xvEPA
United States
Environmental Protection
Agency
Air And Radiation
(6204J)
EPA430-R-95-001
February 1995
Acid Deposition Standard
Feasibility Study
Report To Congress
Draft For Public Comment
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ACKNOWLEDGMENTS
The development of this Acid Deposition Standard
Feasibility Study involved the cooperative efforts
of individuals within the U.S. Environmental
Protection Agency (EPA), especially the Office of
Air and Radiation's Acid Rain Division and the
Office of Research and Development's Effects
Research Laboratory in Corvallis, Oregon, and
Atmospheric Research Exposure and Assessment
Laboratory in Research Triangle Park, North Caro-
lina. Many individuals at EPA, particularly in the
Office of Air Quality Planning and Standards,
contributed useful comments to this report. EPA
also acknowledges The Cadmus Group, Inc., for
their assistance in preparing this report under
Contract Number 68-D2-0168.
in
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TABLE OF CONTENTS
Acknowledgments Hi
List of Exhibits vii
List of Acronyms and Abbreviations xi
Executive Summary xiii
1 Introduction 1
What acid deposition levels are necessary
to protect sensitive regions? 2
What degree of protection is provided by
Title IV? What is the residual risk? What
additional emissions limitations would be
required to protect sensitive regions? 3
How would an acid deposition standard(s)
be implemented? What are the different
implementation approaches? What are the
feasibility and effectiveness relative to
other approaches?
2 Environmental Goals .
.4
.7
2.1 Introduction 7
2.2 Basic Relationships in Surface Water
Acidification and Recovery 8
2.2.1 Relationship of Base Cations,
Sulfur, and Nitrogen in Envi-
ronmental Acidification 10
2.2.2 Episodic Acidification 12
2.2.3 Cumulative Loading Effects 13
2.2.4 Recovery of Acidified Ecosys-
tems 14
2.3 Characterizing Resources at Risk
from Acidic Deposition 15
2.3.1 Defining Sensitive Resources 15
2.3.2 Identifying Resources at Risk 17
2.4 Identification of Resource and Re-
gional Priorities 18
2.4.1 United States 18
2.4.2 Identification of Sensitive
Aquatic Resources in Other
North American Regions 21
2.4.3 Canada 24
2.5 Assessing Protection Needs and Re-
source Responses in the Control of
Acidic Deposition 25
2.5.1 Model Application 25
2.5.2 Direct/Delayed Response Pro-
ject 25
2.5.3 Nitrogen Bounding Study 31
2.5.4 Overview of International and
State Acidic Deposition Crite-
ria and Standards 42
2.5.5 Spatial and Temporal Issues in
Development of a Standard 46
2.6 Controlling Sulfur and Nitrogen to
Reduce Surface Water Acidification 47
3 Source-Receptor Relationships and Depo-
sition Reductions under alternative Emis-
sions Scenarios 49
3.1 Introduction 49
3.2 The Regional Acid Deposition Model
(RADM) 50
3.2.1 Emissions and Atmospheric
Chemistry 53
3.2.2 Modeling Source-Receptor Re-
lationships and Source Attribu-
tion 55
3.2.3 Transport, Chemistry, and
Source-Receptor Relationships 57
3.2.4 Confidence in Results 59
3.3 Source Attribution 59
3.3.1 Changes from 1985 to 2010 59
3.3.2 Regional Emissions Distribu-
tion in 2010 60
3.4 Emissions Reductions Scenarios 62
3.5 Deposition Reductions under Various
National Emissions Reductions Sce-
narios 66
3.5.1 Impact of SO2 Allowance
Trading on Sulfur Deposition 66
3.5.2 Effect of Additional SO2 Emis-
sions Reductions on Sulfur
Deposition 68
3.5.3 Decrease in Total Nitrogen
Deposition from Decreases in
NOX Emissions 69
3.6 Emissions Reductions Strategies to
Achieve Geographically Targeted
Sulfur Deposition Loads 72
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
4 Potential Benefits of an Acid Deposition
Standard on Visibility, Human Health, Ma-
terial, and Cultural Resources 79
4.1 Introduction 79
4.2 Relationship of Visibility to Acidic
Deposition 79
4.2.1 Visibility Impairment 79
4.2.2 Visibility Protection Laws and
Class I Areas 80
4.2.3 Visibility Metrics and the Pro-
jected Impact of 1990CAAA
on Visibility 80
4.2,4 Potential Impact of an Acid
Deposition Standard on Visi-
bility 83
4.3 Relationship of Human Health to
Acidic Deposition 83
4.4 Relationship of Materials Damage
and Cultural Resources to Acidic
Deposition 85
4.4.1 Acidic Deposition Effects on
Materials and Structures 85
4.4.2 Material Life-Cycle and Dam-
age Estimates 86
5 Implementation Issues 89
5.1 Introduction 89
5.2 Targeted Approach 89
5.2.1 Description of Targeted Ap-
proach 89
5.2.2 Integration with Title IV 90
5.2.3 Impediments to Implementa-
tion 90
5.3 Emissions-Based Approach 91
5.3.1 Description of Emissions-
Based Approach 91
5.3.2 Integration with Title IV 91
5.3.3 Impediments to Implementa-
tion 91
5.4 Economic Impacts 91
5.4.1 2010 CAAA Scenario (with
Trading) 92
5.4.2 50 Percent Utility SO2 Reduc-
tion Scenario 92
5.4.3 50 Percent Utility and Indus-
trial SO2 Reduction Scenario 95
5.4.4 Geographically Targeted Re-
duction Scenario 95
5.4.5 NOX Reductions-50 Percent
Utility and Industrial 95
5.4.6 Summary of Economic Im-
pacts 97
5.6 Conclusions 98
6 Integration and Conclusions 99
6.1 Introduction 99
6.2 Establishing Effective Environmental
Coals 99
6.3 Projected Environmental Conse-
quences of Acidic Deposition Reduc-
tion Scenarios 101
6.4 Feasibility of Establishing and Imple-
menting an Acid Deposition Standard 104
Appendices
A Summary of Selected NAPAP Reports A-1
B Selected Plots from EPA's Nitrogen
Bounding Study B-1
C Range of Influence of Emissions from
RADM Tagged Subregions C-1
D Summary of Public Comments Re-
ceived D-1
VI
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LIST OF EXHIBITS
1. Empirically determined relationship
between ANC and pH for three sensi-
tive regions, including cross-regional
mean and standard deviation
2. Principal watershed and surface water
characteristics that influence resource
sensitivity to acidification 17
3. Critical pH for selected taxa in lakes
and streams 21
4. MAGIC 26
5. Study regions included in the Di-
rect/Delayed Response Project and the
Nitrogen Bounding Study 27
6. Target populations included in the
NSWS, DDRP, and NBS Studies 29
7. Impact of CAAA on sensitive surface
waters: NBS model projections for Year
2040 32
8. NBS model projections for year 2040
percentage of target population Adi-
rondack lakes with ANC^O u,eq/l
.33
9. NBS model projections for year 2040
percentage of target population
mid-Appalachian streams with
50 u.eq/1
.34
10. NBS model projections for year 2040
percentage of target population South-
ern Blue Ridge streams with ANC550
u.eq/1 35
11. Interpreting NBS plots 36
12. Time to watershed nitrogen saturation 37
13. Summary of NBS results: Range of
minimum (background deposition) to
maximum (implementation of CAAA)
percentages of acidic and sensitive tar-
get waters 38
14. Surface water responsiveness to reduc-
tions in deposition beyond the CAAA:
Detectible improvements in long-term
ANC by 2040 39
15. LRTAP 44
16. Physical and chemical processes con-
tributing to acidic deposition 51
17. The RADM modeling domain 52
18. Map of annual sulfur emissions density
in 1985 (tons/year) 55
19. Tagged RADM subregions 56
20. Proportion of annual sulfur deposition
contributed by RADM Subregion 15
(OH/WV/PA border region) 57
21. Percentage cumulative range of influ-
ence of RADM Subregion 15
(OH/WV/PA border region) 57
22a. Source-receptor relationships in the
Northeast: Cumulative percent sulfur
deposition 58
22b. Source-receptor relationships in the
lower Ohio Valley: Cumulative percent
sulfur deposition 58
22c. Source-receptor relationships in the
Southeast: Cumulative percent sulfur
deposition 58
23. Percent contribution to sulfur emis-
sions of 53 tagged RADM regions 60
24. Percent reduction in tagged regions
from 1985 to 2010 as a function of
relative contribution of each region to
all tagged emissions 60
25. Percentage of tagged emissions by
tagged regions for 1985 and 2010 61
26. Contribution of top-10 SO2 emitting
regions to sulfur deposition in sensitive
regions 61
27. Comparison of proximate and major
emitting regions to sulfur deposition in
sensitive areas in 2010 62
28. Estimated U.S. SO2 emissions with and
without Title IV from 1980 to 2015 63
29. Predicted SO2 utility emissions from
1990 to 2010 65
30. SO2 emissions in the U.S. RADM do-
main (eastern United States) 66
31. Annual average RADM total sulfur
deposition (kg-S/ha): 1980 66
32. Annual average RADM-predicted total
sulfur deposition (kg-S^a): post-2010
full CAAA implementation 67
33. Percentage reductions in sulfur deposi-
tion from CAAA implementation 67
VII
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
34. Impact of trading on sulfur deposition
in sensitive regions
35. difference in Annual average RADM
total sulfur deposition (kg-S/ha) in 2010
between post-2010 full implementa-
tion and no trading scenarios ...
36. RADM-predicted annual average total
sulfur deposition (kg-S/ha) in 2010 un-
der additional utility SO2 emissions re-
duction scenario
37. RADM-predicted annual average total
sulfur deposition (kg-S/ha) in 2010 un-
der additional utility and industrial SO2
emissions reduction scenario
38. Percentage reductions in sulfur deposi-
tion from post-2010 full implementa-
tion - under additional utility SO2 re-
duction scenario
39. Percentage reductions in sulfur deposi-
tion from post-2010 full implementa-
tion under additional utility and indus-
trial SO2 reduction scenario
40. Sulfur deposition to sensitive regions
under various emissions scenarios
41. Percent reductions in sulfur deposition
to sensitive regions from 1980 levels
under various emissions scenarios
42. Estimated U.S. NOX emissions with and
without Title IV from 1980 to 2015.
43. Percent contribution of utility sources
to nitrogen deposition in 1990
44. Percent contribution of industrial
sources to nitrogen deposition in 1990..
45. Percent contribution of mobile sources
to nitrogen deposition in 1990
46. Annual average RADM total nitrogen
deposition (kg-N/ha) in 1990
47. RADM-predicted annual average total
nitrogen deposition (kg-N/ha) under
utility and industrial NOX emissions
reductions scenario
48. Percentage reductions in nitrogen
deposition under utility and industrial
NOX emissions reductions scenario.
.67
.68
.70
.70
.70
.70
.71
.71
.71
.72
.72
,72
,73
,73
.73
4y. Nitrogen deposition to sensitive re-
gions under base case and additional
utility and industrial NOX emissions
reduction scenario
50. Selection of maintenance loads.
,73
,75
51 a. Geographically targeted additional util-
ity SO2 reduction in contiguous RADM
subregions
.76
51 b. Geographically targeted additional util-
ity SO2 reduction in major RADM
subregions contributing to deposition
(not contiguous)
.76
52. Map of extent of contiguous geo-
graphic regions for achieving targeted
deposition loads equivalent to addi-
tional nationwide utility SO2 reduc-
tions 77
53a. Geographically targeted additional util-
ity and industrial SO2 reduction in con-
tiguous RADM subregions 78
53b. Geographically targeted additional util-
ity and industrial SO2 reduction in ma-
jor RADM subregions contributing to
deposition (not contiguous)
.78
54. Extent of contiguous geographic re-
gions for achieving targeted deposition
loads equivalent to additional nation-
wide utility and industrial SO2 reduc-
tions
55. Geographically targeted reductions
with a maintenance load of 5 kg-S/ha
in major RADM subregions contribut-
ing to deposition (not contiguous)
56. Anthropogenic Contributions to Visibil-
ity Impairment
57. Annual average visual range (km) pro-
jected for 2010 without Title IV:
50th-percentile visibility
58. Annual average visual range (km) pro-
jected for 2010 with Title IV, including
trading: 50th-percentile visibility
59. Annual average improvement in 50th-
percentile visibility (dv) from 1980 to
2010 with Title IV, including trading
60. Specific provisions of Titles I, II, and IV..
61. Average annual visual range estimates
for representative Class I areas in the
Southwest ,
62. Percent increase in visual range from
1985 to 2010 with full CAAA imple-
mentation
63. Percent increase in visual range from
1985 to 2010 with additional SO2 re-
duction beyond CAAA
64. Relationship of acidic deposition proc-
esses to health effects
.78
.78
.79
.81
.81
.81
.82
.83
.83
.84
.84
VIII
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LIST OF EXHIBITS
65. Percentage of metal corrosion attrib-
uted to atmospheric factors 86
66. 2010 annual costs and SO2 emissions
by Census region: CAAA scenario 93
67. 2010 annual costs and SO2 emissions
by Census region: CAAA scenario ver-
sus additional 50 percent utility emis-
sions reduction scenario 94
68. 2010 annual costs and SO2 emissions
by Census region: CAAA scenario ver-
sus additional 50 percent utility and
industrial emissions reduction scenario 96
69a. Annual costs of geographically tar-
geted reductions equivalent to nation-
wide 50% utility SO2 reduction
(contiguous RADM subregions)
69b. Annual costs of geographically tar-
geted reductions equivalent to nation-
wide 50% utility SC>2 reduction: Major
RADM subregions contributing to
deposition (not contiguous)
.97
70. Summary of costs of various emissions
reductions scenarios
.98
71. Year 2040 NBS projections for Adiron-
dack lakes 102
72. Year 2040 NBS projections for mid-
Appalachian streams 103
73. Year 2040 NBS projections for South-
ern Blue Ridge Province streams 103
.97
IX
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LIST OF ACRONYMS
ADIR Adirondack Mountain region
ADOM Acid Deposition and Oxidants Model
ALim inorganic monomeric aluminum
ANC acid neutralizing capacity
AQRV Air Quality Related Values
BACT best available control technology
BEA Bureau of Economic Affairs
Btu British thermal unit
Ca2+ calcium
CAA Clean Air Act
CAAA Clean Air Act Amendments
CCE Coordination Center for Effects
CCT Clean Coal Technology
CEUM Coal and Electric Utilities Model
Cl" chlorine
cm centimeter
CO carbon monoxide
DDRP Direct Delayed Response Project
DOE Department of Energy
DOI Department of the Interior
dv deciview
EMEFS Eulerian Model Evaluation Field Study
EPA U.S. Environmental Protection Agency
FIP Federal Implementation Plan
H+ hydrogen ion
H2O2 hydrogen peroxide
ha hectare
H2SO4 sulfuric acid
HCHO formaldehyde
HNO3 nitric acid
IMPROVE Interagency Monitoring of Protected
Visual Environments
kg kilogram
km kilometer
LNB low-NOx burner
LRTAP Long-Range Transboundary Air Pollution
LTM Long-Term Monitoring
m meter
M-APP mid-Appalachian region
Mg2+
mi
MM4
N
NAAQS
NADB
NAPAP
NAS
NBS
NH3
NH4+
NO
NO2
NO3-
NOX
NRC
NSPS
NSS
NSWS
NURF
NYSDEC
03
PAN
PM10
ppm parts
PSD
RADM
RIA
S
SBRP
SCR
SIP
SNCR
S02
scv-
magnesium
mile
Mesoscale Model
nitrogen
National Ambient Air Quality Standard
National Allowance Data Base
National Acid Precipitation Assessment
Program
National Academy of Sciences
Nitrogen Bounding Study
ammonia
ammonium ion
nitric oxide
nitrogen dioxide
nitrate
nitrogen oxide
National Research Council
New Source Performance Standard
National Stream Survey
National Surface Water Survey
National Unit Reference File
New York State Department of
Environmental Conservation
ozone
peroxyacetyl nitrate
paniculate matter smaller than 10
micrometers
phosphate
per million
prevention of significant deterioration
Regional Acid Deposition Model
Regulatory Impact Analysis
sulfur
Southern Blue Ridge Province region
selective catalytic reduction
State Implementation Plan
selective non-catalytic reduction
sulfur dioxide
sulfate
XI
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
SOMA Sulfur Oxide Management Area ueq/l microequivalents per liter
SOS/T state-of-science/technology ug/l micrograms per liter
SOX sulfur oxide H™ micrometer
UNECE United Nations Economic Commission 4DDA four-dimensional data assimilation
for Europe
yr year
XII
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EXECUTIVE SUMMARY
Scientific evidence has shown that atmospheric
deposition of sulfur and nitrogen compounds can
adversely affect ecosystems. Observed effects in-
clude acidification of surface waters and damage
to high-elevation red spruce forests in the United
States. Title IV of the Clean Air Act Amendments
of 1990 (CAAA or the Act) addresses the problem
of adverse effects on ecosystems from acidic rain
by mandating reductions in emissions of sulfur and
nitrogen oxides the major precursors of acidic
deposition. Coupled with Titles I and II of the Act
which address new and existing stationary and
mobile sources of sulfur and nitrogen oxides, im-
plementation of Title IV is expected to provide
significant benefits to the United States and Can-
ada, including decreases in the acidity of lakes and
streams, concomitant improvements in fish popu-
lation diversity and health, decreases in soil deg-
radation and forest stress, improvements in visibil-
ity (especially to scenic vistas), decreases in dam-
age to materials and cultural resources, and a re-
duction in adverse human health effects. Congress
included Section 404 in Title IV (Appendix B of
the Act) which requires the Environmental Protec-
tion Agency (EPA or the Agency) to provide a re-
port to Congress on the feasibility and effective-
ness of an acid deposition standard to protect sen-
sitive and critically sensitive aquatic and terrestrial
resources. Specifically, Congress listed six areas to
be addressed in the report:
» Identification of sensitive and critically sen-
sitive aquatic and terrestrial resources in
the U.S. and Canada which may be af-
fected by the deposition of acidic com-
pounds;
» Description and specification of a numeric
value for an acid deposition standard suffi-
cient to protect such resources;
» Description of the use of such standard or
standards in other Nations or by any of the
several States in acidic deposition control
programs;
* Description of measures that would be
needed to integrate such standard or stan-
dards with the control program required by
Title IV of the Clean Air Act;
* Description of the state of knowledge with
respect to source-receptor relationships
necessary to develop a control program on
such standard or standards and additional
research that is on-going or would be
needed to make such a control program
feasible;
* Description of impediments to implemen-
tation of such control program and the
cost-effectiveness of deposition standards
compared to other control strategies in-
cluding ambient air quality standards, new
source performance standards and the re-
quirements of Title IV of the Clean Air Act.
This report fulfills the requirement of Section 404
by integrating state-of-the-art ecological effects re-
search, emissions and source-receptor modeling
work, and evaluation of implementation and cost
issues to address the six areas and other issues re-
lated to the feasibility of establishing and imple-
menting an acid deposition standard or standards.
RESOURCES MOST AT RISK FROM ACIDIC
DEPOSITION
The natural resources most at risk from acidic
deposition are aquatic systems and high-elevation
red spruce forests in the eastern United States and
Canada. Although many surface waters in western
North America are equally or more sensitive than
aquatic systems in the East, deposition levels in the
West are sufficiently low that the risk of chronic
acidification to these resources is low at present
and is expected to remain low in the foreseeable
future. Research conducted under the auspices of
the National Acid Precipitation Assessment Pro-
gram (NAPAP) concluded that regions in the
United States most at risk from continued acidic
deposition are located along the Appalachian
Mountain chain stretching from the Adirondacks in
New York to the Southern Blue Ridge in Georgia.
Target populations of Adirondack lakes, mid-Appa-
lachian streams, and Southern Blue Ridge streams,
for which model projections can be reasonably ex-
trapolated, were selected for detailed analysis in
this study because they represent areas that receive
fairly high levels of acidic deposition, have the
best historical data, and have been studied exten-
sively by scientists.
XIII
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
CAAA PROVIDES CLEAR BENEFITS TO
SURFACE WATERS
Modeling analysis indicates that sulfur
deposition reductions mandated by Title
IV of the Act will have clear benefits to
sensitive surface waters. Exhibits HI I
show the percent of target lakes or
streams in each sensitive region pro-
jected to be chronically acidic (acid
neutralizing capacity [ANC]<0 u.eq/1) by
2040 with and without implementation
of the Act. The scenarios are described
according to the extent and rate of ni-
trogen impact on that given watershed.
(See footnote below.)* In each modeled
region, the proportion of targeted acidic
and sensitive surface waters would have
been higher, in some cases significantly,
without the sulfur dioxide (SO2) re-
ductions and nitrogen oxide controls in
the CAAA.
FURTHER REDUCTIONS MAY BE
NECESSARY FOR FULL PROTECTION
Scientific analysis indicates that nitrogen
as well as sulfur deposition plays an im-
portant role in chronic and episodic
acidification of surface waters and full
protection may not be realized without
additional reductions in nitrogen as well
as sulfur deposition. Model projections
indicate that maintaining the proportion
of chronically acidic target surface wa-
ters in the Adirondacks near proportions
observed in 1984 may require reducing
anthropogenic sulfur and nitrogen depo-
sition by 40 to 50 percent or more
below levels achieved by the CAAA
(assuming 100 years to nitrogen
saturation). In the mid-Appalachians,
implementation of the CAAA should
maintain 1985 proportions of
chronically acidic target streams if the time to ni-
trogen saturation is 250 years or longer; more
EXHIBIT I. PERCENT OF TARGETED ADIRONDACK LAKES PROJECTED
TO BE CHRONICALLY ACIDIC (ANCSO U.EQ/L) IN 2040
PERCENT OF TARGET POPULATION ADIRONDACK
LAKES OBSERVED ACIDIC IN 1984 = 19%
TARGET POPULATION = 700 LAKES
Nitrogen Saturation
Model Projections
Never
250 years
100 years
50 years
Percent of Targeted Waters Pro-
jected to be Acidic in 2040
Without CAAA
25%
23%
36%
50%
With CAAA
11%
15%
26%
43%
EXHIBIT II. PERCENT OF TARGETED MID-APPALACHIAN STREAMS PRO-
JECTED TO BE CHRONICALLY ACIDIC (ANC<0 JIEQ/L) IN 2040
PERCENT OF TARGET POPULATION MID-APPALACHIAN
STREAMS OBSERVED ACIDIC IN 1985 = 4%
TARGET POPULATION = 4,300 STREAMS
Nitrogen Saturation
Model Projections
Never
250 years
1 00 years
50 years
Percent of Targeted Waters Pro-
jected to be Acidic in 2040
Without CAAA
8%
21%
23%
33%
With CAAA
0%
4%
5%
9%
EXHIBIT III. PERCENT OF TARGETED SOUTHERN BLUE RIDGE PROVINCE
STREAMS PROJECTED TO BE CHRONICALLY ACIDIC (ANC<0 U.EQ/L) IN 2040
PERCENT OF TARGET POPULATION SOUTHERN BLUE RIDGE PROV-
INCE STREAMS OBSERVED ACIDIC IN 1985 = 0%
TARGET POPULATION = 1,300 STREAMS
Nitrogen Saturation
Model Projections
Never
250 years
1 00 years
50 years
Percent of Targeted Waters Pro-
jected to be Acidic in 2040
Without CAAA
0%
1%
2%
13%
With CAAA
0%
0%
0%
4%
Nitrogen saturation is a measure of the capacity of
biological processes in a watershed to incorporate
nitrogen into organic matter. As this capacity is used
up, nitrogen losses from watersheds increase, princi-
pally in the form of nitrate leaching. The time to ni-
trogen saturation can vary among regions due to
differences in temperature, moisture, length of
growing season, soil fertility, forest age, and historic
nitrogen deposition. Currently, uncertainty regarding
times to nitrogen watershed saturation in each sensi-
tive region is significant.
rapid nitrogen saturation (in the range of 100
years) may require reductions in anthropogenic
sulfur and nitrogen deposition by 25 percent be-
low levels achieved by the CAAA. With imple-
mentation of the CAAA, no chronically acidic
streams are expected within the Southern Blue
Ridge target population. Another useful measure is
the sensitivity of a lake or stream to becoming
acidic (i.e., ANC<50 u.eq/1). Use of this measure
(as described in Chapter 2) also indicated that
further deposition reductions may be necessary for
full protection of target sensitive surface waters.
XIV
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EXECUTIVE SUMMARY
ACIDIC DEPOSITION AND EPISODIC ACIDI-
FICATION
Episodic acidification occurs when pulses of acidic
waters enter lakes and streams during stormwater
runoff and spring snowmelt. Both sulfates and ni-
trates originating from atmospheric deposition
contribute significantly to episodic acidification
events. Rapid, acutely toxic changes in surface
water chemistry often occur at the most biologi-
cally significant time of year (i.e., during spawning
and reproduction). Significantly more lakes and
streams become episodically acidic than are
chronically acidic. Recent analyses have shown
that for the worst episode that may occur during
any year, the number of lakes or streams that were
acidic during that episode in the Adirondacks is
approximately 3.5 times the number found to be
chronically acidic. Thus, for the Adirondacks, ap-
proximately 70 percent of the target population
lakes are at risk of episodic acidification at least
once during each year. For the mid-Appalachian
streams, approximately 30 percent of the target
population stream reaches are likely to be acidic
during the worst episode. This is roughly 7 times
the number of chronically acidic stream reaches.
Due to data limitations, comparable analyses are
not possible for streams in the Southern Blue
Ridge. Lower levels of acidic deposition will lower
the number and severity of acidic and toxic epi-
sodes driven by sulfate and nitrate.
EMISSIONS TRADING is COST-EFFECTIVE AND
MAINTAINS ENVIRONMENTAL BENEFITS
A recently released General Accounting Office
report estimated that the allowance trading
program will reduce control costs by over 40
percent and up to 70 percent if the trading
program is used to the fullest extent, t Atmospheric
modeling of sulfur deposition projects no more
than a 10 percent difference in deposition in 2010
with and without trading. Over most of the eastern
United States, the difference in deposition is less
than 5 percent, and there is no difference
projected for eastern Canada. Exhibit IV is a map
that shows the projected annual average difference
in sulfur deposition between trading and no
trading over the eastern United States and Canada.
Variations in deposition of less than 10 percent are
not projected to result in measurable ecological
impacts. Therefore, while the allowance trading
t U.S. General Accounting Office. December. 1994.
Air Pollution Allowance Trading Offers an
Opportunity to Reduce Emissions at Less Cost.
Washington, DC.
program is expected to reduce costs of control, it is
not projected to have a measurable negative envi-
ronmental impact.
SULFUR EMISSIONS AND DEPOSITION LEVELS
Exhibit V compares deposition levels produced by
several sulfur dioxide emissions scenarios. The
additional reduction scenarios were chosen to
illustrate the effect of further emissions reductions
and to serve as examples for cost and
implementation analyses; they do not represent a
reduction necessary to meet any particular target
load. In comparison with 1980 deposition levels,
implementation of the CAAA is projected to
reduce deposition by 30 to 40 percent by 2010.
Exhibit VI shows the projected percentage de-
crease in sulfur deposition between 1980 and
2010 with full implementation of Title IV. If an
additional 50 percent reduction in utility and in-
dustrial sulfur dioxide emissions beyond the CAAA
were to occur, then sulfur deposition would be re-
duced by about 60 percent compared to 1980.
NATIONAL OR TARGETED EMISSIONS RE-
DUCTIONS
To achieve an acid deposition standard for a par-
ticular sensitive area, some have suggested target-
ing emissions reductions, rather than reducing na-
tional emissions. By 2010, Title IV will produce
the largest emissions reductions in the highest
emitting regions (i.e., Ohio, Indiana, West Vir-
ginia, and western Pennsylvania). An analysis of
geographically targeted emissions reductions using
the Regional Acid Deposition Model (RADM)
shows that to achieve deposition reductions be-
yond the CAAA (equivalent to those achieved by a
50 percent SO2 emissions reduction) in sensitive
receptor regions, zones targeted for emissions re-
ductions would need to include 6 to 11 states and
require source- (region-) specific, sulfur dioxide
reductions of about 95 percent. To achieve
deposition loadings in all 3 sensitive receptor
regions equivalent to that produced by an ap-
proximate 50 percent reduction in sulfur dioxide
emissions, both geographically targeted and
national emissions reductions strategies would
require about the same total emissions reductions
at about the same total cost. Thus, relative to na-
tional emissions reductions, there is no economic
or environmental advantage to geographically
targeting regions for emissions reductions.
xv
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT IV. ANNUAL AVERAGE RADM TOTAL SULFUR DEPOSITION (KC-S/HA) IN 2010: DIFFERENCE
IN DEPOSITION BETWEEN IMPLEMENTATION OF THE CAAA WITH AND WITHOUT TRADING
-0.80 TO -1.16
-0.50 TO -0.80
-0.20 TO -0.50
-0.20 TO 0.20
0.20 TO 0.50
0.50 TO 0.90
0.90 TO 1.53
EXHIBIT V. SULFUR DEPOSITION TO SENSITIVE REGIONS UNDER VARIOUS SO2 EMISSIONS SCENARIOS
Emissions Scenario
1980
1985
2010 after CAAA implementation
CAAA plus additional 50% utility SO? reduction
CAAA plus additional 50% utility and industrial SO? reduction
Annual Avera
Adirondacks
11
9,8
6.9
5.5
4.7
Be Deposition Level (kg-S/ha)
Mid-Appalachi-
ans
19
17
11
8.1
6.9
Southern
Blue Ridge
14
13
9.7
6.8
5.5
XVI
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EXECUTIVE SUMMARY
EXHIBIT VI. PERCENTAGE REDUCTIONS IN SUL-
FUR DEPOSITION FROM 1980 TO 2010
FROM IMPLEMENTATION OF THE CAAA
AND CANADIAN ACID RAIN PROGRAM
IMPLEMENTING AN ACID DEPOSITION
STANDARD
In order to determine the effectiveness of an acid
deposition standard for protecting sensitive re-
sources, it is necessary to describe how the stan-
dard would be implemented.
The study describes two basic approaches to using
an acid deposition standard. Under the first ap-
proach, EPA would set a standard or standards,
either using existing authority or seeking further
authority from Congress to set such standards and
provide deadlines for their attainment. Then, simi-
lar to Title I, states would determine source-spe-
cific limits using source-receptor models and tech-
nical and cost analyses, incorporate those limits in
State Implementation Plans (SIPs), and enforce
them. If one or more states failed to do the above,
EPA would promulgate a Federal Implementation
Plan (FIP).
Under the second approach, Congress would di-
rect EPA to set a deposition standard or standards
and to determine the national (or regional) emis-
sions levels for sulfur dioxide and nitrogen oxides
that would meet those standards. Congress would
then set an emissions cap and allowance alloca-
tions for nitrogen oxides and, if necessary, adjust
the cap for sulfur dioxide in Title IV, and provide a
timetable for meeting the new caps. EPA would
use Title IV provisions to implement the emissions
program.
To provide a rough comparison of the cost-effec-
tiveness of the two approaches for sulfur reduc-
tions, estimates were made of the cost of achieving
the same reduction in sulfur deposition from utility
sources at the three sensitive areas under each ap-
proach. Total compliance costs were similar, al-
though the national emissions reduction approach
resulted in slightly larger and more widespread
emissions reductions which may provide greater
ancillary benefits for human health and visibility
protection. The costs of further emissions reduc-
tions characterized in this report could lead to
costs that are more than double those of the cur-
rent acid rain control program, but the benefits
would be in multiple effects areas.
FEASIBILITY AND EFFECTIVENESS OF AN ACID
DEPOSITION STANDARD
The purpose of this study was to provide Congress
with a report on the feasibility and effectiveness of
an acid deposition standard or standards to protect
sensitive aquatic and terrestrial resources. Based
on current scientific understanding of the effects of
sulfur and nitrogen on aquatic resources, it would
be feasible to set sulfur and nitrogen deposition
standards to protect aquatic resources, but uncer-
tainty remains high as to the impact of nitrogen.
Further research could lower that uncertainty. It
would also be helpful to have guidance from Con-
gress or the public on the degree of protection de-
sired, e.g., to protect every aquatic resource from
any adverse effect, or to protect 95 percent of sen-
sitive resources from chronic anthropogenic acidi-
fication.
The effectiveness of an acid deposition standard
depends heavily on the approach used to imple-
ment it. Although the two basic approaches dis-
cussed in this report could have similar compli-
ance costs and effects on aquatic resources, the
national market-based emissions reduction ap-
proach could have greater benefits for human
health and visibility, is more compatible with the
existing Title IV, and is more likely to be imple-
mented. The likelihood of achieving deposition
reductions is viewed as a critical factor in judging
effectiveness.
XVII
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CHAPTER 1
INTRODUCTION
Title IV of the Clean Air Act Amendments of 1990
(CAAA or the Act) addresses the problem of ad-
verse effects on ecosystems from acid rain by
mandating reductions in emissions of sulfur and ni-
trogen oxides—the major precursors of acid rain.
Coupled with Titles I and II of the Act, which ad-
dress new and existing stationary and mobile
sources of sulfur and nitrogen oxides, implementa-
tion of Title IV is expected to provide significant
benefits to the United States and Canada. Those
benefits include decreases in the acidity of lakes
and streams, concomitant improvements in fish
population diversity and health, decreases in soil
degradation and forest stress, improvements in
visibility (especially to scenic vistas), decreases in
damage to material!; and cultural resources, and a
reduction in adverse human health effects. Con-
gress included Section 404 in Title IV (Appendix B
of the Act) which requires the U.S. Environmental
Protection Agency (EPA or the Agency) to provide
a report to Congress on the feasibility and effec-
tiveness of an acid deposition standard to protect
sensitive and critically sensitive aquatic and terres-
trial resources. Specifically, Congress listed six ar-
eas to be addressed in the report:
* Identification of sensitive and critically sen-
sitive aquatic and terrestrial resources in
the U.S. and Canada which may be af-
fected by the deposition of acidic com-
pounds;
* Description and specification of a numeric
value for an acid deposition standard suffi-
cient to protect such resources;
* Description of the use of such standards or
standards in other Nations or by any of the
several States in acidic deposition control
programs;
* Description of measures that would be
needed to integrate such standard or stan-
dards with the control program required by
Title IV of the Clean Air Act;
* Description of the state of knowledge with
respect to source-receptor relationships
necessary to develop a control program on
such standard or standards and additional
research that is on-going or would be
needed to make such a control program
feasible;
* Description of impediments to implemen-
tation of such control program and the
cost-effectiveness of deposition standards
compared to other control strategies in-
cluding ambient air quality standards, new
source performance standards and the re-
quirements of Title IV of the Clean Air Act.
To achieve significant reductions in emissions of
sulfur and nitrogen oxides, Title IV targets emis-
sions from electric utilities—the major source of
sulfur dioxide (SO2) emissions and a major source
of nitrogen oxide (NOX) emissions. Annual emis-
sions of SO2 are to be reduced by 10 million tons
from 1980 levels through an innovative market-
based allowance trading program. The program es-
tablished an SO2 allowance trading system that al-
lows utilities to minimize the cost of complying
with SO2 emissions reduction requirements, while
maintaining a cap on SO2 emissions from utilities.
The trading program encourages energy conserva-
tion and technological innovation, which should
yield pollution prevention benefits and minimize
compliance costs.
Annual allowances for SO2 emissions have been
allocated to affected utility units based on their
historic emissions and fuel use. Each allowance
permits a utility to emit 1 ton of SO2. Each unit
must hold a sufficient number of allowances at the
end of the year to cover its emissions for that year.
Emissions reductions will be implemented for 263
units under Phase I beginning in 1995 and for
approximately 2,200 units affected under Phase II
in 2000. Utilities may buy, sell, trade, or save al-
lowances for future use. When the program is fully
implemented in 2010, nationwide emissions of
SO2 from affected utilities (i.e., units generating
over 25 MW) will be capped at 8.95 million tons
per year. In addition, nationwide emissions of SO2
from industrial sources are capped at 5.6 million
tons per year. These emissions are not included in
the allowance trading program, but some indus-
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
trial sources may be allowed to elect to participate
in the program under rules currently being devel-
oped by EPA.
Title IV also specifies that standards be set for NOX
emissions from utility boilers, with the goal of re-
ducing nationwide emissions by 2 million tons
from 1980 levels. Unlike the SO2 program, the
NOX program does not use a tradable allowance
system and does not cap emissions, but instead
calls for low-NOx burner technology to reduce
emissions.
Concerns exist regarding how allowance trading
will affect protection of sensitive aquatic and ter-
restrial resources. Some critics argue that the cau-
tious nature of the electric utility industry and lo-
cal pressure in high-sulfur coal regions to continue
use of regional coals to preserve local economies
will not allow trading to achieve economically
meaningful benefits. Others argue that extensive
trading could result in a geographic distribution of
emissions that would prevent achieving the Act's
goal of reducing the effects of acidic deposition in
sensitive areas and that emissions levels resulting
in no adverse effects are necessary for protection.
The process for determining the degree of protec-
tion afforded by Title IV to sensitive and critically
sensitive aquatic and terrestrial resources is com-
plicated by a number of scientific and technical
uncertainties. For example, there are gaps in eco-
logical effects research, particularly regarding ni-
trogen cycling and retention in forested water-
sheds. The scientific community is still learning
how multiple impacts of sulfur and nitrogen acidic
deposition affect ecosystems. In addition, meteoro-
logical variability, uncertainties in emissions in-
ventories, and the complexity of atmospheric
chemistry limit the ability to relate specific ecosys-
tem damage to specific point sources.
The purpose of this study is to integrate state-of-
the-art ecological effects research, emissions and
source-receptor modeling work, and implementa-
tion and cost issues to consider the feasibility of
setting and implementing a standard to protect
aquatic and terrestrial resources from the effects of
acidic deposition. This report addresses three
broad themes:
1. What acidic deposition levels are necessary
to protect sensitive regions?
2. What degree of protection is provided by
Title IV? What is the residual risk? What
additional emissions limitations would be
required to protect sensitive regions?
3. How would an acid deposition standard(s)
be implemented? What are the different
implementation approaches? What are their
relative feasibility and effectiveness?
A common thread running through each is uncer-
tainty in (1) data and models, (2) future ecosystem
behavior, and (3) future economic and policy deci-
sions that may influence decisions regarding feasi-
bility.
WHAT ACIDIC DEPOSITION LEVELS ARE NEC-
ESSARY TO PROTECT SENSITIVE REGIONS?
The report outlined in Section 404 (Appendix B) of
the Act requires identification of sensitive aquatic
and terrestrial resources and description of the na-
ture and numerical value for a deposition standard
that would protect these resources. This report
identifies sensitive resources and describes options
for and the nature of a protective goal (i.e., deposi-
tion standard); scientific uncertainties associated
with the response of specific sensitive regions to
acidic deposition, however, make designation of a
numerical value for a deposition standard that
would protect each sensitive region difficult at this
time. Chapter 2 of this feasibility study brings to-
gether the most current scientific understanding
regarding the relationship between acidic deposi-
tion and ecological effects, specifically effects on
watersheds. The information comes from research
conducted by EPA's Office of Research and De-
velopment, peer-reviewed literature, and efforts to
define appropriate acid deposition standards in the
United States and other countries. Best understood
from the scientific point of view is the relationship
between acidic deposition of sulfur and its effects
on stream and lake ecosystems. Effects of nitrogen
deposition on aquatic systems and of combined
sulfur and nitrogen deposition on terrestrial sys-
tems are less clear and poorly quantified for re-
sources in the United States.'
Regions of North America differ in their sensitivity
to acidic deposition (i.e., ability of a watershed to
buffer acidity) and in the amount of acidic deposi-
tion they receive. Some parts of the eastern United
States are highly sensitive and chronically or epi-
sodically receive damaging concentrations of
acidic deposition. Other sensitive regions, such as
the western United States, are unlikely to suffer
adverse chronic effects at current or projected
rates of acidic deposition. Certain high-elevation
western lakes, however, are subject to episodes of
-------
CHAPTER 1: INTRODUCTION
acidic deposition. Chapter 2 identifies sensitive
aquatic and terrestrial resources in specific regions
of the United States and describes the effects
caused by acidic deposition in each region.
Aquatic resources of concern include fish and
other species, as well as the water quality of lakes
and streams. Terrestrial resources of primary
concern include trees and forest soils.
The relative contributions and importance of sul-
fur- and nitrogen-containing compounds to the ef-
fects of acidic deposition differ among regions.
The importance of each group of compounds de-
pends on its relative deposition level and on the
capacity of individual watersheds to retain depos-
ited nitrogen and sulfur. Sulfur appears to be the
principal cause of ongoing, chronic acidification
of aquatic systems within most affected areas in
eastern North America. The importance of nitro-
gen deposition, however, cannot be overlooked for
several reasons: visibility in many areas of the
West is degraded more by nitrogen than sulfur
deposition; nitrogen, as well as sulfur, produces
episodic surface water acidification effects, espe-
cially during spring snowmelts; and some water-
sheds in the Northeast may be approaching the
limit of their ability to sequester nitrogen, leading
to increased acidification from nitrogen deposi-
tion.
Chapter 2 assesses these issues by analyzing avail-
able scientific data within a risk-based context and
produces a set of environmental goals. The analy-
ses concentrate on the three regions in the United
States most extensively characterized: the Adiron-
dacks, the mid-Appalachians, and the Southern
Blue Ridge Province, each of which is subject to
deposition from sources in the East. Knowledge of
current and historic differences in deposition levels
and watershed sensitivities in these three regions
makes it possible to discern differences in ongoing
effects and remaining risk for each. Risks for other
regions of the United States and Canada are also
described, but in a more qualitative sense. Effects-
based (critical and target load) control strategies
and approaches adopted by Europe, Canada, and
several states in the United States are also dis-
cussed. The importance of considering the effects
of nitrogen deposition on both eutrophication of
estuarine bodies1 and acidification of surface wa-
ters is also discussed. (Note that an effects-based
analysis and development of an acid deposition
standard or standards does not necessarily imply
emissions reductions associated with implementa-
tion of a standard or target loads.)
Although this report focuses on aquatic and terres-
trial systems at risk, acidic deposition and its pre-
cursor emissions also can adversely affect visibil-
ity, human health, and materials. Decreasing
acidic deposition can also provide benefits in
these areas. Visibility, especially in the eastern
United States, is markedly degraded by sulfate par-
ticles in the atmosphere. Human health effects
from exposure to SO2, NO2, and ozone (O3,
formed by chemical reactions involving nitrogen
dioxide) are well known, and effects from particu-
late matter, including acidic aerosols, are docu-
mented as well. Damage to materials and cultural
resources by acidic deposition has been docu-
mented by the National Acid Precipitation As-
sessment Program (NAPAP).2 Visibility degradation
has been addressed more fully in another Report to
Congress3 and is covered under Section 169 A of
the Act. Also, primary National Ambient Air Qual-
ity Standards (NAAQS) have been established to
protect the public health from adverse effects of
criteria pollutants, including SO2, NO2, paniculate
matter (including sulfates and nitrates), and O3.
Nevertheless, any control program or standard es-
tablished to reduce acidic deposition will also
provide benefits in these other related areas. Chap-
ter 4 of this report summarizes these potential
benefits to visibility, human health, and materials.
WHAT DECREE OF PROTECTION is PROVIDED BY
TITLE IV? WHAT is THE RESIDUAL RISK? WHAT
ADDITIONAL EMISSIONS LIMITATIONS WOULD BE
REQUIRED TO PROTECT SENSITIVE REGIONS?
The complex relationship between emissions and
deposition depends on a great number of physical,
chemical, and biological processes. Acidic deposi-
tion results from a complex series of interactions
among chemicals in the atmosphere. Airborne sul-
fur and nitrogen species can be transported hun-
dreds of kilometers by meteorological forces. Dur-
ing transport these species can remain unchanged
or react with other atmospheric pollutants, such as
1 U.S. Environmental Protection Agency. May 1994.
Deposition of Air Pollutants to the Great Waters. First
Report to Congress. EPA-453/R-93-055.
2 Irving, P.M., ed. 1991 Acidic Deposition: State of
Science and Technology. Summary Report. National
Acid Precipitation Assessment Program. Washington,
DC.
3 Office of Air Quality Planning and Standards. Octo-
ber 1993. Effects of the 1990 Clean Air Act Amend-
ments on Visibility in Class I Areas: An EPA Report
to Congress. U.S. Environmental Protection Agency,
Washington, DC.
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
volatile organic compounds (VOCs), to form new
compounds, some of which are acidic. These pol-
lutants are then deposited to the earth through
either wet or dry deposition. To understand the
environmental impact of the Act and to develop
and analyze strategies to reduce the effects of
acidic deposition, the relationship between emis-
sions and deposition (i.e., the source-receptor rela-
tionship) should be addressed not only in the pre-
sent, but also in the future.
SO2 and NOX reduction mandates established by
Title IV provide for a nationwide decrease in
acidic deposition precursors. Geographic or re-
gional restrictions do not exist. The market-based
allowance trading program promotes the most
cost-effective strategy for achieving SO2 reduc-
tions, rather than requiring a specific type of con-
trol on certain sources. Variation in sensitivity to
acidic deposition among geographic regions raises
the question of whether targeted or regional stan-
dards are needed to protect sensitive resources.
Before such a question can be answered, deter-
mining the level of protection that will be pro-
vided by full implementation of Title IV in 2010
and in subsequent years is necessary.
