated Concern
Moderate Concern
Low Concern
< 0 micro equivalent per Liter (ueq/L)
0-50 ueq/L
50-100 ueq/L
> 100 ueq/L
Near complete loss offish populations is expected. Planktonic communities have extremely low diversity and are dominated
by acidophilic forms. The numbers of individuals in plankton species that are present are greatly reduced.
Fish species richness is greatly reduced (more than half of expected species are missing). On average, brook trout populations
experience sub-lethal effects, including loss of health and reproduction (fitness). During episodes of high acid deposition,
brook trout populations may experience lethal effects. Diversity and distribution of zooplankton communities declines.
Fish species richness begins to decline (sensitive species are lost from lakes). Brook trout populations are sensitive and vari-
able, with possible sub-lethal effects. Diversity and distribution of zooplankton communities begin to decline as species that
are sensitive to acid deposition are affected.
Fish species richness may be unaffected. Reproducing brook trout populations are expected where habitat is suitable.
Zooplankton communities are unaffected and exhibit expected diversity and distribution.
Source: EPA, 2010
Critical Loads and Exceedances
The Northeast and Mid-Appalachian Highlands in the east-
ern United States has been strongly affected by acidic depo-
sition. Many of the small forested watersheds, particularly
along the Appalachian Mountain spine, have soils and sur-
face waters that are unable to buffer the acidity from acidic
deposition, causing the ecosystem to acidify. As a result the
health of some tree species and aquatic biota have declined
or species, such as brook trout, are no longer present in the
ecosystem.
Since the early 1980s, acidic deposition has acidified many
lakes and many miles of streams in the Northeast and Mid-
Appalachian Highlands.13 However, with the implementa-
tion of Title IV and other emission reduction programs,
acidic deposition has declined throughout the eastern
United States as emissions of NOX and SC>2 have declined
(see Acid Rain and Related Programs: 2009 Emission, Com-
pliance, and Market Analyses report available at < www.
epa.gov/airmarkets/progress/ARP09_2.html>). Surface
waters across the region have also shown signs of recov-
ery as indicated by declining sulfate concentrations and
increasing ANC levels (see Table 3). In the 2007 and 2008
Acid Rain Progress Reports, critical loads were used to
gauge the extent to which the acid sensitive areas of the
Adirondack Mountains and Central Appalachian Moun-
tains have recovered from acid deposition. Results from
these regions indicate that 10-15% of monitored lakes and
streams now receive levels of acid deposition that are gen-
erally low enough that aquatic ecosystems are protected in
comparison to deposition levels in the early 1990s.
The critical load approach is an assessment tool that can
be used to determine the degree to which air pollution
may be affecting ecological health. A critical load is a quan-
titative estimate of exposure to one or more pollutants
below which significant harmful effects on specific sensi-
tive elements of the environment do not occur according
to present knowledge.14 This approach provides a useful
lens through which to assess the results of current poli-
cies and programs and to evaluate the potential value of
proposed policy options in terms of ecosystem protection.
The critical loads approach has been employed routinely
as an assessment tool for many years in the countries of
the European Union and Canada. Building on past criti-
cal load studies, this analysis explores the extent to which
lakes in the Northeast and streams in the Mid-Appalachian
Highlands in Virginia and West Virginia are protected from
acidifying nitrogen and sulfur deposition as a result of re-
cent emission reductions.
The critical load for a lake or stream provides a benchmark
against which to assess the extent to which a waterbody is
potentially at risk due to current acidic deposition levels.
The analysis focuses on the combined load of sulfur and
nitrogen deposition below which the ANC level would still
support healthy aquatic ecosystems. If pollutant exposure
is less than the critical load, adverse ecological effects
(e.g., reduced reproductive success, stunted growth, loss
of biological diversity) are not anticipated, and recovery is
expected over time if an ecosystem has been damaged by
past exposure. A critical load exceedance is the measure
of pollutant exposure above the critical load. This means
pollutant exposure is higher than, or "exceeds," the critical
load and the ecosystem continues to be exposed to damag-
ing levels of pollutants.