To answer questions regarding the effectiveness of
Title IV in protecting sensitive areas (i.e., the re-
sidual risk after implementation of Title IV) and the
impact of additional control, several alternative
emissions scenarios are developed in Chapter 3:
* A scenario that achieves the SO2 emissions
reductions mandated by the Act,
* A scenario to assess the environmental im-
pact of trading SO2 allowances, and
* Scenarios that achieve additional reduc-
tions of SO2 and NOX emissions from utili-
ties and industrial sources beyond those
required by the Act.
The Regional Acid Deposition Model (RADM)4 is
used to translate each emissions scenario to depo-
sition values for the eastern United States. Deposi-
tion of sulfur and nitrogen species are then com-
pared for each scenario, with particular emphasis
on the three key sensitive areas—the Adirondacks,
Chang, J.S., P.B. Middleton, W.R, Stockwell, C.J.
Walcek, J.E. Pleim, H.H. Lansford, F.S. Binkowski, S.
Madronich, N.L. Seaman, and D.R. Stauffer.
December 1990. The Regional Acid Deposition
Model and Engineering Model. NAPAP SOS/T
Report 4. In: Acidic Deposition: State of Science and
Technology. National Acid Precipitation Assessment
Program.
the mid-Appalachians, and the Southern Blue
Ridge Province.
HOW WOULD AN ACID DEPOSITION STAN-
DARD(S) BE IMPLEMENTED? WHAT ARE THE
DIFFERENT IMPLEMENTATION APPROACHES?
WHAT ARE THE FEASIBILITY AND EFFECTIVENESS
RELATIVE TO OTHER APPROACHES?
In Chapter 5, two broad approaches are reviewed
for degree of protection, geographic coverage, im-
plementation difficulty, and cost. The approaches
follow the emissions reductions scenarios modeled
in Chapter 3. The two broad implementation ap-
proaches are (1) a national, emissions-oriented,
market-based approach and (2) a regional, stan-
dard-oriented, source- (region-) specific limit ap-
proach. Control of both utility and industrial
sources is assessed.
A variety of factors should be considered in im-
plementing an acid deposition standard. To be
successful, an implementation approach must have
clear goals and must provide certainty as to the re-
sponsibilities of the regulated community, EPA,
and states. Chapter 5 identifies- and describes the
factors that may affect implementation of a stan-
dard under both national and regional approaches.
Four general categories of factors are considered:
* STATUTORY AUTHORITY: A first step in assess-
ing the feasibility of an acid deposition
standard is determining whether EPA has
authority to implement a standard under
the existing CAAA. Is existing authority
adequate, or would Congress need to pro-
vide additional authority necessary to im-
plement an acid deposition standard or
standards?
* ADMINISTRATION/COMPLIANCE: How enforce-
able and administratively complex are al-
ternative regulatory approaches? How
would the approach be administered and
enforced? Would new administrative enti-
ties be needed? What level of federal and
state resources would be needed?
* INTERACTION AND INTEGRATION WITH OTHER
ENVIRONMENTAL PROGRAMS: An acid deposi-
tion standard may impose additional limits
on SO2 and NOX emissions from point,
area, and/or mobile sources. Existing fed-
eral, state, and local regulations (including
the Title IV Acid Rain Program, Title I Am-
bient Air Standards, and Title II Mobile
Source regulations at the federal level) ad-
-------
CHAPTER 1: INTRODUCTION
dress emissions from these sources. What munity, as well as the national and local
effect would implementation of an acid economies?
deposition standard have on these and
other environmental programs? Chapter 6 integrates analyses of eny.ronmental
goals, emissions reductions, and implementation
Economic Impacts: What would be the issues and provides conclusions concerning the
costs and economic impacts of an acid feasibility of developing and implementing a stan^
deposition standard to the regulated com- dard or standards for acidic deposition.
-------
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CHAPTER 2
ENVIRONMENTAL COALS
2.1 INTRODUCTION
Title IV of the 1990 Clean Air Act Amendments
(CAAA) addresses the problem of adverse effects to
environmental resources from acidic deposition by
mandating nationwide reductions in emissions of
sulfur and nitrogen oxides from electric utility
generating units, the major contributor to acidic
deposition. While reductions in total emissions
will benefit many aquatic and terrestrial resources,
Congress mandated a study of whether more spe-
cific acid deposition standards may be appropri-
ate. Under Section 404 (Appendix B) of the CAAA,
EPA must assess the feasibility and effectiveness of
establishing an acid deposition standard, or stan-
dards, to protect sensitive aquatic and terrestrial
resources. This chapter addresses three specific
Section 404 requirements:
* Identification of the sensitive and critically
sensitive aquatic and terrestrial resources in
the United States and Canada which may
be affected by the deposition of acidic
compounds;
* Description of the nature and numerical
value of a deposition standard or standards
that would be sufficient to protect such re-
sources;
» Description of the use of such standard or
standards in other Nations or by any of the
several States in acidic deposition control
programs.
Section 2.2 of this report reviews surface water
acidification and recovery processes. The two
most common measures of surface water acidifica-
tion are pH and acid neutralizing capacity (ANC).
Low ANC is a common indicator of sensitivity to
acidification; other parameters, including pH, dis-
solved aluminum, and sensitive species, also pro-
vide useful information on resource health. At-
mospheric deposition of sulfur and nitrogen com-
pounds that form acids is the principal cause of
surface water acidification effects. Most recent at-
tention has focused on the effects and control of
sulfur deposition (Section 2.2.1). While many stud-
ies have focused primarily on long-term acidifica-
tion processes, recent EPA research supports the
contention that short-term acidification caused by
rainstorms and snowmelt may often be the initial
cause of many of the most severe acidification ef-
fects in streams. Consideration of acid deposition
standards may take into account implications to
both the long- and short-term acidification proc-
esses (Section 2.2.2). Recent research also indi-
cates that acidification effects caused by nitrogen
deposition are increasingly important in some ar-
eas. The increasing degree of nitrogen saturation in
some watersheds is leading to long-term and
short-term increases in nitrate concentration and
concomitant acidification of some surface waters
(Section 2.2.3). Section 2.2.4 notes that a number
of studies indicate that surface water acidification
can be reversed by reducing emissions and, at
least temporarily, by practices such as liming (e.g.,
the application of powdered limestone); however,
restoration of ecological systems to their predistur-
bance conditions may not be possible.
Section 2.3 introduces a risk-based approach to as-
sessing the need for an acid deposition standard.
Alternative approaches for defining resource sensi-
tivities are reviewed, and environmental and land-
use characteristics affecting these sensitivities are
described (Section 2.3.1). Section 2.3.2 empha-
sizes that using a risk-based approach requires as-
sessing four central concerns: (1) sensitivities of
potentially affected resources; (2) factors extrinsic
to selected resources that may alter their sensitivi-
ties; (3) geographic location of the selected re-
sources; and (4) exposure intensity, duration, and
timing of acidic deposition at the selected loca-
tions.
Section 2.4 addresses the first Congressional re-
quirement listed above. Section 2.4.1 emphasizes
that assessments of the need for acid deposition
standards should focus primarily on potential
benefits to sensitive aquatic resources in
six regions of the eastern half of the United States
and to sensitive stands of red spruce at high eleva-
tions in two of those regions. An EPA-sponsored
literature review confirmed that the potential ef-
fects of nitrogen deposition is an increasing con-
cern in the western United States as well
(Section 2.4.2). In the area of Canada roughly
south of James Bay and east of the Manitoba-On-
tario border, resource concerns similar to concerns
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
for the eastern United States are emphasized
(Section 2.4.3).
Section 2.5 addresses the second Congressional
requirement by describing the potential need to
protect acid-sensitive resources and the potential
benefits derived from additional control of acidic
deposition. Simulation modeling is the best avail-
able way to project future ecological effects of
possible changes in deposition rates. This ap-
proach is not without limitations, however, as
results carry considerable uncertainty (Section
2.5.1). Two major EPA effects modeling studies
have been completed. The first study, conducted
under the National Acid Precipitation Assessment
Program (NAPAP), projected the impact of sulfur
deposition on long-term soil and surface water
acidification, with consequent loss of aquatic habi-
tat for sensitive fish species, in three broad geo-
graphical regions of the eastern United States. Ad-
verse effects were projected to continue unless sul-
fur deposition was reduced, and sufficient reduc-
tions in sulfur deposition were projected to likely
reverse these effects (Section 2.5.2). The second
major EPA modeling study illustrated the role that
nitrogen deposition may play as an important
cause of soil and water acidification, and its im-
portance was projected as likely to increase in fu-
ture years unless deposition rates decreased
(Section 2.5.3). This study also projected that the
1990 CAAA would provide clear benefits to sur-
face water in three sensitive regions of the eastern
United States. The accuracy of the model projec-
tions is highly uncertain, however, largely because
researchers lack precise estimates of how long it
takes a sensitive watershed to become saturated
with deposited nitrogen.
In addressing the third Congressional requirement,
Section 2.5.4 reviews various acidic deposition
criteria and standards that have been developed
internationally and by individual states, many of
which are being revised as additional and more
accurate information becomes available. This sec-
tion is not, however, a comprehensive list of all
state or international efforts. Important ecological
and geographical concerns that raise questions
about whether the application of deposition stan-
dards should take into account regional and sea-
sonal conditions are discussed in Section 2.5.5. Fi-
nally, assessments of the need and options for de-
veloping deposition standards address concerns re-
lated to both sulfur and nitrogen deposition
(Section 2.6).
2.2 BASIC RELATIONSHIPS IN SURFACE WATER
ACIDIFICATION AND RECOVERY
Understanding the need for and feasibility of es-
tablishing acid deposition standards and under-
standing the sensitivities of resources to acidifica-
tion, requires a knowledge of how environments
assimilate atmospherically deposited acids and
acid-forming chemical compounds. This knowl-
edge is key to assessing whether surface water
acidification effects are occurring in a region,
when these effects began, how extensive the ef-
fects are and may become, and the critical periods
when these effects may be most severe. This sec-
tion briefly reviews the process of surface water
acidification, important considerations in identify-
ing at-risk resources, episodic acidification, and
useful information for setting regional and resource
priorities for acidic deposition controls. The sec-
tion concludes with a brief discussion of the re-
sponses of acidified ecosystems to reductions in
acidic deposition levels. First, a few common
terms and concepts, key to understanding surface
water acidification processes, are introduced.
The common measure of acid-base conditions in
solutions is the pH scale. On this scale, neutrality
(i.e., neither acidic nor basic) occurs at pH 7.0;
acidic conditions have lower values (pH<7.0),
while basic (also termed alkaline) conditions have
higher values (pH>7.0). The most acidic condi-
tions occur near pH 0.0 and the most alkaline
conditions occur near pH 14.0. On this scale,
each full unit decrease in pH (e.g., from 7.0 to 6.0)
represents a ten-fold increase in acidity and in the
concentration of hydrogen ions that cause acidity.
Some surface waters are naturally acidic. This is
largely due to (1) carbon dioxide from the atmos-
phere dissolving to form carbonic acid and (2) or-
ganic acids produced by the decay of dead plant
materials. Waters in some bog lakes, for example,
can have natural pH levels below 5.0. Similarly,
pure rain water and distilled water in equilibrium
with atmospheric concentrations of carbon dioxide
naturally have pH levels near 5.6. Dissolution of
natural organic acids from the atmosphere into
rain water has been found to increase rain water
acidity and lower pH to near 5.2 in some pristine
open ocean areas.
The natural tendency of some surface waters to be
acidic is countered by common, alkaline minerals
such as limestone that dissolve into them. This dis-
solution neutralizes the acidity, often producing
slightly alkaline conditions (alkalinization). The
dissolution of many minerals not only neutralizes
-------
CHAPTER 2: ENVIRONMENTAL GOALS
acidic conditions in waters, it produces a buffering
capacity that enables these waters to maintain
near-ambient pH conditions, while allowing their
mass of dissolved acids to vary within certain lim-
its. Additional buffering capacity can be produced
by solutions of weak acids, including carbonic
acid and many organic acids. The extent of acid-
base buffering within any natural water is deter-
mined by the specific combinations of dissolved
materials.
In total, the interaction of these natural acidifica-
tion, alkalinization, and buffering processes causes
the pH in most surface waters to range from about
6.5 to 8.0. Much of the concern about surface
water acidification focuses on the effects that may
occur with decreases of 0.5 to 2.0 pH units or
more (i.e., increasing acid concentrations in the
environment by 500 to 10,000 percent or more).
Acid neutralizing capacity (ANC) is the term
commonly used to describe the concentration of
dissolved compounds present in fresh water that
collectively tend to neutralize water pH, creating
less acidic and more alkaline conditions. Greater
ANC generally correlates with greater buffer ca-
pacity in the water. In most fresh waters ANC is
determined primarily by concentrations of carbon-
ate and bicarbonate, which generally dissolve into
concentrations of bicarbonates, carbonates, and
hydroxides in water. The total capacity of a sur-
face water to neutralize acidity can include other
chemical and biological processes—the most im-
portant of which are the biologically mediated
processes of sulfate (SO42-) and nitrate (NO3~) re-
duction.
Surface waters with higher ANC are generally
more resistant to acidification and have higher pH
levels. That is, lakes and streams with ANCs
greater than 200 microequivalents per liter (ueq/l)
have significantly moderated potential for pH fluc-
tuations below 6.0. Also, they generally have
minimal development of acidic water qualities that
can be stressful, or even toxic, to aquatic organ-
isms. In turn, waters with ANCs of 50 ueq/l or less
tend to be the most sensitive to severe and
long-term pH depressions below 6.0, which can
produce the most severe effects on aquatic life.
The general empirical relationship between pH
and ANC was determined during EPA's Nitrogen
Bounding Study (NBS) (see Section 2.5.3) for three
regions in the eastern United States that contain
sensitive ecological resources (Exhibit 1). This ex-
hibit shows, for example, that an ANC of 50 ueq/l
correlates approximately to a pH of 6.5 across
these regions.
EXHIBIT 1. EMPIRICALLY DETERMINED RELATIONSHIP BETWEEN ANC AND pH FOR THREE
SENSITIVE REGIONS, INCLUDING CROSS-REGIONAL MEAN AND STANDARD DEVIATION
ANC
(ueq/l)
-10
0
10
40
50
60
Empirical pH for Sensitive Regions
Adirondacks
4.96
5.28
5.69
6.31
6.41
6.50
Mid-
Appalachians
4.98
5.30
5.72
6.36
6.47
6.55
Southern
Blue Ridge
4.95
5.27
5.78
6.53
6.65
6.73
Mean pH
4.96
5.28
5.73
6.40
6.51
6.59
Standard
Deviation
0.02
0.02
0.05
0.12
0.12
0.12
the water from calcium carbonate (the predomi-
nant chemical constituent of limestone, for exam-
ple). Concentrations of borates, phosphates, sili-
cates, sulfides, and organic anions can also con-
tribute to total ANC in surface water. In earlier lit-
erature, the term alkalinity was often used in place
of ANC.5 In most recent literature, however, alka-
linity is used primarily in discussing total dissolved
Drever, J.I. 1982. Geochemistry of Natural Waters.
Prentice-Hall, Inc., Englewood Cliffs, Nj.
Acidic deposition can lead to two kinds of acidifi-
cation processes. First, over the longer term, the
fundamental character of soil and water chemis-
tries can shift to chronically acidic conditions due
to the input and accumulation of deposited acidic
ions. Such conditions can produce adverse envi-
ronmental effects which may be long-term,
chronically toxic, and lethal. Second, acutely
acidic conditions can rapidly develop during peri-
ods leading to, accompanying, or following epi-
sodic events, which primarily accompany dis-
charges of storm and snowmelt water runoff.
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
Pulses of highly acidic water flushing into and
through soils, streams, and lakes often expose soil
and aquatic biota to short-term, acutely toxic, le-
thal chemical conditions.
When considering acidification effects, it is impor-
tant to recognize that the earliest adverse effects to
biological components of an aquatic ecosystem
commonly accompany early episodic acidification
events. For acid-sensitive fish species, for example,
these events often cause complete spawning or re-
cruitment failures. As chronic acidification be-
comes more pronounced, such effects become
more frequent and may result in further impacts to
overall species richness. In contrast, for systems
recovering from acidification, this sequence re-
verses as occasions of episodic effects become less
and less frequent, until acidification effects appar-
ently end.
As more fully discussed in several of the following
sections, available information indicates that sur-
face waters with ANCs<50 ueq/l include the
aquatic resources most sensitive to potential effects
from episodic acidification. Thus, ANC is a very
important response variable for use in evaluating
acidification-related changes in surface waters,
particularly streams. As such, ANC is a primary fo-
cus of discussion throughout this chapter. The next
four subsections review potential chemical rela-
tionships, episodic acidification, cumulative ef-
fects, and recovery processes associated with soil
and water acidification. Biological implications
are discussed in reviewing of identification of re-
source priorities. Additional issues are summarized
in Appendix A, Summary of NAPAP reports.
2.2.1 Relationship of Base Cations, Sulfur,
and Nitrogen in Surface Water
Acidification
Acidic deposition increases total loads of hydrogen
ions (H+) and acidic anions (primarily SO42' and
NO3") in watersheds. A vast majority of these de-
posited ions interact within the watersheds, ex-
changing with and displacing ions of other chemi-
cal species from watershed receptors primarily
through soil weathering and chemical equilibrium
processes in soil waters. Most watershed ions ex-
changed for deposited ions enter soil water solu-
tions and subsequently drain into streams and
lakes.
In a report that provided much of the basis for
EPA's modeling research on watershed responses
to acidic deposition, the National Academy of Sci-
ences (NAS) identified two geochemical processes
as the dominant watershed factors mediating
long-term surface water acidification.6 The first is
the rate at which watershed sources exchange
base cations, especially calcium (Ca2*) and mag-
nesium (Mg2+), for H+ through neutralization and
buffering processes. Essentially all base cations
within a watershed are supplied initially through
the relatively slow process of mineral weathering,
while much more rapid supplies of base cations
can be available through exchange processes
within soil solutions and by soil biota. Acidic
deposition can accelerate each of these processes.
The second dominant factor that NAS identified as
affecting acidification is the capacity of a water-
shed to retain deposited sulfur-containing com-
pounds. This process is important because a vast
majority of atmospherically deposited sulfur is in
the form of SO42" or other inorganic sulfur-contain-
ing compounds that rapidly oxidize to SO42'. The
process of SO42- adsorption by soils directly affects
the mobility of SO42- in watersheds and, thus, the
mobility of assorted base cations and acidic cat-
ions (e.g., H+ and aluminum, AI3+).
The NAS report concluded that the external proc-
ess of acidic ion deposition balanced against in-
ternal watershed processes of base cation supply
(i.e., acid assimilation) and SO42' adsorption (i.e.,
anion mobility) critically determine the rates and
degrees of long-term acidification of soil and
water. The production of base cations and internal
retention of sulfur in watersheds can decrease as
available supplies of base cations and SO42~ ad-
sorption abilities are depleted. Surface waters in
watersheds with insufficiently available base cat-
ion supplies and minimal net annual sulfur reten-
tion tend to be at greater risk from acidic deposi-
tion.
EPA initiated the Direct Delayed Response Project
(DDRP) (see Section 2.5.2) to help understand how
these factors interact and affect environmental re-
sponses to acidic deposition. This project primarily
addressed the question of whether watersheds tend
to acidify immediately in proportion to the inten-
sity of deposition (i.e., "direct" acidification) or lag
6 National Academy of Sciences. 1984. Add Deposi-
tion: Processes of Lake Acidification, Summary of a
Discussion. National Research Council Commission
on Physical Sciences, Mathematics, and Resources.
Environmental Studies Board, Panel on Processes of
Lake Acidification. National Academy Press, Wash-
ington, DC. 11 pp.
10
-------
CHAPTER 2: ENVIRONMENTAL GOALS
in time due to internal watershed processes (i.e.,
"delayed" acidification).
Unquestionably, watershed processes regulating
base cation exchange and retention of atmospheri-
cally deposited sulfur are two primary controls on
surface water acidification rates that must be un-
derstood to allow projection of potential acidic
deposition effects. More recent research shows, in
addition, that direct projection of surface water
acidification rates from acidic deposition also de-
pends upon, and continues to be limited by, major
uncertainties regarding the capacities of water-
sheds to assimilate nitrogen deposition. That is, al-
though NO3~ is often an important acid anion in
acidic deposition, nitrogen is also an essential nu-
trient in high demand by many physiological
processes within organisms. Its frequent scarcity in
many environments, relative to other essential nu-
trients, often limits plant growth and other biologi-
cal activities. Thus, because nitrogen is a fertilizer
that often quickly incorporates into organisms, the
ANC of soils and surface waters is relatively un-
changed. Consequently, only a generally limited
past concern and a narrowly defined research ef-
fort has been focused on ecological effects associ-
ated with nitrogen deposition in watershed acidifi-
cation.
An expanding body of recent research, however,
shows that nitrogen deposition is an important
component and an increasing cause of present and
future acidification in some environments. Specifi-
cally, there are limits to the amount of nitrogen
that can be incorporated into organic matter by
biological processes in watersheds. When these
processes are saturated (i.e., when nitrogen is no
longer the limiting nutrient for biological produc-
tion and growth), nitrogen losses from watersheds
will increase, principally in the form of NCy
leaching. Excess NO3" in watersheds can lead to
depletion of base cations and surface water acidi-
fication through the same processes as those in-
volving excess SO42~. For example, European for-
ests apparently are becoming nitrogen saturated,
and the need for specific additional emissions con-
trols to protect European forests and surface waters
from the detrimental effects of excessive nitrogen
deposition are being evaluated.7
Further, data from Long-Term Monitoring (LTM)
sites in the northeastern United States strongly in-
dicate a regional decrease in lake and stream
water SO42~ through 1989 (the end of the period of
record assessed), suggesting that sulfur deposition
is declining. A concurrent, general increase in lake
and stream water NO3~ concentrations was found
primarily in the Adirondack and Catskill Moun-
tains, suggesting that these watersheds may be
moving toward watershed nitrogen saturation. Sur-
face water NO3- concentrations did not show
marked trends for most other areas of the North-
east. Over this same period in the Northeast, both
pH and ANC tended to increase, but these trends
are weaker than those found for SO42' and NO3"
deposition. For Adirondack lakes, in fact, there
appears to be a possible trend of decreasing ANC.
Coupling these possible regional ANC trends with
the apparent trends for SO42- and NO3" indicates
that surface water acidification effects may be be-
coming more closely tied to NO3" deposition in
the Adirondacks and, possibly, other regions of the
Northeast. Research is continuing to evaluate more
completely this potential shift in surface water
acidification relationships.
Of additional concern are episodes of storm flow
or snowmelt runoff that can expose organisms to
short-term, acutely lethal, acidic water.8 Episodic
events (described in detail in the following section)
occurring during spring snowmelt often tend to be
the most acidic and contain the highest concentra-
tions of inorganic monomeric aluminum, which is
highly toxic to fish.9 NO3~ tends to be more mo-
bile in watershed soils at this time of the year be-
cause most plants are dormant. This fact and the
prevailing cold temperatures through winter and
early spring tend to promote increasing NO3~ ac-
cumulations in soil and overlying snowpack. Espe-
cially during these periods, snowmelt and storm
7 Sullivan, T.J. 1993. Whole-ecosystem nitrogen ef-
fects research in Europe. Environmental Science and
Technology 27(8): 1482-1486.
8 Wigington, P.J., Jr., J.P. Baker, D.R. DeWalle, W.A.
Kretser, P.S. Murdoch, H.A. Simonin, J. Van Sickle,
M.K. McDowell, D.V. Peck, and W.R. Barchet.
1993. Episodic Acidification of Streams in the
Northeastern United States: Chemical and Biologi-
cal Results of the Episodic Response Project.
EPA/600/R-93/190. Office of Research and Devel-
opment, U.S. Environmental Protection Agency,
Washington, DC.
9 Baker, J.P., and S.W. Christensen. 1991. Effects of
acidification on biological communities in aquatic
ecosystems. Pages 83-106 in D.F. Charles (editor).
Acidic Deposition and Aquatic Ecosystems - Re-
gional Case Studies. Springer-Verlag, New York,
NY.
11
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
water runoff can flush NO3" through the watershed
at flow rates that exceed the assimilative capacity
of terrestrial plants to capture the rapidly passing
nutrients. Cold water temperatures also slow the
ability of aquatic organisms to incorporate the
newly added NO3". As a result, NO3~ can be a
significant seasonal cause of episodic acidification
in surface waters in some regions, often occurring
at the most biologically significant time of year
(i.e., during spawning and reproduction).
2.2.2 Episodic Acidification
This report primarily focuses on chronic effects to
surface waters associated with long-term exposure
to acidic deposition, because much past research
has emphasized processes leading to long-term
chronic acidification. In surface waters that have
not completed processes leading to chronic acidi-
fication or are in the process of recovering from
chronic acidification, the largest impacts of acidic
deposition most commonly accompany episodic
acidification. Episodic acidification (temporary
loss of ANC) can occur when pulses of low-ANC
or acidic waters enter streams and lakes as the re-
sult of rainstorms or snowmelt. Acid anions (i.e.,
sulfate and nitrate) that reach surface waters dur-
ing these events may originate from immediate
deposition or, more likely, may be the result of
prior atmospheric deposition (i.e., the previous
year) that has cycled within the watershed and is
flushed from the system during the high storm
flows. Accompanying the acid anions during
leaching are acids or toxic aluminum compounds
leached from soils. Both sulfates and nitrates origi-
nating from atmospheric deposition may contrib-
ute significantly to such episodic acidification
events. Episodic acidification can cause lakes and
streams that have positive ANC during most of the
year to become acidic (ANCSO p.eq/1) and have
high toxic aluminum concentrations for periods of
hours to days.
The most severe episodes of acidification occur in
the spring. The National Surface Water Survey
(NSWS) (described in Section 2.4), however, sur-
veyed lakes in the Adirondacks during the fall.
Significantly, more lakes and streams become epi-
sodically acidic than are chronically acidic. Most
recent estimates indicate that for the worst episode
that may occur during any year, the number of
lakes or streams that were acidic during that epi-
sode in the Adirondacks is approximately 3.5
times the number found to be chronically acidic.
Thus, for the Adirondacks, approximately 70 per-
cent of the target population lakes are at risk of
episodic acidification at least once during each
year. For the mid-Appalachian streams, approxi-
mately 30 percent of the target population stream
reaches are likely to be acidic during the worst
episode. This is roughtly 7 times the number of
chronically acidic stream reaches. Due to data
limitations, comparable anlayses are not possible
for streams in the Southern Blue Ridge. Lower lev-
els of acidic deposition will lower the number and
severity of acidic and toxic episodes driven by sul-
fate and nitrate.
EPA recently completed its Episodic Response Pro-
ject.™ Major conclusions from that project in-
clude:
» Acidic deposition episodes, evidenced by
stream water containing elevated SO42' and
NO3- concentrations during the episodes,
were a common occurrence in the study
streams of all three regions investigated
(i.e., the Adirondack and Catskill
Mountains of New York and the Northern
Appalachian Plateau of Pennsylvania).
* Acidic episodes were common in streams
of each region wherever and whenever
ANC values were 50 ueq/l or less immedi-
ately before the episode. When acidic epi-
sodes occurred, they were accompanied by
depressed pH levels and elevated concen-
trations of inorganic monomeric aluminum
(AW-
* Elevated concentrations of SO42' in Penn-
sylvania streams and of NO3" in Catskill
and Adirondack streams augmented natural
processes during episodes to produce lower
ANC and pH and higher Alini levels than
would have occurred due to natural proc-
esses alone.
* Even when SO42' and NO3" concentrations
did not markedly increase during episodes,
elevated baseline concentrations of SO42-
in all regions and of NO3~ in the Catskill
and Adirondack streams lowered minimum
10 Wigington, P.J., Jr., J.P. Baker, D.R. DeWalle, W.A.
Kretser, P.S. Murdoch, H.A. Sinonin, J. Van Sickle,
M.K. McDowell, D.V. Peck, and W.R. Barchet.
1993. Episodic Acidification of Streams in the
Northeastern United States: Chemical and Biologi-
cal Results of the Episodic Response Project.
EPA/600/R-93/190. Office of Research and Devel-
opment, U.S. Environmental Protection Agency,
Washington, DC.
12
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CHAPTER 2: ENVIRONMENTAL GOALS
ANC and pH below levels naturally ex-
pected during episodes.
* Fish in all three regions studied, exposed to
low pH and high Alim over longer periods
of episodic exposure had higher short-term
mortality rates and showed greater
long-term adverse population-level effects.
Time-weighted median Alim concentration
was the single best predictor of brook trout
mortality found during these studies. Fur-
thermore, it was concluded that stream as-
sessments based solely on chemical meas-
ures during low flow do not accurately pre-
dict the status of fish communities in small
streams.
* The ability of fish to avoid episodic acidic
water conditions by moving to less affected
waters only partially mitigated the adverse
effects in small streams. Such behavioral
adaptations were not sufficient to sustain
fish density or biomass at the levels ex-
pected in the absence of acidic episodes.
* Brook trout density and biomass were not
different between chronically acidic
streams and streams with episodes where
ANC decreased to less than 0 ueq/l. Differ-
ences did exist, however, between streams
with higher ANC with and without epi-
sodes.
This last point supports the hypothesis that epi-
sodic acidification can be the primary cause of ad-
verse effects to brook trout (and other ecological
components) in acid-sensitive streams (i.e., ANC<
50 ueq/l). These episodic effects potentially equal
those seen in chronically acidic streams (ANC<
0 ueq/l). Indeed, effects from severe episodic
acidification (i.e., events leading to ANC<0 ueq/l
in surface waters) are likely the first source of bio-
logical damage to most aquatic populations and
communities inhabiting waters that have become
chronically acidified. The continuing adverse eco-
logical effects from episodic events often blend
with and become indistinguishable from all other
effects accompanying chronic acidification.
The findings of this study and other analyses
clearly point to the importance of considering po-
tential effects of both long-term chronic and
short-term episodic acidification when considering
the effectiveness of an acid deposition standard or
standards.
For most regions of North America at risk from
acidic deposition, the effects from nitrogen deposi-
tion on aquatic systems are more likely to remain
primarily episodic in nature, except when water-
sheds move toward nitrogen saturation, and nitro-
gen increasingly becomes a direct cause of both
episodic and chronic acidification. Currently, data
available for most regions are inadequate to exten-
sively assess episodic effects related to nitrogen
deposition or to assess the potential for and rate of
watershed nitrogen saturation. Furthermore, while
available data on episodic acidification may in-
crease, because of the difficulty, expense, and
often the risk involved in collecting data during
episodic events (e.g., intensive spring sampling in
high-elevation snowmelt areas), data bases for as-
sessing episodic effects are not likely to become
comprehensive. Clearly, both SO42~ and NO3"
deposition can have major influences in surface
water acidification processes. Evaluating the effec-
tiveness of and options for acid deposition stan-
dards should include simultaneous consideration
of both acidification causes.
2.2.3 Cumulative Loading Effects
Deposited sulfur and nitrogen can be incorporated
into watersheds over the long term through a vari-
ety of physical, chemical, and biological proc-
esses. Current evidence suggests that the principal
dynamic mechanism of concern with regard to
watershed sulfur retention is physical/chemical in
nature (i.e., adsorption of inorganic sulfate). On
the other hand, the principal dynamic storage
mechanism of nitrogen retention in watersheds
appears to be biological in nature. Non-biological
removal mechanisms also may incorporate some
nitrogen into long-term storage within some water-
sheds, but this process is poorly understood.
Overall, the dynamics of sulfur adsorption and
desorption and nitrogen retention over landscapes
of different scales (watersheds to regions) can vary
significantly. Considerable uncertainty exists in
understanding these dynamics. Despite this uncer-
tainty, available results strongly indicate that time
is critical in defining sensitivities of resources to
inputs of both sulfur and nitrogen. While most wa-
tersheds can assimilate considerable quantities of
both chemicals without significant adverse effects,
their assimilative capacities are finite. Watershed
assimilative capacities vary with how rapidly de-
posited chemicals are assimilated and the time
over which repeated deposition events impair
these abilities. In other words, there are varying
deposition frequencies, rates, and durations when
13
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
watershed assimilative capacities reach saturation.
This is sometimes called steady state, the point
when the output (loss) of a substance from a wa-
tershed (e.g., sulfur leaving a watershed in the
form of SO42' in stream flows) equals its input
(e.g., as sulfur-containing compounds in deposi-
tion) on an annual basis.
Although sulfur deposition over the long term can
lead to equilibrium or steady state in watersheds,
similar steady-state conditions for nitrogen deposi-
tion are likely to be much less common. This is
because nitrogen uptake dynamics are affected
much more by biological changes within water-
sheds, such as forest cutting and regrowth as well
as natural vegetative succession. Consequently, as-
sessing the history of both sulfur and nitrogen
deposition is important in assessing long-term ef-
fects attributable to cumulative loadings by acidic
deposition.
A resource's or region's current sensitivity to acidic
deposition, therefore, needs to be evaluated with
respect to the historical deposition patterns and re-
sponses. Many regions with ample buffering ca-
pacity and remaining sulfur and nitrogen adsorp-
tion capacities may benefit little from future de-
creases in acidic deposition. Other regions facing
imminent depletion of their buffering or adsorption
capacities, however, would likely be highly re-
sponsive to decreasing deposition rates. The
DDRP, discussed in Section 2.5.2, provides a use-
ful beginning for understanding underlying rela-
tionships and defining remaining uncertainty about
post-deposition dynamics of atmospheric sulfur
deposition in watersheds within three regions of
the eastern United States. The Nitrogen Bounding
Study (NBS), discussed in Section 2.5.3, provides
additional useful results to improve our under-
standing of the influence of nitrogen saturation on
watershed processes affected by combined sulfur
and nitrogen deposition within the three regions
studied in the DDRP.
2.2.4 Recovery of Acidified Ecosystems
Acidified ecosystems can show signs of recovery
following reductions in acidic deposition rates.
Benefits have been demonstrated in (1) regions
where major local source emissions have been re-
duced, (2) experimental aquatic and terrestrial sys-
tems where applied doses of acids have been re-
duced or ended, (3) limed watersheds and surface
waters, and (4) model projections. Varying degrees
of successful recovery in communities of mi-
crobes, algae, higher plants, invertebrates, fish,
and amphibians were noted in the 1990 NAPAP
studies.11 Degrees of ecological recovery, how-
ever, did vary among the species, groups, and
studies reviewed.
Recovery rates depend primarily on three factors:
(1) rates of reduction in emissions and deposition
of SO42' and NO3"; (2) ongoing acid retention
processes in terrestrial environments, including
sulfate adsorption and base cation weathering; and
(3) time lags caused by delayed biological process
responses. In some instances, significant lags are
involved or irreversible changes have occurred.
The influence of such lags accompanying slow,
gradual chemical improvements are well illus-
trated in a series of studies of Ontario lakes follow-
ing reduced emissions from the Sudbury area
smelters.12
Mitigation strategies that attempt to restore ecosys-
tems without reducing deposition (e.g., liming) are
only partially successful in restoring water quality
and recovering biological populations. In fact,
rarely will distressed ecosystems return to their
predisturbance condition after the cause of the dis-
turbance has been removed, because the complex
ecological interrelationships among predisturbance
species are rarely the same following disturbance
of the system and its recovery. When natural evo-
lutionary and successional regimes of predisturbed
systems are disrupted, competition for nutrients
and other habitat resources and predatory relation-
ships among species in recovering systems have
subtle to substantive differences from their predis-
turbance relationships. Thus, restoration to the
"predisturbance condition" is not always possible,
nor is it necessarily an appropriate goal.13 Altema:
tively, rehabilitating or rejuvenating selected at-
11 Baker, J.P., D.P. Bernard, S.W. Christensen, M.J.
Sale, J. Freda, K. Heltcher, D. Marmorek, L. Rowe, P.
Scanlon, C. Suter, W. Warren-Hicks, and P. Wei-
bourn. 1990. Biological Effects of Changes in Sur-
face Water Acid-base Chemistry. NAPAP Report 13.
In: Volume II, National Acid Precipitation Assess-
ment Program, Acidic Deposition: State of Science
and Technology. Superintendent of Documents,
Washington, DC.
12 Keller, W., J.R. Piblado, and J. Carbone 1992.
Chemical responses of acidic lakes in the Sudbury,
Ontario area to reduced smelter emissions, 1931-
1989. Canadian Journal of Fisheries and Aquatic
Sciences 49 (Suppl. 1):25-32.
13 Cairns, J., Jr. 1989. Restoring damaged ecosystems:
Is predisturbance condition a viable option? Envi-
ronmental Professional 11:152-159.
14
-------
CHAPTER 2: ENVIRONMENTAL GOALS
tributes or functions may be all that is required for
restoration to be deemed successful.
A recent review and assessment concluded that
uncertainty remains concerning the definition of
appropriate measures of reversibility and recovery
for acidified ecosystems.14 Differences exist par-
ticularly between setting goals based on hu-
man-centered objectives (e.g., fish production for
human use) versus more intangible ecological and
conservation purposes. Further, assessment of eco-
system recovery following deposition reductions
can be obscured by other environmental perturba-
tions such as climate change and modified land-
use practices.
2.3 CHARACTERIZING RESOURCES AT RISK FROM
ACIDIC DEPOSITION
Relationships of resources to acidic deposition de-
pend on two characteristics: resource sensitivity
and acidic deposition exposure rates. Simultane-
ously considering regional distributions of both
characteristics allows assessments of risk potentials
produced by acidic deposition over discrete geo-
graphic regions. This approach helps to define re-
gional needs for and effectiveness of acid deposi-
tion standards. In this process, sensitivity is an in-
herent attribute of an individual resource that in-
creases its susceptibility to likely adverse effects
due to acidic deposition. Exposure is determined
by the deposition intensity, frequency, duration,
and specific times that acidic deposition falls into
an area. Risk is the probability that exposure to po-
tentially hazardous environmental conditions pro-
duced by acidic deposition will exceed the toler-
ance level for a sensitive resource and cause an
adverse effect. For any sensitive resource to be at
high risk from any hazardous substance or envi-
ronmental condition, it must have a high probabil-
ity of being sufficiently exposed to the substance
or condition, such that its inherent ability to toler-
ate the change will be exceeded and adverse ef-
fects will likely result. Because environmental re-
sources have ranges of sensitivities and risks to po-
tential effects caused by acidic deposition, re-
sources having equivalent sensitivities can have
14 Dise, N, W. Ahlf, C. Brahmer, BJ. Cosby, J. Fott, M.
Hauns, I. Juttner, K. Kreutzer, C.G. Raddum, and
R.F. Wright. 1994. Group Report: Are Chemical
and Biological Changes Reversible? Pages 275-381
in C.E.W. Steinberg and R.F. Wright (editors). Acidi-
fication of Freshwater Ecosystems: Implications for
the Future. J. Wiley and Sons, New York, NY.
different risk potentials for adverse effects depend-
ing on where they are located.
The next two subsections review in more detail the
concepts of resource sensitivity and risk as they
apply to acidic deposition. Understanding these
concepts is essential for determining
* Locations containing sensitive resources at
risk,
* Which sensitive resources may be the app-
ropriate primary focus of protection,
* Appropriate environmental assessment in-
dicators, and
* The extent of protection afforded.
2.3.1 Defining Sensitive Resources
There are different types of sensitivities to acidic
deposition, and a resource can be insensitive to
one effect while being sensitive to others. For ex-
ample, a region with highly alkaline surface waters
may saturate with nitrogen. This saturation could
lead to no change in surface water acidity, but
could lead to significant increases in eutrophica-
tion downstream. Consequently, the term sensitive
can be imprecise and confusing. It can be used to
describe different scales of resolution for different
resource units (e.g., extent of landscape areas,
water chemistry characteristics, or species group-
ings), and different degrees of resource sensitivi-
ties. Also, each use has different scientific and pol-
icy implications. Such differences affect each po-
tential criterion used in determining acid deposi-
tion standards. Thus, when considering potential
resource effects and risks, it is important to care-
fully define the specific ecosystem components
within the region and specific concerns regarding
the sensitivity being addressed.
An early MAS report indicated that lakes and
streams with ANCs of 200 ueq/l or less are sensi-
tive and subject to damage at moderate acidic
deposition rates, whereas surface waters with
ANCs of 40 ueq/l or less are critically sensitive to
such effects.15 Although ANC is an important re-
sponse indicator of potential surface water sensi-
tivity, it is not the only relevant response indicator
of sensitivity. For example, the presence or ab-
sence of acid-sensitive fish, invertebrates, algae,
15 National Academy of Sciences. 1983. Acid Depo-
sition: Atmospheric Processes in Eastern North
America. National Academy Press.
15
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
and higher plant species are other relevant indica-
tors of potential sensitivity and acidification prob-
lems in lakes and streams. Further, knowing the
ANC of surface or ground waters provides little in-
dication of the actual sensitivity of neighboring ter-
restrial resources. For example, injury to red
spruce foliage attributable to acidic deposition
typically has little direct relationship to the ANC of
neighboring soils or waters. Consequently, when
there is a need to assess potential effects of acidic
deposition on terrestrial resources or ecosystems,
assessments should consider other parameters or
indicators of sensitivity in addition to ANC.
Because numerous natural phenomena and proc-
esses influence the sensitivity and potential risk
status of resources, interpretation and projection of
receptor responses to acidification are difficult.
Factors that should be carefully evaluated when
assessing needs for acid deposition standards in-
clude naturally occurring organic acidic systems,
annual and seasonal variabilities in precipitation,
and related climatic variability. Land and resource
use (e.g., changes in fishing pressure, point and
nonpoint nutrient discharges, mining runoff, and
other watershed activities) also potentially con-
found interpretation of acidification sensitivity and
effects. The types of effects caused by many of.
these factors are summarized in Exhibit 2. The list,
although incomplete, shows that a considerable
matrix of factors interact to determine the potential
sensitivity of individual surface waters, water-
sheds, and the natural resources they contain.
These interactions cause differences in sensitivity
and responses to acidic deposition among re-
sources within individual watersheds and among
adjacent watersheds. Many factors summarized in
the exhibit are discussed in greater detail in subse-
quent sections.
Exhibit 2 also suggests a focus on potential acidic
deposition effects linked to terrestrial soils and
aquatic resources. Such a focus is not surprising,
because most acidic deposition eventually flows
through soils and into aquatic systems. Responses
by these resources to acidic deposition are clear,
well understood, and in many cases well docu-
mented. Therefore, most of the following discus-
sion concentrates on concerns associated with en-
vironmental changes in soils, lakes, and streams.
Wherever possible, however, discussion is punctu-
ated with summaries and highlights of potential re-
lationships of acidic deposition to other terrestrial
resources.
Our accumulated knowledge indicates that sensi-
tive resources can be defined over multiple ranges
of temporal, geographic, geochemical, and bio-
logical categories and scales. Also, aquatic re-
sources can be sensitive to episodic short-term
acidification, chronic long-term acidification, or
both. Fundamental factors and attributes associ-
ated with differences in sensitivity include:
« ANC of surface and ground waters;
• Supply of base cations from bedrock and
soil particle weathering;
• Supply of base cations and buffering by soil
solutions;
* Physical and chemical adsorption, desorp-
tion, and reduction of sulfate;
• Biological assimilation of nitrogen within
watersheds; and
» Ability of sensitive resident organisms to
modify physiological processes or behavior
patterns or otherwise escape in response to
habitat changes attributable to acidifica-
tion.
Thus, although considering levels of ANC in as-
sessing sensitive resources is important, consider-
ing the presence and possible implications of other
indicators of potential sensitivity (including the
occurrence of threatened and endangered species)
is often equally important. Appropriate definitions
of sensitivity may therefore vary with precise pol-
icy questions being asked. When the primary con-
cern is protecting water quality in sensitive surface
waters, ANC often can be a useful response indi-
cator. If concern is broadened to include the sensi-
tivity of all natural resources, the approach used to
classify sensitivities also should be expanded.
In congressional discussions regarding the man-
date for an acid deposition standard study, Con-
gress followed the example of the NAS report
noted above in distinguishing between resources
that are "sensitive" and those that are "critically se-
nsitive" to the effects of acidic deposition. Re-
source sensitivity occurs on a continuum. Conse-
quently, rather than refining assessments in the fol-
lowing sections to distinguish among responses for
subcategories of sensitive resources (i.e, sensitive
versus critically sensitive), it is more valuable to
assess the degree to which sensitive resources, in
general, are exposed to different levels of risk from
16
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CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBIT 2. PRINCIPAL WATERSHED AND SURFACE WATER CHARACTERISTICS
THAT INFLUENCE RESOURCE SENSITIVITY TO ACIDIFICATION3
Category
Bedrock geology
Soils:
Buffering capacity
Depth
SO42' adsorption
SO42- reduction
NO3- retention
Topography
Watershed to surface water
area ratio
Lake flushing rate
Watershed vegetation and land use:
Dominant vegetation
Cultural influence
Forest management
Water quality:
Alkalinity/ANC
SO42- reduction
Trophic status
Humic substances
Sphagnum moss
Climate/meteorology.
Precipitation
Snow accumulation
Growing season
Alkaline dusts
Increased Sensitivity
Resistant to weathering
(metamorphic, igneous)
Lower potential
Shallower
Lower potential
Lower potential
Lower potential
Steep-sloped
Lower
Higher
Coniferous
Forested
Reforestation
Lower (< 200 ueq/l)
Lower potential
Highly oligotrophic
Lowest concentrations
Present
Higher
Higher
Shorter
Lower
Decreased Sensitivity
Easily weathered
(sedimentary, calcite containing)
Higher potential
Deeper
Higher potential
Higher potential
Higher potential
Shallow-sloped
Higher
Lower
Deciduous
Agriculture, municipal
Deforestation
Higher (> 200 ueq/l)
Higher potential
Less oligotrophic to eutrophic
Higher concentrations
Absent
Lower
Lower
Longer
Higher
a Modified from Marcus, M.D., B.R. Parkhurst, and F.E. Payne. 1983. An Assessment of the Relationship
among Acidifying Depositions, Surface Water Acidification, and Fish Populations in North America.