The scientific research community has recently completed
and published many peer-reviewed scientific articles that
advance the tools for calculating critical loads in the United
States. Drawing on the methods from the peer-reviewed
scientific literature,15 critical loads were calculated for
over 1,300 lakes and streams using the Steady-State Wa-
12
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Acid Rain and Related Programs: 2009 Environmental Results
Figure 14: Lake and Stream Exceedences of Estimated Critical
Loads (Sulfur + Nitrogen) for Total Nitrogen and Sulfur Deposi-
tion for the Period 1989-1991
Critical Load Exceedances,
1989-1991
Does Not Exceed Critical Load
O Within 10% of the Critical Load
Exceeds Critical Load
..
Source: EPA, 2010
ter Chemistry (SSWC) model and the Model of Acidifica-
tion of Groundwater In Catchments (MAGIC) model. These
critical load estimates represent only lakes and streams
where surface water samples have been collected and do
not represent all types of lakes and streams in the study
region. Water quality chemistry include data collected by
EPA-administered surface water monitoring and assess-
ment programs, such as the National Surface Water Survey
(NSWS), Environmental Monitoring and Assessment Pro-
gram (EMAP), the TIME program, and the LTM program.
The lakes and streams associated with these programs
consist of a subset of lakes and streams that are located in
areas most affected by acid deposition and many sites pro-
vide long term records of surface acidification. The NSWS
and EMAP programs employ probability sampling; each
monitoring site was chosen statistically from a predefined
target population. In New England and the Mid-Appala-
chian Highlands, the target populations include lakes and
streams likely to be responsive to changes in acidic deposi-
tion.
Figure 15: Lake and Stream Exceedences of Estimated Critical
Loads (Sulfur + Nitrogen) for Total Nitrogen and Sulfur Deposi-
tion for the Period 2007-2009
Critical Load Exceedances,
2007-2009
O Does Not Exceed Critical Load
O Within 10% of the Critical Load
Exceeds Critical Load
Source: EPA, 2010
For this particular analysis, the critical load represents the
combined deposition loads of sulfur and nitrogen to which
a lake or stream could be subjected and still have a calcu-
lated ANC of 50 ueq/L or higher. While a critical load can be
calculated for any ANC level, this level was chosen because
it tends to protect most fish and other aquatic organisms,
although systems can become episodically acidic and some
sensitive species maybe lost. Critical loads of combined to-
tal sulfur and nitrogen deposition are expressed in terms of
ionic charge balance as milliequivalents per square meter
per year (meq/m2/yr). When actual measured deposition
of nitrogen and sulfur is greater than the critical load, the
critical load is "exceeded," meaning that combined sulfur
and nitrogen deposition was greater than a lake or stream
could sustain and still maintain the calculated ANC level of
50 ueq/L or above. In order to assess the extent to which
regional lake and stream ecosystems are protected by the
emission reductions achieved by Title IV, this case study
compares the amount of deposition systems can receive
the critical loadto measured deposition for the period
before implementation of the ARP (1989 to 1991) and for
a recent period after ARP implementation (2007 to 2009).
13
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Acid Rain and Related Programs: 2009 Environmental Results
Overall, this critical load analysis shows that emission re-
ductions achieved by the ARP have resulted in improved
environmental conditions and increased ecosystem pro-
tection in the Northeast and Mid-Appalachian Highlands.