EA-3127, Volume 1, Final Report. Electric Power Research Institute, Palo Alto, CA.
acidic deposition across different geographic ar-
eas. As such, the term critically sensitive resource
is not used in this report. Much of the following
discussion does, however, focus on sensitivities
and responses for surface waters projected by EPA
model analyses for lakes and streams having ANCs
of 50 ueq/l or less, a value that approximates the
ANC value of 40 ueq/l considered by MAS and
Congress to distinguish between their two sensitiv-
ity groupings of concern. Therefore, discussions in
the following section regarding acidification rela-
tionships in lakes and streams with ANCs<50 ueq/l
can generally be interpreted as applying to
"critically sensitive" resources.
2.3.2 Identifying Resources at Risk
Sensitivity, as noted above, is only one determi-
nant of potential risk. For a resource to be at risk, it
must be sensitive to a potential stressor and must
have an actual or reasonable possibility of expo-
sure to the stressor in a magnitude sufficient to
cause an adverse effect.16 Sensitive resources are
at low risk when located where acidic deposition
16 Risk Assessment Forum. 1992. Framework for Eco-
logical Risk Assessment. EPA/630/R-92/001. U.S.
Environmental Protection Agency, Washington, DC.
17
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
loads are currently below and are projected to re-
main below thresholds likely to cause adverse ef-
fects. For example, NAPAP studies reviewed in
Appendix A generally indicate that many surface
waters in western North America are likely to be
more sensitive to adverse effects from acidic depo-
sition than are similar resources in eastern North
America. Because current deposition levels in the
West are generally below thresholds that produce
long-term surface water acidification, however,
the present risk to these resources from chronic
acidification is low. If the intensity of western
deposition increases, adverse chronic acidification
effects in the West might exceed those in the East.
Consequently, the potential for high future risk to
sensitive western resources remains a concern.
Risk assessment, therefore, must address not only
whether a location now receives sufficient acidic
deposition to produce adverse effects, but also the
likelihood that the intensity and composition of the
deposition may change in the future, thereby
changing future exposure and potential risks to re-
ceptor resources. Consequently, it is necessary to
determine what residual risks to sensitive resources
remain after implementing emissions controls re-
quired by the CAAA, where these risks may be lo-
cated, and their significance. For example, EPA
modeling analyses indicate the degree to which
sensitive resources will be protected when current
Title IV requirements are fully implemented (i.e.,
emissions reductions of 10 million tons SO2 and
2 million tons NOX). But, how might sensitive re-
sources benefit from further decreases in deposi-
tion rates or implementation of a deposition stan-
dard? How might these benefits change with vary-
ing additional reductions of sulfur or nitrogen
deposition? How do possible changes in sulfur or
nitrogen retention within watersheds affect these
possible relationships? The DDRP and NBS, as dis-
cussed in Sections 2.5.2 and 2.5.3, begin to an-
swer these questions.
Resources potentially sensitive to and at risk from
acidic deposition occupy vast expanses of water-
sheds and forests, multitudes of lakes, and miles of
streams in regions scattered across North America.
A complete census of these resources would be
prohibitively time consuming and costly. Instead,
assessments of locations containing sensitive re-
sources must depend on statistically based surveys,
using proven methods to characterize populations
within an estimated probability of error. Regional
and resource priorities for an acid deposition stan-
dard or standards should be based on such sur-
veys. Information summarized in Section 2.4 re-
views survey results useful in targeting geographic
regions at risk and identifying sensitive resources.
2.4 IDENTIFICATION OF RESOURCE AND
REGIONAL PRIORITIES
2.4.1 United States
Scientific information from the extensive research
efforts supported and reviewed by NAPAP directly
apply to setting needs and priorities for protecting
resources and regions sensitive to acidic deposi-
tion. Appendix A summarizes the major conclu-
sions from 10 of NAPAP's State of Science and
Technology reports. This section synthesizes gen-
eral findings from these reports.17 Also summa-
rized here is additional information from more re-
cent research regarding identification of regions
sensitive to the effects of acidic deposition in the
United States18 and Canada.19
The NAPAP studies, which provide much of the
best information currently available for the United
States, contain clear implications for identifying
resources most at risk from atmospheric deposition
of acidic compounds. Of all effects to environ-
mental resources from acidic deposition, the scien-
tific community best understands changes in lakes,
streams, rivers, and soil chemistries. The rate and
extent of acidic deposition effects on other re-
sources are less clear.
1. SOIL CHEMISTRY EFFECTS: In the eastern
United States, concentrations of sulfur in
soils generally follow trends in sulfur depo-
sition. In some regions, soil concentrations
of calcium and magnesium are inversely
related to sulfur deposition loads resulting
in soil nutrient depletion. Further, a recent
study suggests that most calcium and mag-
nesium in the soil of the spruce-fir ecosys-
tem in the Northeast was lost 20-40 years
17 This review is primarily drawn from conclusions pre-
sented by P.M. Irving (editor). 1991. Acidic Depo-
sition: State of the Science and Technology - Sum-
mary Report of the U.S. National Acid Precipitation
Assessment Program. National Acid Precipitation
Assessment Program, Washington, DC.
18 The primary source for this additional summary in-
formation is NAPAP. 1992. Report to Congress.
National Acid Precipitation Assessment Program,
Washington, DC.
19 Brydges, T.C. 1991. Critical loads, reversibility and
irreversibility of damage to ecosystems. Pages 245-
260 in Electricity and the Environment, International
Atomic Energy Agency, Vienna, Austria.
18
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CHAPTER 2: ENVIRONMENTAL GOALS
ago due to acidic deposition, when
deposition rates were increasing rapidly.
While control studies quantitatively link
changes in soil chemistries to tree and
other plant responses, similar studies link-
ing acidic deposition effects on soils to ac-
tual plant effects in nature remain incon-
clusive.20 Many important studies are con-
tinuing, however. The most apparent influ-
ence of soil chemistry responses attribut-
able to acidic deposition are seen in effects
on surface waters (see Item 3, below).
2. FOREST AND AGRICULTURE CROP EFFECTS: De-
veloping evidence indicates that acidic
cloud water, in combination with other
stresses, likely increases winter injury, and
reduces tree vigor and growth, and causes
crown damage and death to high-elevation
red spruce forests in the United States, par-
ticularly in the northern Appalachians and
high-elevation regions of the Northeast.
Evidence of acidic deposition involvement
in the decline of red spruce in the southern
Appalachians is less substantial. Involve-
ment of acidic deposition in the decline of
sugar maples in parts of the northeastern
United States and eastern Canada has not
been demonstrated but cannot be ruled out
on the basis of available information. Re-
cent information, in fact, indicates an ap-
parent improvement in sugar maple tree
health since 1988, with no visible effects
from sulfate deposition, with the exception
of observed decline in health of the On-
tario maple.21 The vast majority of forests
in the United States and Canada have not
declined. Within forested regions, acidic
deposition primarily exerts its stress on nu-
trient cycling. Some evidence suggest that
lichen communities and chemistries may
be useful early indicators of forest health
effects. Ambient acidic deposition levels
have not been shown to be responsible for
agricultural crop yield reductions.
20 Brandt, C.J. 1994. Acidic Deposition and Forest
Soils: Potential Changes in Nutrient Cycles and Ef-
fects on Tree Growth. Report to Watershed Re-
sponse Program, Environmental Research Laboratory,
U.S. Environmental Protection Agency, Corvallis,
OR.
21 U.S. EPA. 1994. U.S. Canada Air Quality Agree-
ment Progress Report.
3. SURFACE WATER ACIDIFICATION: Numerous
lines of evidence support the fact that
acidic deposition can acidify surface wa-
ters and that acidification attributable to
acidic deposition has occurred in sensitive
aquatic systems during this century (see
Appendix A). Most sensitive lakes and
streams in the United States—especially
those that have current ANCs of 50 ueq/l or
more—probably have not experienced re-
cent chronic declines in pH or ANC asso-
ciated with acidic deposition.
4. REGIONS CONTAINING SENSITIVE SURFACE WA-
TERS: The National Surface Water Survey
(NSWS) conducted under the auspices of
NAPAP in 1984-85, identified six "high-in-
terest areas" containing most of the surface
waters surveyed (95 percent of the lakes
and 84 percent of the stream reaches) that
were chronically acidified as indicated by
concentrations of inorganic anions, pre-
dominately SO42-, NO3-, and Ch. These
areas include the southwest Adirondack
Mountains, New England, mid-Appala-
chian Region, Atlantic Coastal Plain,
northern Florida Highlands, and low-silica
lakes in the eastern Upper Midwest.
Historical evidence supports the premise
that acidic deposition undoubtedly is
related to surface water acidification in the
Adirondacks, the Pocono/Catskill subre-
gion, mid-Appalachians, eastern portion of
the Upper Midwest, the New Jersey Pine
Barrens, and, to a lesser extent, the Florida
panhandle. (Other areas of the Mid-
Atlantic Coastal Plain appear to be affected
more by organic acidification and land-use
activities such as acid mine drainage.)
Chronic acidification of western lakes from
acidic deposition appears not to have oc-
curred. The following subsections further
describe several of the regions containing
sensitive surface waters.
5. CHARACTERISTICS OF WATERSHEDS CON-
TAINING SENSITIVE SURFACE WATERS: Surface
waters most sensitive to acidic deposition
are often located in watersheds having
shallow acidic soils with rapid, shallow
subsurface flows. Acidic lakes and streams
tend to occur in smaller watersheds and, in
regions where significant elevation gradi-
ents exist, at the higher elevations (e.g.,
watersheds less than 30 km2 and elevations
greater than 300 m in the mid-Appalachian
19
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
region and the Pocono/Catskill subregion).
It must be noted, however, that these rela-
tionships are derived from studies empha-
sizing watershed responses to sulfate depo-
sition. Other, primarily biological, relation-
ships exist where deposition of acidifying
nitrogen compounds are a significant or
predominant concern.
6. RESPONSES BY SENSITIVE AQUATIC SPECIES AND
ECOSYSTEMS: Acid-sensitive species occur in
all major groups of aquatic organisms, but
most is known about responses by fish and
aquatic invertebrates. In general, sensitive
aquatic species inhabiting surface waters
that have low calcium concentrations
(<100-150 ueq/l) begin to be affected by
acidification processes as pH decreases be-
low about 6.0-6.5 (Exhibit 3) and as inor-
ganic monomeric aluminum concentra-
tions increase above 30-50 ug/l. These
changes affect these species first by de-
creasing their ability to survive, reproduce,
or compete in acidic surface waters. Such
responses can eliminate affected species
and reduce species richness (i.e., the num-
ber of species living within a surface
water). Such changes typically occur first
in affected surface waters during episodic
runoff events (i.e., when storm water or
snowmelt runoff causes short-term flushes
of acutely toxic water chemistries to enter
receiving waters). System-level processes
such as composition, nutrient cycling, oxy-
gen usage, and photosynthetic rate are
fairly robust and are affected only at rela-
tively high levels of acidity (e.g., chronic
pH less than 5.0-5.5).22
These findings lead to several working conclusions
regarding sensitive resources and regions at poten-
tially greatest risk from acidification, and the
maximum degree of protection that may be neces-
sary. It is on these resources, regions, and protec-
tion goals that considerations should focus regard-
ing potential necessity and benefits of acid deposi-
tion standards:
1. SENSITIVE RESOURCES OF PRIMARY CONCERN:
Considering that the natural resources most
sensitive to acidic deposition would exhibit
22 Schindler, D.W. 1987. Detecting Ecosystem Re-
sponses to Anthropogenic Stress. Canadian Journal
of Fisheries and Aquatic Sciences 44(Suppl.):6-25.
the strongest responses and provide the
most conclusive evidence of effects, it is
reasonable to conclude that the natural re-
sources most sensitive to acidic deposition
are aquatic systems and high-elevation red
spruce forests. Therefore, possible future
acid deposition standards focused on pro-
tecting sensitive aquatic resources in the
eastern United States and red spruce forests
in the northern Appalachians and high-ele-
vation regions of the Northeast should pro-
vide adequate protection for most sensitive
natural resources at risk. Protection of sens-
itive aquatic resources should particularly
focus on lakes and streams located where
watersheds are smaller, have shallow
acidic soils with rapid, shallow subsurface
flows, and are at higher elevations.
2. REGIONAL PRIORITIES FOR PROTECTION: In the
eastern United States, the 1990 CAAA and
any future acidic deposition controls are
most likely to reduce the threats of acidic
deposition to surface water resources in
these regions: Adirondack Mountains, Po-
cono and Catskill Mountains,
mid-Appalachian Region, the Southern
Blue Ridge Province, New Jersey Pine Bar-
rens, northern Wisconsin and Michigan's
Upper Peninsula, and, possibly, northern
Florida. The first three regions apparently
are now at continuing risk from acidifica-
tion effects. Other regions with sensitive
resources should be monitored and as-
sessed to evaluate whether continuing
acidic deposition will affect those re-
sources. These areas include parts of
Maine, New Hampshire, Vermont, Massa-
chusetts, Connecticut, and Rhode Island;
northern Minnesota; parts of the Ozark
Mountains, Ouachitas Mountains, the
Carolina Piedmont, and the Atlantic
Coastal Plain; and parts of the Rocky
Mountains, Sierra Nevada Mountains, and
Cascade Mountains.
3. PROTECTION GOALS FOR AQUATIC SPECIES: The
biological effects of inorganic monomeric
aluminum associated with acidic deposi-
tion are minimized as the level of acidic
deposition is decreased and pH and ANC
levels in sensitive waters are kept relatively
high. Based on studies of sensitive aquatic
species, to protect aquatic resources in se-
nsitive watersheds from the effects of
long-term, chronic acidification, a general
20
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CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBIT 3. CRITICAL pH FOR SELECTED TAXA IN LAKES AND STREAMS*^
Critical pH Levels for Selected Aquatic Organisms
ao 5.5 5.0 45
4.0
Yellow Perch
Brook Trout
Lake Trout
Smallmouth Bass
Rainbow Trout
Common Shiner
American Toad*
Wood Frog-
Leopard Frog*
Spotted Salamander*
Crayfish'*
Mayfly-
Clam"
Snail"
From National Acid Precipitation Assessment Program. 1991. 7990 Integrated Assessment Report. NAPAP Of-
fice of the Director, Washington, DC.
Solid symbols for each type of organism are placed in favorable pH ranges; shaded symbols are placed in less
favorable ranges. No symbol is placed in pH ranges that generally do not support populations of a particular
type of organism.
Embryonic life stages.
Selected species.
goal is to maintain the pH of sensitive lakes
above pH 6.0-6.5 and inorganic mono-
meric aluminum below 30-50 ug/l. To pro-
tect these resources from the potential ef-
fects of episodic, acute acidification, sur-
face water ANC should be maintained at or
above 50 ueq/l. No single water quality
goal, however, addresses all needs to pro-
tect sensitive surface water resources.
Goals to protect aquatic resources may
also address site-specific needs to maintain
sensitive aquatic species, species of special
concern (e.g., listed threatened or endan-
gered species), and species richness in
these sensitive surface waters. This effort
certainly must include recognition that pH
levels less than 6.0 and ANC less than
50 ueq/l occur in some naturally acidic
surface waters, and that levels of pH less
than 6.0 can occur naturally in some loca-
tions accompanying periods of episodic
stormwater and snowmelt runoff. The spe-
cific environmental objectives of any acid
deposition standard should accommodate
the ranges of chemical qualities occurring
in natural waters. Furthermore, they should
protect those special biological communi-
ties evolved to inhabit naturally acidic sur-
face waters.
2.4.2 Identification of Sensitive Aquatic
Resources in Other North American
Regions
Most of this chapter reports quantitative results
based on EPA model analyses for three case study
regions: the Northeast (including the Adirondack
Mountains), the mid-Appalachian Region, and the
Southern Blue Ridge Province. Similar model
analyses were not performed for other sensitive re-
gions of North America due to data and budget
limitations. Instead, and in addition to the results
of the NAPAP National Surface Water Survey, EPA
supported a recent report that reviewed the re-
21
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
sponses of aquatic resources to acidic deposition
in four other regions of North America identified
in previous studies as holding sensitive aquatic re-
sources: the mountainous western United States,
upper midwestern United States, and northern
Florida.23 That review addressed effects from
acidic deposition, specifically sensitive aquatic re-
sources in each region. The approach incorporated
key results available from past research and as-
sessment efforts in North America and Europe. The
major conclusions derived during this review that
specifically related to the four regions assessed are
presented below. (Some conclusions from this re-
view regarding general deposition and response re-
lationships duplicate the findings of other studies
reported above and are not repeated in this sec-
tion.)
Western United States
* Most low-ANC lakes in the West are con-
fined primarily to glaciated, higher eleva-
tion, mountainous regions. These water-
bodies can be generally consolidated into
five lake populations, based on their loca-
tions within similar geomorphic units:
(Dthe Sierra Nevada in California; (2) the
Cascade Mountains in California, Oregon,
and Washington; (3) the Idaho Batholith in
Idaho and Montana; (4) the mountain
ranges of northwestern Wyoming; and
(5) the Rocky Mountains in Colorado.
* Results of the NSWS indicate that although
no lakes in this region are chronically
acidic, many lakes were found to have very
low ANC (i.e., 17 percent have ANC<
50 ueq/l), and should be considered sus-
ceptible to acidification, particularly epi-
sodic acidification.
* Watersheds in the alpine areas of these five
regions generally include broad expanses
of exposed bedrock, which is often highly
resistant to weathering, and contain little
soil or vegetative cover to neutralize acidic
inputs. Consequently, these regions include
a significant portion of the region's aquatic
23 Sullivan, T.J., and J.M. Eilers. 1994. Assessment of
Deposition Levels of Sulfur and Nitrogen Required to
Protect Aquatic Resources in Selected Sensitive Re-
gions of North America. Final Report. Environ-
mental Research Laboratory-Corvallis, U.S. Environ-
mental Protection Agency, Corvallis, OR.
resources that are the most sensitive to
acidic deposition.
* Natural characteristics of these watersheds
particularly predispose the surface waters
they contain to episodic acidification ef-
fects: (1) low-water retention capacities of
most watersheds; (2) ultra-low concentra-
tions of base cations; (3) low ANC in sur-
face waters throughout the year; (4) large
snowpack accumulations and substantial
base cation dilution during runoff;
(5) frequent, periodic, heavy rain storms
with high runoff events; and (6) short water
retention times and high flushing rates for
most lake basins.
* Nitrate concentrations in the majority of
the western lakes are virtually undetectable
during the fall. The NSWS samples, how-
ever, revealed that a relatively large num-
ber of lakes in northwestern Wyoming, Si-
erra Nevada, and Colorado Rockies con-
tained high concentrations of NO3~. These
concentrations were sufficiently high to in-
dicate that many watersheds in these re-
gions may have little remaining capacity to
assimilate excess NO3" deposition.
• No extant data suggest that lakes in the
West have experienced chronic acidifica-
tion. It is likely, however, that episodic ef-
fects have occurred in some lakes under
some current deposition regimes and that
deposition concentrations of NO3" espe-
cially have caused small, chronic losses of
ANC in some high-elevation watersheds.
As previously described, both nitrogen and
sulfur have the potential to contribute to
episodic acidification.
* Fish population data, although very lim-
ited, do not indicate a significant effect due
to acidification. However, trout species na-
tive to Western lakes such as rainbow trout
and cutthroat trout are considered more
sensitive to low pH and elevated aluminum
than brook trout native to eastern lakes and
streams.
* Although potentially difficult based on
wide ranges in precipitation volume, an
event-based deposition standard may be a
consideration to adequately address the po-
tential effects of episodic acidification in
the West. Another consideration which
combines concerns for both total mass and
22
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CHAPTER 2: ENVIRONMENTAL GOALS
maximum concentration of pollutants de-
posited, is an acid deposition standard
which establishes limits based on the
maximum allowable annual-weighted pol-
lutant concentrations or based on total an-
nually deposited chemical mass loading.
Upper Midwestern United States
* The Upper Midwest is characterized by
numerous lakes created by repeated glacia-
tion, little topographic relief, deep glacial
overburden, and rarely exposed bedrock.
In this region the primary aquatic resources
sensitive to acidic deposition are seepage
lakes with low base cation concentrations
that receive nearly all of their water inputs
as precipitation directly onto the lake sur-
face. These lakes have generally long water
retention times, which provide opportuni-
ties for in-lake SO42" reduction and NO3~
assimilation processes to neutralize most
acidic inputs and to prevent the concentra-
tion of SO42' through evaporation.
* The Upper Midwest has a large population
of low-ANC lakes, but relatively few acidic
(ANC<0 ueq/l) lakes. Paleolimnological
evidence suggests that some of these lakes,
particularly in Michigan's Upper Peninsula,
have developed low ANC or become
acidic, consistent with historic trends for
sulfur deposition since preindustrial times.
It should be noted that land use changes
and other human disturbances in the wa-
tersheds have exerted greater influences on
the acid-base chemistry in more sensitive
lakes of this region than has acidic deposi-
tion.24
* The NSWS indicated that 19 percent of the
lakes in this region have ANC<50 ueq/l
(only 3 percent of that figure is ANC<0).
Historical data are too limited to determine
the degree to which acidic deposition has
impacted fish populations in this region
such as yellow perch, bass, and others.
However, lakes with low pH in northeast-
ern Wisconsin and upper Michigan support
24 Sullivan, T.J. 1990. Historical Changes in Surface
Water Acid-Base Chemistry in Response to Acidic
Deposition. SOS/Til, National Acid Precipitation
Assessment Program, Washington, DC. 212 pp.
fewer fish species than expected for their
size and lake type.
* Concentrations of inorganic nitrogen are
uniformly low in surface waters throughout
this region. Most nitrogen is efficiently re-
tained by terrestrial and aquatic organisms.
Snowmelt has not been shown to provide
any significant influx of NO3- to these lakes
because most snowmelt water percolates
through the soil prior to entering surface
waters, allowing terrestrial organisms to as-
similate the deposited nitrogen. Therefore,
the key concern for this region is chronic,
sulfur-driven acidification. If recent trends
of decreasing sulfur deposition in the Up-
per Midwest were to reverse, lakes with
ANCs near zero in this region could be ex-
pected to acidify.
Northern Florida
• Florida lakes are located on marine sands
overlying carbonate bedrock; where
groundwater interacts with the deeper
aquifer, surface water can be highly alka-
line. Lakes that receive input waters only
from shallow aquifers in highly weathered
sands, however, can be quite acidic and
sensitive to acidic deposition. In fact,
northern Florida contains one of the largest
populations of acidic lakes in the United
States. Seventy-five percent of the Panhan-
dle lakes are acidic, as are 26 percent of
the lakes in the northern peninsula.
* The NSWS determined that approximately
60 percent of the acidic lakes in Florida,
primarily in the northcentral peninsula, are
acidic due to acidic deposition. Subsequent
scientific study however, suggest that the
role of the natural sulfate bearing ground
water, significant land use changes in the
region, and marine sources likely have
greater effects on the acidic water chemis-
tries of these systems than was previously
estimated. Therefore, the extent of possible
water quality changes due to acidic deposi-
tion alone in Florida cannot now be
quantified, but is likely lower than the
NSWS estimate. The best evidence that
acidic deposition effects have altered the
surface water chemistry exists for the Trail
Ridge region in northeastern Florida.
Available data currently indicate that there
has been no widespread biological damage
23
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
due to acidic deposition within the sens-
itive regions studies in northern Florida.
2.4.3 Canada
The area of Canada considered to be at greatest
risk from acidification (i.e., the region having
minimal ability to neutralize incoming acids and
receiving elevated deposition of potentially acid-
forming chemicals), includes the region east of the
Manitoba-Ontario border and roughly south of 52°
N latitude (near the southern limit of James Bay).25
(Known threats to forests in this regions, as sum-
marized by NAPAP, were reviewed earlier in this
section.) This area contains more than
700,000 lakes covering about 160,000km2
(excluding the Great Lakes). Extrapolation of sur-
vey information indicates that 14,000 lakes are
presently acidic. Modeling projections for eastern
Canada indicate that at least an additional
10,000 to 40,000 lakes would become chronically
acidic at 1985 deposition levels, as watershed in-
put-output budgets reach equilibrium over time
with concentrations of atmospherically deposited
acid-forming ions.
Four important relationships primarily influence
the surface water chemistry of these Canadian
lakes and their potential sensitivity to acidification:
1. Because of the predominance of silicate
bedrock, thin overburden, and generally
high precipitation volumes (approximately
100 cm/yr or greater), nearly all lakes in
eastern Canada can be hydrologically
characterized as drainage lakes.26 (The
acidification processes that dominate seep-
age lakes in the upper Midwest and Florida
lack importance in most Canadian lakes.)
2. Most glacially deposited soil covering east-
ern Canada has essentially no capacity for
SO42' adsorption. Because there appears to
be no significant geological sources of
25 Information presented in this section regarding sen-
sitive aquatic resources in Canada, unless otherwise
cited, comes from the summary of Jefferies, D.S.
1991. Southeastern Canada: An Overview of the
Effects of Acidic Deposition on Aquatic Resources.
Pages 273-286 in D,F, Charles (editor). Acidic
Deposition and Aquatic Ecosystems - Regional Case
Studies. Springer-Verlag, New York, NY.
26 Drainage lakes are lakes with permanent surface
water inlets and, usually, outlets. Seepage lakes are
lakes with no permanent surface water inlets or out-
lets.
SO42- in this region, limited adsorption ca-
pacity indicates that existing SO42- levels
are principally controlled by atmospheric
inputs. Note that most glacially deposited
soil covering eastern Canada is similar to
the soils in the northeast United States (i.e.,
Adirondacks) in that they have very limited
SO42- adsorption capacity, and acidifica-
tion is primarily controlled by atmospheric
inputs in both regions. 27
3. Base cation production of principally Ca2*
and Mg2+ by primary weathering or cation
exchange in the surrounding terrestrial wa-
tershed provide most of the ANC of all Ca-
nadian lakes. Thus, the variable mineralogy
in glacial overburden surrounding the lakes
dominates control of subregional variability
in sensitivity to acidification.
4. Organic acid anions appear to be impor-
tant in some waters, particularly in the ex-
tensive wetland areas of Nova Scotia,
Newfoundland, Labrador, and northwest-
ern Ontario; the occurrence of these ani-
ons, however, are generally not the pri-
mary cause of acidity in all lakes with ANC
of 0 ueq/l or less and pH less than 6.0.
Almost all known acidification related losses of
lake trout (Salvelinus fontinalis), smallmouth bass
(Micropterus dolomieui), and walleye (Stizostedion
vitreum) in Ontario surface waters have occurred
in the Sudbury area and are related to emissions
from the Sudbury area smelters; complete or bio-
logically significant reversal of acidic conditions in
these waters, however, may depend on continued
reductions in emissions over a wider region.28
Concern also continues regarding probable epi-
sodic influence of acidic deposition on Atlantic
salmon in tributary streams along the Atlantic
coast from Maine northward.29
27 Sullivan, T.J. 1990. Historical Changes in Surface
Water Acid-Base Chemistry in Response to Acidic
Deposition. SOS/T 11, National Acid Precipitation
Assessment Program, Washington, DC. 212 pp.
28 Keller, W. 1992. Introduction and overview to
aquatic acidification studies in the Sudbury, Ontario,
Canada, area. Canadian Journal of Fisheries and
Aquatic Sciences 49(Suppl. 1 ):3-7.
29 Lacroix, G.L. 1989. Ecological and physiological
responses of Atlantic salmon in acidic organic rivers
of Nova Scotia, Canada. Water, Air, and Soil Pollu-
tion 46:375-386.
24
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CHAPTER 2: ENVIRONMENTAL GOALS
2.5 ASSESSING PROTECTION NEEDS AND
RESOURCE RESPONSES IN THE CONTROL OF
ACIDIC DEPOSITION
Determining the potential for future benefits from
additional control of acidic deposition involves as-
sessing changes over broad geographical and eco-
logical scales. Projecting potential future relation-
ships is best done using models to simulate possi-
ble future scenarios. Simulation models are, in
fact, conceptualizations of the way things "work."
Due to the very simple character of their control-
ling process, some models can be very precise and
can be verified repeatedly by comparisons of pro-
jections to subsequent observations (e.g., projec-
tions of solar eclipses). In contrast, models of wa-
tersheds and of surface water responses to envi-
ronmental perturbations (e.g., acidic deposition)
are much more difficult to design and test.
This section briefly reviews the use and constraints
of simulation models. It presents detailed results
from two major EPA modeling studies and an ex-
tensive literature review aimed at increasing the
understanding of how acid-sensitive soil and
aquatic resources are affected by both sulfur and
nitrogen deposition. International and state effects-
based efforts to regulate acidic deposition are also
described. Findings from a recent EPA investiga-
tion, previously described in Section 2.2.2, show
the key involvement of rainstorm and snowmelt
events in lake and stream acidification and are
summarized here as well.
2.5.1 Model Application
MAGIC (Model for Acidification of Groundwater
in Catchments) is currently the model of choice for
assessing many watershed processes associated
with acidic deposition (Exhibit 4). It provided the
primary analytical basis for EPA's DDRP and the
NBS, as summarized in the next two subsections.
MAGIC has been tested more than any other
acidic deposition effects model. Results from these
tests (including some still underway) indicate that
MAGIC correctly projects the direction of change
of watershed responses and accurately projects the
magnitudes of rates of change for surface water
ANC and pH. MAGIC reasonably represents sulfur
retention within watersheds and the generation
and leaching of cations from watersheds, two
functions generally acknowledged to be the most
important of the modeled processes.
In recognizing that all models have strengths and
weaknesses, it is obviously unreasonable to expect
that MAGIC (or any other watershed acidification
model) will predict accurately exact values of
ANC or pH for any individual lake or stream in the
distant future (e.g., 50 years or more) under condi-
tions of significant dynamic change. Rather, the
appropriate use of MAGIC and other such models
is to project the direction and magnitude of possi-
ble chemical changes and to compare the relative
potential effects of different scenarios of acidic
deposition. MAGIC appears to be reasonably well
suited for such tasks. In reviewing model projec-
tions from these two studies on potential effects at-
tributable to future sulfur and nitrogen deposition,
it remains important to keep in mind the associ-
ated uncertainties that are highlighted in the fol-
lowing sections.
2.5.2 Direct/Delayed Response Project
As introduced in Section 2.2.1, a 1984 NAS panel
identified two geochemical processes as the domi-
nant watershed factors affecting or mediating
long-term surface water acidification: (1) the rates
at which a watershed supplies base cations
through neutralization and buffering processes in
response to its assimilation of deposited acids, and
(2) the capacity of a watershed to retain deposited
sulfur-containing compounds. (This NAS report in-
cluded minimum concern regarding effects from
nitrogen deposition). The capabilities of water-
sheds to perform these processes decrease over
time. Consequently, based largely on the NAS
conclusions, defining needs to protect various
aquatic and terrestrial resources from acidic depo-
sition depended on whether acidification is imme-
diately proportional to the intensity of the deposi-
tion (i.e., "direct") or lags in time (i.e., "delayed")
through such watershed processes.
The DDRP was designed to begin assessing the
state and influence of these processes. The man-
date of the DDRP was to make comparative re-
gional projections of the future effects of sulfur
deposition on long-term surface water chemistry in
the eastern United States, based on the best avail-
able data and most widely accepted hypotheses of
the acidification process related to atmospheric
deposition.
Two principal reports produced by the DDRP as-
sessed potential long-term effects of sulfur deposi-
tion on lake and stream water chemistry in the
eastern United States. The first report focused on
analysis of lake resources in the Northeast and
stream resources in the Southern Blue Ridge Prov-
25
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT 4. MAGIC
MAGIC is a lumped parameter model of intermediate complexity that was originally developed to project
long-term effects (i.e., decades to centuries) caused by acid deposition on surface water chemistry. The
model uses a minimum number of critical chemical and hydrological processes in watersheds to simulate
soil solution and surface water chemistry, and to project average monthly or annual concentrations of acid-
base chemistry in surface water. MAGIC was introduced in a pair of 1985 articles by B.J. Cosby, G.M.
Hornberger, J.N. Galloway, and R.F. Wright:
* Time scales of catchment acidification: A quantitative model for estimating freshwater acidification.
Environmental Science and Technology 19:1144-1149; and
* Modeling the effects of acid deposition: Assessment of a lumped parameter model of soil water and
stream water chemistry. Wafer Resources Research 21:51 -63.
Church et al. (see footnote 30) summarize various studies using MAGIC. Recent modifications of the model
are summarized by Sullivan, T.J., B.J. Cosby, C.T. Driscoll, H.F. Hemond, D.F. Charles, S.A. Norton, and
J.M. Eilers (1993. The influence of naturally occurring organic acids on model estimates of lake water
acidification using the Model of Acidification of Groundwater in Catchments (MAGIC). Report
DOE/ER/30196-3. U.S. Department of Energy, Washington, DC).
MAGIC has been tested more than any other acidic deposition effects model. Those tests indicate that its
projections are reasonably reliable:
* Individual process formulations in the model have been tested against laboratory experiments with
soils.
* Model hindcasts (i.e., backward predictions) of historical lake chemistries in the Adirondacks have
been made and compared with values inferred from lake sediment records.
* Numerous predictions of the effects from whole-watershed manipulations have been compared to
observed effects.
The MAGIC model, as all models, illustrates problems associated with uncertainty, parameterization, and
validation. For example, MAGIC currently does not explicitly represent detailed cycling or processes
affecting the rate of nitrogen uptake and release. In fact, processes (and their governing factors) that control
the transition of a watershed to a state of nitrogen saturation are poorly known and, as yet, not represented
in any complete or tested watershed model. Better nitrogen models to address the questions are being
developed, however. The Nitrogen Bounding Study developed for this report used a series of four scenarios
for time-to-nitrogen-saturation to "bound" the possibilities. The NBS represents the first time a nitrogen
component has been added and effectively used with the MAGIC model for assessments at regional scales.
In analyses completed for the DDRP and the NBS, it is impossible to know precisely what deposition levels
will be over the next 50 years or more. Therefore, the NBS approach assessed a range of deposition levels to
evaluate potential effects of possible sulfur and nitrogen deposition combinations. This approach indicates
why model runs are more correctly termed "projections" rather than "predictions." The latter implies an
exact knowledge of model inputs and system dynamics. The NBS projects watershed acidification responses
for possible alternative acidic deposition rates in the year 2040.
ince (SBRP).30 The second report addressed poten-
tial stream chemistry effects in the mid-Appala-
chian Region, and summarized and integrated
30 Church, M.R., K.W. Thornton, P.W. Shaffer, D.L.
Stevens, B.P. Rochelle, C.R. Holdren, M.G. Johnson,
J.J. Lee, R.S. Turner, D.L. Cassell, D.A. Lammers,
W.G. Campbell, C.I. Liff, C.C. Brandt, L.H. Liegel,
C.D. Bishop, D.C. Mortenson, S.M. Pierson, and
D.D. Schmoyer. 1989. Direct/Delayed Response
Project: Future Effects of Long-term Sulfur Deposi-
tion on Surface Wafer Chemistry in the Northeast
and Southern Blue Ridge Province. EPA/600/3-
89/026a-d. U.S. Environmental Protection Agency,
Washington, DC. 887 pp.
conclusions from the three regional analyses.31
Exhibit 5 shows the locations of three study
regions. General characteristics and sizes of target
31 Church, M.R., P.W. Shaffer, K.W. Thornton, D.L.
Cassell, C.I. Liff, M.G. Johnson, D.A. Lammers, J.J.
Lee, G.R. Holdren, J.S. Kern, L.H. Liegel, S.M. Pier-
son, D.L. Stevens, B.P. Rochelle, and R.S. Turner.
1992. Direct/Delayed Response Project: Future Ef-
fects of Long-term Sulfur Deposition on Stream
Chemistry in the Mid-Appalachian Region of the
Eastern United States. EPA/600/R-92/186. U.S. En-
vironmental Protection Agency, Washington, DC.
384 pp.
26
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CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBITS. STUDY REGIONS INCLUDED IN THE DIRECT/DELAYED
RESPONSE PROJECT AND THE NITROGEN BOUNDING STUDY
Northeast-
Adi rondacks
achi an
on
Southern Bl ue Ridge
Provi nee
27
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
surface water populations for all regions included
as part of the NSWS and the DDRP and NBS
studies are presented for comparison in Exhibit 6.
Specific characteristics of the three DDRP study
areas and their surface waters are summarized in
the following.
Northeast32
This region includes lakes potentially sensitive to
acidic deposition over the near- to long-term and
covers an area extending from northeast Pennsyl-
vania and northern New Jersey through the entire
State of Maine (Exhibit 5). Bedrock and surface
physiographic characteristics in these subregions
help to limit supplies of base cations draining from
these glaciated and predominately forested water-
sheds. Seepage lakes are uncommon, representing
only 7 percent of the lakes classified by hydrologic
type, but seepage lakes also generally had the
lowest values of ANC and pH of any lake type in
this region. Although the NSWS included lakes
with areas only between 4 and 2,000 ha (see Ex-
hibit 6), there may be from one to four times as
many lakes with areas less than 4 ha in the North-
east. Because of their smaller sizes and higher
rates of water turnover, such lakes are likely to be
more highly susceptible to acidic deposition ef-
fects. Concentrations of nutrients (i.e., NO3",
NH4+, and PO43+) were low for most of lakes sam-
pled in this region.
This region includes the highest dissolved concen-
tration of SO42' observed during the NSWS. Acidic
lakes were also characterized by high concentra-
tions of extractable aluminum.
The Adirondack subregion, including Adirondack
State Park, which was emphasized during the NBS
(see Section 2.5.3), has the highest number and
percentage of acidic (ANC<0 ueq/l) lakes (14 per-
cent) found for any NSWS subregion, except Flor-
ida. Approximately half of the Adirondack lakes
having pH 5.0 were organic acid dark-water lakes,
while the remainder were clear water acid lakes.
This indicates that inorganic ions, including min-
eral acids, were likely the primary cause of their
acidity. Both pH and ANC tended to decrease as
the lake elevation increased, a relationship not ob-
served in other NSWS subregions of the Northeast.
Drainage lakes were the most common type of
lake (77 percent of the target population). Most
lakes with areas of less than 4 ha in the Adirond-
acks are more boglike and more strongly influ-
enced by organic acidity, compared to the larger
lakes in this subregion.
Mid-Appalachian Region33
This region included most stream reaches potenti-
ally sensitive to acidic deposition within the area
from central and eastern Pennsylvania through
western Maryland and Virginia and into eastern
West Virginia (Exhibit 5). This DDRP study region
included the mountainous physiographic provinces
of the Mid-Atlantic Appalachian Mountains, in-
cluding the northern Blue Ridge Mountains, Valley
and Ridge Province, and Appalachian Plateau.
This area includes Shenandoah National Park in
Virginia. Much of the area extends over bedrock
that is relatively resistant to weathering. Rates of
sulfur deposition in the mid-Appalachians are
much greater than in the Northeast or SBRP.
Based on the National Stream Survey (NSS)
streams acidified by acidic deposition (ANC<0
ueq/l) accounted for 4 percent of the target stream
length in this region; 18 percent of the upstream
ends and 7 percent of the downstream ends had
ANC<50 ueq/l. These estimates excluded streams
acidified by mine drainage (e.g., coal mining).
Mine drainage was responsible for acidifying four
times as many downstream reaches as acidic
deposition.
More than 99 percent of the acidic target streams
(ANC<0 ueq/l) within the mid-Appalachians were
located in watersheds with at least 85 percent for-
est cover. Many more streams with very low ANC
(<50 ueq/l) are found in these forested areas, com-
pared to those in mixed forest or open areas. This
situation probably is not due to the fact that forests
control ANC and acidic levels; rather, most re-
maining forested areas that were never clear cut
lie in the less-weatherable, less-fertile uplands un-
suitable for agriculture. Areas where forests were
32 This summary is primarily drawn from Linthurst,
R.A., D.H. Landers, J.M. Eilers, D.F. Brakke, W.S.
Overton, E. P. Meier, and R.E. Crowe. 1986. Char-
acteristics of Lakes in the Eastern United States, Vol-
ume I: Population Descriptions and Physico-Chemi-
cal Relationships. EPA/600/4-86/007a. U.S. Envi-
ronmental Protection Agency, Las Vegas, NV.
33 This summary is primarily drawn from Herlihy, A.T.,
P.R. Kaufmann, M.R. Church, P.J. Wigington, Jr., J.R.
Webb, and M.J. Sale. 1993. The effects of acidic
deposition on Streams in the Appalachian Mountain
and Piedmont Regions of the Mid-Atlantic United
States. Water Resources Research 29(8):2687-2703.
28
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CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBIT 6. TARGET POPULATIONS INCLUDED IN THE NSWS, DDRP, AND NBS STUDIES
Target population refers to the number of systems for which
model projections can be reasonably extrapolated. Studied tar-
get populations of surface waters generally became refined and
smaller in each of these successive studies, allowing acidic
deposition research efforts to focus increasingly on relation-
ships in more sensitive surface waters (see figure—not drawn to
scale). As a consequence of narrowing research efforts, propor-
tions of sensitive surface waters and the magnitude of the po-
tential response to acidic deposition by these respective target
populations tend to increase through subsequent studies. Gen-
eral characteristics of these target populations are presented be-
low.
As part of the NSWS, the Eastern Lake Survey (ELS) includes lakes between 4 ha (10 acres) and
2,000 ha (5,000 acres) in size that have ANC<400 ueq/l, excluding non-freshwater lakes (i.e., small
ocean bays and estuaries); broad waters with apparent flows (reservoirs were included, however);
marshes or swamps; and water-bodies surrounded by urban, industrial, or agricultural activities (i.e.,
lakes with extensive cultural disturbance in their watersheds). In turn, the National Stream Survey
(NSS) included target stream reaches in sensitive regions not sampled during the National Lake Survey
(NLS) that had drainage areas <155km2 (<60 mi2) and showed as "blue line" streams on
1:250,000-scale U.S. Geological Survey topographic maps. Such streams were judged to be large
enough to be important for fish habitat, yet small enough to be susceptible to potential effects of
acidic deposition. At least 50 percent of the stream reach had to be within the designated region to be
included. Among the stream reaches excluded from the target population were, for example, reaches
affected by gross pollution (e.g., mine or oil-field drainage), highly urbanized development, or tidal
influence.
DDRP target lakes in the Northeast included a subset of NLS target lakes by excluding lakes <1.5 m
deep. DDRP target stream reaches included a subset of the NSS target reaches by excluding streams
with ANC>200 ueq/l in the mid-Appalachian region (Southern Blue Ridge Province streams had no
additional restriction on ANC); drainage areas >3,000 ha (>7,400 acres); and all watersheds in the
Southern Piedmont Regions and Coastal Plain and north of the maximum extent of glaciation.
NBS target lake and stream popluations represent a sensitive subset of the lakes and streams within the
three regions:
« ADIRONDACK LAKES: About 700 lakes, approximately 45 percent of the lakes in the Adirondack
region meeting ELS sampling requirements
* MID-APPALACHIAN STREAMS: About 4,300 stream reaches, approximately 17 percent of the re-
gion's total stream reaches
* SOUTHERN BLUE RIDGE PROVINCE STREAMS: About 1,300 stream reaches, approximately 65 per-
cent of the region's stream reaches meeting the sampling requirements described above
The target populations used in the NBS therefore, represent the best available data for case studies of
sensitive regions in the United States.
historically cleared for agriculture predominate
along the more-weatherable, more-fertile valleys.
Southern Blue Ridge Province
This region includes potentially sensitive stream
reaches in the extreme western portions of North
Carolina, South Carolina, eastern Tennessee, and
northern Georgia (Exhibit 5).34 The SBRP includes
34 This summary for the Sourthern Blue Ridge Province
is primarily drawn from Elwood, J.W., M.J. Sale, P.R.
Kaufman, and G.F. Cada. 1991. The Southern Blue
(continued)
29
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
a steep mountainous region characterized by high
rainfall, highly weathered base-poor soils, and
relatively unreactive bedrock. Target surface
waters of this region contain some of the lowest
concentrations of dissolved solids of any region
sampled in the United States, and among the high-
est deposition rates for H+, SO42-, and NO3~. This
area includes the Great Smoky Mountains Na-
tional Park. Although no acidic streams (ANC<0 u
eq/l) were found during the NSS, statistical analy-
sis of the results from this study indicated that a
small number representing less than 1 percent of
the streams in the region may be acidic or be af-
fected acidic episodes. Also, a separate non-ran-
dom survey during 1982-1984 found 3 percent of
the small streams in the region to be acidic; no
larger acidic streams were reported in this study.35
Watershed retention of SO42' and NO3" is the ma-
jor process generating ANC in drainage lakes
within this region, exceeding base cation mobili-
zation in importance. Sulfur and nitrogen retention
capacities are generally similar and provide rela-
tively consistent sources of ANC across the region.
The primary cause of ANC differences in these
streams appears to be different rates of acidic cat-
ion mobilization from the region's watersheds.
Dissolved organic carbon concentrations are typi-
cally low and do not appear to provide significant
contributions to stream acidity.