For the period from 2007 to 2009, 26% of the waterbodies
examined received levels of combined sulfur and nitrogen
deposition that exceeded the critical load (Figure 15). This
is an improvement when compared to the 1989-1991 pe-
riod before implementation of Title IV, during which 42%
of waterbodies exceeded the critical load (Figure 14). Thus,
during the 2007 to 2009 period, 37% of those waterbod-
ies exceeding their critical load in the previous period
were no longer receiving sulfur and nitrogen deposition
loads that threaten the health of these ecosystems. Areas
with the largest concentration of lakes where acid depo-
sition currently is greater than or exceeds estimated
critical loads include the Adirondack mountain region in
New York, southern New Hampshire and Vermont, north-
ern Massachusetts, northeast Pennsylvania, and the cen-
tral Appalachian Mountains of Virginia and West Virginia
(Figure 15).
Reductions in acidic deposition have occurred over the
past decade, as demonstrated by the deposition maps in
Figures 5 and 6 on page 5. However, this comparison of
past and current total deposition estimates with critical
loads estimates indicates that acid-sensitive ecosystems in
the Eastern United States are still at risk of acidification at
current deposition levels. As a result, additional reductions
in acidic deposition from current levels might be necessary
to fully protect these ecosystems.
Ammonium Deposition in the Eastern United States
Ammonium (NH4+) forms when ammonia gas (NHs) reacts
with sulfur, nitrogen and other acidic compounds in the
atmosphere. Ammonium is of concern because it contrib-
utes to the formation of fine particles, which have negative
human health effects and can cause reduced visibility. Wet
deposition of NH4+ is measured at NTN sites across the US
while dry deposition of NH4+ is estimated from weekly fil-
ter pack concentrations measured at CASTNET sites and
estimated deposition velocities. While total nitrate deposi-
tion fluxes have decreased 24% in the eastern US, wet de-
position has only decreased 4% since 1990. The 36 CAST-
NET sites used for this analysis are shown in Figure 16.
Figure 17 shows the wet and dry NH4+ deposition flux from
36 CASTNET sites between 1990 and 2009. The three year
average for wet and dry NH4+ flux between 1990-1992 is
2.34 and 0.42 kg-N/ha, respectively. The most recent three
year average, 2007-2009, the wet NH4+ flux is 2.24 and the
dry flux is 0.31 kg-N/ha. While there is a small decrease
in dry deposition (0.11 kg-N/ha), the 3 year average total
nitrogen deposition decreased from 7.62 to 5.80 kg-N/ha
(1.82 kg-N/ha), a larger reduction likely attributed to the
large reductions we have seen in particulate nitrate (NOy
) deposition fluxes. The NH4+ ion is the basic component
of PM2.5 formation, causing negative health effects and re-
duced visibility. In addition, NH4+ leads to eutrophication
of ecosystems. While NOX emissions under the Acid Rain
Program, the NOX Budget Trading Program, and CAIR have
resulted in significant improvements in air quality, gaseous
emissions have been increasing. This will provide a
Figure 16: Eastern U.S. CASTNET Monitoring Locations Analyzed
for Ammonium Deposition
Source: EPA, 2010
Figure 17: Measured Wet and Estimated Dry Deposition of
Ammonium at Eastern CASTNET sites, 1990-2009
3.0
2.5
2.0
1.5
j?
c
g
1 1.0
CL
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Acid Rain and Related Programs: 2009 Environmental Results
challenge in the future for understanding changes in atmo-
spheric chemistry and nitrogen deposition to forests, lakes,
streams and other sensitive areas. Therefore, it is impor-
tant to continue monitoring long-term trends in air qual-
ity and deposition to further understand pollutants which
contribute to poor air quality.
Online Information, Data, and Resources
The availability and transparency of data, from emission
measurement to allowance trading to deposition monitor-
ing, is a cornerstone of effective cap and trade programs.
CAMD, in the Office of Air and Radiation's Office of Atmo-
spheric Programs, develops and manages programs for col-
lecting these data and assessing the effectiveness of cap and
trade programs, including the Acid Rain Program. CAMD
then makes these data available to the public in readily us-
able and interactive formats. The CAMD website provides
a public resource for general information on how market-
Figure 18: US S02 Emissions and Sulfate Concentrations, 1990
based programs work and what they have accomplished,
along with the processes, information, and tools necessary
to participate in any of these market-based programs.
To increase data transparency, EPA has created supple-
mentary maps that allow the user to display air market
program data geospatially on an interactive 3D platform.
Figures 18 and 19 are examples of these maps. The maps
come in the form of a KMZ file (a compressed KML file) that
is downloaded directly to the user's computer. Data can be
explored in new and meaningful ways by turning differ-
ent layers on and off, overlaying data points and satellite
imagery, and using navigation tools to change the view of
the Earth's surface. KMZ/KML files are supported by pro-
grams such as Google Earth, ESRI Arc Explorer, and NASA
WorldWind View. These interactive mapping applications
provide a unique way to identify environmental trends and
track the progress of various EPA programs, such as the
ARP. For more information or to utilize this tool, visit the
website at .
Note: This example depicts 1990 S02 emissions from ARP sources along with 1990
sulfate concentration data as measured by the CASTNET monitoring program.
Source: EPA 2010
15
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Acid Rain and Related Programs: 2009 Environmental Results
In another effort to increase data transparency, EPA reg-
ularly posts updates of quarterly SC>2 and NOX emissions
data from coal-fired power plants controlled under the
ARP and other programs to make it easy for the public to
track changes in emissions from these sources (available
at: ).
The data presented on the quarterly emissions tracking
website compare emissions, emission rates, and heat input
from power plant units in the ARP. These files graphically
and spatially compare quarterly emission data from the
most recent completed quarter of 2010 with data for the
same quarter from 2008.
Interactive motion charts are a key feature on the quarterly
tracking website. Figure 20 on page 17 shows examples of
motion charts created to show changes in ARP SC>2 emis-
sions and SC>2 emission rates over time (from 1990 to
Figure 19: US S02 Emissions and Sulfate Concentrations, 2009
2009). These motion charts show, historically, how coal-
fired power plants have responded to the ARP. Each circle
on the motion chart represents a facility in the ARP with
one or more units that burn coal to create electricity. The
size and color of these circles tell us something about the
facility. To the right of the motion chart you'll find two leg-
ends. The color spectrum at the top represents the emis-
sions generated per unit of fuel (also known as the SC>2
emission rate), with warmer colors (yellow through red)
representing a high emission rate and cooler colors (green
through blue) representing a low emission rate. The size
of the circle on the chart is proportional to the emissions
from that plant. On the interactive mapping website, the
user can watch this data move through time by clicking the
play button.
For more information or to utilize these and other tools,
visit the website at < www.epa.gov/airmarkets/>.
Note: This example depicts 2009 S02 emissions from ARP sources along with 2009
sulfate concentration data as measured by the CASTNET monitoring program.
Source: EPA, 2010
16
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Acid Rain and Related Programs: 2009 Environmental Results
Figure 20: Motion Charts of Annual ARP Coal-Fired Emissions,
S02 Emission Rates and Heat Input over Time, 1990 and 2009
Source: EPA, 2010
Notes
1 Driscoll, C.T., Lawrence, G., Bulger, A., Butler, T, Cronan, C, Eagar, C,
Lambert, K.F., Likens, G.E., Stoddard, ]., and Weathers, K. 2001. Acid
deposition in the Northeastern U.S.: Sources and Inputs, Ecosystem
Effects, and Management Strategies. Bioscience, 51:180-198.
2 An equivalent is a measure of a substance's ability to combine with
other substances. The equivalent is formally defined as the amount
of a substance, in moles, that will react with one mole of electrons. A
microequivalent is one millionth of an equivalent.
3 Monteith, D.T., Stoddard, J.L., Evans, C.D., Heleen, A., Forsius, M.,
H0gasen, Wilander, A., Skjelkvale, B. L., Jeffries, D. S., Vuorenmaa, ].,
Keller, B., Kopacek, J. and Vesely, J. 2007. Dissolved organic carbon
trends resulting from changes in atmospheric deposition chemistry.