Concentrations of SO42- have increased at an an-
nual rate of approximately 1 ueq/l during the
10 years prior to 1993 in selected streams draining
both high- and low-elevation watersheds. Over the
same period both ANC and base cation concentra-
tions declined, indicating that base cation mobili-
zation is not keeping pace with acidic deposition.
Despite this, no adverse biological effects from
acidic deposition on streams and lakes have been
conclusively demonstrated for the SBRP.
The DDRP projected changes in target surface
water chemistry for one or two sulfur deposition
scenarios, while holding nitrogen deposition and
retention constant, using up to three watershed
Ridge Province. Pages 319-364 in D. F. Charles
(editor). Acidic Deposition and Aquatic Ecosystems
Regional Case Studies. Springer-Verlas. New York,
NY.
35 Winger, P.V., P.]. Lasier, M. Hudy, D.L. Fowler, and
M.J. Van Den Avyle, 1987. Sensitivity of high-eleva-
tion streams in the Southern Blue Ridge Province to
acidic deposition. Water Resources Bulletin 23:379-
386.
models. Model projections were compared among
the three DDRP regions.
Results for the Northeast indicated that these target
lakes would likely respond relatively rapidly to
changes in sulfur deposition, because Northeast
watersheds appear, on average, to be near sulfur
steady state. That is, annual loads of atmospheric
sulfur deposited into most watersheds approxi-
mately equal loads discharged with waters drain-
ing from the watersheds. Remaining sulfur reten-
tion capacities of Northeast soils appear to be gen-
erally limited. In contrast, DDRP projected that at
either current or increased sulfur deposition load-
ings, it might take 150-200 years, on average, be-
fore SBRP watersheds attain sulfur steady state.
That is, sulfur retention potentials appear much
greater in the SBRP than in the Northeast.
Watersheds in the mid-Appalachians have sulfur
retention characteristics similar to some Northeast
and some SBRP watersheds. As such, they appear
to represent a transition region where some sys-
tems will likely respond relatively rapidly to
changes in sulfur deposition rates, whereas other
systems may respond more slowly. Projections of
times necessary to reach sulfur steady state aver-
aged about 50 years for the mid-Appalachians wa-
tersheds. Further, in contrast to the lack of cur-
rently acidic SBRP streams, about 4 percent of the
DDRP target population stream reaches in the
mid-Appalachians are now chronically acidic.
Acidic sulfur deposition appears to be the most
likely cause of surface water acidity in
mid-Appalachian streams; sulfate from atmos-
pheric sources dominates the strong acid anion
component in these streams.
Results of the DDRP model projections for the tar-
get populations in each of the three regions stud-
ied and for the sulfur deposition scenarios mod-
eled for purposes of the NAPAP 1990 Integrated
Assessment Report can be summarized as follows.
For lakes in the Northeast, the 1990 CAAA was
projected to reduce the loss of habitat for sensitive
fish species between 1990 and 2030 by 16 percent
to 18 percent from that which was projected to
occur without the CAAA. The results also pro-
jected that a decrease in sulfur deposition of
30 percent from 1985 levels would lead to in-
creases in lake ANC and decreases in the numbers
of chronically and episodically acidic lakes in the
Northeast through the last year of the DDRP
model projection, 2030.
30
-------
CHAPTER 2: ENVIRONMENTAL GOALS
The DDRP model projections for the SBRP indi-
cated that continued sulfur deposition at 1985 lev-
els for 50 years would increase stream sulfur con-
centrations and decrease stream ANC, with the re-
sult that a small percentage of the DDRP target
population stream reaches might become chroni-
cally acidic in 50 years. There might also be a
slight increase in the number of stream reaches
susceptible to acidic episodes.
In the mid-Appalachians at the 1985 rate of sulfur
deposition, the DDRP model projected that in 50
years the proportion of target acidic (ANC<0 ueq/l)
stream reaches would increase between 3 percent
and 11 percent. This scenario also is projected to
double, from 25 percent to 54 percent, the target
population of stream reaches that have an ANC
less than 50 ueq/l. This would also double the
number of stream reaches potentially susceptible
to acidic episodes. Models of a deposition scenario
involving a 50 percent decrease from 1985 sulfur
deposition rates projected that increases in stream
ANC would occur in 50 years across the DDRP
target population (although statistically significant
changes in the number of acidic reaches or
reaches with ANC less than 50 ueq/l were not ex-
pected).
2.5.3 Nitrogen Bounding Study
Evidence and concern regarding long-term surface
water acidification associated with nitrogen depo-
sition and NO3~ leaching from watersheds is in-
creasing as some watersheds appear to be ap-
proaching nitrogen saturation (see Section 2.2.1).
The relative importance of nitrogen deposition is
also becoming more apparent as adverse effects
from sulfur deposition are apparently easing in re-
sponse to the 1990 CAAA and earlier SO2 emis-
sions reductions.
Exhibit 7 presents NBS model projections of per-
centages of acidic and sensitive surface waters
(ANC<0 ueq/l and ANC<50 ueq/l) in three regions
of the eastern United States in the year 2040 under
three scenarios: (1) without the 1990 CAAA (sulfur
deposition held constant at 1993 levels and a 1
percent annual increase in nitrogen deposition af-
ter the year 2000); (2) without the 1990 CAAA sul-
fur reductions (sulfur deposition held constant at
1993 levels) but with the nitrogen controls
(nitrogen deposition held constant at 1985 levels);
and (3) with implementation of the CAAA sulfur
and nitrogen controls (nitrogen deposition held
constant at 1985 levels). Results of these three
scenarios illustrate the proportion of surface waters
that would likely have been acidic or sensitive to
becoming acidic had there been no CAAA. These
modeling projections are subject to the uncertain-
ties described previously. As such, they indicate
approximate proportions of the surface water tar-
get populations projected to have ANC<0 or 50 u.
eq/l for the indicated deposition scenarios. The
scenario depicting no sulfur reductions and a 1
percent annual increase in nitrogen deposition is
intended to be representative of a situation without
CAAA reductions (no sulfur reductions and a con-
tinuing increase in nitrogen deposition). The ex-
hibit shows that the reduction in sulfur deposition
levels resulting from the 1990 CAAA are projected
to provide clear benefits in improving ANC and re-
ducing acid stress in the lakes and streams of the
three regions that hold a major proportion of sens-
itive aquatic resources in the eastern United States.
EPA designed the NBS to begin providing a more
complete understanding of potential effects attrib-
utable to nitrogen deposition during surface water
acidification.36 This study examined the combined
effects on surface water chemistry due to potential
changes in the deposition rates of total sulfur and
total nitrogen, and due to possible alternative rates
of nitrogen saturation within watersheds. Projected
surface water chemistry for two target years (2015
and 2040) were assessed, with the assumption that
emissions reductions mandated by the 1990 CAAA
(the permanent 10 million tons of SO2 and the
temporary 2 million tons of NOX) were fully im-
plemented.
The study evaluated target populations of surface
waters in three regions: lakes in the Adirondack
Region and stream reaches in the mid-Appalachi-
ans and the Southern Blue Ridge. Target popula-
tions of waters modeled are described in Exhibit 6.
The NBS model projections were completed using
a modification of the MAGIC model (see Ex-
hibit 4). The primary input data came from the
NSWS, DDRP studies, and updated deposition in-
formation from the EPA atmospheric modeling
studies discussed in Chapter 3. The NBS results
represent responses for proportions of NBS mod-
eled systems; they do not represent responses for
either all surface waters or for all NSWS sampled
surface waters in the modeled regions.
36 Van Sickle, J., and M.R. Church. 1995. Methods for
Estimating the Relative Effects of Sulfur and Nitrogen
Deposition on Surface Water Chemistry. U.S. Envi-
ronmental Research Laboratory, Corvallis, OR.
31
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT 7. IMPACT OF CAAA ON SENSITIVE SURFACE WATERS: NBS MODEL PROJECTIONS FOR YEAR 2040
PROPORTIONS OF TARGET POPULATION SURFACE WATERS IN TWO ANC CATEGORIES
FOR THREE DEPOSITION SCENARIOS (SEE TEXT FOR ADDITIONAL DETAILS)
Region
ADIR
M-APP
SBRP
Criterion
ANCSO ueq/l
ANC<50 ueq/l
ANC^O ueq/l
ANC<50 ueq/l
ANC^O ueq/l
ANC£50 (jeq/l
Deposition Scenario
w/o CAAA: 1% N increase*3
w/o CAAA: Constant Nc
CAAAd
w/o CAAA: 1 % N increase1"
w/o CAAA: Constant Nc
CAAAd
w/o CAAA: 1 % N increase6
w/o CAAA: Constant Nc
CAAAd
w/o CAAA: 1% N increase1"
w/o CAAA: Constant Nc
CAAAd
w/o CAAA: 1 % N increase1"
w/o CAAA: Constant Nc
CAAAd
w/o CAAA: 1 % N increase1"
w/o CAAA: Constant Nc
CAAAd
Observed
Proportion*
19
55
4
27
0
6
Proportions (Percentages) at Modeled
Times to Watershed Nitrogen Saturation
50 years
52
50
43
77
74
67
42
33
9
76
67
41
14
13
4
31
20
16
100 years
39
36
26
59
58
57
28
23
5
66
54
37
7
2
0
22
17
16
250 years
23
23
15
55
55
54
23
21
4
65
48
28
2
1
0
17
15
14
Never
24
25
11
55
55
54
21
8
0
49
38
23
0
0
0
15
15
11
a Observed in 1984 in Adirondacks and 1985 in Mid-Appalachians and Southern Blue Ridge.
b Sulfur deposition held constant at 1993 levels; nitrogen deposition increases 1% per year after 2000.
c Sulfur deposition held constant at 1993 levels; nitrogen deposition held constant at 1985 levels.
d Reflects only decreases in sulfur deposition from implementation of Title IV; nitrogen deposition held constant at
1985 levels.
The NBS study is the most recent major study hav-
ing important implications for considering the fea-
sibility of aquatics-based acid deposition standards
in the United States. Therefore, it is valuable to
understand clearly the nature of the results prod-
uced by this study. Exhibits 8-10 present 3 of over
60 similar sets of four plots presenting NBS model
results. These sets of plots show modeled re-
sponses for percentages of the target population of
Adirondack Region lakes projected to meet the cri-
terion of ANC of 0 ueq/l or less in the year 2040
and the percentages of target populations of mid-
Appalachians and SBRP stream reaches projected
to meet the criterion of ANC of 50 ueq/l or less in
year 2040, Exhibit 11 provides guidance for inter-
pretation of the NBS plots presented in Exhib-
its 8-10 and in Appendix B.
Times to watershed nitrogen saturation in these
systems remain a major uncertainty. Some model-
ing and empirical analyses (for example, at Hub-
bard Brook in New Hampshire and the Harvard
Forest in Massachusetts) indicate rather long times
to nitrogen saturation, while other results from the
experimentally manipulated watersheds of Bear
Brook in Maine and Fernow in West Virginia indi-
cate shorter response times to increased nitrogen
additions. Also, nitrate concentrations have no-
ticeably increased recently in some surface waters
draining the Catskills, Adirondacks, and the high-
est elevation spruce stands in the Great Smoky
Mountains, suggesting that some watersheds in
these regions are moving toward or have reached
nitrogen saturation. Consequently, evaluating cur-
rent trends for nitrogen saturation on a regional
basis remains very difficult. To accommodate this
uncertainty, the NBS model projections assumed
constant rates of nitrogen assimilation (i.e., no
change from present) and included scenarios of
time to watershed nitrogen saturation of 50, 100,
and 250 years. Additional considerations regarding
32
-------
CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBIT 8. NBS MODEL PROJECTIONS FOR YEAR 2040 PERCENTAGE
OF TARGET POPULATION ADIRONDACK LAKES WITH ANC^O U.EQ/L
I
o
12 -
10 -
8 -
8 e -I
a. o -
o
Q
4 -
o
I
« 2 -
£
0 -
6%
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
12 -
J 10 -
I 8-
o.
4)
Q
o
z
I
6 -
4 -
2 -
0 -
3.4%
i
0
8
r
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
CD
O
Q
0>
I
12 -
10 -
8 -
6 -
4 -
2 -
0 -
0%
i
0
i
2
i
4
i
6
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
12 -
10 -
8 -
8 R -i
Q. 0 ~
o
8s
£
"o
4 -
2 -
0 -
0%
8
i
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
33
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT 9. NBS MODEL PROJECTIONS FOR YEAR 2040 PERCENTAGE OF TAR-
GET POPULATION MID-APPALACHIAN STREAMS WITH ANC< 50 U.EQ/L
14 H
1 12 H
3 10 H
I 8H
i
I
6 -
4 -
2 -
0 -
5.3%
8
i
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
14 -
12 -
j| 10 -
1 8
| 6H
O
I 4H
I 2-
0 -
4.8%
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
14 H
•& 12 H
8» 10 H
o
o
o°
8 -
6 -
4 -
2 -
0 -
4.6%
8
I
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
14 -
I ,2-
.§» 10 -
I SH
Q 6 -
§
O A. ™
^ "*
3Z
2
5 2 H
0 -
3.9%
i
0
i
2
6. 8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
34
-------
CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBIT 10. NBS MODEL PROJECTIONS FOR YEAR 2040 PERCENTAGE OF TAR-
GET POPULATION SOUTHERN BLUE RIDGE STREAMS WITH ANC<50
a
12 -
10 -
6 -
4 -
3.8%
T i 1 r
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
o>
o
0>
O
0>
"
•s
12 -
10 -
8 -
6 -
4 -
2 -
0 -
3.4%
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
I
o>
O
I
12 H
10 H
6
4
2 -
0 -
I
0
2%
2
I
8
i
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
35
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT 11. INTERPRETING NBS PLOTS
To illustrate interpretation of the NBS plots, the four individual plots in Exhibit 8 show projected per-
centages of NBS target population lakes in the Adirondack Mountains with ANC of 0 ueq/l or less,
where watershed nitrogen saturation is assumed to occur at 50, 100, and 250 years, as well as where
watershed assimilation rates for nitrogen remain constant at recently estimated rates (i.e., watershed
nitrogen saturation will never occur). Response contours for each plot show how percentages of target
waters are projected to vary with changes in total sulfur and nitrogen deposition throughout the mod-
eled ranges of deposition. These ranges begin at projected background deposition rates for sulfur and
nitrogen (1 kg S/ha/yr and 4 kg N/ha/yr) and extend to their maximum modeled concentrations for
2040 (7.5 kg S/ha/yr and 11.3 kg N/ha/yr), i.e., the rates projected to accompany implementing the
1990 CAAA (see Chapter 3). Thus, for the upper right plot of Exhibit 8, which shows projections based
on an assumed 100 years to nitrogen saturation, the model projects that approximately 26 percent of
the target Adirondack lakes may be acidic (ANC 0 ueq/l) at year 2040 for modeled sulfur and nitrogen
deposition rates projected to accompany implementation of the 1990 CAAA. With only background
deposition of sulfur and nitrogen, 3.4 percent of these target lakes are projected to be acidic at 2040.
Several general observations apply to these plots:
* The slopes of contour lines in each plot reflect the relative importance of sulfur and nitrogen in
causing the projected response relationships. Nearly vertically plotted response contours indi-
cate that the projected ANC response is attributable primarily to sulfur deposition. Nearly
horizontal plotted response contours indicate the plotted ANC response is attributable primar-
ily to nitrogen deposition. A forty-five degree diagonal contour indicates equal contributions
by both sulfur and nitrogen depositions.
• Changes in the spacing between individual response contours within each plot appears to be
attributable to patterns in sample weighting during model projections, rather than due to some
intrinsic character of the deposition-response relationships.
* The density of response contours across the modeled deposition ranges for each plot directly
relates to the potential average responsiveness by target water-bodies to potential changes in
sulfur and nitrogen deposition rates on the specified water quality classification variable mod-
eled (e.g., ANC<0 ueq/l). Therefore, plots with a high density of contour lines depict a high
level of responsiveness to changes in deposition.
possible regional difference in times to nitrogen
saturation are presented in Exhibit 12.
For these plots, deposition rates of sulfur and ni-
trogen were assumed to be those projected to ac-
company implementation of the 1990 CAAA to the
year 2010. At that time, different sulfur and nitro-
gen deposition scenarios were defined and mod-
eled. Some modeled scenarios maintained the
2010 deposition rates, while some alternative
modeled scenarios declined to background deposi-
tion rates. Rates for still other scenarios reduced to
levels between these extremes. (Background depo-
sition rates include only natural, agricultural fertil-
izer, and domestic livestock sources.) Each mod-
eled deposition rate was then assumed to remain
constant at the modeled 2020 rate until the year
2040, the end of the model projection period.
Sets of plots similar to those shown in Exhib-
its 8-10 have been produced through this study
projecting year 2040 proportions of NBS target
surface waters within each of the three modeled
regions meeting five evaluation criteria: ANCS
0 ueq/l, ANCS50 ueq/l, pH<5.0, pH<5.5, and pH<
6.0. Similar plots for all four water chemistry crite-
ria were also produced for the year 2015. All other
NBS plots showing results of both ANC and pH
projections are presented in Appendix B.
Because of uncertainties associated with the com-
plex chemical relationships modeled in MAGIC's
derivations for pH, ANC projections are consid-
ered to be more reliable than those for pH. As pre-
viously described, ANC is an important indicator
of sensitivity to acidification (see Section 2.3.1).
Furthermore, projected water quality changes are
36
-------
CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBIT 12. TIME TO WATERSHED NITROGEN SATURATION
Present scientific knowledge does not allow quantifying the
time to nitrogen saturation for any of the three study regions
considered in the NBS, and no scientific consensus exists re-
garding actual times to nitrogen saturation for watersheds
within these three regions or any others regions. Indeed,
NBS investigators and most of this project's technical re-
viewers deem it scientifically premature to define specific
times to saturation for any region. Yet, it is reasonable to
suggest that times to saturation do vary among regions. This
variation is due primarily to differences in temperature,
moisture, soil fertility, primary production rates, decomposi-
tion rates, forest age, and the accumulation of plant biomass,
along with different histories of nitrogen deposition among
the regions. Further, given historic and current nitrogen
deposition rates in these three regions, it is reasonable to as-
sume that forested watersheds would eventually reach nitro-
gen saturation (barring major disturbances such as logging,
major fires, blow downs, and insect infestations).
As a speculative example, watersheds in the Adirondacks
have cooler annual temperatures, shorter growing seasons,
lower inherent productivity potentials, restrictive logging
practices and fire control policies, very mature old-growth
forest stands, and long histories of elevated deposition rates
of sulfur and nitrogen. Consequently, watersheds in these ar-
eas may include those having the shortest remaining times to
nitrogen saturation. It has been suggested that saturation
times in this region may average less than 100 years; some
even suggest the range of 25 to 75 years. In comparison,
moving to a more southerly region, watersheds in the
Mid-Appalachians have generally warmer annual tempera-
tures, longer growing seasons, less restrictive forestry prac-
tices, and greater inherent productivity potentials, while also
having high nitrogen deposition rates. Watersheds in this re-
gion may have somewhat longer remaining times to nitrogen
saturation. Finally, watersheds in the Southern Blue Ridge
Province have even warmer annual temperatures, longer av-
erage growing seasons, relatively the greatest productivity
potentials, the fastest decomposition rates, historically lower
nitrogen deposition rates, much smaller pools of soil nitro-
gen, and generally very low stream nitrogen concentrations.
Here, the remaining time to watershed nitrogen may be
greater still. Some areas in this region such as those found in
the Great Smoky Mountains National Park also include ma-
ture old-growth forests and other forests which have sus-
tained previous damage (i.e., insect damage). Time to nitro-
gen saturation would likely be shorter in these areas. Esti-
mated times to watershed nitrogen saturation for each region
are uncertain, but the relative ranking of these times is likely
appropriate across the three broad regions modeled.
likely to be highly transient in nature
for the year 2015, largely because
potential benefits from implementa-
tion of 1990 CAAA requirements will
still be accruing at that time. There-
fore, this section focuses primarily on
projected ANC changes in the
year 2040. For those also interested
in projected changes in pH, the ANC
changes discussed can be related to
corresponding pH changes using the
empirical relationships between
these variables presented in Ex-
hibit 1: on average, across the three
NBS regions, ANC<0 ueq/l and ANC
<50 ueq/l approximate pH<5.3 and
pH<6.5, respectively. NBS plot pro-
jection for pH changes are presented
in Appendix B.
Summary of NBS Results
Exhibit 13 summarizes the observed
and modeled percentages of surface
waters in each NBS region target
population for both ANC criteria.
The observed values were those
measured during the 1984 NSWS
studies in the Adirondacks and the
1985 studies in the other two re-
gions. For example, 19 percent of the
target lakes in the Adirondacks used
during the NBS were observed to be
acidic (ANC<0 ueq/l) during the
1984 NSWS. Note, however, that the
target population of the NBS
modeling included generally more
sensitive subsets of target population
surface waters than were included in
the NSWS (see Exhibit 6).
The modeled projections summa-
rized in Exhibit 13 indicate propor-
tions of surface waters in the two
ANC categories by the year 2040
under the assumed times of 50 to
250 years and never for watershed
nitrogen saturation for each region.
This range brackets the modeled
times for watershed nitrogen satura-
tion occurring across the three NBS
regions for proportions of waters
within each ANC category. The per-
centages presented encompass the
range of NBS results for modeled
37
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT 13. SUMMARY OF NBS RESULTS: RANGE OF MINIMUM (BACKGROUND DEPOSITION) TO
MAXIMUM (IMPLEMENTATION OF CAAA) PERCENTAGES OF ACIDIC AND SENSITIVE TARGET WATERS
Acidic (ANC<0 peq/l)
Observed*
Projected 2040, nitrogen saturation in 50 yr
Projected 2040, nitrogen saturation in 100 yr
Projected 2040, nitrogen saturation in 250 yr
Projected 2040, nitrogen saturation never
Sensitive (ANC<50 ueq/l)
Observed3
Projected 2040, nitrogen saturation in 50 yr
Projected 2040, nitrogen saturation in 100 yr
Projected 2040, nitrogen saturation in 250 yr
Projected 2040, nitrogen saturation never
Percentage of Target Waters
Adirondacks
19
6-43
3-26
0-15
0-11
55
53-67
51-57
44-54
44-54
Mid-
Appalachians
4
0-9
0-5
0-4
0-0
27
5-41
5-37
5-28
4-23
Southern
Blue Ridge
0
0-4
0-0
0-0
0-0
6
4-16
4-16
3-14
2-11
a Observed 1984 for Adirondack lakes and 1985 for mid-Appalachian and Southern Blue Ridge streams.
minimum (background) and modeled maximum
(projected 1990 CAAA deposition) rates for both
total sulfur and nitrogen. For example, with an as-
sumed time to watershed nitrogen saturation of
100 years, the upper right plot of Exhibit 8 shows
that background total sulfur and nitrogen deposi-
tion in the Adirondacks is projected to result in
3.4 percent of the target lakes having ANC of
0 ueq/l or less in the year 2040. Similarly, the
maximum deposition for both anions likely under
implementation of the 1990 CAAA are projected
to result in about 26 percent of the target lakes in
this water quality class. Also, under this same
range of deposition scenarios, when the time to
watershed nitrogen saturation is assumed to equal
250 years, the lower left plot in Exhibit 8 shows
that the model projects that between 0 percent and
15 percent of these same target Adirondack lakes
will have ANCs of 0 ueq/l or less. Exhibit 13
shows these two ranges and summarizes all other
similar NBS projections for ANC by the year 2040
for all three modeled regions.
The numerical ranges in the model projections
presented in Exhibit 13 provide an indication of
the extent of uncertainty associated with each set
of model projections for each region. For example,
with the modeled rates of sulfur and nitrogen
deposition expected to accompany implementa-
tion of the 1990 CAAA, the percentage of target
lakes in the Adirondacks with ANCs of 0 ueq/l or
less would likely range from about 15 percent to
43 percent, depending on whether the true time to
watershed nitrogen saturation is nearer 250 or
50 years, respectively. As discussed previously, a
variety of sources of variability and uncertainty af-
fect the overall uncertainty of these model projec-
tions. If these sources were included in an overall
evaluation of uncertainty, the associated uncer-
tainty could be greater, with projections of future
responses by target waterbody populations
potentially falling beyond either end of all mod-
eled ranges presented in Exhibit 13. Therefore,
while the NBS projections of change represent the
best currently available techniques for projecting
environmental changes associated with acidic
deposition, the projections are best used as indica-
tors of general direction and magnitude of possible
water quality changes associated with changes in
total sulfur and total nitrogen deposition rates (See
Exhibit 6).
As noted in Exhibit 11, the density of contours
across the modeled deposition ranges in NBS plots
for ANC, including those in Exhibits 8-10, appears
to relate to the potential average responsiveness of
target waterbpdies to potential changes in deposi-
tion rates. (Vertical and horizontal contours indi-
cate a strong role of sulfur or nitrogen, respec-
tively.) Based on this relationship, all regional
plots for alternative projected times to watershed
nitrogen saturation were categorized into one of
three generalized levels of projected response sen-
sitivities. These categories provide a basis for
evaluating the relative confidence that reducing
sulfur or nitrogen depositions below levels pro-
jected to accompany the 1990 CAAA would pro-
duce detectable improvements in ANC within the
38
-------
CHAPTER 2: ENVIRONMENTAL GOALS
NBS target surface waters. Exhibit 14 presents the
results of the surface water responsiveness catego-
rization for the three modeled regions. The follow-
ing summary of regional relationships to acidic
deposition rates is drawn from Exhibits 13 and 14
and from the individual plots for all three NBS
study regions.
Regional Summaries37
Adirondack Region
ROLES OF SULFUR, NITROGEN AND NATURAL ACIDITY:
For the NBS target population of Adirondack lakes,
after implementation of the 1990 CAAA, sulfur
deposition appears to continue to be the primary
cause of the present chronically acidic surface
EXHIBIT 14. SURFACE WATER RESPONSIVENESS TO REDUCTIONS IN DEPOSITION BE-
YOND THE CAAA: DETECTIBLE IMPROVEMENTS IN LONG-TERM ANC BY 2040a-b
Region
ADIR
M-APP
SBRP
Deposition
Parameter
Reduced
Sulfur
""Sulfur
Nitrogen
Nitrogen
Sulfur
Sulfur
Nitrogen
Nitrogen
Sulfur
Sulfur
Nitrogen
Nitrogen
Criterion
(ANC)
<0jieq/l
<50 u.eq/1
<0jieq/l
<50 tieq/l
<0jiec|/l
<50 u.eq/1
<0jieq/l
<50 ueq/l
<0 u.eq/1
<50 neq/l
<0 u.eq/1
<50 u.eq/1
Estimated Time to Watershed Nitrogen Saturation
50 Years
A
•
A
A
O
A
•
A
O
O
O
•
100 Years
A
O
A
O
O
A
O
A
0
•
O
•
250 Years
A
•
O
O
0
A
O
A
0
A
O
O
Never
A
•
O
O
O
A
O
A
O
•
O
O
a Key to symbols:
O Additional decrease in acidic deposition of any magnitude below 1990 CAAA requirements is
unlikely to produce improved conditions.
• Additional decrease in acidic deposition of at least 50% below 1990 CAAA requirements may
produce improved conditions. Lesser reduction in deposition is unlikely to produce improved
conditions.
A Additional decrease in acidic deposition of at least 50% below 1990 CAAA requirements is
likely to produce improved conditions. Lesser reduction in deposition may produce improved
conditions.
b Improved conditions is defined as decreases by greater than 5% of the target population (e.g., from
30% to 24%) meeting the specified criterion (e.g., ANC<50 ueq/l), assuming the specified time as
the region average for watershed N saturation. Reduced deposition may lead to environmental im-
provement which does not meet the definition of "improved conditions" described above.
Because the strongest scientific data collected on
acidity in the eastern lakes and streams come from
the 1984 NSWS, the water quality conditions
found in that survey serve as a model for protec-
tive goals used in this report.
water chemistry when longer modeled times to ni-
trogen saturation (>250 years) are assumed. Nitro-
gen and sulfur deposition are projected to share
relatively equal future roles in affecting modeled
ANC when watershed nitrogen saturations are as-
37 Most of this discussion was developed from evalu-
ations of results from the National Surface Water
Survey and model projections from the Nitrogen
Bounding Study.
39
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
sumed to occur within 100 years. And, when
50 years is assumed as the time to nitrogen satura-
tion, the future importance of nitrogen deposition
as a direct cause of surface water acidification is
projected to be greater.
Proportions of ANC<50 ueq/l lakes in this region
are projected as likely to show very small changes
due to deposition reductions by the year 2040.
This relatively small potential for change indicates
that this region has a high proportion of lakes
which naturally have ANCs of 50 ueq/l or less
without the influence of acidic deposition. These
lakes will continue to have low ANC levels regard-
less of acidic deposition rates. This condition is not
detrimental in itself, but makes these waters highly
sensitive to adverse effects from episodic acidic
events.
CHRONIC ACIDIFICATION: Under an assumed time to
watershed nitrogen saturation of 50 years and un-
der the deposition reductions projected from 1990
CAAA implementation, the proportion of chroni-
cally acidic (ANC<0 ueq/l) Adirondack target lakes
is projected to increase by about 50 percent in
2015 and may double by 2040, relative to 1984
proportions. Assuming 100 years to nitrogen satu-
ration, NBS modeling projects that with imple-
mentation of the 1990 CAAA, proportions of ANC
<0 ueq/l lakes in the NBS target population may
increase from 19 percent in 1984 to 26 percent in
2040. These increased proportions appear due
largely to the increased effects of nitrogen deposi-
tion. If, however, the time to nitrogen saturation
equals or exceeds 250 years, the model projects a
slight reduction in the proportion of acidic lakes in
2040 with the implementation of the 1990 CAAA
(i.e., from 19 percent to 15 percent or less). The
uncertainty regarding time to watershed nitrogen
saturation remains an important consideration.
EPISODIC ACIDIFICATION:38 Two parallel but inde-
pendent estimates place the number of Adirondack
lakes within the NBS target population that may
become acidic (i.e., ANC<0 ueq/l) during snow-
melt or heavy storm flow events at least once per
year at about 3.5 times the number of chronically
acidic lakes. The 1984 proportion of NBS target
population lakes estimated to be at risk of episodic
acidification at least once per year is 73 percent,
compared to 19 percent estimated to be chroni-
cally acidic. Because episodes are driven princi-
pally by deposition acidity, reductions in acidic
38 Also see Section 2.2.2 on episodic acidification.
deposition rates for either sulfur, nitrogen, or both
can be expected to significantly reduce the occur-
rence of acidic episodes in the target population of
Adirondack lakes. This would be expected to oc-
cur at a more rapid rate than the reduction in pro-
portions of chronically acidic lakes because depo-
sition reductions are likely to have the greatest
immediate influence in reducing the mass of acids
and acid anions deposited by major storms.
RESPONSIVENESS TO DEPOSITION REDUCTIONS: Model-
ing results for the NBS target population of Adir-
ondack lakes indicate a reasonable expectation
that additional reductions in sulfur deposition
rates, beyond those projected to accompany the
1990 CAAA, would likely produce detectable
long-term improvements in ANC, regardless of the
time to nitrogen saturation for the NBS target wa-
tersheds. It is also reasonable to expect that re-
duced nitrogen deposition would produce detect-
able ANC changes in these lakes, but primarily if
times to nitrogen saturation for these watersheds
average 100 years or less.
While considerable uncertainty regarding time to
watershed nitrogen saturation exists, if the average
time for Adirondack watersheds to reach nitrogen
saturation is close to 100 years or less, the model
predicts that maintaining the proportion of chroni-
cally acidic (ANC<0 u.eq/1) target population Adir-
ondack lakes near their 1984 proportions in 2040
may require reducing anthropogenic sulfur and ni-
trogen deposition by 40-50 percent or more be-
low the reductions projected to accompany the
1990 CAAA. The model projects that reductions in
sulfur and nitrogen deposition of about 4.5 kg-
S/ha/yr and 7.5 kg-N/ha/yr, are projected as neces-
sary to maintain proportions of sensitive lakes
(ANC<50 ueq/l) near their 1984 levels (i.e, 55 per-
cent) if the time to watershed nitrogen saturation
approaches 50 years or less. If the time to satura-
tion actually is 100 years or longer, the model pro-
jects that deposition reductions accompanying the
1990 CAAA will allow proportions of Adirondack
lakes with ANCS50 u.eq/1 to maintain their ap-
proximate 1984 levels to the end of the projection
interval at the year 2040.
Mid-Appalachian Region
ROLES OF SULFUR AND NITROGEN: For the NBS target
population of mid-Appalachians stream reaches
assessed, model projections indicate that sulfur
and nitrogen deposition appear about equally im-
portant in surface water acidification processes for
this region (Exhibit 9).
40
-------
CHAPTER 2: ENVIRONMENTAL GOALS
CHRONIC ACIDIFICATION: As progressively shorter
times to watershed nitrogen saturation are as-
sumed, adverse effects associated with nitrogen
deposition are projected to increase, essentially
offsetting reduced proportions resulting from im-
plementation of the 1990 CAAA sulfur reductions
in mid-Appalachians target streams. Under as-
sumptions of 250 years or less as the time to wa-
tershed nitrogen saturation, no net change in the
proportion of acidic (ANC<0 ueq/l) streams in the
NBS target population is projected to accompany
implementation of the 1990 CAAA.
EPISODIC ACIDIFICATION: The estimated number of
mid-Appalachians stream reaches in the NBS tar-
get population that are episodically acidic (ANC<
0 ueq/l) at least once per year under 1985 deposi-
tion levels is approximately six times the number
estimated to be chronically acidic (i.e., approxi-
mately 23 percent of the target stream reaches
likely experience acidic episodes). Reducing depo-
sition of sulfur, nitrogen, or both would be ex-
pected to reduce the number of episodically acidic
stream reaches in the mid-Appalachians target
population faster than the rate of reduction for
chronically acidic reaches, for reasons similar to
those concluded for Adirondack lakes, above.
RESPONSIVENESS TO DEPOSITION REDUCTIONS: Poten-
tial benefits from additional deposition reduction
beyond the 1990 CAAA is projected to have bene-
fits for the target population of mid-Appalachians
stream reaches having ANCs of 50 ueq/l or less,
regardless of the time to nitrogen saturation. The
nature of these benefits should be viewed not so
much as potentially reducing chronically acidic
conditions in these target streams (although this is
likely), but as potentially reducing the susceptibil-
ity of sensitive streams to episodic acute acidifica-
tion effects (i.e., decreasing the proportion of
stream segments with ANC less than 50 ueq/l).
The 1985 Eastern Stream Survey found 27 percent
of the NBS target streams in the mid-Appalachians
had ANCs of 50 ueq/l or less. NBS projections in-
dicate that if the average time to watershed nitro-
gen saturation approximates 250 years or greater,
implementation of the 1990 CAAA would likely
result in target stream reaches maintaining their
1985 proportions of chronically acidic (ANC<0 ji
eq/l) as well as sensitive (ANC<50 u.eq/1) stream
reaches in the year 2040. If, instead, average time
to watershed nitrogen saturation approximates
100 years or less, the model projects that reducing
either sulfur or nitrogen deposition by about
25 percent below projected CAAA reductions, or
some lesser combined deposition reduction for
both chemicals, could be necessary to maintain
proportions of target stream reaches in the
year 2040 near their 1985 conditions. That is, the
deposition rates may have to be reduced by about
3.5 kg-S/ha/yr or 3 kg-N/ha/yr, or some combina-
tion leading to reduced deposition for both acidify-
ing chemicals, if the time to nitrogen saturation
approximates 100 years to maintain 1985 propor-
tions.
Southern Blue Ridge Province
ROLES OF SULFUR AND NITROGEN: In the SBRP, like
the mid-Appalachians, projected deposition effects
after implementation of the 1990 CAAA appear
about equally attributable to sulfur and nitrogen
deposition. This is probably due to the relatively
high remaining potentials for SO42~ adsorption in
soils and NO3~ retention in watersheds of the
SBRP. NBS modeling results project a relatively
minor response to deposition changes during the
period modeled; more discernible water quality
changes related to either sulfur or nitrogen deposi-
tion may occur after the year 2040.
CHRONIC ACIDIFICATION: Generally, the NBS target
population of stream reaches in this region pres-
ently appears to be little affected by chronic acid-
ity. Six percent of the NSWS stream reaches in this
region had ANC of 50 ueq/l or less. Under the
1990 CAAA and under all four assumptions of time
to watershed nitrogen saturation, the acid chemis-
try in most target stream reaches are projected to
change generally little by 2040 from 1985 condi-
tions. A marked exception to this pattern is that,
even with CAAA implementation, the proportion
of target stream reaches with ANCs of 50 ueq/l or
less are projected to approximately double, reach-
ing 11 percent to 16 percent by 2040, under the
modeled times to nitrogen saturation of 250 years
or greater. In turn, if nitrogen saturation occurs at
about 50 years, about 4 percent of stream reaches
might become acidic (ANC<0 ueq/l) where none
previously had been acidic.
EPISODIC ACIDIFICATION: Potential changes in epi-
sodic effects within the SBRP were not modeled
during the NBS because appropriate model cali-
bration data were lacking, and therefore, no avail-
able numeric estimates exist for the percentage of
stream reaches in this NBS target population that
may become episodically acidic by either 2015 or
2040. Nevertheless, as the number of stream
reaches in the SBRP target population with chronic
ANC of 50 ueq/l or less increases, the possibility of
41
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ACID DEPOSITION STANDARD.FEASIBILITY STUDY
episodically acidic conditions increases substan-
tially and can be reasonably expected to occur in
these target streams prior to (and at greater per-
centages than) the occurrence of chronically
acidic conditions.
RESPONSIVENESS TO DEPOSITION REDUCTIONS: Model
projections indicate that deposition reductions ac-
companying the CAAA would likely prevent long-
term acidification (ANC<0 ueq/l) of sensitive
streams at least until 2040, if the time to nitrogen
saturation is 100 years or longer. Modeling projec-
tions also indicate that additional reductions in
sulfur or nitrogen deposition in the SBRP, beyond
those expected to accompany the CAAA, could
reduce the proportion of target stream reaches
with ANCs of 50 ueq/l or less. If the time to water-
shed nitrogen saturation in this region is near
250 years or longer, the NBS modeling projects
that sulfur deposition would need to be reduced by
about an additional 25 percent to maintain the
ANCs in the target stream reaches near their 1985
conditions. Whereas, if the time to watershed ni-
trogen saturation is nearer 100 years or less, a
65 percent total decrease in both sulfur and nitro-
gen deposition beyond the CAAA is projected as
necessary to maintain ANC in these target stream
reaches near their 1985 conditions in the year
2040. NBS projections for this potential time to ni-
trogen saturation (100 years or less) indicate that
deposition may have to be reduced below 4 kg-
S/ha/yr and below 5 kg-N/ha/yr to maintain pro-
portions of stream reaches in the SBRP target
population with ANC<50 ueq/l at 1985 values.
Implications for an Acid Deposition Standard
The potential sensitivities of target aquatic re-
sources, their potential responses, and time course
of responses to changes in acidic deposition rates
as well as the relative current and potential roles
of sulfur and nitrogen clearly differ among regions.
This strongly supports the development of a site-
specific deposition standard or target load.
The acidifying effects of nitrogen deposition
should be considered when evaluating options and
potential needs for acid deposition standards. Spe-
cifically, NBS modeling indicates that nitrogen
deposition appears to produce important conse-
quences for the future acidification rates of surface
waters. Additionally, for many watersheds the ef-
fect of nitrogen deposition could be a greater con-
cern than are the effects of sulfur deposition alone.
The wide-ranging projections of possible benefits
associated with implementation of the 1990 CAAA
for each of these regions illustrate the need to im-
prove the ability to quantify watershed nitrogen
saturation rates. Until watershed nitrogen satura-
tion is better understood, significant uncertainty
will continue to accompany surface water benefits
analyses of potential reductions in sulfur and ni-
trogen deposition. Despite the uncertainty, how-
ever, it is useful to recognize that any reductions
in nitrogen deposition would not only reduce total
acidic deposition rates, but also reduce the actual
times to watershed nitrogen saturation. This proc-
ess is similar to the ongoing process whereby re-
ductions in sulfur deposition due to the 1990
CAAA are likely extending times for water sulfur
saturation within watersheds.
2.5.4 Overview of International and State
Acidic Deposition Criteria and
Standards
International consideration of ecologically based
standards to address air pollution problems origi-
nated in the mid-1960s. Driven primarily by the
acid rain debate over the next 30 years, the origi-
nal concept of using concentration-based criteria
gave way to using uniform maximum allowable
mass deposition rates, with 20 kg-wet SO42Yha/yr
(6.7 kg-S/ha/yr) being the first widely recognized
interim target load. Subsequently, site-specific
critical loads were increasingly emphasized. Their
development is generally attributed to Swedish re-
search efforts in the late 1960s.39
Critical loads are estimates of the maximum pol-
lutant loadings that environmental resources can
absorb on a sustained basis without experiencing
measurable degradation. Only inherent ecological
properties are included in site-specific critical load
determinations. Steps involved in defining and im-
plementing critical loads usually include
(1) resource identification and characterization,
(2) identification of regions or functional subre-
gions, (3) characterization of deposition within
subregions, (4) definition of assessment endpoint(s)
(see below), (5) selection and application of mod-
els, and (6) mapping projected environmental re-
sponses.40
39 Nilsson, j. and P, Crennfelt (editors). 1988. Critical
Loads for Sulphur and Nitrogen Report from a Work-
shop Held at Skokoster, Sweden, 19-24 March 1988,
UN/ECE and Nordic Council of Ministers.
40 Strickland, T.C., G.R. Holdren, Jr., P.L. Ringold, D.
Bernard, K. Smythe, and W. Fallen. 1993. A Na-
tional Critical Loads Framework for Atmospheric
(continued)
42
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CHAPTER 2: ENVIRONMENTAL GOALS
Target loads differ from critical loads in that their
definitions incorporate social, policy, economic,
and related considerations along with scientific
findings. An example of a target load would be an
acidic deposition level adequate to maintain pro-
portions of ANC<50 ueq/l waters at or below the
proportions found during the 1984-85 NSWS for
one or more of the surveyed regions. (This exam-
ple is illustrated in Section 3.6.) Other possible
target loads could include, for example, a deposi-
tion level to produce a specified percentage reduc-
tion in the 1984-85 proportions of ANC<50 ueq/l
waters.
Assessment endpoints are formal expressions of the
environmental value(s) to be protected. They can
include thresholds for "deleterious conditions"
(commonly some adverse ecological condition)
that a standard would attempt to prevent. Assess-
ment endpoints should be biologically relevant,
operationally definable, accessible to prediction
and measurement, and sensitive to the pollutant(s)
of concern. From a policy perspective, assessment
endpoints also should be socially relevant; that is,
they should be environmental characteristics mu-
tually understood and valued by the public and by
decision makers (e.g., populations of crops, trees,
fish, birds, or mammals). When the most appropri-
ate sensitive species or other endpoint used is not
socially valued, then their link to valued species or
other valued environmental attributes should be
explicitly demonstrated to simplify understanding
of why using such an endpoint is useful. Using
endpoints that have social relevancy helps to unify
scientific and social concerns in commonly shared
objectives.
The first and still dominant ecological assessment
endpoint used for critical and target load estima-
tion is freshwater aquatic responses, most com-
monly manifested as changes in pH or ANC. Of
particular interest here is that pH or ANC changes
themselves are often a relatively minor concern,
but the influence of such changes on biological
species is of considerable importance. Therein is a
defining attribute of how the concept of critical
loads has developed in its international use, i.e.,
critical loads of chemicals (e.g., SO42' and NO3")
are surrogates for biological concerns. The key
biological concern most often focused upon is fish
viability.
A critical load value can be viewed as a single,
especially important point along a continuous
range of values representing an ecological loss or
damage function. Viewing a critical load as part of
an ecological loss function is especially important
because that view has shaped much of the Euro-
pean debate over the appropriate and scientifically
defensible uses of critical loads in acid rain control
policy development. Further, these loss functions
are particularly important when it is recognized (as
it has been in Europe, Canada, and the United
States) that significant numbers of highly sensitive
receptor locations have associated critical loads
that likely cannot be met, even with widespread
and high levels of acidic deposition reduction em-
ploying the limits of technological feasibility. Loss
functions provide a mechanism to aggregate eco-
logical damage across regions and integrate a
quantitative understanding of acidic deposition to
ecological damage relationships. Without these
functions, more qualitative, often highly subjec-
tive, aggregation approaches are used to place
critical load concepts into policy-relevant con-
texts. The results of such approaches most often
are termed target loads, as described above.
Most countries of western Europe have adopted
the system for estimating critical loads developed
by the Coordination Center for Effects (CCE) of the
United Nations Economic Commission for Europe
(UNECE) under the auspices of the UNECE Con-
vention on Long-Range Transboundary Air Pollu-
tion (LRTAP) (Exhibit 15). A recently published
manual presents improved methods that are being
widely applied across Europe for mapping both
critical loads to protect sensitive resources and
critical levels of allowable atmospheric concentra-
tions of acidic pollutants.41 In this system, critical
loads are developed for individual cells of the
mapping grid based on the potential sensitivity to
acidification of forest soils and surface waters.
Critical loads for sulfur, nitrogen, and total acidity
have been mapped across Europe. Although a va-
riety of models were used, nearly all countries that
participated in the European mapping effort em-
ployed the simple mass balance steady-state
method as the underlying approach to estimate
critical loads. Several countries also used dynamic
models and other methods. In the CCE approach,
Deposition Effects Assessment: I. Method Summary.