Nature, 450: 537-541.
4 Evans, C.D., Monteith, D.T., and Cooper, D.M. 2005. Long-term in-
creases in surface water dissolved organic carbon: Observations,
possible causes and environmental impacts. Environmental Pollu-
tion, 137: 55-71
5 Findlay, S.E.G. 2005. Increased carbon transport in the Hudson
River: unexpected consequence of nitrogen deposition? Frontiers in
Ecology and the Environment. 3: 133137.
6 "Most", "many", "some", "few" improving indicate greater than 75%,
from 50-75%, from 25-50%, and less than 25% of lakes or streams
with a statistically significant trend at the 95% confidence level re-
spectively.
7 Townsend, PA, Eshleman, K.N., and Welcker, C. 2004. Remote sens-
ing of gypsy moth defoliation to assess variations in stream nitrogen
concentrations. Ecological Applications, 14(2): 504-516.
8 Scanlon, T.M., Ingram, S.M., and Riscassi, A.L. 2010. Terrestrial and
in-stream influences on the spatial variability of nitrtae in a forested
headwater catchment. Journal of Geophysical Research, 115, G02022.
8 Monteith, D.T., Stoddard, J.L., Evans, C.D., Heleen, A., Forsius, M.,
H0gasen, Wilander, A., Skjelkvale, B. L., Jeffries, D. S., Vuorenmaa, J.,
Keller, B., Kopacek, J. and Vesely, J. 2007. Dissolved organic carbon
trends resulting from changes in atmospheric deposition chemistry.
Nature, 450: 537-541.
9 Stager, J. C., McNulty S., Beier, C., and Chiarenzelli, J. 2009. Histori-
cal patterns and effects of changes in Adirondack climates since the
early 20th century . Adirondack Journal of Environmental Studies,
15(2): 14-24.
10 Schaberg, P.G., J.W Tilley, G.J. Hawley D.H. DeHayes, and S.W Bailey.
2006. Associations of calcium and aluminum with the growth and
health of sugar maple trees in Vermont. For. Ecol. Manage. 233:159-
169.
11 Warby, R. A. F, Johnson, C. E., & Driscoll, C. T. (2009). Continuing
acidification of organic soils across the northeastern USA: 1984-
2001. Soil Science Society of America Journal, 73,273-284.
12 Bailey, S.W, S.B. Horsley, and R.P. Long. 2005. Thirty years of
change in forest soils of the Allegheny Plateau, Pennsylvania. Soil Sci.
Soc. Am. J. 69:681-690.
12 SanClements, M.D.m I. J. Fernandez, S.A. Norton. 2010. Soil chemi-
cal and physical properties at the Bear Brook Watershed in Maine,
USA. Environ Monit Assess DOI 10.1007/sl0661-010-1531-3
13 Stoddard, J. L; etal. (2003) Response of Surface Water Chemistry
to the Clean Air Act Amendments of 1990; EPA/620/R-03/001; U.S.
EPA: Washington, DC.
14 Nilsson, J. & Grennfelt, P. (Eds) (1988) Critical loads for sulphur
and nitrogen. UNECE/Nordic Council workshop report, Skokloster,
Sweden. March 1988. Nordic Council of Ministers: Copenhagen.
15 EPA, 2008 and DuPont J., T.A. Clair, C. Gagnon, D.S. Jeffries, J.S. Kahl,
and S.J. Nelson, and J. M. Peckenham. (2005). Environmental Moni-
toring and Assessment 109: 275-291
15 Sullivan T.J., B.J. Cosby, J.R. Webb, R.L. Dennis, A.J. Bulger, and FA.
Deviney, Jr. 2007. Streamwater acid-base chemistry and critical loads
of atmospheric sulfur deposition in Shenandoah National Park, Vir-
ginia. Environmental Monitoring and Assessment 137:85-99
17
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