Environmental Management 17:329-324.
41 Task Force on Mapping. 1993. Manual on Mapping
Critical Levels/Loads. Coordination Center for Ef-
fects, U.N. Economic Commission for Europe. Ber-
lin, Germany.
43
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT 15. LRTAP
In 1981, the United States became party to
the UNECE Convention on Long-Range
Transboundary Air Pollution (LRTAP). Signa-
tory countries include most western Euro-
pean countries, several newly Independent
States, Canada, and the United States. The
Convention symbolizes a formal recognition
on the part of signatory countries that the
transboundary flow of air pollution is an im-
portant issue that merits formal international
cooperation. In 1985, the first Sulfur Proto-
col under the Convention committed those
countries who signed to a 30 percent reduc-
tion in emissions of sulfur dioxide from 1980
levels. (The United States did not sign based
on significant sulfur dioxide emission reduc-
tion efforts already undertaken in the 1970s.)
Soon thereafter, emissions reductions based
on achievement of critical loads became the
focus of efforts under LRTAP. In 1994, most
countries signed the Second Sulfur Protocol
requiring mapping of critical loads for all af-
fected countries. It is the expectation that
this effort will lead to emission reductions
based on the critical loads. A Nitrogen Ox-
ide Protocol was signed in 1988 by most
countries, including the United States. The
NOX Protocol outlined steps to reduce na-
tional annual NOX emissions. It also initi-
ated research and cooperative efforts on
critical loads for nitrogen.
the indicator used to estimate critical loads for for-
est soils (using the simple mass balance steady-
state model) is the concentration of aluminum in
the soil solution required to maintain pH above
4.0. Some countries varied the basis for their map-
ping procedures based on the availability of the
data collected by the individual .countries, and the
regional and national concerns regarding the sensi-
tivity of specific sensitive resources. The majority
of critical load values in Europe reflect the sensi-
tivity of forest soils. Critical loads in Finland, Nor-
way, Sweden, Switzerland, and several of the
newly Independent States reflect forests and sur-
face waters.42
The single most important technical attribute
around which European activities on acid deposi-
tion standards have revolved is associated with de-
fining the spatial resolution used. Interestingly, the
early decision to use a 150 km by 150 km square
grid as the fundamental spatial assessment unit for
acidic deposition control strategies had no direct
connection to spatial levels of resolution deemed
appropriate for critical load estimation. In fact, the
grid was in place well before the critical loads
concept achieved common usage. This relatively
coarse grid size, however, often allows for signifi-
cant spatial variation in environmental types and
designated critical load alternatives within indi-
vidual cells. This leads to difficult questions re-
garding spatial estimation of specific critical loads
appropriate for supporting deposition-based con-
trol policy and measurement of maintenance
and/or exceedance levels.
To provide a reasonable level of protection for
more sensitive ecological resources within each
grid cell, the European approach uses cumulative
distributions of critical load values and selects
from this distribution a non-exceedance level for
each cell. Under this approach two loadings are
calculated: one that would protect 95 percent of
sensitive ecological resources within the grid (i.e.,
the 5-percentile load), and one that would protect
99 percent of the resources (i.e., the 1-percentile
load). This procedure reconciles some of the basic
problems that arise when point estimates are used
to represent regional concerns. But the approach
still holds difficulties related largely to the process
of selecting appropriate critical load values from
the resulting distribution functions. Specifically, it
is sometimes difficult to determine the rationale by
which individual critical load values are selected
among the different cells. Qualitative considerat-
ions, which often stem from political agreements,
also have a role in this process.43 Despite the dif-
ferent sensitivities of various ecosystems, most
critical loads developed in Europe are very low
when compared to present deposition. This has
given some countries the impetus to seek greater
emissions reductions than were already planned.
Thus, most of the reductions of sulfur emissions
42 Coordination Center for Effects, National Institute of
Public Health and Environmental Protection. 1991.
(continued)
Mapping Critical Loads for Europe. CCE Technical
Report No. 1. U.N. Economic Commission for
Europe, Bilthoven, Netherlands.
43 Henriksen, A., and D.F. Brakke. 1988. Sulfate
deposition to surface waters. Environmental Science
and Technology 22(1 ):8-14.
44
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CHAPTER 2: ENVIRONMENTAL GOALS
under the second Sulfur Protocol fall in the range
of 50-80 percent. The European community as a
whole is projecting emissions decreases of over 60
percent by the year 200 compared to 1980 levels.
The four European countries that signed the sec-
ond Sulfur Protocol and their commitments to re-
ductions (relative to 1980) are:44
* France: 74% by 2000; 78% by 2010
« Germany: 83% by 2000; 87% by 2005
* Italy. 65% by 2000; 73% by 2005
« United Kingdom: 50% by 2000; 80% by 2010
The French reductions translate into an emissions
level of approximately 825,000 tons of sulfur diox-
ide; emissions reductions in the other three coun-
tries are around 1.1 million tons. These emissions
reductions will occur in an area that is in general
much less energy intensive than in the two North
American members of LRTAP (the United States
and Canada), so it is difficult to argue that the
Europeans are simply getting easy, low-cost reduc-
tions. Taken together, these four European indus-
trialized countries represent a population very
close to that of the United States. By 2010, their
emissions of sulfur dioxide will be less than 5 mil-
lion tons, while the United States is projected to
have emissions of around 15 million short tons.
Canada committed to reducing its emissions by 46
percent within a Sulfur Oxide Management Area
(SOMA), which represents a targeted approach to
the acidification problem in Eastern Canada. Can-
ada's population is about 10 percent that of the
United States; it is committed to a national cap of
3.2 million metric tons (about 3.5 million short
tons) in the year 2000.
Canada adopted 6.7 kg-S/ha/yr (wet deposition) in
the early 1980s as what would now be termed a
target load. This value was based on available data
indicating that loss of sport fish would occur at pH
less than 5.3, and this loss would produce signifi-
cant economic and social impacts. This target load
was not, however, intended to protect extremely
sensitive areas. Canadian policy makers concluded
that additional research was necessary to deter-
mine appropriate loading limits to completely pro-
tect all sensitive Canadian ecological resources.
The target load was used as a goal in developing
the Canadian acid rain control program and in
discussions with the United States on transbound-
ary air pollution. As a result of current U.S. and
Canadian acid rain control programs, most areas
of Canada are expected to reach the 6.7 kg-S/ha/yr
target.
Subsequently, in 1990, critical loads for water-
sheds in eastern Canada were calculated using
steady-state water chemistry models that projected
sulfur deposition necessary to maintain 95 percent
of the lakes at pH 6.0 or greater.45 Included in this
approach were allowances for maintaining condi-
tions in naturally acidic surface waters. Resulting
critical load estimates ranged from less than 2.7 to
more than 6.7 kg-wet S/ha/yr, with the variation
primarily dependent on geological characteristics.
The eastern region of Canada, including the Atlan-
tic provinces, Labrador, and eastern Quebec, was
determined to require the lowest critical loads (less
than 2.7 kg-S/ha/yr), which are close to back-
ground deposition levels. Critical loads estimated
for Quebec ranged from 3 to more than 6.7 kg-
S/ha/yr, and for Ontario they ranged from 2.7 to
more than 6.7 kg-S/ha/yr. In a separate analysis, a
simple mass balance approach was used to deter-
mine that a loading of 5 to 6.7 kg-S/ha/yr wet
deposition would maintain surface water pH
greater than 5.3 on an annual basis in watersheds
that have lakes with ANCs of 200 ueq/l or greater
in regions of low runoff.
Using the initial Canadian effort as an example,
the New England states and New York adopted
Canada's first target load of 6.7 kg-S/ha/yr (wet
deposition) as a level adequate to protect moder-
ately sensitive ecological resources from additional
damage caused by acidic deposition.46 This level
was not viewed, however, as adequate to protect
the most sensitive resources within these regions.
44 United Nations Economic Commission for Europe.
1994. Protocol to the 1979 Convention on Long-
Range Transboundary Air Pollution on Further Re-
duction of Sulphur Emissions. ECE/EB,AIR/40. Ge-
neva.
45 Federal/Provincial Research and Monitoring Coor-
dination Committee. 1990. The 1990 Canadian
Long-Range Transport of Air Pollutants and Acid
Deposition Assessment Report. 8 parts. Research
and Monitoring Coordination Committee, Canada.
46 New England Governor's Conference. 1985. His-
tory and the Development of the New England Posi-
tion on Acid Rain. New England Governor's Confer-
ence, Inc.
New York State Department of Environmental Con-
servation (NYSDEC). 1985. A Policy for New York
State to Reduce Sulfur Dioxide Emissions: The Sulfur
Deposition Control Program. Final Environmental
Impact Statement. NYSDEC, Albany, NY.
45
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
Maryland developed critical loads based on the
sensitivity of individual streams to acidification.47
This effort included as its overall goal an assess-
ment of the extent to which the state could meet
or surpass its ecological objectives to minimize
potential acidic deposition effects. Calculated
critical loads for areas within Maryland ranged by
region from less than 8 to more than 64 kg-S/ha/yr.
These loads were developed using (1) two models
(PROFILE and MAGIC), (2) pH limits required to
protect the most sensitive life stages of biological
indicator species, and (3) a complex of specific
physical, chemical, and biological factors that
potentially affect soil and water chemistries. Acid
sensitivities for three indicator fish species were
used across the different regions assessed: blue-
back herring (pH=6.2), smallmouth bass (pH=5.8),
and brook trout (pH=5.75). The assessment re-
vealed that critical loads at several sensitive recep-
tor locations could not be met for any plausible
emissions control scenario. These locations were
thus deemed possible candidates for site-specific
mitigation measures, principally stream liming.
Minnesota is the only state with an established
deposition standard for sensitive areas.48 Sensitive
areas are defined based on lake ANC, with the
state's deposition standard of 3.7 kg-S^a/yr (wet
deposition) established to protect lakes whose
ANC is less than 40 ueq/l. The standard was de-
rived using regression techniques to relate deposi-
tion SO42' concentrations and acidity to the ability
of sensitive Minnesota lakes (ANC<40 ueq/l) to as-
similate additional acid loadings. This deposition
standard may also be viewed as equivalent to the
critical load for this region because maps of sens-
itive soils show that the majority of areas with
potentially the most sensitive soils correspond to
those regions with sensitive lakes.
In contrast to regions east of the Mississippi River,
deposition of nitrogen has long been viewed as a
more significant problem than deposition of SO2 in
much of .the western United States. For example,
NO3- approximately equals SO42' deposition in
some areas of California. Consequently, critical
47 Sverdrup, H., P. Warfvinge, M. Rabenhorst, A. Jan-
icki, R, Morgan, and M. Bowman. 1992. Critical
Loads and Steady-State Chemistry for Streams in
Maryland. Environmental Pollution 77': 195-203.
48 Minnesota Pollution Control Agency. 1985. State-
ment of Need and Reasonableness: Proposed Acid
Deposition Standard and Control Plan. State of Min-
nesota Pollution Control Agency, St. Paul, MN.
loads for nitrogen deposition have been estimated
for California,49 but similar loads for sulfur have
not. Studies show that loadings of 10 to 20kg-
N/ha/yr would likely protect California forests.
Critical loadings recommended to protect sensitive
California resources ranged from 5 to 45 kg-
N/ha/yr, depending on the region. Other state ef-
forts are currently underway.
2.5.5 Spatial and Temporal Issues in
Development of a Standard
Information discussed in Sections 2.3, 2.4, 2.5.2,
and 2.5.3 clearly demonstrate that regions of
North America differ in both their potential sensi-
tivity and risk to adverse effects caused by sulfur
and nitrogen deposition. These differences provide
a strong scientific justification for setting different
standards that recognize variations among and
within sensitive regions. Regions covered by any
individual standard would be larger than most
states and undoubtedly smaller than the nation as
a whole. It is also clear that appropriate and scien-
tifically justifiable environmental goals could differ
across areas within individual states.
Sensitive resources tend to cluster within relatively
easily defined geographic areas often associated
with specific mountain ranges and other areas hav-
ing relatively unique geologic attributes. Potential
protection requirements for ecological resources in
sensitive regions can be identified, categorized,
and aggregated across several levels of organiza-
tion. These include regions, landscapes, ecosys-
tems, communities, populations, and individual
site-specific measures (e.g., critical stream habitat
for a listed endangered fish species). In general,
the smaller the area of concern, the greater the
precision required in establishing the basis for
standards and in determining the boundaries
where standards would apply. Under a critical
loads approach, appropriate ecological rationales
would need to be developed for whatever scale is
targeted for protection by a standard. Furthermore,
there is significant variation in spatial scale of ex-
posure (i.e., wet and dry deposition) on a regional
as well as site-specific level. Deposition and effe-
cts monitoring (further described in the following
49 Takemoto, B.K., M. Bergen, N. Motallebi, M. Muel-
ler, H. Margolis, and S. Prasad. 1992. The Atmos-
pheric Acidity Protection Program: Annual Report to
the Governor and Legislature. Draft report. State of
California Air Resources Board, Research Division,
Sacramento, CA.
46
-------
CHAPTER 2: ENVIRONMENTAL GOALS
chapter) is an essential component in the standard
setting and implementing process.
Beyond these considerations regarding differences
in the spatial scale are important considerations
regarding the temporal scale. For example, the po-
tential for exposure to, and risk from, acidic condi-
tions is often highest during the spring due to the
mobilization of the winter accumulations of
deposited acids and the activation of seasonal bio-
logical growth process. Also, the sensitivity of
many resources changes over time. For example,
the most sensitive life stages of many fish species
are hatching eggs and newly hatched fry. The pe-
riods of greatest sensitivity for many species are
spring and fall when most fish species hatch. Simi-
larly, spring budding periods for flowers and
leaves and initial root growth by seedlings are par-
ticularly sensitive periods for many terrestrial
plants. In contrast, many resources have low sensi-
tivity during the winter, when their biological ac-
tivity is low. Patterns in weather variations may
change from year to year and would also have to
be considered in a standard-setting process. Op-
tions that could be evaluated for appropriate aver-
aging periods which accommodate temporal issues
include single-event loadings, seasonal loadings,
total annual loadings, average annual loadings,
10-year average loadings and 50-year average
loadings. The timing of a standard, therefore, ulti-
mately has significant implications for develop-
ment and implementation of an acid deposition
standard.
Determining appropriate averaging periods must
also include consideration of possible temporally
delayed effects. For example, the chemistry of
spring meltwaters may better reflect accumulated
winter deposition than springtime deposition. Such
processes could indicate the need in some regions
for more stringent winter deposition standards.
Such standards might aim to minimize over-winter
accumulations of strong-acid anions in snow
packs, thereby minimizing the potential acidity of
spring meltwaters and their potential adverse effe-
cts on receiving waters. Likewise, in those areas
where episodic pulses of nitrogen are the primary
concern, temporal considerations may best be fo-
cused on a particular pollutant. Consideration of
effects that are cumulative in nature and potential-
ly significantly delayed in time is a complex yet
critical component in the development of an acid
deposition standard.
2.6 CONTROLLING SULFUR AND NITROGEN TO
REDUCE SURFACE WATER ACIDIFICATION
Atmospheric deposition of sulfur- and nitrogen-
containing compounds, primarily SO42~ and NO3",
contributes to the acidic deposition problem, as
reviewed in Section 2.2.1. The relative contribu-
tion of sulfur and nitrogen to this problem differs
among regions, depending not only on external
differences in the deposition rates of these chemi-
cals, but also on differences among the capacity of
receptor watersheds to retain sulfur and nitrogen,
as discussed in Sections 2.2.1, 2.5.2, and 2.5.3.
Such differences have led some authorities (e.g.,
those in California) to date to focus solely on the
need to control nitrogen deposition (see Sec-
tion 2.5.4) while others have focused primarily on
sulfur.
For most regions of North America affected by and
at risk from chronic effects of acidic deposition,
the principal present concern is sulfur deposition.
As more fully discussed in Sections 2.2.1, 2.5.2,
and 2.5.3, sulfur deposition appears to be the pri-
mary cause of long-term chronic acidification in
all affected sensitive areas. The NBS illustrates that
for the near term sulfur deposition is likely to rem-
ain the overriding acidification problem in most
sensitive areas of eastern North America. This
likely will remain the case until annual retention
of nitrogen decreases sufficiently and the full po-
tential acidifying influence of nitrogen deposition
commences. At these times, sulfur and nitrogen
are projected to have approximately equal roles in
surface water acidification. Thus, for most areas,
where current or near-term needs for additional
acidic deposition control are projected, and where
watershed nitrogen saturation is not likely immi-
nent, the greatest potential benefits will come pri-
marily from control of sulfur emissions and deposi-
tion.
A significant and growing body of scientific re-
search indicates, however, that nitrogen deposition
is a major and important contributor to the acidic
deposition problem. First, many areas of the West
are more affected by nitrogen deposition than by
sulfur deposition. Second, as briefly reviewed in
Section 2.2.1, nitrogen (in the form of nitrate an-
ion) frequently has been found to be a significant
contributor to episodic events in streams and lakes
in some parts of the Northeast.50 In these areas, as
50 This does not imply that sulfur deposition is not often
a key component of episodic acidification, because
sulfur has often been found to be the primary cause
(continued)
47
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
adverse effects accompanying chronic acidificat-
ion due to sulfur deposition are reduced, overall
adverse effects due to episodic acidification would
likely continue to impair the water quality in many
of these surface waters, but the extent of these eff-
ects would likely be reduced because reducing the
chronic sulfur effects also decreases potential epi-
sodic effects as well. Third, some watersheds of
the Northeast (e.g., in the Catskill Mountains of
New York) and the mid-Appalachians may be
moving toward nitrogen saturation. For these re-
gions, nitrogen deposition is now or would likely
become a more direct cause of chronically acidic
conditions in sensitive waters, with potential
adverse effects caused by acidic sulfur and nitro-
gen deposition becoming approximately equal and
directly additive. In fact, additional limits on nitro-
gen deposition would likely produce a two-fold
potential benefit by both reducing acidic deposi-
tion rates and lengthening average times to water-
shed nitrogen saturation. These benefits would ef-
fectively allow a greater mass of NO3" to be de-
posited over longer periods without significantly
increasing surface water acidification processes.
Scientific uncertainties regarding regional rates
and differences in processes affecting watershed
assimilation of acid-forming sulfur and nitrogen
compounds preclude defining either national or
regional protection levels below which deposition
of either chemical would produce no adverse im-
pact. Available information does indicate, how-
ever, that additional deposition reduction through-
out the range of potential reductions in sulfur
and/or nitrogen depositions down to background
deposition loads would likely reduce regional pro-
portions of chronically acidic surface waters (ANC
<0 ueq/l) or proportions of surface waters potenti-
ally most sensitive to episodic effects (ANC<50
ueq/l) or proportions of both groups. The magni-
tude of these potential benefits to each group of
surface waters varies considerably by region. NBS
projections indicate for some regional surface
water groupings that potential benefits may
amount to a few percentage points shift in propor-
tions of acidic or sensitive surface waters benefit-
ing, while for other groupings in other regions po-
tential benefits from deposition reductions could
benefit 20 percent or more of the acidic or sens-
itive waters. Note, however, that even a few per-
centage points may mean many lakes or stream
reaches. Now, however, even a sound qualitative
ranking of these differences awaits resolution of
key scientific unknowns, exemplified by the
marked uncertainty associated with quantifying re-
gional differences in their remaining times to wa-
tershed nitrogen saturation.
of episodic acidification in areas both within and
outside the Northeast. For example, see A.K.
O'Brien, K.C. Rice, M.M. Kennedy, and O.P.
Bricker. 1993. Comparison of episodic acidification
of mid-Atlantic upland and coastal plain streams.
Water Resources Research 29(91:3029-3039.
48
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CHAPTER 3
SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSITION
REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
3.1 INTRODUCTION
Acidic deposition results from a complex series of
interactions among chemical compounds present
in the atmosphere. The two most important groups
of chemicals are sulfur- and nitrogen-containing
compounds. These chemicals are emitted to the
atmosphere from natural and anthropogenic
sources, with anthropogenic sources dominating.
Natural sources include vegetative emissions, geo-
thermal activities, forest fires, lightning, soil, and
salt-water organisms. Anthropogenic sources in-
clude point sources such as utilities, industrial
boilers and other industrial processes, mobile
sources including automobiles, trucks, and off-
highway vehicles, and area sources such as resi-
dential boilers which are too small and numerous
to track individually.
Sulfur and nitrogen compounds in the atmosphere
can be transported hundreds to thousands of kilo-
meters by meteorological forces. During transport
sulfur dioxide (SO2) and nitrogen oxides (NOX), the
primary emissions of these species, are oxidized in
the air or in cloud-water to form other, secondary
compounds. The great majority of these com-
pounds, particularly sulfate and nitrate, are acidic.
The oxidizers, such as the hydroxyl radical, hydro-
gen peroxide, and ozone are produced by reac-
tions of volatile organic compounds (VOC) and
NOX. The sulfur and nitrogen pollutants are depos-
ited to the earth by either dry or wet deposition.
Dry deposition occurs when particles settle out of
the air onto the earth or when gases or fine parti-
cles directly impact land, plants, or water, or when
plant stomata take up gases, such as SO2. In wet
deposition, pollutants are removed from the at-
mosphere by rain or snow. Fine particles or secon-
dary aerosols formed by these same processes scat-
ter or absorb visible light and thus impair visibility.
When inhaled these secondary aerosols and their
gaseous precursors can also cause adverse human
health effects. Potential benefits to visibility, hu-
man health, materials, and cultural resources from
controlling acidic deposition are discussed in
Chapter 4.
The complex relationship between emissions and
deposition depends on a large number of physical,
chemical, and biological processes. To understand
the environmental impact of the CAAA and to de-
velop and analyze strategies to reduce the effects
of acidic deposition, the relationship between
emissions and deposition must be understood not
only for the present, but also for future years. To
predict deposition a model must be able to de-
scribe the transformation of anthropogenic and
biogenic emissions by atmospheric processes to
wet and dry deposition. The goal of deposition
modeling is to simulate the source-receptor rela-
tionships that translate emissions into deposition
values in space and over time. An understanding
of these complex interactions is necessary to de-
velop a comprehensive approach to achieving the
environmental goals discussed in Chapter 2.
This chapter addresses the following requirement
of Section 404 (Appendix B) of Title IV of the
CAAA:
* Description of the state of knowledge with
respect to source-receptor relationships
necessary to develop a control program on
such standard or standards and additional
research that is on-going or would be
needed to make such a control program
feasible
Section 3.2 describes state-of-the-art atmospheric
modeling techniques, the uncertainties associated
with predictive modeling, precursor emissions and
emissions inventories, and deposition species. This
section describes RADM, the atmospheric model
used for this study, and examines its use in model-
ing of acidic deposition and the results of its
evaluation.
Section 3.3 presents RADM results that explore
and define source-receptor relationships. Relation-
ships prior to implementation of the 1990 CAAA
are compared and contrasted to those expected af-
ter full implementation of the Act in 2010. The
model discussion is followed in Section 3.4 by a
description of the inventories used to evaluate al-
ternative emissions control scenarios.
49
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ACID DEPOSITION FEASIBILITY STUDY
In Section 3.5, source-receptor relationships from
RADM are used to assess the environmental im-
pact of the CAAA and to predict deposition values
for acidic sulfur and nitrogen under various alter-
native emissions scenarios for the year 2010. Emis-
sions scenarios analyzed include full implementa-
tion of Title IV with trading, full implementation of
Title IV without trading, national reductions in util-
ity and industrial SO2 emissions beyond Title IV,
and national reductions in utility and industrial
NOX emissions beyond Title IV. Regionally tar-
geted emissions reductions for SO2 are examined
in Section 3.6.
3.2 THE REGIONAL ACID DEPOSITION MODEL
RADM has been developed over the past 10 years
under the auspices of the National Acid Precipita-
tion Assessment Program (NAPAP) to address pol-
icy and technical issues associated with acidic
deposition. The model is designed to provide a
scientific basis for predicting changes in deposition
resulting from changes in precursor emissions, to
predict the influence of emissions sources in one
region on acidic deposition in other geographic
regions, and to predict the levels of acidic deposi-
tion in certain sensitive receptor regions.
A key requirement for the model selected for NA-
PAP was the ability to assess changes in sulfate in
response to projected changes in SO2 emissions.
Based on knowledge gained from models and ex-
perimental measurements, a reduction in SO2
emissions is expected to lead to a less than equiva-
lent reduction in sulfate deposition. This non-
equivalency between emissions reductions and
decreases in deposition is due to the nonuniform
spatial distribution of emissions reductions and the
complexities of atmospheric chemistry. While
simpler models can predict the nonequivalency
due to spatial nonuniformities, a complex model
such as RADM is needed to calculate the addi-
tional affects of atmospheric chemistry on deposi-
tion.
The development, application, and evaluation of
RADM has been documented extensively by NA-
51,52,53 RADM continues to undergo periodic
peer reviews, evaluations, and improve-
ments.54'55-56 Understanding and modeling acidic
deposition requires consideration of a complex
range of physical and chemical processes and their
interactions, including:
* The emissions of precursor chemicals that
produce and regulate acidity in atmos-
pheric deposition;
* The meteorological processes that transport
and mix emitted species in space and time;
• The physical and chemical transformations
that alter the physical phases and chemical
properties of emitted species;
* The meteorological factors and properties
of the Earth's surface that lead to deposi-
tion of acidic substances.
RADM is an Eulerian model in which concentra-
tions of gaseous and particulate species are calcu-
lated for specific fixed positions in space (grid
cells) as a function of time. The concentration of a
specific pollutant in a grid cell at a specified time
is determined by: the emissions input rate; the
transport of that species by wind into and out of
the grid in three dimensions; movement by turbu-
lent motion of the atmosphere; chemical reactions
that either produce or deplete the chemical; the
change in concentration due to vertical transport
by clouds; aqueous chemical transformation and
51 Renne, D., et al. December 1989. Models Planned
for Use in the NAPAP Integrated Assessment. Sec-
tion 4: Atmospheric Models. National Acid Precipi-
tation Assessment Program.
52 Chang, J.S., P.B. Middleton, W.R. Stockwell, C.J.
Walcek, J.E. Pleim, H.H. Lansford, F.S. Binkowski,
(continued)
S. Madronich, N.L. Seaman, and D.R. Stauffer. De-
cember 1990. The Regional Acid Deposition Model
and Engineering Model. SOS/T Report 4. In: Acidic
Deposition: State of Science and Technology. Na-
tional Acid Precipitation Assessment Program.
53 Dennis, R.L., W.R. Barchet, T.L. Clark, and S.K.
Seilkop. September 1990. Evaluation of Regional
Acid Deposition Models (Part I). SOS/T Report 5. In:
Acidic Deposition: State of Science and Technology.
National Acid Precipitation Assessment Program.
54 Dennis, R.L., J.N. McHenry, W.R. Barchet, F.S.
Binkowski, and D.W. Byun. 1993. Correcting
RADM's sulfate underprediction: Discovery and cor-
rection of model errors and testing the corrections
through comparisons against field data. Atmospheric
Environment 27A(6):975-997.
55 McHenry, J.N., and R.L. Dennis. 1994. The relative
importance of oxidation pathways and clouds to at-
mospheric ambient sulfate production as predicted
by the Regional Acid Deposition Model. Journal of
Applied Meteorology 33 (7): 890-905.
56 External Review Panel report on RADM evaluation
for the Eulerian Model Evaluation Field Study Pro-
gram.
50
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
scavenging; and removal by dry deposition. These
physical and chemical process are depicted in Ex-
hibit 16.
sional data assimilation (4DDA) to produce the
most accurate recreation of past weather.57 An ag-
gregation technique, described fully by Dennis et
EXHIBIT 16. PHYSICAL AND CHEMICAL PROCESSES CONTRIBUTING TO ACIDIC DEPOSITION
SOURCES
NO,
/
/
voc
'.'
1 ' /
1 / '
/
/ Gaseous
Pollutants
Atmosphere
l^__^ §
[*»• Dry Deposition
}
Depo
Ui
\/
Pollutants
Cloud Wat
and
Precipitatu
Wet
sition , ,
M. 1 A M.
\
Particulate
Pollutants.
in
Atmosphere
/ B
o
«7S
O
in o.
er §
Dn §
\
* __-•
.
Natural
RECEPTORS
The version of RADM used for NAPAP and for the
analyses presented in this report covers an area of
2,800 by 3,040 km east of central Texas and south
of James Bay, Canada to the southern tip of Flor-
ida. RADM divides this area into 80 by 80 km grid
cells. For sulfur deposition modeling the distance
from ground level to 16 km in altitude was re-
solved into 6 vertical layers; for nitrogen deposi-
tion 15 layers were used. The RADM domain, pic-
tured in Exhibit 17, consists of 35 by 38 horizontal
grid cells. The model is run with either 7,980 or
19,950 cells depending upon whether 6 or 15 ver-
tical layers are employed. For each grid cell, pre-
dictions are generated at dynamically determined
time-steps of seconds to minutes and are output
hourly by RADM with 41 chemical species being
transported. Hourly wet and dry deposition values
are also generated for each surface cell for 12 spe-
cies (6 wet and 6 dry).
The meteorological fields used to drive the RADM
are from the Pennsylvania State University-Na-
tional Center for Atmospheric Research Mesoscale
Model (MM4). The MM4 is run using 4-dimen-
al.,53 developed during NAPAP is used to develop
annual estimates of acidic deposition. Meteoro-
logical cases with similar wind flow patterns were
grouped by applying cluster analysis to classify the
wind flow patterns from 1982 to 1985, resulting in
19 sampling groups, or strata. Meteorological
cases were randomly selected from each stratum;
the number selected was based on the number of
wind flow patterns in that stratum relative to the
number of patterns in each of the other strata, to
approximate proportionate sampling. A total of 30
cases were used in the current aggregation ap-
proach. Deposition results for these cases were
weighted according to the strata sampling fre-
quencies to form annual averages.
57 Seaman, N.L., and D.R. Stauffer. 1989. Develop-
ment of Four-Dimensional Data Assimilation for
Regional Dynamic Modeling Studies. Final Report to
the U.S. Environmental Protection Agency, Contract
CR-814068-01, the Pennsylvania State University,
102 pp.
51
-------
ACID DEPOSITION FEASIBILITY STUDY
EXHIBIT 17. THE RADM MODELING DOMAIN
Development and implementation of an acidic
deposition control strategy requires an understand-
ing of the interaction among emissions of several
chemical species, the spatial and temporal patterns
of those emissions, and atmospheric transport of
those species to regions where deposition occurs.
It is important, therefore, to understand the uncer-
tainty and reliability of model predictions. The fol-
lowing subsections briefly describe the key input
and functional components upon which RADM
predictions are based. The discussion on emissions
and atmospheric chemistry explains why certain
emissions are important, distinguishes anthropo-
genic (controllable) from natural (background
sources), and describes why detailed spatially and
temporally resolved emissions inventories are
needed to accurately predict deposition. Subsec-
tions 3.2.2 and 3.2.3 describe the development of
source-receptor relationships and how these rela-
tionships can be used to identify emissions sources
responsible for deposition, whether proximate or
hundreds of kilometers distant. The reliability and
52
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
confidence of the scientific community in using
modeling results for decision-making is discussed
in subsection 3.2.4.
3.2.1 Emissions and Atmospheric Chemistry
The principal acids in deposition are sulfuric
(H2SO4) and nitric (HNO3) acids. Thus, emissions
of compounds containing sulfur and nitrogen have
received primary emphasis in acidic deposition
control strategies. However, emissions of VOCs
and their oxidation products are extremely impor-
tant because they are involved in reactions that
produce the oxidizing species that lead to forma-
tion of sulfuric and nitric acids in the atmosphere.
Key environmentally important species predicted
by RADM are:
* AMBIENT CONCENTRATIONS: SO2, NO, NO2,
HN03, 03, H2O2, NH3, PAN, HCHO, CO,
aerosol SO42'
WET DEPOSITION: SO42-, NO3- as HNO
NH3, H+
3,
* DRY DEPOSITION: SO2, SO42-, HNO3, O3,
NO2
The RADM chemistry component consists of 140
reactions among 60 species, 40 of which are or-
ganic compounds. Chemical decomposition by so-
lar radiation (photolysis) is included in the model
chemistry as are aqueous-phase reactions which
occur in clouds. These latter reactions are particu-
larly important in sulfuric acid formation. The
chemically derived nonequivalency between
emissions reduction and deposition decreases is
due in part to local depletion of hydrogen perox-
ide, a compound produced by atmospheric photo-
chemical (involving sunlight) reactions that is im-
portant in the oxidation of aqueous-phase SO2 to
sulfate.58 This nonlinear oxidant limitation affects
only wet deposition of sulfate. Details of the
RADM chemistry are described in NAPAP State of
Science and Technology Report 4.52
Inputs to RADM include hourly emissions of SO2,
sulfate, nitric oxide (NO), nitrogen dioxide (NO2),
ammonia (NH3), carbon monoxide (CO), particu-
58 McHenry, J.N., and R.L. Dennis. 1994. Cloud and
chemistry pathway characterization of the nonlinear
response of sulfur deposition and sulfate air concen-
trations to changes in SO2 Emissions in the RADM.
In Atmospheric Chemistry Extended Abstracts. AMS
Conference held January 1994, Nashville, TN.
pp. 203-208.
late matter, and 15 classes of VOCs from natural
and anthropogenic sources. Emissions inventories
used for RADM were derived from the 1985 NA-
PAP Emissions Inventory, the most comprehensive,
highest quality air emissions inventory ever as-
sembled. (RADM inputs have since been updated
to include more recent sector-specific inventories
such as the National Allowance Data Base devel-
oped under the Acid Rain Program for utility emis-
sions.) The basis and key assumptions for the 1985
NAPAP Emissions Inventory are described below.
A detailed description of this inventory and the
data processing for use in RADM are described in
NAPAP State of Science and Technology Re-
port 1.59
Natural Emissions Sources
Natural emissions of acidic precursor species, or-
ganic matter, and alkaline materials (dust) are gen-
erated by vegetative matter, microbes, geothermal
activity, natural combustion (such as forest fires),
lightning, and salt-water organisms. These sources,
in contrast to anthropogenic sources, are widely
distributed, small, sporadic, and subject to large
seasonal and weather-related variations. Total
VOC emissions from biogenic sources are esti-
mated to be of the same order of magnitude as
VOC emissions from anthropogenic sources.60
Principal biogenic sources are trees, shrubs,
grasses, agricultural crops, decaying leaf litter, and
vegetation in fresh and salt water. Some biogenic
compounds are very reactive in the atmosphere;
others are relatively inert. Emissions for individual
grid cells and specific VOCs or VOC classes are
calculated from estimates of biomass data
(vegetation type, species, land-use coverage, and
leaf area index), adjusted for seasonal variation,
temperature, solar intensity, soil conditions includ-
ing moisture, and elevation.
Natural emissions of SO2, sulfates, and nitrogen
oxide have been found to be less important than
anthropogenic sources. Natural emissions of sulfur,
which are not well understood, are estimated to be
about 6 percent of anthropogenic emissions and
59 Placet, M., R.E. Battye, and F.C. Fehsenfeld. De-
cember 1990. Emissions Involved in Acidic Depo-
sition Processes. SOS/T Report 1. In: Acidic Deposi-
tion: State of Science and Technology. Volume I.
National Acid Precipitation Assessment Program.
60 Novak, J.H., and T.E. Pierce. 1993. Natural
Emissions of Oxidant Precursors. In Water, Air, and
Soil Pollution. Vol. 67, pp. 57-77.
53
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ACID DEPOSITION FEASIBILITY STUDY
are not included in the inventory. Natural emis-
sions of nitrogen compounds, which are also
poorly characterized, are estimated to be 1 to 2
percent of total nitrogen emissions. The 1985 NA-
PAP inventory does include nitrogen emissions
from soil because these emissions have been stud-
ied, although large uncertainties in estimates still
exist. Lightning-generated NOX may be episodi-
cally important, but is not understood well enough
to be included. Alkaline dust emissions (from
sources such as erosion and unpaved roads) are
also not included in the inventory. The principal
natural source of ammonia, an important nitrogen-
containing atmospheric reactant, is animal excre-
ment, from feedlots, farm animals, and wildlife;
however, the paucity of data on wildlife densities
does not allow this source to be estimated with
any degree of reliability. "Natural" emissions of
ammonia are included in the inventory, but the es-
timates have a large degree of uncertainty. Am-
monia is considered in RADM simulations because
it is an important contributor to nitrogen deposi-
tion and affects rainwater pH and the production
of secondary sulfate and nitrate aerosols.
Anthropogenic Emissions Sources
Acidic deposition precursor species and reactive
atmospheric chemicals are generated by energy
production, industrial processes, mobile sources,
and waste disposal. Major sources (such as power
plants) that emit large quantities of pollutants at
specific, well-defined locations are called "point
sources." Small emissions sources such as residen-
tial boilers and mobile sources are grouped to-
gether as "area sources." In general, emissions from
point sources are reported to EPA by states, while
EPA calculates emissions from area sources. The
RADM model includes a species allocation mod-
ule61 which accounts for the small amount of pri-
mary sulfate (about 1 percent of SO2 emissions)
that is emitted with SO2. This primary sulfate is
negligible in comparison to sulfate formed in the
atmosphere. Similarly, the allocation module splits
NOX emissions into a 5:95 ratio of NO2 to NO. As
is the case with biogenic VOC emissions, some
VOC species are very reactive in the atmosphere,
others are not. The model calculates VOC species
or classes by grid ceil based on source category.
Major emissions source categories in the United
States and their contribution to total emissions of
each pollutant in the year 1992 are summarized
below for SO2, NOX, and VOC.62
* SO2
• Electric Utility Fuel Combustion (69.7%)
• Industrial Fuel Combustion (13.6%)
• Metals Processing (3.8%)
» Highway Vehicles (3.5%)
• Other Fuel Combustion (2.6%)
» All Other Sources (6.8%)
* NOX
• Electric Utility Fuel Combustion (32.3%)
• Highway Vehicles (32.3%)
» Industrial Fuel Combustion (15.2%)
• Off-Highway Vehicles (12.3%)
» Other Fuel Combustion (3.2%)
» All Other Sources (4.7%)
» VOC
» Highway Vehicles (26.8%)
» Solvent Utilization (26.7%)
• Waste Disposal and Recycling (10.2%)
» Off-Highway Vehicles (9.4%)
» Storage and Transport (8%)
» All Other Sources (18.9%)
Temporal and Spatial Allocation of Emissions
Data
To create modeling inventories the annual inven-
tories must be resolved spatially, temporally, and
by chemical species. Allocation of emissions to
grid cells for point sources is relatively straightfor-
ward. The geographic (latitude-longitude) location
of each point source determines its grid cell
placement. Since area sources are too small to be
included individually in the annual inventory,
emissions from these sources are calculated by
multiplying an emissions factor by an activity pa-
rameter which reflects the operating rate of each
source. The activity parameter is determined from
surrogates such as population (number of gasoline
service stations), housing (residential fuel combus-
tion), and agricultural land area (ammonia fertil-
izer application). A process was developed by
61 Walters, R., and M. Saeger. 1990. The NAPAP Emis-
sions Inventor/: Development of Species Allocation
Factors. EPA-600/7-89-01 Of. U.S. Environmental
Protection Agency, Research Triangle Park, NC.
62 Office of Air Quality Planning and Standards. Octo-
ber 1993. National Air Pollutant Emissions Trends,
1900-1992. EPA Report No. 454/R-93-032. U.S.
Environmental Protection Agency. Research Triangle
Park, NC.
54
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
EPA53 to apportion area source emissions to indi-
vidual grid cells from county-level data using these
surrogate parameters. Exhibit 18 is a density map
of SO2 emissions for the RADM region.
EXHIBIT 18. MAP OF ANNUAL SULFUR
EMISSIONS DENSITY IN 1985 (TONS/YEAR)
43000 • 160000
>160000
Day- and hour-specific gridded emissions are cre-
ated for each of the 30 RADM aggregation cases.
Temporal allocation factors were developed for
NAPAP64 that provide day-specific estimates based
on tabulation of representative relative diurnal
emissions patterns by day of the week and by sea-
son for each source. Where emissions strongly re-
spond to temperature and other meteorological
conditions, the day-specific meteorology is used in
the emissions estimation procedure. Examples in-
clude volatile organics that evaporate easily,
sources for which ambient conditions affect per-
formance and hence emissions, such as mobile
sources, and sources for which temperature affects
biological processes, such as biogenic emissions,
Plume rise from major industrial and utility sources
63 Modica, L, and D.R. Dulleba. April 1990. The 1985
NAPAP Emissions Inventory: Development of Spatial
Allocation Factors. EPA Report No. 600/7-89-01 Ob.
Air and Energy Research Laboratory, U.S. Environ-
mental Protection Agency, Research Triangle Park,
NC.
64 Fratt, D., D.F. Mudgett, and R.A. Walters. 1990. The
1985 NAPAP Emissions Inventory: Development of
Temporal Allocation Factors. EPA Report No. 600/7-
89-01 Od. U.S. Environmental Protection Agency,
Research Triangle Park, NC.
is also computed hourly, based on the hourly me-
teorology of each day.
3.2.2 Modeling Source-Receptor Relation-
ships and Source Attribution
Eulerian, or fixed-grid models, are very suitable for
representing the full, complex nonlinearity of the
photochemistry involved in the oxidation of pri-
mary emitted species to acidic substances. The
gas- and aqueous-phase oxidation of sulfur is non-
linear; the nonlinearity comes from competition
for scarce oxidizers, such as hydrogen peroxide.
The most accurate modeling of source-receptor re-
lationships must maintain the overall competition
for oxidants represented by the concentrations
produced by all the SO2 emissions, while tracking
the particular emissions from the SO2 sources of
interest. In other words, the influence of SO2
sources cannot be studied individually, but must
be considered altogether. This is because, for a
nonlinear system, the sum of the deposition from
individual point source emissions is not expected
to be the same as the total deposition from all
point emissions computed simultaneously.
Eulerian models have not historically been used to
study source-receptor relationships. The Tagged
Species Engineering Model65 was developed under
NAPAP to study such relationships. The Tagged
Species Model gives the Eulerian RADM modeling
system the capability to identify, for assessment
purposes, the concentration and deposition fields
attributable to specified SO2 emissions source re-
gions in the presence of the full concentration
fields. The Tagged Model preserves the oxidant
competition across space and time. A tagging con-
cept is applied in which additional, identical mass
conservation equations are solved for a portion of
the sulfur concentration field that originates from
specific geographical locations within the full
modeling domain. This allows tagged concentra-
tion fields and tagged wet and dry deposition to be
identified and tracked in the model separate from,
yet as portions of, the total sulfur chemical envi-
ronment that is nonlinear and that produces the
complete concentration and deposition fields. Ex-
hibit 19 shows the tagged RADM regions created
for the Engineering Model and their geographical
65 McHenry, J.N., F.S. Binkowski, R.L. Dennis, J.S.
Chang, and D. Hopkins. 1992. The tagged species
engineering model (TSEM). Atmospheric Envi-
ronment 26A(8):1427-1443.
55
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ACID DEPOSITION FEASIBILITY STUDY
EXHIBIT 19. TAGGED RADM SUBREGIONS"
a Geographical description of numbered subregions:
1 Montreal Area 28
2 NH/MA Border 29
3 NYC/CT 30
4 Southeast NY 31
5 Southern Tier NY 32
6 Niagara Area 33
7 Detroit Area 34
8 Central/Southern NJ 35
9 Eastern PA 36
10 Southern PA/MD 37
11 Central PA 38
12 Northwest PA 39
13 Southwest PA/Northern WV 40
14 Cleveland Area 41
15 OH/WV/PA Border 42
16 Northewest OH/Eastern IN 43
17 Chicago Area 44
18 Eastern VA 45
19 Western VA/Eastern WV 46
20 OH/WV/KY Border 47
21 KY/WV/VA Border 48
22 Cincinnati Area 49
23 Central KY 50
24 Central IL/IN Border 51
25 Southwest IN/KY Border 52
26 St. Louis Area 53
27 Northeast NC
Northwest NC
Blue Ridge Area
NC/TN/GA Border
Central TN
Central TN/KY Border
Central TN/AL Border
IL/MO/TN Border
Memphis Area
Southeast SC
Central SC/NC Border
Northeast GA
Northeast AL/Northwest GA Border
Western AL
Mobile Area
Baton Rouge Area
Northeast TX
Lake Ontario/NY Shore
Adirondacks
VT/NH
Southeast NC
Southeast GA
Southern AL/GA Border
Northern MS
Norther FL Pennisula
FL Panhandle
Southern MS
56
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
description. The use of the Tagged Model in this
study represents the first extensive use of a Eule-
rian model to study source-receptor relationships.
3.2.3 Transport, Chemistry, and Source-Re-
ceptor Relationships
The lifetime in the atmosphere of the sulfur diox-
ide from an emissions source is several days. The
main loss mechanisms are the incorporation of
SO2 and sulfate in clouds and the subsequent wet
deposition and dry deposition of these species.
Thus, SO2 is transported over many hundreds of
kilometers before concentrations from a source are
substantially reduced by wet and dry deposition.
As a result, the deposition at any one receptor area
is coming from a very large number of sources,
spread over a large geographic area. Source re-
sponsibility cannot be determined, therefore, from
monitoring data. Too much is mixed together. Al-
though rough estimates of source-type responsibil-
ity can be developed with source apportionment
techniques using unique chemical "fingerprints,"
all assessments of source responsibility or source
attribution require the use of an air quality model.
Calculations from the Tagged Species Model illus-
trate the distances over which SO2 emissions
sources can have an influence. A map of the com-
puted proportion of total annual sulfur deposition
contributed by the Ohio/West Virginia/Pennsylva-
nia source subregion (RADM Subregion 15) along
the upper Ohio River Valley is shown in Ex-
hibit 20. Exhibit 21 shows the distance covered to
deposit increasing fractions of the total deposition
in the eastern United States. The subregion's range
of influence is more than 1,000 km. Typically, the
range of influence of a sub-region extends out to
between 500 and 1,200 km. The Tagged Species
Model analyses indicate that about two-thirds of
the total sulfur deposition from major sources
along the Ohio River Valley occurs within 500 to
700 km. For the southern source regions, the dis-
tance to deposit about two-thirds of a source's spa-
tially integrated deposition is somewhat less, about
300 to 500 km. The difference in scale of influ-
ence is primarily due to meteorology.
A number of meteorological factors influence the
existence of dominant transport directions and de-
termine how a group of SO2 emissions sources in-
fluence nearby regions. Key factors are the posi-
tion of the jet stream, which moves storms across
the upper Mid-West; the influence of the Appala-
EXHIBIT 20. PROPORTION OF ANNUAL
SULFUR DEPOSITION CONTRIBUTED BY RADM
SUBREGION 15 (OHAW/PA BORDER REGION)
\
•4,.
EXHIBIT 21. PERCENTAGE CUMULATIVE RANGE
OF INFLUENCE OF RADM SUBREGION 15
(OH/WV/PA BORDER REGION)
chian Mountains on winds and rainfall patterns;
the Bermuda highs (stagnation) that move Ohio
River Valley emissions in a clockwise direction;
and the ocean and Gulf Coast weather that
produces lighter winds and more convective con-
ditions, including a typically large proportion of
57
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ACID DEPOSITION FEASIBILITY STUDY
convective clouds across the southern states. As a
result, source-receptor relationships are skewed to
the northeast (Exhibit 22a) starting in northern
West Virginia and show a predominantly northerly
to easterly quadrant of flow along the lower
Ohio River Valley (Exhibit 22b), yet are nearly
symmetrical in the Southeast (Exhibit 22c). Thus,
the patterns and ranges of source influence can
vary. Models, such as those in the RADM system,
help to interpret and explain the deposition at re-
ceptors of interest.
An alternative approach to determining efficient
and cost-effective strategies to achieve deposition
targets relies on the use of optimization models.66
An optimization model for acidic deposition could
simultaneously minimize SO2 removal cost and
average exceedance of target deposition rates over
the receptor model domain.67 Such a model could
calculate costs and emissions reductions necessary
to achieve a regionally averaged target load com-
parable to the average annual deposition level
calculated by RADM. This model was investigated
for this report but not used because of the compu-
EXHIBIT 22A. SOURCE-RECEPTOR
RELATIONSHIPS IN THE NORTHEAST:
CUMULATIVE PERCENT SULFUR DEPOSITION
66 Streets, D.G., D.A. Hanson,, and L.D. Carter. 1984.
Targeted strategies for control of acidic deposition.
Journal of the Air Pollution Control Association
34(12):1187-1197.
67 Ellis, J.H. 1988. Multiobjective mathematical pro-
gramming models for acid rain control. European
Journal of Operational Research 35(3):365-377.
EXHIBIT 22s. SOURCE-RECEPTOR
RELATIONSHIPS IN THE LOWER OHIO VALLEY:
CUMULATIVE PERCENT SULFUR DEPOSITION
EXHIBIT 22c. SOURCE-RECEPTOR RELATIONSHIPS IN THE
SOUTHEAST: CUMULATIVE PERCENT SULFUR DEPOSITION
tational difficulty in combining the nonlinear
RADM transfer coefficients into a linear
programming optimization model. Although an
optimization model could have been employed
using linear transfer coefficients, at the time this
report was being developed, no linear transfer
coefficients that approximated the RADM transfer
coefficients were available. Optimization models
are, however, used extensively when important
58
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
critical or target load decisions have already been
made (e.g., in European countries). Development
of linear transfer coefficients consistent with
RADM are beyond the scope of this study.
3.2.4 Confidence in Results
The United States and Canada have been cooper-
ating in a bi-national Eulerian Model Evaluation
Field Study (EMEFS) and model evaluation since
1985.53 U.S. data collection for EMEFS was con-
ducted under the auspices of NAPAP. Phase 1 of
the evaluation of the two advanced acidic deposi-
tion models ADOM (Acid Deposition and Oxi-
dants Model) and RADM (Regional Acid Deposi-
tion Model) has been completed and described.68
Phase 1 was primarily based on 1988 EMEFS data.
Improvements to RADM resulted and uncertainty
analyses regarding the scientific assumptions in the
model were prepared that went beyond the earlier
NAPAP results (outlined in reference 3). Phase 2,
the final phase of the evaluation, has been com-
pleted and is based on the 1990 EMEFS data as
well as data of high diagnostic value from the
1988 period. A cooperative effort in the evaluation
of the advanced models will continue under the
U.S.-Canada Air Quality Agreement.
At the end of Phase 1 and Phase 2, the evaluation
results were subjected to extensive external peer
review. The review focused on how well the
evaluation had followed the preset protocol, the
resulting credibility of the models, and the appro-
priateness of model applications. The review panel
members determined they would have confidence
in the ability of the models to represent (1) total
sulfur loading of the atmosphere, suggesting that
the emissions inventory and average lifetimes of
sulfur species are roughly correct; (2) annual sulfur
deposition, although there is some seasonal bias,
with deposition being underestimated in summer
and overestimated in winter; and (3) total nitrogen
loading of the atmosphere, suggesting that the ni-
trogen budget and average lifetime of nitrogen
species is roughly correct.
From the Phase 1 review, the external review
panel felt that the models could be used for esti-
mating annual deposition of sulfur and nitrogen.
68 Pacific Northwest Laboratories. 1991. The Eulerian
Model Evaluation Field Study. Interim report PNL-
7914. Prepared for the U.S. Environmental Protec-
tion Agency, Research Triangle Park, NC. IAG
DW89933040-01, 81 pp.
The reviewers concluded that, while both models
were adequate for the study of large-scale and
long-distance source-receptor relationships, the
models are so complex that their application to the
problem of source-receptor relationships might be
more limited. A valuable application would be for
the complex models to serve as a check on the ac-
curacy of the simpler, but much faster, Lagrangian
methods or on the source-receptor matrices pro-
duced by the Lagrangian methods. Regarding non-
linearities in the sulfur deposition, the reviewers
agreed that they may be of the order of 10 percent
to 15 percent. However, they noted there is no ob-
servational means of fully testing the validity of the
model estimates of the relatively modest nonlinear
effects. The panel concluded that a great deal had
been accomplished and significant improvements
had been made in the models during the evalu-
ation process. The protocol and evaluation have
been essential in winning scientific support for the
models, and the models have shown evidence of
converging towards operational use. This process,
however, is more advanced for sulfur than for ap-
plications involving nitrogen.69
As part of the model evaluation process, bounding
studies were performed to assess the risk that the
predicted changes in air concentrations and depo-
sition would be sensitive to uncertainties in the
scientific descriptions in RADM. The bounded
range of RADM predictions is roughly 10 percent
around the best estimate of deposition change.
There is greater confidence in the upper bound,
but less in the lower bound because it is affected
by our lack of complete understanding of the non-
linear processing affecting sulfur deposition. The
narrow range would suggest that there is little risk
that the model will misguide users regarding the
predicted change in sulfur deposition, despite
shortcomings uncovered in the model evaluation.
This appraisal of the bounding results may change
if significant new insight or knowledge develops in
the future.
3.3 SOURCE ATTRIBUTION
3.3.1 Changes from 1985 to 2010
Fifty-three tagged regions were identified in Ex-
hibit 19 for which the major point source SO2
emissions (utility and major industry) have been
69 U.S. Environmental Protection Agency. 1994. Pro-
gress Report for the U.S.-Canada Air Quality
Agreement.
59
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ACID DEPOSITION FEASIBILITY STUDY
explicitly tracked, using the RADM Tagged Spe-
cies Engineering Model. These regions account for
84 percent of the major point source emissions in
the United States during 1985 and 76 percent in
2010. Exhibit 23 shows the percentage contribu-
tions of the 53 tagged regions to U.S. major point
SO2 emissions in the RADM domain, to total U.S.
SO2 emissions in the RADM domain, and to all
the top 10 in 1985 are still in the top 10 in 2010.
But, the top 10 emitters are responsible for a
smaller fraction of the deposition in the sensitive
regions in 2010 than they were in 1985. The frac-
tion of the total deposition attributable to the top
10 emitters goes down by 32 percent, 24 percent,
and 52 percent for the Adirondacks, the mid-Ap-
palachians, and the Southern Blue Ridge, respec-
EXHIBIT 23. PERCENT CONTRIBUTION TO SULFUR EMISSIONS OF 53 TAGGED RADM REGIONS
Year
1985
2010
Year
1985
2010
RADM SO2 Emissions Data
53 Regions: Percent of
U.S. Major Point Sources
83.6%
75.7%
53-Region Total
Tagged Emissions
15,420,000 tons
9,265,000 tons
53 Regions: Percent
of Total U.S. Sources
74.8%
63.6%
Total Major U.S.
Point Emissions
1 8,452,000 tons
12,245,000 tons
53 Regions: Percent of Total
North American Sources
66.6%
54.3%
Total Emissions from
All U.S. Sources
20,323,000 tons
14,557,000 tons
North American SO2 emissions in the Northeast,
respectively. Thus, although the 53 tagged regions
accounted for three-quarters of the 1985 total SO2
emissions, by 2010, after implementation of the
1990 CAAA, they will account for about
60 percent of the U.S. emissions of SO2 from all
sources.
The listing of emissions by tagged-source region
shows that emissions per unit area are being lev-
eled out by Title IV. This is shown in Exhibits 24
and 25. Exhibit 24 quantifies this by comparing the
percent reduction between 1985 and 2010 as a
function of the 1985 SO2 emissions contributed by
each of the 53 regions. The top 8 emitting regions
in 1985 will have the largest percent reductions by
2010, around 50 to 60 percent. A major fraction of
the regions, those with emissions between 100,000
and 450,000 tons/year, have their SO2 emissions
reduced between 10 and 50 percent, forming a
second tier of reductions. Emissions from most of
the smallest emitting regions (<100,000 tons/year)
actually increase (negative percent change) be-
tween 1985 and 2010, thereby increasing in im-
portance in analyses regarding additional emis-
sions reductions beyond the CAAA. The result is a
modest leveling out of responsibility for the emis-
sions, as shown in Exhibit 25.
Exhibit 26 shows the percentage contributions of
the top 10 emitting regions of 1985 and 2010 to
deposition in the three sensitive regions. Eight of
lively. The change is largest for the Southern Blue
Ridge, resulting in the top 10 emitting regions be-
ing responsible for only 16 percent of the sulfur
deposition in 2010.
EXHIBIT 24. PERCENT REDUCTION IN TAGGED
REGIONS FROM 1985 TO 2010 AS A FUNCTION
OF RELATIVE CONTRIBUTION OF EACH
REGION TO ALL TAGGED EMISSIONS
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n n -
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Percent Reduction from 1985 to 2010
.• 53 Emissions Regions
3.3.2 Regional Emissions Distribution in
2010
With greater emissions reductions coming from the
heavier-polluting regions, the relative importance of
60
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
EXHIBIT 25. PERCENTAGE OF TAGGED EMISSIONS BY TAGGED REGIONS FOR 1985 AND 2010
10.0
0.0
13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53
Region in Rank Order
201°
! 985
EXHIBIT 26. CONTRIBUTION OF TOP-! 0 SO2 EMITTING
REGIONS TO SULFUR DEPOSITION IN SENSITIVE REGIONS
Year
1985
2010
Top-1 0 SO2 Emitting Regions3
1 5,26,39,1 3,22,20,25,32,7,24
1 5,1 3,22,20,51 ,26,25,39,7,1 7
Adirondacks
30.2%
20.6%
Mid-
Appalachians
49.4%
37.3%
Southern
Blue Ridge
30.8%
16.1%
a See Exhibit 19 for geographical descriptions of RADM subregions.
long-range transport is expected to decrease in
2010 compared to 1985. The character of the
source contributions in 2010 is important to any
analyses of further emissions control to reduce
deposition. Two aspects stand out: first, in the mid-
Appalachians and Southern Blue Ridge significant
contributions to sulfur deposition come from
sources near the sensitive aquatic regions; second,
the local versus long-range character of the
sources of deposition changes when moving south
from the Adirondacks to the mid-Appalachians and
the Southern Blue Ridge. The importance of the
top emitting regions decreases as one moves north
or south away from the mid-Appalachians. This re-
sults from a combination of meteorology (transport
directions), proximity to large emissions sources,
and the pattern of emissions in 2010.
Exhibit 27 shows that, as one moves from the Adi-
rondacks to the mid-Appalachians and Southern
Blue Ridge, emissions sources near sensitive areas
are responsible for a greater percentage of deposi-
tion relative to the contributions from the top 10
emissions regions. This can be seen by noting the
contribution to deposition from source regions as
one moves from row A to row D. For the Adirond-
acks, the 10 regions contributing the most to depo-
sition are responsible for about 3.5 times more
deposition than do nearby sources (row D versus
61
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ACID DEPOSITION FEASIBILITY STUDY
EXHIBIT 27. COMPARISON OF PROXIMATE AND MAJOR EMITTING
REGIONS TO SULFUR DEPOSITION IN SENSITIVE AREAS IN 2010
Geographic Source of Deposition
A. Sources local to sensitive region
B. Sources local and contiguous to sensitive
regions
C. Sources from top-1 0 deposition contributing
regions (utilities only)
D. Sources from top-1 0 deposition contributing
regions (utilities plus industry)
Adirondacks
3.7%
7.8%
20.0%
28.0%
Mid-
Appalachians
24.0%
28.0%
37.0%
46.0%
Southern
Blue Ridge
12.0%
26.0%
34.0%
41.0%
row B). In the mid-Appalachians and the Southern
Blue Ridge the top emitting regions contribute
only 60 percent more deposition than do nearby
regions. In these two receptor regions several of
the regions contributing large amounts of deposi-
tion are either local or contiguous to the receptor
regions. The importance of the contribution from
the top emitting regions (primarily located along
the Ohio River Valley) to the deposition in the
sensitive regions, and hence, the importance of
long-range transport, decreases as one moves from
north to south. This source attribution insight is
used in Section 3.6 to analyze a regionally tar-
geted emissions reduction approach to achieving
deposition reductions of sulfur.
3.4 EMISSIONS REDUCTIONS SCENARIOS
This section describes emissions scenarios created
to evaluate the impact of Title IV on sensitive re-
gions and the environmental impact of additional
emissions reductions beyond those mandated by
Title IV. This analysis concentrates on emissions of
SO2 and deposition of sulfur because Title IV fo-
cuses on SO2 emissions and because emissions in-
ventories and source-receptor relationships are
better characterized for sulfur than for nitrogen,
the other key pollutant contributing to acidic
deposition. A scoping analysis of nitrogen deposi-
tion is included. The scenarios are used in this
chapter to compare deposition levels in sensitive
regions. Cost and economic impacts of sulfur re-
ductions scenarios are presented in Chapter 5, Im-
plementation.
Two sets of scenarios for SO2 emissions in 2010
(the year Title IV will be fully implemented) were
developed. The first was created to evaluate the
environmental impacts (i.e., changes in deposition)
resulting from the trading of SO2 emissions allow-
ances. The second represents additional SO2 emis-
sions reductions beyond those mandated by Title
IV. The environmental impacts of these scenarios
are compared to the pre-CAAA case (1980). One
scenario was developed to compare NOX re-
ductions with 1990 baseline emissions levels. Base
years for SO2 and NOX emissions were selected
based on the availability of data at the time of this
analysis.
Emissions for each scenario were projected from
existing EPA emissions inventories. The National
Allowance Data Base (NADB) was used as the ba-
sis for electric utility SO2 emissions estimates and
projections. EPA developed the NADB to allocate
and track SO2 allowances issued under Title IV.
The NADB was prepared by updating the utility
emissions data base (the 1985 National Unit Ref-
erence File or NURF) included in the 1985 NAPAP
Emissions Inventory. The basis for non-utility SO2
emissions estimates is the 1985 NAPAP Emissions
Inventory for Canadian and U.S. emissions. The
basis for NOX emissions estimates for utilities and
industrial sources is EPA's 1990 Interim Emissions
Inventory.70 As with the NADB, the 1990 Interim
Inventory was developed by updating the 1985
NAPAP Emissions Inventory. The 1985 inventory
was updated for 1990 using industry growth rates,
EPA's Mobile 4.1 model for mobile sources, and
by adding electric utility units that became opera-
tional between 1985 and 1990. The 1980 SO2
emissions inventory used to calculate 1980 sulfur
70 U.S. Environmental Protection Agency. June 1992.
Regional Oxidant Modeling—Emissions Inventory
Development and Emission Control Scenarios.
U.S. Environmental Protection Agency, May 1989.
Regional Ozone Modeling for Northeast Trans-
port—Development of Base Year Anthropogenic
Emissions Inventory.
62
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
deposition was developed for EPA's retrospective
cost/benefit analysis of the CAA conducted pursu-
ant to section 812 of the 1990 Amendments. This
inventory was constructed by backcasting emis-
sions from the 1985 NAPAP emissions inventory.
The base year for evaluating the environmental
impacts of the CAAA and the benefits of additional
SO2 emissions reductions beyond the CAAA is
2010. Utility emissions for 2010 were forecasted
in EPA's 1993 Base Case Analysis,71 which was
developed to support rulemaking under Title IV.
Existing and planned electric utility boilers identi-
fied in NADB Version 3.11 plus generic plants re-
quired to meet growth in electricity demand were
used as a basis for the SO2 forecasts. For EPA's
1993 Base Case Analysis, electric utility SO2 emis-
sions were projected from the
NADB inventory using ICF's
Coal and Electric Utilities Model
(CEUM). EPA's analysis assumed
full implementation of
allowance trading (i.e., electric
utilities would engage in
allowance trading in order to
minimize the overall cost of
reducing SO2 emissions by 10
million tons below 1980 levels).
Projections of 2010 non-utility
SO2 emissions from the 1990 In-
terim Inventory were based on a
straightforward approach devel-
oped by EPA.72 First, emissions
from the 1990 Interim Inventory
were grown according to the Bu-
reau of Economic Affairs (BEA)
industrial earnings growth factor
(i.e., by 2-digit SIC code and
state). Next, the grown emissions
were adjusted to reflect the re-
tirement of existing sources, new
emissions (assumed to be subject
to New Source Performance
Standards [NSPS]) to replace
those lost due to retirement, and
the application of additional controls required by
the CAAA. In total non-utility SO2 emissions did
not change significantly between 1990 and 2010.
Projected nationwide total annual emissions of
SO2 with and without implementation of Title IV
are shown in Exhibit 28. The projections are based
on CEUM predictions of utility emissions with and
without Title IV and predictions of non-utility
emissions calculated as described in the previous
paragraph. Annual SO2 emissions decreased by
about 14 percent between 1980 and 1990. With-
out Title IV annual emissions would slowly begin
to increase after 1990 and almost reach 1980 lev-
els by 2010. Under Title IV SO2 emissions will de-
crease dramatically after 1990, achieving a 10 mil-
lion ton reduction from 1980 levels by 2010.
EXHIBIT 28. ESTIMATED U.S. SO2 EMISSIONS
WITH AND WITHOUT TITLE IV FROM 1980 TO 2015
20
s
18
16
14
1980
2010 2015
71 ICF Resources, Inc. February 1994. Economic
Analysis of the Title IV Requirements of the 1990
Clean Air Act Amendments. Prepared for
U.S. Environmental Protection Agency, Office of Air
and Radiation, Acid Rain Division.
72 U.S. Environmental Protection Agency, May 1993.
Regional Interim Emissions Inventories (1987-
1991). Volume I: Development of Methodologies.
Under Title IV utilities are allowed to defer the use
of allowances to future years, i.e. bank allow-
ances. The Nitrogen Bounding Study, discussed in
Chapter 2, calculates the aquatic impact of deposi-
tion in the year 2040 based on deposition values
between 2010 and 2040. EPA has projected SO2
emissions only through 2010 because the assump-
63
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ACID DEPOSITION FEASIBILITY STUDY
tions underlying the CEUM are assumed to be
valid only through that year. Exhibit 29 shows na-
tional utility SO2 emissions predicted by CEUM for
the years 1990-2010. The model predicts that sev-
eral million allowances will be banked in the early
years of the allowance trading program (resulting
in early emissions reductions) for use between the
years 2000 and 2010, with almost 1 million
banked allowances used in 2010. Thus, SO2 emis-
sions in the year 2010 will be almost 1 million
tons higher than in subsequent years. A no-bank-
ing (post 2010 full implementation) scenario was
created for purposes of this analysis to represent
typical emissions for the years 2010 through 2040,
and thus to better reflect deposition modeled in
the Nitrogen Bounding Study.
The emissions inventories for the alternative sce-
narios for SO2 in 2010 were developed for the
RADM domain (eastern United States, see Ex-
hibit 17) as follows and are summarized in Ex-
hibit 30:
* 2010 SCENARIO: This scenario represents
utility SO2 emissions in 2010 forecast by
the CEUM in the 1993 Base Case Analysis
described above. Utility emissions in this
scenario total 9.5 million tons in 2010. The
following two scenarios were calculated
from this inventory.
* PosT-2010 FULL IMPLEMENTATION SCENARIO:
Utility SO2 emissions from the 2010 sce-
nario were reduced to allowance levels in
2010 (the 8.95 million ton cap mandated
by Title IV) by removing emissions banked
from previous years (see Exhibit 29). This
scenario was developed to represent full
implementation of Title IV after 2010. Util-
ity emissions in years beyond 2010 should
remain near this level. Non-utility emis-
sions were unchanged from the 2010 sce-
nario.
* 2010 NO-TRADING SCENARIO: Existing units
operating in 2010 were forecast to emit
SO2 at their allowance levels. Allowances
issued to units retiring before 2010 (0.14
million) were assigned to new units. This
scenario also assumes (based on the EPA
2010 Base Case Forecast Without Title IV)
that oil and gas-fired units, which were al-
located allowances, would have emissions
that are 0.3 million tons less than their al-
lowance allocations. Of the total of 0.44
million tons of allowances, 0.19 million
tons were assumed to be allocated to new
units, leaving 0.25 million tons unused.
Thus total nationwide utility SO2 emissions
in this scenario are 8.95 million tons less
the 0.25 unused allowances or 8.7 million
tons. Non-utility emissions were un-
changed from the 2010 scenario.
ADDITIONAL UTILITY SO2 REDUCTION
SCENARIO (UTILITY SO2 EMISSIONS REDUCED BY
50 PERCENT FROM THE POST-2010 FULL IM-
PLEMENTATION SCENARIO): Total electric util-
ity SO2 emissions were reduced by
50 percent from the Post 2010 Full Imple-
mentation Scenario. To allocate this reduc-
tion units included in the baseline scenario
able to achieve significant reductions in
SO2 emissions (i.e., those with emissions
rates forecast to be greater than 0.8 pounds
of SO2 per million British thermal unit
[Ibs/MMBtu]) were identified. From this
group, the set of boilers able to most cost-
effectively reduce SO2 were selected based
on Cadmus' Generic Retrofit Scrubbing
Cost Model. The 50 percent utility reduc-
tion was then pro-rated by state. Baseline
emissions for the set of boilers with SO2
emissions rates greater than 0.8 Ibs/MMBtu
were reduced in proportion to the pro-rated
state reduction.
ADDITIONAL INDUSTRIAL SO2 REDUCTION
SCENARIO (INDUSTRIAL SO2 EMISSIONS
REDUCED BY 50 PERCENT FROM THE POST-2010
FULL IMPLEMENTATION SCENARIO): SO2 emis-
sions in the RADM domain for the 2010
projection described above were estimated
to total about 4 million tons. A 2 million
ton reduction was achieved from industrial
boilers and major process industries
(categories projected to emit more than
10,000 tons in 2010). As described in more
detail in Chapter 5, cost-effectiveness
measures (annual cost per ton of SO2 re-
moved) were calculated for each sector to
allocate the 2 million ton reduction. Based
on this analysis, industrial boilers ac-
counted for 63 percent and industrial
sources 37 percent of the emissions re-
moved.
ADDITIONAL UTILITY AND INDUSTRIAL SO2
REDUCTION SCENARIO (UTILITY AND IN-
DUSTRIAL SO2 EMISSIONS REDUCED BY
50 PERCENT FROM THE POST-2010 FULL IMPLE-
64
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
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ACID DEPOSITION FEASIBILITY STUDY
EXHIBIT 30. SO2 EMISSIONS IN THE U.S.
RADM DOMAIN (EASTERN UNITED STATES)
Scenario
1980
1985NAPAP
2010
Post-2010 full implementation scenario
No Trading
Additional utility SO2 reduction
Additional utility and industrial SO2 reduction
SO2 Emissions
(million tons)
24.8
20.3
14.6
14.0
13.7
9.7
7.8
MENTATION SCENARIO): Combination of the
two previous scenarios. As shown in Sec-
tion 3.2.1, utility and industrial sources ac-
count for 87 percent of total nationwide
SO2 emissions. Thus, a 50 percent reduc-
tion in utility and industrial SO2 emissions
represents a 43.5 percent reduction in total
SO2 emissions.
* ADDITIONAL UTILITY AND INDUSTRIAL NOX
REDUCTION SCENARIO (UTILITY AND IN-
DUSTRIAL NOX EMISSIONS REDUCED BY 50
PERCENT FROM THE 1990 INTERIM EMISSIONS
INVENTORY): 1990 emissions from each unit
were reduced by half. As shown in Section
3.2.1, utility and industrial sources account
for 47.5 percent of total nationwide NOX
emissions. Thus, a 50 percent reduction in
utility and industrial NOX emissions
represents only a 24 percent reduction in
total NOX emissions.
3.5 DEPOSITION REDUCTIONS UNDER VARIOUS
NATIONAL EMISSIONS REDUCTIONS
SCENARIOS
Total sulfur or nitrogen deposition to each 80 by
80 km grid cell in the RADM region was calcu-
lated for each scenario in Exhibit 30. Sulfur depo-
sition in the units of kilograms of sulfur per hectare
per year (kg-S/ha/yr) is the sum of wet and dry sul-
fate; nitrogen deposition (in units of kg-N/ha/yr)
consists of wet and dry nitrate and wet and dry
ammonia. This section is divided into three parts:
sulfur deposition reductions under Title IV and an
analysis of trading; additional sulfur deposition
emissions beyond Title IV; and nitrogen deposition
reductions.
3.5.1 Impact of SO2 Allow-
ance Trading on Sulfur
Deposition
Exhibit 31 is a map of the spatial
distribution of total sulfur deposi-
tion in the RADM region for the
pre-CAAA year of 1980. The
highest sulfur deposition levels,
more than 20 kg-S/ha/yr, were in
the industrial area encompassing
parts of West Virginia, Pennsyl-
vania, Ohio, Kentucky, and Indi-
ana and in the mid-Appalachians.
Deposition levels were about
20 percent lower in the Southern Blue Ridge and
Pocono Mountains and about 40 percent lower in
the Adirondacks. Thus, the highest deposition lev-
els are in or just downwind of the highest emitting
areas.
EXHIBIT 31. ANNUAL AVERAGE RADM
TOTAL SULFUR DEPOSITION (KG-S/HA): 1980
Exhibit 32 is the same map projected for the post
2010 full implementation scenario. The spatial dis-
tribution of emissions in 2010 is similar to that of
1980. In 2010 deposition levels in the Southern
Blue Ridge are projected to be about 15 percent
lower and those in the Adirondacks about
40 percent lower than in the industrial mid-West
and mid-Appalachians. Similar spatial distributions
are also found for the 2010 and 2010 no-trading
scenarios. Exhibit 33 is a map of percentage reduc-
tions in sulfur deposition from 1980 to the post
2010 full implementation scenario. This map again
demonstrates that the largest deposition reductions
66
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
EXHIBIT 32. ANNUAL AVERAGE RADM-PREDICTED
TOTAL SULFUR DEPOSITION (KG-S/HA): POST-
201 0 FULL CAAA IMPLEMENTATION
EXHIBIT 33. PERCENTAGE REDUCTIONS IN SULFUR
DEPOSITION FROM CAAA IMPLEMENTATION
EXHIBIT 34. IMPACT OF TRADING ON SULFUR DEPOSITION IN SENSITIVE REGIONS
Emissions Scenario
1980
1985 NAPAP
201 0 (with trading)
Post-2010 full implementation
201 0 (without trading)
Annual Average Deposition Level (kg-S/ha)
Adirondacks
11.0
9.8
7.1
6.9
6.8
Mid-
Appalachians
19.0
17.0
12.0
11.0
11.0
Southern
Blue Ridge
14.0
13.0
9.9
9.7
9.2
tions, over 40 percent, are in the highest emitting
regions.
Exhibit 34 compares annual average sulfur deposi-
tion for the 2010 (with trading), no-trading, and
post 2010 full implementation scenarios for the
three sensitive regions analyzed for this report. The
1980 and 1985 deposition values are included for
reference. The most consistent comparison to as-
sess the impact of allowance trading is between
the post 2010 full implementation and no-trading
scenarios. For the post 2010 full implementation
scenario sulfur deposition in the Adirondacks and
mid-Appalachians is reduced about 40 percent
from 1980 and reduced about 30 percent in the
Southern Blue Ridge. The deposition values and
percent reductions are essentially the same for the
no-trading case in the Adirondacks and the mid-
Appalachians. The lower total RADM-wide SO2
emissions for the no-trading case shown in Ex-
hibit 30 most likely accounts for the 0.1 kg-S/ha
difference in deposition in the Adirondacks. In the
Southern Blue Ridge the deposition (9.2 kg-S/ha) is
lower and the percent reduction (34 percent) is
higher for the no-trading case.
A more detailed spatial view of the differences in
sulfur deposition between the post 2010 full im-
plementation and no-trading scenarios for 2010 is
given in the map in Exhibit 35. The areas shaded
diagonally and in solid black depict areas where
projected regional deposition would be higher
with trading. The areas shaded with a plaid pattern
are those where projected deposition would be
lower with trading. The largest increases associ-
ated with trading, somewhat more than 1.2 kg-
S/ha or about 10 percent of total deposition, are
near the Southern Blue Ridge and in northern Ala-
bama. Most of the increases are 0.2-0.8 kg-S/ha,
less than 10 percent of the total sulfur deposition.
Note that the allowance trading program cannot
result in exceedance of a National Ambient
Air Quality Standard
(NAAQS), and thereby
will not create or inten-
sify local air quality or
public health problems,
because sources must
always comply with the
requirements of the
NAAQS as well as the
67
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ACID DEPOSITION FEASIBILITY STUDY
EXHIBIT 35. DIFFERENCE IN ANNUAL AVERAGE RADM TOTAL SULFUR DEPOSITION (KG-S/HA)
IN 2010 BETWEEN POST 2010 FULL IMPLEMENTATION AND NO TRADING SCENARIOS
-0.80 TO -1.16
-0.50 TO -0.80
-0.20 TO -0.50
-0.20 TO 0.20
0.20 TO 0.50
0.50 TO 0.90
0.90 TO 1.53
Prevention of Significant Deterioration Program
(e.g., in National Parks and Wilderness Areas).
There are a few small regions where deposition is
projected to decrease due to trading. These are in
southern New England, southern Florida, and
northwestern Tennessee. Exhibit 35 shows that the
lack of a difference in deposition in the mid-
Appalachians between post 2010 full implementa-
tion and no-trading scenarios listed in Exhibit 34 is
due to increases in one area being offset by de-
creases in another. Nonetheless, the modeling es-
timates of the differences in sulfur deposition be-
tween the post 2010 full implementation and no-
trading scenarios indicate that they are expected to
be less than 10 percent, and in many cases much
less in the sensitive aquatic regions. It is also ap-
parent from Exhibit 35 that trading has virtually no
impact on sulfur deposition in Canada.
3.5.2 Effect of Additional SO2 Emissions Re-
ductions on Sulfur Deposition
This section presents deposition values for the
additional SO2 reduction scenarios described
above and compares each to the post 2010 full
implementation scenario. The additional reduc-
tions scenarios were chosen to demonstrate the
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
magnitude of further emissions reductions and the
spatial distributions associated with each. Exhib-
its 36 and 37 are sulfur deposition maps for
50 percent additional utility SO2 reductions, and
50 percent utility plus 50 percent industrial SO2
reduction scenarios. Exhibits 38 and 39 are maps
of percentage reductions in sulfur emissions rela-
tive to the post 2010 full implementation case for
each scenario. Exhibit 40 summarizes the annual
average deposition to the three sensitive regions
for each scenario, and Exhibit 41 lists the percent-
age decrease in sulfur deposition to each region
relative to 1980.
By comparing the sulfur deposition maps for the
additional reductions scenarios (Exhibits 36 and
37) to the deposition map for the post 2010 full
implementation case (Exhibit 32), it is apparent
that the general spatial distribution of sulfur depo-
sition remains similar with the highest deposition
values falling in the industrial areas of the mid-
West, the regions with the highest emissions. Ex-
hibits 38 and 39 show that the percentage reduc-
tions increase as one moves south, with the largest
percentage reductions occurring from the lower
Ohio Valley through Florida, with significant re-
ductions in central New York State. The reduction
in industrial emissions has the greatest impact in
the Southern Blue Ridge and South Atlantic re-
gions.
3.5.3 Decrease in Total Nitrogen Deposi-
tion from Decreases in NOX Emissions
Nitrogen deposition is primarily the sum of wet
and dry deposition of nitrate and ammonia. (There
is also a small contribution from dry NO2.) As
noted in Section 3.2.1, emissions estimates for
ammonia are highly uncertain both in terms of
magnitude and source. Therefore, RADM replaces
predicted ammonia with observed ammonia depo-
sition. In deposition modeling the contribution of
ammonia to nitrogen deposition is treated as back-
ground along with nitrogen deposition from oxi-
dized species from natural sources and agricultural
emissions. Background from these sources is esti-
mated to be 4, 4.3, and 2.8 kg-N/ha in the Adiron-
dacks, mid-Appalachians, and Southern Blue
Ridge, respectively.73 Therefore, the scenario ana-
73 Van Sickle, J., M.R. Church. 1995. Methods for Es-
timating the Relative Effects of Sulfur and Nitrogen
Deposition on Surface Water Chemistry. Environ-
mental Research Laboratory, Corvallis, OR.
lyzed in this section focuses on anthropogenic
NOX emissions reductions.
Exhibit 42 shows historical and projected nation-
wide total NOX emissions for the period 1980 to
2010 with and without implementation of Title IV.
The emissions projections include Title I (i.e., rea-
sonably achievable control technologies) and mo-
tor vehicle NOX reductions mandated by Title II.
The difference in the two projections is the 2 mil-
lion ton per year reduction in utility NOX emis-
sions estimated to come from Title IV. Unlike util-
ity SO2 emissions, NOX emissions are not capped
by the CAAA.
As shown in Section 3.2.1, utilities and highway
vehicles are each responsible for about one-third
of nationwide emissions of NOX and industrial
sources for about one-sixth of nationwide NOX
emissions. Exhibits 43-45 show nitrogen deposi-
tion maps of the RADM region for each of these
source categories in 1990. Deposition resulting
from utility emissions is strongly concentrated in
the Ohio Valley and falls away almost uniformly
with distance. Industrial deposition is concentrated
on the Gulf Coast, reflecting the concentration of
high NOX emitting industry in that area. Deposi-
tion from mobile sources is concentrated along the
East Coast as a result of automobile emissions in
urban areas. The principal contributors to deposi-
tion in the Southern Blue Ridge and mid-Appala-
chians are utility and industrial emissions.
The Adirondacks are affected about equally by
utility and mobile source emissions.
It is estimated that under Title II of the Act, mobile
source emissions in 2010 in the eastern half of the
United States will be reduced by about 15 percent
from 1990 levels. Since mobile sources represent
about one-third of total NOX emissions, this reduc-
tion corresponds to a reduction in total NOX of
about 5 percent. Under Title IV, NOX emissions
rates from utility boilers will be reduced based on
the degree of reduction available through the ap-
plication of control technology. EPA estimates that
implementation of these regulations will result in
utility NOX emissions in 2010 which are approxi-
mately 20 percent less than would have existed
without Title IV. Implementation of Title I to
achieve the ozone standard in the Northeast and
other nonattainment areas is expected to involve
significant reductions in NOX emissions from util-
ity, industrial, and mobile sources. These reduc-
tions are likely to be region specific.
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ACID DEPOSITION FEASIBILITY STUDY
EXHIBIT 36. RADM-PREDICTED ANNUAL AVERAGE
TOTAL SULFUR DEPOSITION (KG-S/HA) IN
2010 UNDER ADDITIONAL UTILITY SO2
EMISSIONS REDUCTION SCENARIO
EXHIBIT 37. RADM-PREDICTED ANNUAL AVERAGE
TOTAL SULFUR DEPOSITION (KG-S/HA) IN
2010 UNDER ADDITIONAL UTILITY AND
INDUSTRIAL SO2 EMISSIONS REDUCTION SCENARIO
EXHIBIT 38. PERCENTAGE REDUCTIONS IN SULFUR
DEPOSITION FROM POST-2010 FULL IMPLEMENTATION -
UNDER ADDITIONAL UTILITY SO2 REDUCTION SCENARIO
EXHIBIT 39. PERCENTAGE REDUCTIONS IN
SULFUR DEPOSITION FROM POST-2010 FULL
IMPLEMENTATION UNDER ADDITIONAL UTILITY
AND INDUSTRIAL SO2 REDUCTION SCENARIO
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
EXHIBIT 40. SULFUR DEPOSITION TO SENSITIVE REGIONS UNDER VARIOUS EMISSIONS SCENARIOS
Emissions Scenario
1980
1985 NAPAP
Post-2010 full implementation
CAAA implementation plus additional
utility SO2 reduction
CAAA implementation plus additional
utility and industrial SC>2 reduction
Annual Average Deposition Level (kg-S/ha)
Adirondacks
11.0
9.8
6.9
5.5
4.7
Mid-
Appalachians
19.0
17.0
11.0
8.1
6.9
Southern
Blue Ridge
14.0
13.0
9.7
6.8
5.5
EXHIBIT 41. PERCENT REDUCTIONS IN SULFUR DEPOSITION TO SENSITIVE
REGIONS FROM 1980 LEVELS UNDER VARIOUS EMISSIONS SCENARIOS
Emissions Scenario
Post-2010 full implementation
CAAA implementation plus additional
utility SC>2 reduction
CAAA implementation plus additional
utility and industrial SC>2 reduction
Percent Reduction
Adirondacks
39
51
58
Mid-
Appalachians
41
56
63
Southern
Blue Ridge
31
52
60
EXHIBIT 42. ESTIMATED U.S. NOX EMISSIONS
WITH AND WITHOUT TITLE IV FROM 1980 TO 2015
14
1980 1985 1990
1995
Year
2000 2005 2010
71
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ACID DEPOSITION FEASIBILITY STUDY
EXHIBIT 43. PERCENT CONTRIBUTION OF UTILITY
SOURCES TO NITROGEN DEPOSITION IN 1990
EXHIBIT 45. PERCENT CONTRIBUTION OF MOBILE
SOURCES TO NITROGEN DEPOSITION IN 1990
EXHIBIT 44. PERCENT CONTRIBUTION OF INDUSTRIAL
SOURCES TO NITROGEN DEPOSITION IN 1990
A detailed inventory of NOX emissions in the year
2010 suitable for RADM modeling which consid-
ered full implementation (all Titles) CAAA was not
available for this report. As a scoping exercise for
this report two emissions scenarios were used: the
baseline 1990 Interim Inventory and an inventory
in which NOX emissions from industrial and utility
sources is reduced by 50 percent from 1990 levels.
This reduction inventory, as described in Section
3.4, corresponds to a 24 percent decrease in total
NOX emissions. Nitrogen deposition from ammo-
nia was assumed to remain constant. Exhibits 46
and 47 are maps of nitrogen deposition from the
1990 Interim Inventory and from the utility and in-
dustrial emissions reduction scenario. Exhibit 48
is a map of percentage reductions in nitrogen
deposition due to the control scenario. Exhibit 49
summarizes nitrogen deposition and percentage
reductions for the baseline and control scenarios.
Percentage reductions are less than for the similar
SO2 scenario for two reasons: ammonia emissions,
which account for between one-fifth and one-
quarter of nitrogen deposition in the eastern
United States, are assumed to be constant and util-
ity and industrial sources account for only about
50 percent of total NOX emissions, whereas these
categories account for 87 percent of SO2 emis-
sions. The greatest percentage reduction is found
in the mid-Appalachians, followed by the Southern
Blue Ridge, and then the Adirondacks, reflecting
the relative importance of utility and industrial
NOX emissions to deposition in these regions.
3.6 EMISSIONS REDUCTIONS STRATEGIES TO
ACHIEVE GEOGRAPHICALLY TARGETED
SULFUR DEPOSITION LOADS
In the previous section deposition values were pro-
jected for nationwide emissions control scenarios.
That is, similar to Title IV, the reductions resulted
from control approaches which specified emis-
sions reductions by source category not by geo-
graphic location. This section describes and ana-
lyzes geographically targeted emissions reductions
to achieving deposition reductions. Analysis of
72
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
EXHIBIT 46. ANNUAL AVERAGE RADM TOTAL
NITROGEN DEPOSITION (KG-N/HA) IN 1990
EXHIBIT 47. RADM-PREDICTED ANNUAL
AVERAGE TOTAL NITROGEN DEPOSITION
(KG-N/HA) UNDER UTILITY AND INDUSTRIAL
NOX EMISSIONS REDUCTIONS SCENARIO
EXHIBIT 48. PERCENTAGE REDUCTIONS IN NITROGEN
DEPOSITION UNDER UTILITY AND INDUSTRIAL
NOX EMISSIONS REDUCTIONS SCENARIO
EXHIBIT 49. NITROGEN DEPOSITION TO SENSITIVE REGIONS UNDER BASE CASE AND
ADDITIONAL UTILITY AND INDUSTRIAL NOX EMISSIONS REDUCTION SCENARIO
Emissions Scenario
1 990 Base Case
Additional utility and industrial NOX
reductions from 1 990 base case
(% reduction from base case in parenthesis)
Annual Avera
Adirondacks
9.5
8.1
(14%)
ge Deposition Level (kg-N/ha)
Mid-
Appalachians
14.3
11.3
(21%)
Southern
Blue Ridge
11.9
9.9
(16%)
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ACID DEPOSITION FEASIBILITY STUDY
these reductions relies on RADM source-receptor
relationships to target source regions for emissions
reductions needed to achieve a specified target
load.
The assessment of geographically targeted reduc-
tions strategies relies on the ability of the RADM
Tagged Engineering Model to identify the contribu-
tion of emissions from tagged source regions to
deposition in selected target regions. This assess-
ment is based on results for the 53 tagged regions
shown in Exhibit 19. To illustrate the potential ap-
plicability of this geographic targeting, deposition
goals for the three sensitive aquatic regions were
selected to be equal to the deposition levels
achieved by the source-category specific SO2 re-
duction scenarios analyzed in Section 3.5. The
maintenance load scenario is described in Ex-
hibit 50.
As noted in Section 3.3, implementing Title IV will
result in a higher proportion of sulfur deposition in
sensitive aquatic regions coming from nearby
sources in 2010 than was the case in 1985. At the
same time, the top 10 emitting regions' contribu-
tions to deposition in those areas will decrease in
importance from 1985 to 2010. To assess the fea-
sibility of defining geographically targeted emis-
sions reductions for given deposition targets, two
potential scenarios were selected. The first limits
the selection to obtaining the required emissions
reductions from sources near a sensitive region.
The other focuses on securing emissions reductions
from those RADM subregions most responsible for
deposition in a sensitive region.
Exhibit 40 lists the deposition level for each sensi-
tive aquatic region achieved by a further
50 percent reduction in utility SO2 emissions be-
yond the CAAA from utility sources in the United
States and the deposition level achieved by a fur-
ther 50 percent reduction in utility and industrial
sources. These deposition levels were selected to
illustrate target loads in order to compare nation-
wide and target emissions reductions. Exhibit 27
shows that in 2010 after implementation of the
CAAA, local and contiguous sources will contrib-
ute 7.8 percent of deposition in the Adirondacks,
28 percent in the mid-Appalachians, and
26 percent in the Southern Blue Ridge. Thus, up to
a point it should be feasible to develop geographi-
cally restricted targeted areas for emissions reduc-
tions in the mid-Appalachians and the Southern
Blue Ridge as long as the reductions demanded
are significant, such as 95 percent or greater. Be-
cause of the relatively smaller contribution of local
sources to deposition in the Adirondacks, reduc-
tions in emissions from a large geographic area
would be required to achieve targeted levels of
deposition.
To achieve the same deposition levels as the sce-
nario defined by a 50 percent reduction in utility
SO2 emissions beyond that achieved by the CAAA,
targeted source regions were identified by sequen-
tially removing 95 percent of the SO2 utility emis-
sions (remaining after implementation of Title IV)
from each subregion until the deposition levels
listed in Exhibit 40 were achieved. For the con-
tiguous scenario, emissions were removed starting
at the center of the sensitive region and continued
outward until the deposition goal was reached. For
the noncontiguous scenario, emissions were re-
moved by subregion in order of contribution to
deposition to the sensitive region.
Exhibits 51 a and 51 b show the results of targeted
emissions reductions to achieve the same deposi-
tion as the 50 percent additional SO2 utility case.
The exhibits show that geographically targeted re-
ductions can be achieved for all of the sensitive
aquatic areas. As anticipated, it takes a larger
number of tagged-regions and greater emissions
reductions to achieve the target for the Adirond-
acks than for the other two regions. By comparing
the emissions reductions required in the contigu-
ous vs. non-contiguous scenarios, it is apparent
that only in the Southern Blue Ridge are contigu-
ous reductions more efficient, requiring 25 percent
fewer emissions reductions. This difference for the
Southern Blue Ridge can be attributed to the inclu-
sion of RADM Subregion 51 (Northern Florida), a
major emitting region, in the non-contiguous re-
ductions. For the other two sensitive regions both
contiguous and non-contiguous reductions rely es-
sentially on the same geographic areas of the
country. It is also interesting to note that the total
emissions reductions to achieve all three target
loads simultaneously are essentially equal and are
less (i.e., amount of total emissions reductions)
than 10 percent more efficient than a nationwide
50 percent reduction in utility SO2 emissions.
74
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
EXHIBIT 50. SELECTION OF MAINTENANCE LOADS
Critical load definitions are founded on scientific determinations of quantitative pollutant loadings be-
low which no significant harmful effects occur to critical ecological processes. Critical load definitions
depend solely on inherent ecological properties. Target load definitions differ in that they incorporate
social, policy, economic, and related considerations along with the scientific observations. An
example of a possible target load would be a decision to regulate acidic deposition at levels adequate
to maintain proportions of ANC<50 ueq/l waters, roughly equivalent to pH>6.5, at or below the pro-
portions found during the 1984-85 National Surface Water Surveys (NSWS) for each of three regions
in the eastern United States discussed here. Other possible goals for defining target loads could in-
clude, for example, restricting deposition to produce a reduction in the 1984-85 proportions of ANC<
50 ueq/l waters by a specific percentage or to some fixed proportion endpoint. Also, the definition
could be expanded to include a larger geographical region.
For purposes of illustration, assume that social, policy, economic, scientific considerations led to a se-
lection of the first of the above options as a generally acceptable goal for a target load: to limit sulfur
deposition rates sufficiently to maintain proportions of target population surface waters having ANC<
50 ueq/l in these regions through the year 2040 at proportions no greater than those found for the
same subpopulation by the 1984-85 NSWS. This maintenance load also would allow for a possible
restoration of prior ANC levels and reduction in acidic conditions in some of these waters. The nu-
meric proportions of target population waters with ANC<50 ueq/l found in 1984-85 that would be
used to evaluate attainment of this goal in each of the three regions are <55 percent in the Adirond-
acks, <27 percent in the Mid Appalachians, and <6 percent in the Southern Blue Ridge. It is important
to note, however, that the percentages reported here represent sensitive subpopulations of the overall
number of surface waters in those regions.
The Nitrogen Bounding Study (NBS) projections, described in detail in Chapter 2, can be used to de-
termine sulfur deposition values necessary to attain this goal by locating these percentages on the NBS
plots shown in Appendix B. For each region, projections of sulfur deposition loadings necessary to
maintain the 1984-85 NSWS proportions of ANC<50 ueq/l waters depends on the assumed time to ni-
trogen saturation for that region. For the purposes of this analysis, it has been assumed that nitrogen
deposition rates will remain unchanged for the 1984-2040 period. In considering this approach, it is
critical to recognize that, while the NBS modeling results are the best available information, there are
significant uncertainties in the projected relationships (see Section 2.6.3). Consequently, the magni-
tude of the uncertainties in sulfur deposition maintenance loads used in this analysis cannot be quanti-
fied at this time.
Current scientific uncertainty does not allow quantifying the time to nitrogen saturation for any of the
three sensitive regions nor for any other regions. However, it is reasonable to suggest that times to
saturation do vary among regions due to differences in temperature, moisture, soil fertility, forest age,
history of nitrogen deposition, and other variables. Watersheds in the Northeast have cooler annual
temperatures, snorter growing seasons, and long histories of elevated nitrogen and sulfur deposition
levels. Consequently, watersheds in the Adirondacks may include those having the shortest remaining
times to nitrogen saturation. Watersheds in the Mid-Appalachians and Southern Blue Ridge Province
may have longer remaining times to nitrogen saturation.
For illustrative purposes in this analysis, if the time to nitrogen saturation is between 75 and 150 years
in the Mid-Appalachians, and between 200 and 300 years in the SBRP, then a sulfur deposition load of
about 5 kg/ha/yr is projected by the NBS results as potentially maintaining surface waters with ANC<
50 ueq/l at 1984-85 proportions. NBS modeling suggests that for a time to nitrogen saturation for the
Adirondacks of between 25 and 75 years, a greater than 50 percent reduction in both sulfur and nitro-
gen may be necessary to maintain the 1984 proportion of ANC<50 ueq/l lakes; however, a 5 kg/ha/yr
sulfur load would likely provide some benefits.
This example of a loadings approach (i.e., maintenance load) concentrates only on sulfur deposition,
while nitrogen deposition is held constant. However, NBS results for all three regions indicate that re-
ducing nitrogen deposition rates is projected to provide likely benefits in reducing proportions of ANC
<0 ueq/l and ANC<50 ueq/l surface waters that may equal or exceed the potential benefits obtainable
from reducing sulfur deposition alone. The amount of benefit would depend on the actual amount of
reduction in sulfur and nitrogen deposition obtained, and on the actual time to watershed nitrogen
saturation within each region. Any efforts to develop acid deposition standards would likely include
both sulfur and nitrogen.
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ACID DEPOSITION FEASIBILITY STUDY
EXHIBIT 51 A. GEOGRAPHICALLY TARGETED ADDITIONAL
UTILITY SO2 REDUCTION IN CONTIGUOUS RADM SUBREGIONS
Deposition
Subregions
SO2 Emissions
Reduction (tons)
Sensitive Region
Adirondacks
5.5 kg-S/ha
45,44,5,4,2,3,9,
11,12,14,7,8,10,
13,15,18,19,20,22
3,018,000
Mid-
Appalachians
8.1 kg-S/ha
13,19,28,20,
21,27,15,22
1,952,000
Southern
Blue Ridge
6.8 kg-S/ha
29,30,31,39,38,
37,28,21,23,27,
36,32,33,40
1,508,000
All Three
Receptor
Regions
—
4,526,000a
Nationwide
Utility
—
5,047,000
a This emissions total was derived without double counting those subregions contributing to deposition in
more than one receptor region.
EXHIBIT 51 B. GEOGRAPHICALLY TARGETED ADDITIONAL UTILITY SO2 REDUCTION
IN MAJOR RADM SUBREGIONS CONTRIBUTING TO DEPOSITION (NOT CONTIGUOUS)
Deposition
Subregions
SO2 Emissions
Reduction (tons)
c
Adirondacks
5.5 kg-S/ha
15,13,22,20,
14,7,10,12,25,
3,9,5,2,17,44
3,160,000
»ensitive Region
Mid-
Appalachians
8.1 kg-S/ha
13,20,15,
22,28,27,10
2,080,000
Southern
Blue Ridge
6.8 kg-S/ha
31,29,39,38,28,
37,40,51,20,22
2,081,000
All Three
Receptor
Regions
—
4,658,000a
Nationwide
Utility
—
5,047,000
a This emissions total was derived without double counting those subregions contributing to deposition in
more than one receptor region.
Exhibit 52 is a map of the geographically targeted
reductions for the contiguous reductions for all
three receptor regions. RADM subregions required
for the Adirondacks (indicated by short dashes) ex-
tend from the Canadian border south through Vir-
ginia and Kentucky and from the Eastern seaboard
west to the middle of Indiana. Source regions for
the mid-Appalachians (solid line) are fairly sym-
metric around the region and extent from the mid-
dle of Ohio south through central North Carolina
and west from the North Carolina Coast to the
middle of Indiana. For the Southern Blue Ridge
source regions (longer dashed lines) include most
of North and South Carolina, Kentucky, Tennes-
see, and Alabama and the northern half of Geor-
gia. The map shows the striking overlap between
tagged regions required for achievement of the tar-
get load in the mid-Appalachians and the other
two source regions and the lack of overlap be-
tween the Adirondack and Southern Blue Ridge
source regions. Achieving the target loads in the
Adirondacks and Southern Blue Ridge would also
achieve the target load in the mid-Appalachians.
A similar analysis was conducted to determine if
the maintenance loads developed for illustrative
purposes and loads defined by the deposition
levels achieved by the nationwide 50 percent
utility plus industrial SO2 emissions reduction
could be achieved by targeting only utility
emissions in the 53 tagged subregions. It was only
possible to achieve the deposition targets in the
mid-Appalachians. It was possible to achieve the
maintenance load in the Adirondacks, but not in
the mid-Appalachians or the Southern Blue Ridge,
when only considering 95 percent reductions of
utility emissions in the targeted regions. Thus, tar-
76
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
EXHIBIT 52. MAP OF EXTENT OF CONTIGUOUS
GEOGRAPHIC REGIONS FOR ACHIEVING TARGETED
DEPOSITION LOADS EQUIVALENT TO ADDITIONAL
NATIONWIDE UTILITY SO2 REDUCTIONS
geting scenarios that included industrial emissions
reductions were then analyzed.
For the 50 percent utility and industry SO2 reduc-
tions beyond the post 2010 full implementation
scenario, targeted source subregions were identi-
fied by sequentially removing 95 percent of the
SO2 major source (utility plus large industrial
sources) emissions from each subregion. Results
are shown in Exhibits 53a and 53b. It appears fea-
sible to have geographically targeted controls if
reductions are from both utility and industrial
sources. For these targets, there is little geographic
"efficiency" between constraining the tagged
subregions to be contiguous or focusing on the re-
gions most responsible for deposition. Thirty-one
emissions subregions are included in the former
case for all three sensitive receptor regions and 29
in the latter. As shown in Exhibit 54 the tagged
subregions cover the same geographical expanse
as those involved in utility reduction targeted sce-
nario (compare Exhibit 52). The principal differ-
ence is that the overlap among regions is more ex-
tensive with reductions in some source regions re-
quired to achieve target loads in all three sensitive
regions. Again, emissions reductions from sources
in the western part of the RADM domain are not
required.
For the maintenance load analysis, a 95 percent
reduction in major source emissions was used. The
results for the case focusing on emissions reduc-
tions from regions most responsible for deposition
are shown in Exhibit 55. Compared to the number
of tagged emissions regions shown in Exhibit 53b,
the number is somewhat less for the Adirondacks,
somewhat more for the Southern Blue Ridge, and
almost 3 times larger for the mid-Appalachians.
Nevertheless, the total number of tagged regions is
comparable, 31 for the 50 percent utility plus in-
dustrial source scenario and 33 for the mainte-
nance load scenario). Two-thirds of the subregions
contributing to the Adirondacks and two-thirds of
the subregions contributing to the Southern Blue
Ridge also are major sources of deposition in the
mid-Appalachians. (A similar analysis using only
contiguous subregions shows a greater than
90 percent overlap between the Southern Blue
Ridge and mid-Appalachians). There is no overlap
of the two sets of subregions identified for the Adi-
rondacks and the Southern Blue Ridge. Thus, these
regions could be individually targeted; however,
attempting to achieve the maintenance load for
the mid-Appalachians may be best done as part of
a strategy to achieve it for all three sensitive
regions at the same time. Using a maintenance
load chosen for illustrative purposes in this report,
it appears not to be very advantageous to
geographically target regions individually to
achieve a particular load.
77
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ACID DEPOSITION FEASIBILITY STUDY
EXHIBIT 53A. GEOGRAPHICALLY TARGETED ADDITIONAL UTILITY AND
INDUSTRIAL SO2 REDUCTION IN CONTIGUOUS RADM SUBREGIONS
Deposition
Subregions
Adirondacks
4.7 kg-S/ha
45,44,5,4,2,3,9,
11,12,14,8,10,13,
15,7,18,20,22
Sensitive Region
Mid- Appalachians
6.9 kg-S/ha
13,19,20,10,
15,28,27,18,22
Southern
Blue Ridge
5.5 kg-S/ha
29,30,31,39,38,
37,28,21,19,36,27,
15,20,22,23,40
EXHIBIT 53s. GEOGRAPHICALLY TARGETED ADDITIONAL UTILITY AND
INDUSTRIAL SO2 REDUCTION IN MAJOR RADM SUBREGIONS
CONTRIBUTING TO DEPOSITION (NOT CONTIGUOUS)
Deposition
Subregions
Adirondacks
4.7 kg-S/ha
15,13,5,14,7,20,
22,44,10,12,45,
17,9,25,26,3
Sensitive Region
Mid- Appalachians
6.9 kg-S/ha
13,20,15,22,19,
28,27,18,10
Southern
Blue Ridge
5.5 kg-S/ha
29,31,39,38,37,
28,40,20,51,27,
21,30,22,15,25
EXHIBIT 54. EXTENT OF CONTIGUOUS GEOGRAPHIC
REGIONS FOR ACHIEVING TARGETED DEPOSITION
LOADS EQUIVALENT TO ADDITIONAL NATIONWIDE
UTILITY AND INDUSTRIAL SO2 REDUCTIONS
EXHIBIT 55. GEOGRAPHICALLY TARGETED REDUCTIONS WITH A
MAINTENANCE LOAD OF 5 KG-S/HA IN MAJOR RADM SUBREGIONS
CONTRIBUTING TO DEPOSITION (NOT CONTIGUOUS)
Subregions
Adirondacks
15,13,5,14,7,20,
22,44,10,12,45,17
Sensitive Region
Mid- Appalachians
13,20,15,22,19,28,
27,10,18,31,21,12,
25,14,37,39,29,26,
7,51,34,32,23,9
Southern Blue Ridge
29,31,39,38,37,28,40,
20,51,27,21,30,
22,15,25,41,32,36
78
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CHAPTER 4
POTENTIAL BENEFITS OF AN ACIDIC DEPOSITION STANDARD ON
VISIBILITY, HUMAN HEALTH, MATERIAL, AND CULTURAL RESOURCES
4.1 INTRODUCTION
Chapter 2 of this report focused on the effects of
acidic deposition on sensitive aquatic and terres-
trial resources. This chapter describes how an acid
deposition standard, in this case aquatics-based,
could improve visibility, protect human health,
and preserve material resources of functional and
cultural importance.
4.2 RELATIONSHIP OF VISIBILITY TO ACIDIC
DEPOSITION
In certain areas of the United States, visibility is a
significant environmental indicator of air quality.
Visibility impairment and subsequent improve-
ment is therefore a strong measure of effectiveness
and benefits. This section identifies regions in the
United States subject to visibility degradation and
describes how these areas could benefit from an
acid deposition standard that, in this case, is de-
signed to protect sensitive aquatic resources.
4.2.1 Visibility Impairment
Visibility refers to the degree to which the atmos-
phere is transparent to visible light. Fine particles
in the atmosphere absorb and scatter light, thereby
limiting visual range, decreasing color discrimina-
tion, and obscuring details of distant objects. Im-
pairment of visibility depends on several factors,
especially the size and composition of particles in
the viewing path. Some gases absorb visible light
and can impair visibility. Visibility is also affected
by the angle of sunlight and so varies with time of
day and season. Humidity can reduce visibility
when hygroscopic particles absorb water and in-
crease in size; larger particles scatter more light.
Thus, natural visibility in the humid East is gener-
ally poorer than in the more arid West.
Visibility can be impaired by natural and anthro-
pogenic sources. Natural sources include fog, pre-
cipitation, sea mist, windblown dust, volcanic
emissions, and forest fires. Visibility impairment
from these sources varies by season and meteoro-
logical condition. Anthropogenic sources include
gaseous and particulate emissions from stationary
and mobile sources. Most visibility impairment
can be traced to one gas, nitrogen dioxide, and
five particulate substances: sulfates, nitrates, or-
ganics, elemental carbon, and soil dust.
The National Academy of Sciences (NAS) esti-
mated the contribution of anthropogenic air pol-
lutants to visibility impairment in three areas of the
country: the East (i.e., states east of the Missis-
sippi), the Southwest (i.e., California, Nevada, Ari-
zona, New Mexico, Utah, and Colorado), and the
Northwest (i.e., Oregon, Washington, and
Idaho).74 Exhibit 56 summarizes findings for rural
regions in each area. NAS also calculated that an-
thropogenic sources are responsible for seven-
eighths of the visibility impairment in the East,
five-eighths in the Northwest, and three-eights in
the Southwest.
EXHIBIT 56. ANTHROPOGENIC CONTRIBU-
TIONS TO VISIBILITY IMPAIRMENT74
Contaminant
Sulfates
Organics
Elemental Carbon
Suspended Dust
N itrates
Nitrogen Dioxide
Percent
East
65
14
11
2
5
3
Southwest
39
18
14
15
9
5
Northwest
33
28
15
7
13
4
The exhibit clearly indicates that most visibility
impairment in the East is caused by sulfates
(transformation products of sulfur dioxide, the ma-
jority of which is emitted from power plants). No
single source category dominates visibility im-
pairment in the West, although sulfur compounds
play a significant role. In relatively clean areas of
the West, small increases in pollutant levels can
markedly degrade visibility. Thus, visibility in
74 Committee on Haze in National Parks and Wilder-
ness Areas. 1993. Protecting Visibility in National
Parks and Wilderness Areas. National Research
Council and National Academy of Sciences, Wash-
ington, DC.
79
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ACID DEPOSITION FEASIBILITY STUDY
Class I areas in the West is especially sensitive to
increased levels of pollution.
4.2.2 Visibility Protection Laws and Class I
Areas
Section 169A of the Clean Air Act (CAA) of 1977
established as a national goal "the prevention of
any future and the remedying of any existing im-
pairment of visibility in mandatory Class I areas
which impairment results from man-made air pol-
lution." Class I federal areas are defined in CAA
Section 162(a) as international parks, national wil-
derness and memorial parks exceeding 5,000
acres, and national parks exceeding 6,000 acres in
existence in 1977. EPA, in conjunction with the
Department of the Interior (DOI), has designated
158 mandatory Class I areas where visibility is im-
portant.
States that either have Class I areas or contain
sources that may contribute to visibility impair-
ment of these areas are required to include in their
state implementation plans (SIPs) a long-term strat-
egy for making reasonable progress toward reduc-
ing impairment. Major stationary sources reason-
ably expected to contribute to visibility impair-
ment in a Class I area must install best available
retrofit technology.
The prevention of significant deterioration (PSD)
provision in Sections 160-169 of the CAA also
applies to visibility protection. The PSD program,
which is directed toward new sources, requires
that major emitting facilities seeking to locate in
clean-air areas (i.e., areas meeting the National
Ambient Air Quality Standard [NAAQS] for a par-
ticular pollutant) use best available control tech-
nology (BACT). The source must also comply with
air quality increments that specify the maximum
permissible increase in ambient pollutant levels for
SO2, NO2, and particulate matter. Class I areas are
further protected by the designation of Air Quality
Related Values (AQRV) for several parameters, in-
cluding visibility. In addition to complying with
BACT and increment requirements, new sources
must demonstrate that they will not adversely af-
fect an area's AQRV.
When the PSD program was created in 1977, large
national parks and wildernesses were designated
as Class I areas to provide them with special air
quality protection. Other parks and wilderness ar-
eas have been designated Class I in succeeding
years. About two-thirds of the current Class I areas
are west of the Mississippi. Nearly one-quarter are
located in four southwestern states: Utah, Colo-
rado, Arizona, and New Mexico. Monitoring visi-
bility conditions at some sites was initiated by the
National Weather Service in 1978. At approxi-
mately 43 other sites, visibility monitoring began
in 1987 and continues under a multi-agency pro-
gram called the Interagency Monitoring of Pro-
tected Visual Environments (IMPROVE). A rule-
making effort on regional haze protection recently
initiated by EPA will further examine visibility im-
pacts on and protection of Class I areas.
4.2.3 Visibility Metrics and the Projected
Impact of the CAAA on Visibility
No standard or EPA-approved method for measur-
ing optical air quality exists. Visibility has been
measured and reported in several ways. Standard
visual range is based on human perception of a
large black object placed in the sky and is reported
in kilometers or miles. A more scientific measure
of visibility impairment is light extinction. An ex-
tinction coefficient is proportional to the attenu-
ation of light per unit distance due to absorption
and scattering of light by particles or gases. Extinc-
tion coefficients are a function of particle size and
shape and the gaseous chemicals present. A third,
recently developed measure is the deciview scale
(analogous to the decibel scale for sound), which
provides a haziness index designed to be linear to
humanly perceived changes in visibility caused
solely by air quality changes. The deciview (dv)
scale is near zero for pristine atmospheric condi-
tions and increases as visibility degrades; a 1 dv
change corresponds to a 10 percent change in
light extinction and approximates a minimum,
commonly observable visibility change.
Several recent visibility studies have been con-
ducted to assess the impact of the CAAA on visibil-
ity improvement; some analyses specifically as-
sessed the impact of Title IV.
Eastern United States
A recent analysis compared standard visual range
with and without Title IV of the CAAA to assess
economic benefits of improvements in visibility.75
The visual range maps (Exhibits 57 and 58) illus-
75 Chestnut, L.C., R.L. Dennis, and D.A. Latimer. 1994.
Economic benefits of improvements in visibility:
Acid rain provisions of the 1990 Clean Air Act
Amendments. Presented at Aerosols and Atmos-
pheric Optics: Radiation Balance and Visual Air
(continued)
80
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CHAPTER 4: POTENTIAL BENEFITS OF A STANDARD ON VISIBILITY,
HUMAN HEALTH, MATERIAL, AND CULTURAL RESOURCES
EXHIBITS?: ANNUAL AVERAGE VISUAL
RANGE (KM) PROJECTED FOR 2010 WITHOUT
TITLE IV: SOTH-PERCENTILE VISIBILITY
EXHIBIT 58. ANNUAL AVERAGE VISUAL RANGE (KM)
PROJECTED FOR 2010 WITH TlTLE IV, INCLUDING
TRADING: SOTH-PERCENTILE VISIBILITY
trate impressive changes in visibility associated
with Title IV. Although results are preliminary, the
economic analysis indicates potentially significant
Quality, Air & Waste Management Association Inter-
national Specialty Conference, Snowbird, Utah,
September 30.
monetary benefits to residential areas of 31 eastern
states in the United States and to national parks in
the southeastern United States.
An analysis was conducted for this study to com-
pare current and future perceptible visibility deg-
radation in the East, assuming implementation of
Title IV in the year 2010. The assessment indicated
a noticeable improvement in visibility across the
eastern United States (Exhibit 59) from the 1980
base year, with most of the change occurring in
the warm seasons.
EXHIBIT 59. ANNUAL AVERAGE IMPROVEMENT IN SOTH-
PERCENTILE VISIBILITY (DV) FROM 1980 TO 2010 WITH
TITLE IV, INCLUDING TRADING
A 1993 EPA Report to Congress presented visibility
improvements to Class I areas that could be ex-
pected to accompany implementation of the 1990
CAAA.76 The analysis evaluated impacts of control
provisions for NOX, SO2, and paniculate matter by
assuming implementation of Titles I, II, and IV of
the CAAA. Exhibit 60 lists specific provisions of
each title.
Because sulfates dominate visibility impairment in
the East, and no single chemical species dominates
in the Southwest, EPA modeled each region sepa-
76 Office of Air Quality Planning and Standards. Octo-
ber 1993. Effects of the 1990 Clean Air Act Amend-
ments on Visibility in Class I Areas: An EPA Report to
Congress. U.S. Environmental Protection Agency,
Washington, DC.
81
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ACID DEPOSITION FEASIBILITY STUDY
EXHIBIT 60. SPECIFIC PROVISIONS OF TITLES I, II, AND IV
Title I
Application of reasonably available control technology (RACT) NOX control for ozone-
moderate areas (or worse) and ozone transport regions
Enhanced motor vehicle inspection and maintenance (I&M) for areas with conditions
classed as ozone serious (or worse) and carbon monoxide-moderate (or worse)
New Source Performance Standards (NSPSs) for NOX
Title II
» 1995 tailpipe standards for NOX reduction
• Using oxygenated fuels in nonattainment areas for carbon monoxide
Title IV
Implementation of Phase I and II SO2 limits, where emissions are based on projections
from the Coal and Electric Utilities Model for the EPA Regulatory Impact Analysis (RIA)
Implementation of NOX controls
rately. EPA used the Regional Acid Deposition
Model (RADM) post-processor (EM-VIS) to calcu-
late visibility for each RADM cell in 1985 and
2010. The 1985 NAPAP emissions inventory was
used as the basis for 1985 values. For the year
2010, implementation of Title IV was assumed, but
without SO2 emissions trading, because an
emissions scenario depicting trading was unavail-
able at that time. (Note, however, that the analysis
of the environmental impact of trading allow-
ances, described in Section 3.5.1, found only
minimal differences in deposition due to trading.)
Reductions in Canadian SO2 emissions predicted
by Environment Canada as part of the 1990 NA-
PAP Integrated Assessment were also used in the
2010 modeling.
Extensive comparisons of percent change in visual
range were made using annual average change
and 90th-percentile worst days (i.e., only
10 percent of days have worse visibility) and
10th-percentile best days (i.e., only 10 percent of
days have better visibility). For 50th-percentile
visibility days (half the days have better and half
worse visibility), the percent increase in visual
range in the East ranged from 10 to 20 percent in
Florida, New England, and just east of the Missis-
sippi to 30 to 40 percent in the mid-Appalachians
and the Ohio Valley. The largest improvement in
50th-percentile visibility range in Class I areas was
predicted to be in Shenandoah National Park in
the mid-Appalachians.
Western United States
To illustrate the visibility impact on western Class I
areas in the 1993 Report to Congress, EPA con-
ducted a comprehensive analysis of changes in
visual range resulting from implementation of the
1990 CAAA, including the development of emis-
sions inventories for anthropogenic sources of
NOX, SOX, and particulates for 1988, a 2005
base-case scenario, and a 2005 CAAA-imple-
mented scenario.77 The 1985 NAPAP emissions
inventory served as the basis for the annual inven-
tories. Electric utility emissions estimates in the
NAPAP inventory were replaced by emissions
from the more up-to-date National Allowance
Data Base (NADB). Emissions estimates for two
large smelters near the border in Mexico were also
included in the inventories. These emissions
inventories were used to model projected air
quality changes from 1988 to 2005. (EPA's 1993
Visibility Report to Congress contains a detailed
description of models used and assumptions
made.)
Comparing emissions for SO2, NOX, and fine par-
ticulates revealed only minor differences in the
total emissions of each species between 1988 and
2005. Emissions for the three scenarios by source
category for SO2 and NOX show that SO2 emis-
sions are equally distributed among area, utility,
and other point sources (e.g., smelters, refineries,
and pulp mills). Of the 100,000-ton decrease in
utility emissions between 1988 and 2005, over
half is a result of scrubbing at the Navaho station.
NOX emissions are primarily attributable to motor
vehicles and area sources. Mobile sources account
for most of the decrease in NOX emissions,
whereas utility emissions increase slightly.
77 Visibility modeling described here was conducted
before this study was initiated; thus, inventories used
are slightly different from those described in Chapter
3. Differences in the inventories should not signifi-
cantly affect qualitative conclusions, however.
82
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CHAPTER 4: POTENTIAL BENEFITS OF A STANDARD ON VISIBILITY,
HUMAN HEALTH, MATERIAL, AND CULTURAL RESOURCES
Three-hour average visual range estimates
were developed for representative Class I
areas in six geographic regions: Central
Coast (California), Sierra, Southern
California, Desert Southwest, Golden Circle
(Arizona), and Rockies. Exhibit 61
summarizes annual visual range estimates
for each inventory scenario calculated from
the 3-hour averages.
EXHIBIT 61. AVERAGE ANNUAL VISUAL RANGE ESTIMATES
FOR REPRESENTATIVE CLASS I AREAS IN THE SOUTHWEST
Exhibit 61 indicates that neither the growth
in emissions between 1988 and 2005 nor
implementation of the 1990 CAAA at
sources in the Southwest will have an
appreciable effect on visual range in Class I
areas. The insensitivity of predicted visibility
changes between the years 1988 and 2005, even
with implementation of the 1990 CAAA in the
latter year, is clearly consistent with the relatively
small changes in SO2, NOX, and particulates
during this period.
4.2.4 Potential Impact of an Acidic Deposi-
tion Standard on Visibility
For this study, visibility in the East was calculated
using the RADM EM-VIS model for two SO2 emis-
sions scenarios described in Chapter 3. Visible
ranges for the post-2010 full implementation sce-
nario and for the additional utility and industrial
SO2 reduction scenario (approximately 44 percent
decrease in SO2 emissions beyond CAAA reduc-
tions) were calculated for 90th-percentile worst
days. Maps in Exhibits 62 and 63 show percentage
changes in annual average visibility for these two
scenarios between 1985 and 2010. To assess the
impact of changes in visibility due only to de-
creases in ambient sulfate concentration, visibility
impairment from other ambient species remained
constant in the models.
The greatest improvements in visual range be-
tween 1985 and the baseline scenario in 2010 lie
in a band from northern Mississippi to southwest-
ern New York State. Improvements in visibility for
Class I areas in the mid-Atlantic region, which in-
cludes the Great Smoky Mountains and the
Shenandoah Valley, range between 30 percent and
more than 40 percent. For the additional SO2 re-
duction case, improvements in visual range of
greater than a factor of two are predicted for these
key mid-Atlantic areas. Increases in visual range of
60 to 100 percent (i.e., the ability to see twice as
far) are predicted for coastal areas and New Eng-
land.
Geographic Region
Central Coast
Sierra
Southern California
Desert Southwest
Golden Circle
Rockies
Representative
Class I Areas(s)
Pinnacles
Yosemite
San Gorgon io
Chiricahua
Grand Canyon
Arches
Rocky Mountain
Bandelier
Visual Range (km)
1988
96
104
68
118
134
116
121
119
2005
CAAA
94
101
66
115
132
115
120
116
The study conducted by EPA for Class I areas in
the Southwest demonstrates that no single pollut-
ant or source category is responsible for most of
the visibility impairment in that region. Thus,
EXHIBIT 62. PERCENT INCREASE IN VISUAL RANGE FROM
1985 TO 2010 WITH FULL CAAA IMPLEMENTATION
while an acid deposition standard could reduce
ambient sulfate, nitrate, or NO2 levels, projection
of potential improvements in visibility for specific
Class I areas would require additional model
analysis.
4.3 RELATIONSHIP OF HUMAN HEALTH TO
ACIDIC DEPOSITION
Exposure to SO2, paniculate matter (including
acidic aerosols), NO2, and ozone (O3) in ambient
air can cause adverse health effects. (Ozone is a
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ACID DEPOSITION FEASIBILITY STUDY
EXHIBIT 63. PERCENT INCREASE IN VISUAL RANGE
FROM 1985 TO 2010 WITH ADDITIONAL
SO2 REDUCTION BEYOND CAAA
related concern for acidic emissions and deposi-
tion standards because NO2 is a major precursor in
O3 formation.) Possible health effects related to
acidic deposition and its precursors are quite com-
plex because of the variety of pollutants, possible
routes of exposure, and mechanisms involved
(Exhibit 64).
EXHIBIT 64. RELATIONSHIP OF ACIDIC DEPOSI-
TION PROCESSES TO HEALTH EFFECTS
Nitrogen Oxides
VOCs
Nitrates
Sulfates
The potential health benefits derived from reduc-
tions in fine particulate mass, SO2, and NOX emis-
sions resulting from Title IV as well as additional
reductions beyond the CAAA have not been mod-
eled for this study. Several health issues are briefly
outlined here, however, because of the potential
benefit of an acid deposition standard. Current ap-
plicable standards include National Ambient Air
Quality Standards (NAAQS) for SO2, NO2, and O3.
The 10-million ton SO2 emissions reduction from
1980 levels under Title IV is expected to result in
human health benefits and potentially high mone-
tary savings due to reduced mortality and morbid-
ity effects associated with SO2 and fine particle
exposures.
The following subsection summarizes health ef-
fects and potential risks associated with airborne
acidic pollutants. Current knowledge on possible
future risks to human health associated with
changes in acidic deposition rates is summarized
in the second subsection.
Several respiratory problems can be caused by
ambient air concentrations of SO2, particulate
matter (including acidic aerosols), NO2, and O3
(separately and in combination). Effects include
chronic bronchitis, bronchoconstriction, other
pulmonary function impairments, chest discom-
fort, cough, lung inflammation, increased inci-
dence of infectious respiratory disease, and in-
creased mortality rates. The elderly, the very
young, and individuals with pre-existing respira-
tory diseases, such as asthma, are at
greatest risk and would benefit most from
reductions in the atmospheric
concentrations of these pollutants.
Under Sections 108 and 109 of the CAAA,
EPA establishes primary NAAQSs, which
protect the most sensitive segments of the
population, with an adequate margin of
safety. The additional emissions reductions
achieved by an acid deposition standard
would facilitate the attainment and mainte-
nance of the primary NAAQS established
under Sections 108 and 109 of the Act.
A number of recent epidemiological studies
have associated particle pollution with
excess mortality and morbidity at levels
below the existing 24-hour particulate
matter NAAQS. For example, a decade-
long, six-city study provided evidence of a
strong association between fine particulates,
including sulfate, and mortality in humans and
also indicated that acidic aerosol concentrations
were directly associated with increased prevalence
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CHAPTER 4: POTENTIAL BENEFITS OF A STANDARD ON VISIBILITY,
HUMAN HEALTH, MATERIAL, AND CULTURAL RESOURCES
of bronchitis in children.78 Clinical studies suggest
that asthmatics may exhibit sensitivities to short-
term exposures to acidic aerosols. As a result, the
EPA has initiated the review of the air quality
criteria and standards for particulate matter,
including acidic aerosols. Additional research
shows that sulfate aerosols comprise the majority
of acidic aerosols in ambient air and a large share
of total ambient inhalable particulate matter in the
eastern United States.79
Based on the available data, many in the scientific
community believe that if the mortality and mor-
bidity effects observed in these studies are causal,
the agent(s) is more likely to be fine particles (<2.5
Urn) than coarse particles (2.5 to 10 Lim). If it is de-
termined, after completion of the ongoing review,
that a new fine particle standard(s) is appropriate,
the associated control strategies will focus on the
control of the precursors (e.g., SO2, NOX, ammo-
nia, and condensible hydrocarbons) to secondary
fine particles. Thus, the emissions reduction
objectives of an acid deposition standard would be
compatible with those of a potential new fine
particle NAAQS.
Insofar as acidic SO42' trends roughly parallel total
SCVtrends, NAPAP estimated that between 2000
and 2020, the region generally incorporating the
states of Ohio, Indiana, West Virginia, Pennsylva-
nia, New Jersey, Maryland, Virginia, North Caro-
lina, Kentucky, Tennessee, and northern Georgia
could experience the greatest decrease in acidic
sulfate levels from implementation of the CAAA.80
The upper Midwest (Michigan and Wisconsin) and
the upper Northeast (Maine and New Hampshire),
which had lower ambient 1-hour sulfate levels, are
estimated to have only slightly improved atmos-
78 D.W. Dockery, C.A. Pope, X. Xu, J.D. Spengler, J.H.
Ware, M.E. Fay, B.C. Ferris, and F.E. Speizer. 1993.
An association between air pollution and mortality
in six U.S. cities. New England Journal of Medicine
329:1753-1759.
79 L.C. Chestnut and A. Patternson. 1994. Human
Health Effects Benefits Assessment of the Acid Rain
Provisions of the 1990 Clean Air Act Amendments.
(Draft methodology report.) Prepared for the Acid
Rain Division, U.S. Environmental Protection
Agency, Washington, DC. (Currently being prepared
for peer review.)
80 This section is drawn primarily from the National
Acid Precipitation Assessment Program, 1991 (7990
Integrated Assessment Report. NAPAP Office of the
Director, Washington, DC.).
pheric concentrations in the years 2000 and 2020
under this scenario. Several ongoing benefit as-
sessments will address the extent of monetary
health benefits associated with implementation of
Title IV.
With respect to NO2, no area of the United States
presently exceeds the NAAQS of 0.053 ppm, an-
nual average. The attainment of the annual stan-
dard also significantly limits the likelihood and
magnitude of short-term 1-hour peak NO2 levels.
NO2 and its transformation products, however, are
precursors to O3 formation. Consequently, reduc-
tions in NO2 or NOX emissions are key compo-
nents of the O3 control strategies.
4.4 RELATIONSHIP OF MATERIALS DAMAGE AND
CULTURAL RESOURCES TO ACIDIC
DEPOSITION
All materials exposed to the outdoor environment
are subject to degradation caused by natural
weathering processes involving moisture, heat,
oxygen, solar radiation, bacteria, and fungi. Ad-
verse effects from these processes can be acceler-
ated by deposition of wet and dry acidic air pol-
lutants. Several NAPAP reports, including the State
of the Science and Technology Report No. 19, the
1990 Integrated Assessment Report, and the 1992
Report to Congress, considered the delivery of wet
and dry deposition to various types of material sur-
faces, particularly in urban areas.
4.4.1 Acidic Deposition Effects on Materials
and Structures
Research conducted by NAPAP and other pro-
grams confirms that certain metals such as galva-
nized steel are corroded (Exhibit 65), surface coat-
ings are degraded, and carbonate stone is dis-
solved by SO2, wet-deposited acids, and natural
atmospheric acidity from dissolved CO2. Spotting
of automotive finishes can also occur from acidic
deposition.
To illustrate the potential role of acidic deposition,
Exhibit 65 shows that an estimated 31 to
78 percent of the dissolution of galvanized steel
and copper continues to be attributable to wet and
dry acidic deposition. Acidic deposition can also
accelerate deterioration of stone through three
processes: dissolution and erosion of material or
surface features; blackening of the stone surface;
and cracking, splintering, and chipping of the
stone surface. One of the most visible and docu-
mented forms of pollutant damage to limestone
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ACID DEPOSITION FEASIBILITY STUDY
EXHIBIT 65. PERCENTAGE OF METAL CORROSION ATTRIBUTED TO ATMOSPHERIC FAcroRsa-b
Metal
Study Region
Galvanized Steel
Adirondack Mtns, NY
Washington, DC
Steubenville, OH
Copper
Adirondack Mtns, NY
Washington, DC
Steubenville, OH
Corrosion Rate
(um/yr)
0.62±0.26
1.01 ±0.42
1.47±0.21
0.37±0.14
0.83±0.19
0.88±0.29
Dry Deposition
of Sulfur
6%
52%
56%
10%
38%
57%
Acidity
(hydrogen ion con-
centration)
25%
23%
22%
25%
25%
20%
Other Corrosion
Factors
69%
25%
22%
65%
37%
23%
a Source: NAPAP. 1993. 1992 Report to Congress. National Acid Precipitation Assessment Program, Washington,
DC.
b Corrosion rates are mean measurements from NAPAP field sites.
and marble is the accumulation of dark gypsum (a
mineral formed from calcium sulfate and water)
crusts in areas sheltered from rain. Sulfur deposited
onto carbonate stone (e.g., marble) reacts with
calcite to form a black crust containing a mixture
of gypsum, fly ash particles, soot, and biological
growth, all of which can cause decay deeper into
the stone. Laboratory and field studies show a cor-
relation between dry deposition of SOX and a thin
black accumulation on masonry materials, includ-
ing sandstone, granite, and brick. European studies
show that SO42- and NO3- concentrations in stone
occur in proportion to atmospheric concentrations
of SO2 and NOX. Erosion rates of stone in Europe,
however, are significantly greater than those found
in North America because of the higher ambient
concentrations of acidic deposition precursors in
Europe.
Rates of damage to materials associated with
acidic deposition depend on atmospheric and
structural factors that influence delivery of the pol-
lutant to a material's surface, i.e., regulate its
"dose." Wet deposition delivers atmospheric pol-
lutants to surfaces of buildings, structures, and
other objects primarily through rainfall, snowfall,
fog, dew, and frost. Dry deposition provides a
more constant delivery of pollutants as large parti-
cles fall with gravity and small particles and trace
gases are delivered by atmospheric turbulence.
Pollutants delivered by turbulent processes can po-
tentially damage a greater proportion of material
surfaces than can large particles. Also, a damp sur-
face provides a much more effective sink for sol-
uble trace gases (e.g., SO2) than does the same sur-
face when dry. Thus, dry deposition can often be
intimately linked with the processes by which ma-
terial surfaces are wetted.
The key effect of concern for cultural materials is
physical damage, often expressed in terms of the
time it takes for the material to lose its unique
qualities. For construction materials, the key effect
of concern is the expenditure to maintain an ac-
ceptable level of functionality and appearance
over the life of the structure.
NAPAP studies reveal that U.S. regions with the
largest number of cultural and historical monu-
ments also often have the highest levels of acidic
deposition, including areas of the Northeast having
long settlement histories and the greatest number
of pre-Civil War buildings and tombstones. Most
historic battlefields, especially those with com-
memorative monuments, are located east of the
Mississippi. Also, material selection has changed
over time; more durable materials have replaced
acid-sensitive marbles, sandstones, and zinc,
which were predominantly used prior to large-
scale western expansion and late 19th-century
population increases. Distributions of acid-sensi-
tive cultural materials are therefore expected to be
less dense west of the Mississippi.
4,4=2 Material Life-Cycle and Damage
Estimates
Potential acidic deposition-related damage to both
function (e.g., building material, bridges, and
automobile and other exterior coatings) and cul-
tural value (e.g., historical monuments and sculp-
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CHAPTER 4: POTENTIAL BENEFITS OF A STANDARD ON VISIBILITY,
HUMAN HEALTH, MATERIAL, AND CULTURAL RESOURCES
tures) potentially represent a very large overall cost
to society. Acidic deposition control can therefore
be linked to potentially large monetary benefits.
Relating acidic deposition-induced physical dam-
age to the shortened usefulness of materials re-
mains an important area of research. Quantifying
changes in maintenance and replacement cycles
attributable to changes in acidic deposition is nec-
essary for estimating the economic consequences
of physical deterioration. The complexity of this
linkage involves three primary areas:
* Extrapolation of laboratory findings to
doses on large structures caused by ambi-
ent exposure;
* Tolerance to decay by acidic deposition,
which varies with the specific function of
the material within the structure; and
* Maintenance and replacement cycles,
which can be affected by a great many fac-
tors other than acidic deposition, including
market factors.
Clearly, emissions reductions can minimize the
need for or frequency of public and private main-
tenance, repair, and replacement. EPA and NAPAP
are working on approaches to estimate the damage
to materials from acidic deposition. Assessing ma-
terial effects present an array of options for valuing
damages, costs, and benefits of emissions reduc-
tions in physical terms (e.g., corrosion rates and
reduced service life), market terms (e.g., life-cycle
costing and shifts in material selection and market
share), and nonmarket terms (e.g., heritage valu-
ation of damage to historical monuments and
buildings). Several efforts have been initiated to
determine the material damage costs associated
with acidic deposition; damage to automotive
coatings are highlighted here as one example of
analyses. EPA and NAPAP have begun to investi-
gate the costs of damage to automotive finishes
and subsequent savings attributable to CAAA im-
plementation.
Spotting of automotive finishes can occur from
acidic deposition. This effect is most pronounced
on dark finishes and in warmer climates, because
the rate of chemical reactions causing spotting
increases with temperature. The damage, typically
appearing as irregularly shaped etched areas, oc-
curs after evaporation of a moisture droplet.
Automotive coatings may be damaged by all forms
of acidic deposition, particularly when dry deposi-
tion is mixed with dew or rain. It has been diffi-
cult, however, to quantify the specific contribution
of acidic deposition to paint finish damage relative
to other forms of environmental fallout, the im-
proper application of paint, or deficient paint for-
mulations.
Although the existence of damage to automotive
coatings has been well documented, there has
been little analysis of the economic costs imposed
by this damage. Such an analysis may include in-
vestigation of issues such as actions taken to pre-
vent the negative effects of pollution (i.e., actions
taken by car and truck manufacturers) and the
market value of a car or truck damaged by acidic
deposition (i.e., actions taken by automobile deal-
ers regarding damage which has occurred). The
scoping exercise conducted by EPA and NAPAP
begins to illustrate the potentially large costs asso-
ciated with this type of damage and therefore the
significant benefits of the Acid Rain Program.81 For
example, estimates of annual costs to manufactur-
ers of cars and trucks for including acid-resistant
features can be as high as $400 million. Estimates
of the value of potential annual residual damage to
cars and trucks in the eastern United States may
range from $50 million to over $400 million. Total
annual costs could be $96 million to $850 million.
Additional materials damage and pollution reduc-
tion benefit efforts are also underway to determine
the benefits of acidic deposition control on a func-
tional item such as steel bridges as well as the po-
tential benefits of control to preserve cultural re-
sources of historical importance. Damage to cul-
tural resources can result in potentially high repair
and maintenance costs, replacement costs, and the
value placed on one-of-a-kind resources. These
and other costs associated with acidic deposition-
induced damage would likely decrease with im-
plementation of an acid deposition standard.
81 ICF Incorporated. September 30, 1994. Add Rain
Program Evaluation: Valuing Potential Reductions in
Automobile Finish Damages-Scoping Study.
87
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CHAPTER 5
IMPLEMENTATION ISSUES
5.1 INTRODUCTION
In order to determine the effectiveness of an acid
deposition standard for protecting sensitive re-
sources, it is necessary to know not only the bene-
fits that would result from a given standard if it
were achieved, but also to know how the standard
would be implemented.
In Section 404 of the CAAA, two requirements that
Congress laid out for this acid deposition standard
feasibility study dealt with implementation issues.
The statutory language calls for:
* Description of the measures that would
need to be taken to integrate such standard
or standards with the control program re-
quired by Title IV of the Clean Air Act; and
* Description of the impediments to imple-
mentation of such control program and the
cost-effectiveness of deposition standards
compared to other control strategies in-
cluding ambient air quality standards, new
source performance standards and the re-
quirements of Title IV of the Clean Air Act.
This chapter describes two basic approaches to
implementing an acid deposition standard. Under
the first approach, EPA would set a standard or
standards, either using existing authority or seeking
further authority from Congress to set such stan-
dards and provide deadlines for their attainment.
Then, similar to Title I, states would determine
source-specific limits using source-receptor models
and cost analyses, incorporate those limits in state
implementation plans (SIPs), and enforce them. If
one or more states failed to do the above, EPA
would promulgate a Federal implementation plan
(FIP).
Under the second approach, Congress would di-
rect EPA to set a deposition standard or standards
and to determine the national (or regional) emis-
sions levels for sulfur dioxide and nitrogen oxides
that would meet those standards. Congress would
then set an emissions cap and allowance alloca-
tions for nitrogen oxides and, if necessary, adjust
the cap for sulfur dioxide in Title IV; and provide a
timetable for meeting the new caps. EPA would
use Title IV provisions to implement the emissions
programs.
For these two basic approaches, this chapter will:
* Describe how each would be imple-
mented, including any measures that
would need to be taken to integrate it with
Title IV;
* Describe any impediments to implementa-
tion, including the need for any additional
statutory authority; and
* Discuss their relative cost-effectiveness.
To provide a rough comparison of the cost-effec-
tiveness of the two approaches for sulfur reduc-
tions, estimates are made of the cost of achieving
the same reduction in sulfur deposition at the three
sensitive areas under each approach. The example
used is based on a 50 percent reduction in na-
tional utility SO2 emissions beyond that required
by the 1990 CAAA (as described and modeled in
Chapter 3). The use of this example in no way
suggests that such a reduction is necessary, appro-
priate, or sufficient, but is merely put forth for il-
lustrative purposes.
There are, of course, many variations on these ba-
sic approaches, and probably other ways to realize
the goals of an acid deposition standard. The pur-
pose here is not to provide a complete examina-
tion of this issue, but to show how existing ap-
proaches could be adapted to implement such a
standard and to compare costs.
5.2 TARGETED APPROACH
5.2.1 Description of Targeted Approach
EPA would either set national standards for sulfur
and nitrogen deposition, or set different regional
standards for sulfur and nitrogen based on the dif-
ferent sensitivities of different regions to sulfur and
nitrogen. (See 5.2.3 for a discussion of statutory
authority.) EPA would also establish deadlines for
the attainment of such standards, unless such
deadlines were established through new statutory
authority.
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
States would determine the form and level of
emissions limits for the sources of sulfur and nitro-
gen that they determine relevant to the attainment
of the national acid deposition standards. Their de-
terminations would probably be based on atmos-
pheric models and technical and cost analyses of
the sources within their jurisdictions.
The process for making these determinations and
incorporating such limits into SIPs would vary
from state to state. Some states may wish to pro-
vide flexibility to their sources by allowing various
forms of emissions trading. EPA would need to re-
view each SIP and determine that it, in conjunc-
tion with other relevant SIPs, would attain and
maintain the specific acid deposition standard or
standards. States would be responsible for collect-
ing emissions information and enforcing limits. If
states failed to carry out these responsibilities, EPA
would impose sanctions and/or prepare and im-
plement a FIP. EPA would implement such a pro-
gram through Titles I, II, and V. The program
would be evaluated through deposition and effects
monitoring.
5.2.2 Integration with Title IV
Setting an acid deposition or standards would not,
by itself, directly affect other environmental pro-
grams unless it required emissions reductions. The
specific sources and level of emissions reductions
would determine the direct impact on other pro-
grams and usefulness of coordination and integra-
tion.
If no changes were made to Title IV, the allocation
and transfer of allowances would not be restricted
by state actions to set new emissions limits, but
sources would not be able to emit more SO2 than
their state limits allow, regardless of how many al-
lowances they might hold. This is the same situ-
ation as currently exists, but if more stringent limits
were imposed on a large number of sources, the
demand for and price of allowances would de-
cline. In fact, taken to extreme if the aggregate ef-
fect of new source-specific state limits were to re-
duce utility SO2 emissions below the current 8.95
million allowance cap and the SIPs did not allow
emissions trading, the price of allowances should
theoretically drop to zero because they would be
of no use.
Both in setting the level of the standard and in its
implementation, an acid deposition standard or
standards should be coordinated and potentially
integrated with several other environmental pro-
grams, particularly attainment and maintenance of
primary and secondary National Ambient Air
Quality Standards (NAAQS) for SO2, NO2, O3, and
PM10; visibility protection; new source review; and
new source performance standards under Title I of
the CAAA. It would also be useful to coordinate
acid deposition standard-setting and implementa-
tion with water quality programs, particularly in-
volving eutrophication of estuaries. Substantial re-
ductions in SO2 or NOX could assist in the
achievement of the goals of these programs.
5.2.3 Impediments to Implementation
It may be possible to set acid deposition standards
under existing statutory authority. At this time, no
definitive determination has been made. However,
clear direction from Congress in this area would
certainly make implementation more feasible and
effective. For example, implementation would be
facilitated by explicit authority to set deposition
standards, to set regionally different nitrogen
and/or sulfur standards, to set deadlines for attain-
ment, to require uniform measurement and report-
ing of emissions for sources not already affected by
Title IV, and to establish uniform procedures for
interstate trading of NOX emissions. It would prob-
ably not be possible for sources to conduct effi-
cient interstate trading of NOX emissions without
federal legislation.
Time and resource issues may be significant if, for
example, a regionally targeted implementation
approach shared some of the same administrative
complexities as the current SIP process to meet the
NAAQS. For instance, the development of state-
specific regulations by states for emissions limita-
tions, followed by EPA review and approval, can
be a resource-intensive and lengthy process. EPA is
currently working to streamline this process. An-
other concern is the incomplete and sometimes
inconsistent state emissions inventory data upon
which compliance and effectiveness is deter-
mined. Furthermore, if a state is not in attainment
of the NAAQS, it is commonly a long period of
time until state-specific air quality levels are
achieved. Compliance deadlines for certain
NAAQS have been refined and revised with each
amendment to the CAA. The uncertainties of this
process are not usually conducive to long-range
planning and cost-effective compliance by the
regulated community.
The 1990 CAAA recognized the role of long-range
pollutant transport in ozone formation and author-
ized EPA to create "ozone transport regions" where
nonattainment in one state may be the result of
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CHAPTER 5: IMPLEMENTATION ISSUES
emissions in another state. The newly created
Northeast Ozone Transport Region extends from
Northern Virginia to Maine. The Northeast Trans-
port Commission is currently developing plans to
achieve attainment of the ozone standard by de-
termining both local and transport-region wide
limits on nitrogen oxides emissions. The Commis-
sion is also considering market-based approaches
(e.g., NOX trading within the transport region) to
achieve maximum protection at least cost. This ef-
fort represents a possible variation on the targeted
regional approach.
Implementation of a targeted approach for deter-
mining and maintaining acid deposition standards
would require, at a minimum, an enhancement of
existing deposition monitoring and atmospheric
modeling abilities. Program requirements would
need to identify pollutants to be monitored, and
determine standard procedures for measuring wet
and dry deposition, spatial resolution, and tempo-
ral requirements. Enhanced effects monitoring
(i.e., surface water monitoring) would be desirable
to evaluate effectiveness of the standard or stan-
dards.
5.3 EMISSIONS-BASED APPROACH
5.3.1 Description of Emissions-Based Ap-
proach
Congress would direct EPA to provide (1) a range
of target loads and emissions levels of sulfur and
nitrogen designed to provide a range of ecosystem
protection (and other benefits), (2) levels of na-
tional and regional sulfur and nitrogen emissions
that met those target loads, and (3) estimates of the
benefits and costs of meeting those emissions lev-
els.
Taking this information into account, Congress
would amend Title IV of the CAAA by setting an
allowance cap for nitrogen, revising the cap for
SO2 (if necessary), including any new source cate-
gories (if necessary), allocating allowances, and
providing timetables for the achievement of the
new caps. EPA would implement through con-
forming changes to the Title IV rules. The program
would be evaluated through deposition and effects
monitoring.
5.3.2 Integration with Title IV
Depending on any statutory changes enacted, EPA
would make conforming changes to the Title IV
implementing regulations, 40 CFR Parts 72-78. Ti-
tle IV permitting, allowance trading, emissions
monitoring, penalty provisions and data systems
would be used to ensure compliance. Since the in-
formation that EPA would provide Congress as in-
put to their deliberations in setting allowance caps
would include all human health and ecosystem
impacts from sulfur and nitrogen known to EPA at
the time, coordination with standard-setting and
implementation of the other air and water pro-
grams cited above would be greatly facilitated. If
the number of SO2 allowance were lowered, al-
lowance prices would rise. In certain cases, com-
pliance strategies of affected sources may change.
5.3.3 Impediments to Implementation
New statutory authority would be needed. Title IV
allowance levels for SO2 cannot be changed with-
out Congressional actions, and there is currently
no allowance program for NOX.
Administrative impediments would be limited to
any difficulty posed by the statutory changes. Cur-
rently, Title IV does not appear to have any signifi-
cant administrative or compliance impediments.
Enhanced effects monitoring (i.e., surface water
monitoring) would be desirable to track the effec-
tiveness of deposition reductions.
5.4 ECONOMIC IMPACTS
If an acid deposition standard involves emissions
reductions and requires stricter SO2 and NOX point
source emissions controls, that standard would re-
sult in direct cost increases to utilities and indus-
trial sources. The magnitude of cost increases, and
the sectors most affected would vary depending on
the regulatory approach selected, the quantity of
emissions reduced, the sources affected, and the
timing of the reductions. For the scenarios consid-
ered in this study, at least two comprehensive sec-
tor models—one for the electric utility industry
and one for the industrial sector—would need to
be applied to fully understand the wide ranging
impacts that would result from an acid deposition
standard. While such models exist, budget con-
straints did not allow for their use for this study.
An expanded economic analysis would consider
variations in emissions and the timing of achieving
emissions reductions.
While the use of more precise sector models may
become necessary if further emissions reductions
are pursued, the scale of the economic analysis
presented here is scoping in nature and limited in
its degree of detail. Instead, this study uses:
91
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
» Analyses of the electric utility sector that
have already been conducted by EPA to
support Title IV.
* A spreadsheet-based scoping model to esti-
mate broadly the range of costs associated
with different regulatory approaches.
In Chapter 3, the impacts of various SO2 control
strategies on deposition in sensitive regions were
evaluated for the year 2010. The costs of the key
emissions control strategies described in Chapter 3
are evaluated below. Costs are estimated for the
following control scenarios for the year 2010.
* 2010 CAAA Scenario,
» Additional 50 Percent Utility SO2 Reduc-
tion Scenario,
» Additional 50 Percent Utility and Industrial
SO2 Reduction Scenario, and
* Geographically Targeted Utility SO2 Re-
duction Scenarios.
The costs of additional utility and industrial NOX
reductions are described qualitatively. With the
exception of the geographically targeted utility
SO2 reduction scenario, all cost scenarios are
based on a national emissions-based approach.
5.4.1 2010 CAAA Scenario (With Trading)
The 2010 scenario with trading described in Chap-
ter 3 is used as the baseline scenario for compar-
ing costs of alternative emissions reductions sce-
narios. The costs of this scenario were developed
as part of the 1993 EPA Base Case Analysis82 used
to support rulemaking under Title IV and have
thus, been reviewed extensively. EPA has esti-
mated that compliance with Title IV of the 1990
CAAA will cost electric utilities about 2.2 billion
dollars in the year 2010 (Exhibit 66). SO2 emis-
sions forecasts project that SO2 emissions decrease
by about 9.2 million tons in 2010. This means that
the average cost of reducing SO2 is about $240 per
ton SO2 removed. The marginal SO2 removal cost
(i.e., the cost of reducing one additional ton of
SO2) is forecast to be much higher, however,
about $500 per ton SO2 removed.
82 Economic Analysis of the Title IV Requirements of
the 1990 Clean Air Act Amendments. 1994. U.S.
EPA, Office of Air and Radiation, Acid Rain
Division, February.
With Title IV fully implemented, electric utility
SO2 emissions are forecast to equal about 9.5 mil-
lion tons in 2010. This is higher than the 8.95 mil-
lion ton SO2 allowance cap that is binding in 2010
because:
It is forecast that about 0.52 million tons of allow-
ances would be "banked" between 1995 and 2009
and used in 2010; and
Units not affected by Title IV (i.e., those with
nameplate capacity less than 25 megawatts) would
emit about 0.05 million tons of SO2 in 2010.
Exhibit 66 shows utility costs and SO2 emissions
forecasts in 2010 by U.S. census region. As can be
seen in the exhibit, the majority of SO2 emissions
reductions and compliance costs (about 60 percent
of the U.S. total) are expected to occur in the cen-
tral United States (i.e., East North Central and
West South Central Census Regions).
5.4.2 50 Percent Utility SO2 Reduction Sce-
nario
For the 50 Percent Utility SO2 Emissions Reduction
Scenario, the 8.95 million ton electric utility SO2
emissions cap was cut in half (i.e., set equal to
4.48 million tons SO2). Under this scenario, costs
are estimated to increase to $4.8 billion and SO2
emissions are estimated to decrease by an addi-
tional 5 million tons in 2010 relative to the CAAA
Scenario (Exhibit 67). The average cost of reducing
SO2 emissions by 5 million tons is about $955 per
ton SO2 removed, which is almost four times the
average cost of emissions reductions forecast in
the CAAA Scenario (i.e., $240 per ton SO2 re-
moved). The marginal cost of reduction is also
much greater than in the CAAA scenario (i.e.,
about $1,225 per ton SO2 removed versus $500
per ton SO2 removed).
Costs and unit level SO2 emissions for the 50 per-
cent electric utility reduction scenario were fore-
cast using EPA retrofit scrubber cost assumptions
and unit level SO2 emissions forecast in the CAAA
Scenario. It was assumed that SO2 emissions re-
duction would be achieved by unscrubbed coal-
fired units not already forecast to use low sulfur
coal in the CAAA scenario (i.e., unscrubbed units
forecast to have an SO2 emissions rate greater than
0.8 Ibs SO2 per million Btu). Using EPA scrubber
cost assumptions used in the 1993 Base Case
Analysis, costs were estimated for achieving the
5.0 million ton SO2 emissions reduction.
92
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CHAPTER 5: IMPLEMENTATION ISSUES
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CHAPTER 5: IMPLEMENTATION ISSUES
5.4.3 50 Percent Utility and Industrial SO2
Reduction Scenario
Under this scenario, the 50 percent utility SO2
emissions reduction is no different than described
above. However, it is supplemented by a 50 per-
cent reduction from industrial sources. As a result,
costs are estimated assuming electric utility and
industrial sources achieve a total reduction in SO2
emissions of about 7 million tons in 2010 relative
to the CAAA Scenario (Exhibit 68). Under the
CAAA Scenario, utility and industrial sources are
estimated to emit about 9.5 and 4.0 million tons of
SO2 in 2010 respectively. Commercial/institutional
sources are expected to emit an additional 0.2 mil-
lion tons. The cost of achieving the 50 percent
electric utility and industrial source SO2 emissions
reduction is estimated to be about $6.5 billion
annually. This corresponds to an average SO2
emissions reduction cost of about $926 per ton
SO2 removed. As described in the previous sec-
tion, the average cost of reductions for electric
utilities is about $955 per ton SO2 removed, and
the average cost for industrial sources is estimated
to be about $850 per ton SO2 removed.
5.4.4 Geographically Targeted Reductions
Scenario
Costs were calculated for both the contiguous and
noncontiguous geographically targeted reduction
scenarios corresponding to the deposition levels
achieved by the 50 percent nationwide SO2 utility
reduction scenario for all three sensitive receptor
regions and for all three regions together. These
scenarios were constructed by sequentially remov-
ing 95 percent of utility SO2 emissions (remaining
after implementation of the Title IV) from subre-
gions (either contiguously or in order of contribu-
tion to deposition) until the deposition loads were
achieved. Ninety-five percent SO2 emissions re-
moval was necessary given the smaller number of
sources from which to draw emissions. Costs were
estimated by applying the EPA retrofit scrubbing
cost assumptions to achieve 95 percent SO2 re-
moval from utility boilers identified in each subre-
gion. Exhibit 69a provides costs for achieving the
target deposition loads for each receptor region
individually and for all three regions simultane-
ously for the contiguous approach. Exhibit 69b
provides the same information for the non-con-
tiguous case.
The total tons removed, about 4.6 million tons, the
annual costs, about $4.6 billion, and the cost-ef-
fectiveness, about $1,000 per ton removed, are es-
sentially identical for the contiguous and non-con-
tiguous cases. Thus, no increase in efficiency
would be gained by choosing the more complex
non-contiguous approach over sequential contigu-
ous geographical targeting.
Costs were not estimated for the geographically
targeted utility and industrial scenario because
specific cost functions for SO2 removal from indi-
vidual industrial sources were not available. De-
veloping cost functions for individual industrial
sources would require an extensive cost develop-
ment effort beyond this scoping study.
5.4.5 NOX Reductions—50 Percent Utility
and Industrial
EPA is currently developing regulations for the
control of NOX emissions from electric utilities af-
fected under Title IV of the CAAA. Regulations for
Group I boilers in Phase I were promulgated on
March 22, 1994.83 Regulations for Group II boilers
are under development and the costs and emis-
sions reductions expected from these regulations
were not available for this report. Preliminary in-
formation on the cost of controlling NOX emissions
from various types of electric utility boilers is
available, however, from a recent EPA report.84
These costs vary significantly depending on the
type of technology applied, NOX control effi-
ciency, and boiler specific parameters. Some of
the results presented in that report are described
below.
Title IV of the CAAA requires EPA to set emissions
limits for Group 1 boilers (i.e., dry bottom wall-
fired and tangentially fired boilers) based on the
application of low-NOx burners (LNB) at affected
electric utility units. In the RIA, which covers
Group 1 boilers, EPA estimated that NOX emis-
sions would decrease by about 1.9 million tons
annually and cost about $0.3 billion annually (an
average cost of about $159 per ton NOX re-
moved).85 The RIA considers a variety of NOX con-
83 See 59 Fed. Reg. 13538-80. On November 29,
1994, the U.S. Court of Appeals for the District of
Columbia circuit vacated regulations and remanded
them to EPA for further action. Alabama Power Co.
v. U.S. EPA. No. 941170 (1994).
84 Alternative Control Techniques Document - NOX
Emissions from Utility Boilers, March 1994, U.S.
EPA, Office of Air Quality Planning and Standards,
EPA-453/R-94-023.
85 Regulatory Impact Analysis of NOX Regulations,
February 1994, U.S. EPA Office of Atmospheric and
Indoor Air Programs, Acid Rain Division.
95
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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96
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CHAPTER 5: IMPLEMENTATION ISSUES
EXHIBIT 69A: ANNUAL COSTS OF GEOGRAPHICALLY TARGETED REDUCTIONS EQUIVALENT TO NATION-
WIDE 50% UTILITY SO2 REDUCTION (CONTIGUOUS RADM SUBREGIONS)
Emissions Reduction (tons)
Cost ($ million)
Cost- Effectiveness ($/ton)
Sensitive Region
Adirondacks
3,018,000
3,131
1,037
Mid- Appalachi-
ans
1 ,952,000
2,127
1,089
Southern
Blue Ridge
1,508,000
1,610
1,068
All Three
Receptor Regions
4,526,000
4,741
1,048
EXHIBIT 69s: ANNUAL COSTS OF GEOGRAPHICALLY TARGETED REDUCTIONS EQUIVALENT TO NATIONWIDE 50%
UTILITY SO2 REDUCTION: MAJOR RADM SUBREGIONS CONTRIBUTING TO DEPOSITION (NOT CONTIGUOUS)
Emissions Reduction (tons)
Cost ($ million)
Cost-Effectiveness ($/ton)
Sensitive Region
Adirondacks
3,160,000
3,151
997
Mid- Appalachi-
ans
2,079,000
2,193
1,054
Southern
Blue Ridge
2,081,000
1,952
938
All Three
Receptor Regions
4,658,000
4,523
971
trol technologies.86 Applied to pre-New Source
Performance Standard (NSPS) coal-fired electric
utility boilers, these technologies are capable of
achieving NOX emissions reductions of about 10 to
60 percent. On a boiler operating in baseload,
these technologies are estimated to cost from
about $100 to $1,000 per ton NOX removed.
To reduce electric utility NOX emissions by more
than required by Title IV, it would be necessary to
apply technologies with NOX removal efficiencies
and costs greater than LNB technology. It would
be necessary for sources would be required to ap-
ply selective catalytic reduction (SCR) or selective
non-catalytic reduction (SNCR) technologies. SCRs
can achieve NOX removal efficiencies ranging
from 75 to 85 percent. In combination with LNB
technologies, SCRs can reach removal efficiencies
of 85 to 95 percent. EPA estimates that a stand
alone SCR would cost from $810 to $2,490 per ton
NOX removed at a coal-fired unit operating in
baseload. In combination with LNB technology,
SCR application could cost about $1,300 to
86 The Court held in Alabama Power Co. v. U.S. EPA
that EPA's definition of low-NOx burner in the
March 22, 1994, regulations was too broad. That
ruling does not affect the use of information
developed for the RIA in this scoping economic
analysis.
$2,490 per ton NOX removed. These cost would
be expected to decline with the wide-scale appli-
cation of SCR throughout the electric utility indus-
try, based on economies of scale.
As with utility boilers, a wide variety of NOX con-
trol technologies are applicable to industrial boil-
ers. These include LNB technologies, SNCR, and
SCR. It is usually more cost-effective to apply these
technologies to electric utility boilers than to in-
dustrial boilers because electric utility boilers gen-
erally provide more suitable operating conditions
and economies of scale.
5.4.6 Summary of Economic Impacts
Exhibit 70 summarizes the total costs and costs per
ton of SO2 removed for each reduction strategy for
which costs were developed. The additional 50
percent nationwide SO2 utility reduction scenario
is a factor of four less cost-effective than the CAAA
baseline. This is not surprising since the allowance
trading program was designed to achieve maxi-
mum cost- effectiveness, and, thus, utility reduc-
tions beyond that required by the Act would nec-
essarily be less economically attractive. Additional
emissions reductions would likely impact the
emissions trading program and limit the compli-
ance flexibility inherent in the current program.
From Exhibit 70 it is apparent that difference in
97
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT 70. SUMMARY OF COSTS OF VARIOUS EMISSIONS REDUCTIONS SCENARIOS
Scenario
CAAA baseline
50% utility SO2 removal
Targeted utility SO2 removal
(contiguous)
Targeted utility SO2 removal
(not contiguous)
50% utility and industry SO2
removal
SO2 Removed
(Tons x 1000)
9,166
5,047
4,526
4,658
7,047
Annual Cost
($ Billion)
2.2
4.8
4.7
4.5
6.5
Cost-Effectiveness
($/Ton SO2 Rem.
in 1994 dollars)
240
955
1,048
971
926
costs between the nationwide and geographically
targeted SO2 emissions reductions strategies to
achieve the same level of deposition are not sig-
nificant. Therefore, there does not appear to be a
significant cost advantage to adopting a geo-
graphically targeted approach to achieving the
deposition levels attained by the nationwide 50
percent SO2 utility reduction scenario. Interest-
ingly, the 50 percent utility and industrial reduc-
tion scenario is about equal in cost-effectiveness to
the 50 percent utility reduction scenario. This indi-
cates that emissions reductions from major indus-
trial sources would be as cost-effective as addi-
tional utility reductions.
The costs presented in this economic analysis were
based on conventional SO2 scrubbing and are
scoping in nature/These costs should be viewed as
conservative (i.e., over-stated) in that within the
time frame in which acid deposition standards
could be developed, newer innovative technolo-
gies may become available which may be less
costly than conventional scrubbing, including
clean coal technologies, repowering with natural
gas, or increased use of pollution prevention tech-
nologies such as renewal energy and conservation.
If further emissions reductions were mandated by
Congress in order to implement an acid deposition
standard, there may be a greater impetus for the
commercial deployment of clean coal technolo-
gies in the U.S. market place as well as other re-
powering or pollution prevention technologies.
5.5 CONCLUSIONS
In this chapter, the discussion has been focused on
implementation issues associated with an acid
deposition standard. Assuming that a decision
were made to reduce emissions of sulfur dioxide
and/or nitrogen oxides beyond the current Clean
Air Act to address the acidification of surface wa-
ters and/or the multiple effects associated these
pollutants, it is feasible to implement such an ap-
proach. There are different approaches that could
be taken and various factors to be considered
(administrative complexity, resource demands on
the government and regulated industry, costs, in-
teractions with other programs). Based on the mul-
tiple effects of acidic deposition and its sulfur and
nitrogen precursors, it is recommended that if fur-
ther emissions reductions are pursued, they be as
broad as possible; either a national approach or a
regional approach that incorporated a large area of
the country (e.g., east of the Mississippi River).
Furthermore, Title IV is an administratively effi-
cient way to achieve emissions reductions with the
basic infrastructure already set up under Title IV
being well-suited to incorporate further sulfur di-
oxide and nitrogen oxides emissions reductions.
The costs of further emissions reductions charac-
terized in this report could lead to costs that are
more than double those of the current acid rain
control program, but the timing of those reductions
would affect the costs and the benefits. The bene-
fits would likely be in multiple effects areas. Com-
pliance costs could be significantly impacted by
the timing of any further emissions reductions. Any
additional reductions which may be required later
rather than earlier may cost less based on cost-sav-
ing technologies demonstrated through clean coal
technology and pollution prevention efforts and
based on the replacement of existing sources by
new, lower emitting sources.
98
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CHAPTER 6
INTEGRATION AND CONCLUSIONS
6.1 INTRODUCTION
Section 404 Appendix B of the Clean Air Act di-
rects EPA to assess the feasibility and environ-
mental effectiveness of an acid deposition standard
or standards to protect sensitive ecosystems. This
chapter summarizes and integrates the findings
from the previous chapters showing how scenarios
for potentially reducing acidic deposition de-
scribed in Chapter 3 address environmental goals
defined in Chapter 2. Chapter 4 described
potential benefits to visibility, human health,
material, and cultural resources accompanying
additional reduc-tions in acidic deposition.
Chapter 5 addressed feasibility and effectiveness of
implementing a deposition standard.
Section 6.2 highlights the conclusions that are key
to establishing effective environmental goals and
efficient deposition control strategies. Section 6.3
then integrates these findings to show the potential
surface water benefits from a selection of alterna-
tive emissions reduction scenarios, considering a
range of times to watershed nitrogen saturation.
6.2 ESTABLISHING EFFECTIVE ENVIRONMENTAL
GOALS
Defining appropriate environmental goals and pro-
jecting future effectiveness of various control op-
tions requires characterizing potential environ-
mental effects and benefits over a range of sulfur
and nitrogen deposition loadings. A regionally spe-
cific approach to determining a deposition
standard or standards can provide the basis for
implementation under either regional or national
approach. Resource-specific goals can be used to
determine what emissions and deposition reduc-
tions would likely be needed. A regionally specific
environmental goal (i.e., acid deposition standard)
can be used to achieve effective and efficient
environmental protection of those resources and
ecosystems most sensitive to adverse effects and
most likely to benefit from acidic deposition
control.
Establishing appropriate environmental goals for
an acid deposition standard or standards requires
selection of appropriate ecological endpoint crite-
ria and indicator measures. Such measures must
provide information to accurately judge how suc-
cessfully the key ecosystems and resources of con-
cern are being protected. The applicability of these
measures varies among regions and, in some
cases, among individual systems (e.g., watersheds,
lakes, or streams). While the analysis presented in
this chapter focuses on changes in surface water
quality reflected by two ANC measures within
these waters, other endpoints may be equally or
more appropriate for protecting sensitive resources
of local interest, such as individual stands of red
spruce forests or populations of listed threatened or
endangered species. Potentially useful endpoints
can include the "most sensitive" systems or species
within an environment or some defined index of
ecological structure.
Several major points and conclusions from the re-
search and analyses presented in the previous
chapters are key to the process of integrating our
understanding of environmental effects and
source-receptor relationships related to acidic
deposition and setting effective environmental
goals. The following points highlight the processes
and relationships described earlier in this report.
* CHEMICAL EMISSIONS AND ATMOSPHERIC
PROCESSES AFFECTING DEPOSITION ACIDITY:
The principal acids in deposition are sulfu-
ric (H2SO4) and nitric (HNO3) acids. Thus,
emissions of compounds containing sulfur
and nitrogen have been the primary focus
in acidic deposition control strategies.
Volatile organic compounds (VOCs) and
their oxidation products are also important
because they often control reactions that
produce the oxidizing species that lead to
formation of sulfuric and nitric acids in the
atmosphere. In total, production of atmos-
pheric acids is a complex process, involv-
ing 140 known reactions among 60 chemi-
cal species, 40 of which are organic com-
pounds.
» NATURAL EMISSIONS SOURCES: Natural emis-
sions of acid precursor species, organic
matter, and alkaline materials (dust) are
99
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
generated by vegetative matter, soil and
saltwater microbes, geochemical activity,
lightning, and natural combustion (e.g.,
forest fires). Natural emissions of SO2, sul-
fates, and nitrogen oxides are significantly
less important than anthropogenic sources
in their impact on sensitive ecological re-
sources.
ANTHROPOGENIC EMISSIONS SOURCES: Acidic
deposition precursor species and reactive
atmospheric chemicals are generated by
energy production, industrial processes,
mobile sources, and waste disposal. Cur-
rent estimates indicate that of the total
emissions of SO2, electric utility fuel com-
bustion contributes 70 percent, industrial
fuel combustion contribute 14 percent, and
the balance comes from other sources. For
emissions of NOX, both electric utility fuel
combustion and highway vehicles each are
estimated to contribute 32 percent, indus-
trial fuel combustion provides 15 percent,
and off-highway vehicles produce 12 per-
cent, with the balance coming from other
sources. Potential future needs to effec-
tively and efficiently further reduce anthro-
pogenic source emissions and deposition
would likely focus primarily on limiting
emissions from these major sources.
CHEMICAL CAUSES OF ACIDIC DEPOSITION
EFFECTS: Accumulating scientific evidence
verifies that deposition of acid-forming sul-
fur and nitrogen compounds both can be
significant causes of surface water acidifi-
cation effects. Although past research and
control efforts have primarily focused on
the control of sulfur emissions and deposi-
tion, recent research indicates that nitrogen
deposition often may be an equally and
sometimes more important cause of some
surface water acidification effects. For ex-
ample, considerable evidence indicates
that nitrogen deposition is generally a
greater acidification concern in the western
United States and that nitrogen deposition
as well as sulfur deposition can be a sig-
nificant contributor to episodic acidifica-
tion of surface waters in the East.
WATERSHED NITROGEN SATURATION: There
are limits to the amount of nitrogen that
can be sequestered (i.e., in organic matter)
in watersheds. As these systems approach
saturation, nitrogen losses from watersheds
will increase leaching of nitrate. This can
lead to acidification of surface waters.
Times to nitrogen saturation vary among
regions due to differences in temperature,
moisture, length of growing season, soil
fertility, forest age, history of nitrogen
deposition, and other variables. Significant
variability and uncertainties remain in de-
termining the time to nitrogen saturation
for specific watersheds across regions. Ni-
trogen saturation is a potentially significant
concern that contributes to the acidifica-
tion process, even if total saturation never
occurs.
* MOST SENSITIVE REGIONS AT RISK: Based on
the NAPAP National Surface Water Survey,
six regions contain 95 percent of the lakes
and 84 percent of the stream reaches that
were chronically acidified [i.e., having an
acid neutralization capacity (ANC) of 0
ueq/l or less] due to inorganic ions, pre-
dominantly SO42', NCy, and CK These
areas include the southwest Adirondack
Mountains in New York, New England,
mid-Appalachian Region, Atlantic Coastal
Plain, northern Florida Highlands, and low-
silica lakes in the upper Midwest. Com-
piled evidence indicates that acidic deposi-
tion most likely caused significant acidifi-
cation of surface waters in the Adirond-
acks, the Pocono/Catskill subregion, mid-
Appalachians, eastern upper Midwest, the
New Jersey Pine Barrens, and, to a lesser
extent, northeastern Florida. These regions,
therefore, require greatest consideration
when determining the need for protection
from future acidic deposition loadings.
* MOST SENSITIVE RESOURCES AT RISK: An acid
deposition standard or standards intended
to prevent adverse effects should provide
adequate protection for the most sensitive
resources at greatest risk. The predominant
natural resources that appear to be both
most sensitive to and at greatest potential
risk from acidic deposition are aquatic sys-
tems and high-elevation red spruce forests.
» NATURALLY ACIDIC SURFACE WATERS: Evalu-
ation of acidic deposition should include
the realization that all regions hold natu-
rally acidic surface waters. For example,
about 40 to 50 percent of the target
population surface waters in the Adi-
rondacks with ANC of 50 ueq/l or less (i.e.,
100
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CHAPTER 6: INTEGRATION AND CONCLUSIONS
sensitive) are likely to persist even with
complete elimination of anthropogenic
acidic deposition. Certain biota evolve to
live in naturally acidic systems. Manage-
ment and policy decisions should
recognize the existence of these systems
and consider protecting populations and
communities that have naturally evolved as
part of these ecosystems.
ENVIRONMENTAL GOALS TO PROTECT SENSITIVE
AQUATIC RESOURCES: The biological effects
of inorganic monomeric aluminum associ-
ated with acidic deposition are minimized
as the level of acidic deposition is de-
creased and pH and ANC levels in sensi-
tive waters are kept relatively high. Based
on laboratory and field studies of sensitive
aquatic species, a general goal is to main-
tain surface water pH above 6.0. Greatest
protection of sensitive aquatic resources
occurs in surface waters where ANC is
generally maintained above 50 ueq/l.
EPISODIC ACIDIFICATION: Short term acute ef-
fects occur when pulses of acidic waters
enter lakes or streams with storm runoff
and snowmelt. The resulting potentially
acutely toxic changes in surface water
chemistry often occur at the most biologi-
cally significant time of year. The projected
number of systems at risk of episodic acidi-
fication is substantially higher than the
number of chronically acidic systems.
CRITICAL AND TARGET LOADS: A critical load
is a quantitative pollutant loading below
which no significant harmful effects occur
to ecological processes; a critical load de-
pends solely on inherent ecological proper-
ties. A target load may incorporate social,
policy, economic, and other considerations
along with the scientific observations. Pro-
tection approaches emphasizing critical
and target loads are currently used exten-
sively in most European countries and
Canada. A critical and/or target load ap-
proach is conceptually similar to the depo-
sition standard approach discussed in this
report for determining the most appropriate
level of protection.
This report does not develop or set critical
or target loads. It does, however, provide
the scientific basis upon which to deter-
mine critical loads. The scientific uncer-
tainty regarding watershed nitrogen
saturation makes determining a standard
difficult at this time.
* MONITORING TO ASSESS EFFECTIVENESS AND
BENEFITS OF CONTROLS: Although the analy-
ses presented in this report focused on en-
vironmental goals appropriate for reducing
regional proportions of surface waters with
ANC below 0 ueq/l and maintaining sur-
face water ANC above 50 ueq/l, monitor-
ing to assess the actual effectiveness of any
emissions or deposition controls should as-
sess not only the potential benefits of con-
trols on surface water ANC, but also on
other resources of concern. Such concerns
include possible changes in the stand con-
dition within red spruce forests at potential
risk, visibility impairment particularly in
National Parks, and degradation of materi-
als and cultural resources.
* REGIONALLY BASED RESEARCH: Outside the
context of an acid deposition standard, re-
gionally based ecological knowledge can
be used to help guide efforts to improve or
monitor the ecological health of sensitive
areas.
6.3 PROJECTED ENVIRONMENTAL CONSE-
QUENCES OF ACIDIC DEPOSITION RE-
DUCTION SCENARIOS
From the above summary of findings, the surface
waters in the United States at greatest apparent
continuing risk from acidic deposition extend from
the Adirondacks south along the Appalachian
chain into northern Florida. Past research has
primarily focused on understanding acidic
deposition relationships within three representative
case-study regions along this area, particularly
lakes in New York's Adirondack Mountains, acid
sensitive stream reaches in the mid-Appalachian
Region (portions of New York, New Jersey,
Pennsylvania, Maryland, West Virginia, and
Virginia) and Southern Blue Ridge Province
(portions of North Carolina, South Carolina,
Tennessee, and Georgia). These representative re-
gions receive fairly high levels of acidic deposi-
tion, have the best historical data and are best un-
derstood by scientists.
Potential benefits of additional sulfur and nitrogen
deposition reductions to the three sensitive aquatic
resource regions were projected using relation-
ships defined through the Nitrogen Bounding
Study (NBS) discussed in Chapter 2 and shown in
Appendix B. Specifically, the scenarios presented
101
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
in Exhibits 40 and 49 in Chapter 3 that provide for
the maximum reduction in acidic deposition levels
by the year 2010 represent approximately a 44
percent decrease in SO2 emissions from all sources
and a 24 percent decrease in projected NOX
emissions beyond those achieved by the 1990
CAAA. These deposition reductions were
produced by reducing both sulfur and nitrogen
emissions from utility and industrial combustion
sources by 50 percent. The NBS projections were
then used to relate the resulting sulfur and nitrogen
deposition levels to probable proportions of
surface waters in the two ANC groupings. Because
times to watershed nitrogen saturation are not
certain for these three or any other region,
projections using all four possible times for
watersheds to reach nitrogen saturation modeled
by NBS were prepared.
Exhibits 71-73 present actual and modeled pro-
portions of lakes and streams for two ANC group-
ings (ANC<0 ueq/l and ANC<50 ueq/l). Projected
proportions in both of these ANC categories are
shown for each of the three study regions under
each of four possible times to watershed nitrogen
saturation for the following six scenarios:
* Actual 1984 or 1985,
* NBS projections of surface waters in the
year 2040 if the 1990 CAAA had not been
implemented,
* NBS projections of surface waters in the
year 2040 with full implementation of the
1990 CAAA.
* NBS projections of surface waters in the
year 2040 with additional reductions in
utility and industrial emissions of sulfur be-
yond the CAAA (see Exhibit 40).
* NBS projections of surface waters in the
year 2040 with additional reductions in
utility and industrial emissions of nitrogen
beyond the CAAA (see Exhibit 48).
* NBS projections of surface waters in the
year 2040 with additional reductions in
utility and industrial emissions of both sul-
fur and nitrogen beyond the CAAA (see Ex-
hibits 40 and 49).
EXHIBIT 71. YEAR 2040 NBS PROJECTIONS FOR ADIRONDACK LAKES
50 Yr lOOYr 250 Yr Never 50 Yr 100Yr 250 Yr
Time to Watershed Nitrogen Saturation
• 1984 Base
Qw/oCAAA
• 1990 CAAA
H CAAA-additional S
I I CAAA-additional N
• CAAA-additional S+N
Never
102
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CHAPTER 6: INTEGRATION AND CONCLUSIONS
EXHIBIT 72. YEAR 2040 NBS PROJECTIONS FOR MID-APPALACHIAN STREAMS
50 Yr lOOYr 250 Yr Never 50 Yr lOOYr 250 Yr
Time to Watershed Nitrogen Saturation
Never
EXHIBIT 73. YEAR 2040 NBS PROJECTIONS FOR SOUTHERN BLUE RIDGE PROVINCE STREAMS
JU -
aj 25 -
u
as
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
The following points highlight the major relation-
ships shown in the projections for the year 2040.
In reviewing these projections, it is important to
remember that these projections are for changes in
proportions representing the NBS modeled sub-
populations of the "most sensitive surface waters"
within each of these regions; these proportions do
not apply to all the surface waters or some other
subset of surface waters in these regions.
* The 1990 CAAA markedly reduces the pro-
portions of surface waters in all three re-
gions projected to be acidic (i.e., ANC<0
ueq/l) by 2040, relative to conditions
projected without its implementation. This
is seen by comparing the second and third
vertical plotted bars shown for each region
and across each assessed time to watershed
nitrogen saturation.
* The 1990 CAAA markedly reduces the pro-
portions of stream reaches in the mid-Ap-
palachians projected to become increas-
ingly sensitive to potential effects from epi-
sodic acidification (i.e., ANC<50 ueq/l) by
2040. This benefit is projected to be lesser
in magnitude for the lakes in the Adirond-
acks and stream reaches in the Southern
Blue Ridge.
• Sensitivities of target aquatic resources and
their potential responses to changes in
acidic deposition clearly differ among the
modeled regions.
* The benefits to sensitive surface waters
from sulfur deposition reductions mandated
by the 1990 CAAA may be lessened due to
future increases in nitrogen leaching
caused by continuing nitrogen deposition
and saturation of watersheds with
deposited nitrogen. This is shown by the
projected increasing proportion of ANC<0
ueq/l and ANC<50 ueq/l surface waters at
shorter times to watershed nitrogen
saturation.
* Uncertainty related to actual times to wa-
tershed nitrogen saturation within these re-
gions causes projections of relationships
between deposition and ANC responses to
range by factors of about two or greater.
Longer times to nitrogen saturation lead to
fewer projected acidic and sensitive
aquatic systems.
» Significant uncertainty continues to accom-
pany the rate of watershed nitrogen satura-
tion, contributing to uncertainty in
projecting the impact of additional
reductions in sulfur and nitrogen deposi-
tion.
* Any reductions in nitrogen deposition
would not only reduce total acidic deposi-
tion, but would also increase the actual
times to watershed nitrogen saturation. This
is similar to sulfur deposition reductions
that are now likely extending times for
watershed sulfur saturation.
6.4 FEASIBILITY OF ESTABLISHING AND
IMPLEMENTING AN ACID DEPOSITION
STANDARD
A variety of factors could affect the implementa-
tion of an acid deposition standard. The effect of
these factors on implementation will depend on
the approach selected. To be successful, an im-
plementation approach must be clear and unambi-
guous, and provide certainty as to the responsibili-
ties of the regulated community, EPA, and states.
Two basic approaches were evaluated in this re-
port. First, a regional targeted approach would
involve establishing deposition standards for
specific geographic regions, and require geograph-
ically targeted reductions in emissions of SO2
and/or NOX. EPA would either set a standard or
standards using existing authority or would seek
additional congressional authority and timetables.
Source-specific limits would be determined using
source-receptor models, and limits would be
incorporated into State Implementation Plans (SIPs)
and enforced by states. Second, a national,
emissions-based approach which would involve
congressional direction to EPA to set an acid
deposition standard or standards and determine
emissions for SO2 and NOX to meet the standards
within a certain time frame. An emissions cap and
allowance allocations would have to be set for
NOX and, as appropriate, adjusted for SO2. The re-
gional approach would be similar to the SIP pro-
gram used to implement Title I of the Act regard-
ing attainment of National Ambient Air Quality
Standards (NAAQS) while the national approach is
similar to the current Acid Rain Program.
Two emissions reductions scenarios were devel-
oped to compare national and targeted reduc-
tions—a nationwide 50 percent reduction beyond
the CAAA of SO2 emissions from utilities and a
geographically targeted strategy which removed
95 percent of utility SO2 emissions from geo-
graphically constrained regions. Both approaches
104
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CHAPTER 6: INTEGRATION AND CONCLUSIONS
were analyzed for emissions reduction efficiency
and cost-effectiveness. The following points high-
light the findings and conclusions from these
analyses.
* Geographic targeting can be used to
achieve target loads in each receptor re-
gion and in all three regions simultane-
ously. Delineation of targeted emissions
reduction areas is made complex, how-
ever, by potentially large target areas, sig-
nificant source-specific emissions reduc-
tions requirements (i.e., 95 percent SO2
reductions), and impairment to the cost
savings of the current Acid Rain Program
due to regional restrictions on allowance
trading.
* The total emissions reductions required to
achieve the deposition loads in all three
receptor regions simultaneously did not dif-
fer significantly between the nationwide
and targeted approaches. Costs of control
and cost-effectiveness for the nationwide
emissions reduction scenario and the geo-
graphically targeted scenario were ap-
proximately equal. (To assess the ap-
proaches, each scenario was designed to
achieve the same environmental goal.)
There is no environmental or economic
gain by geographically targeting source re-
gions.
• Title IV will produce the largest emissions
reductions in the highest source regions
(i.e., Ohio, Indiana, West Virginia, and
western Pennsylvania) making control
efforts beyond the CAAA more effective if
focused on broad geographic regions, or
nationally.
* The cost of additional emissions reductions
addressed in this report are, at a minimum,
double the cost of the current Acid Rain
Program. The benefits, although not
quantified here, would be in multiple ef-
fects areas such as human health, visibility,
and materials, as well as aquatic systems.
Based on scientific understanding of the effects of
sulfur and nitrogen, it would be feasible to set
sulfur and nitrogen deposition standards to protect
aquatic resources. However, uncertainty regarding
the impact of nitrogen remains high, making it
difficult to determine the appropriate level of a
standard or standards at this time.
105
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APPENDIX A
SUMMARY OF SELECTED NAPAP REPORTS
-------
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APPENDIX A
SUMMARY OF SELECTED NAPAP REPORTS
INTRODUCTION
This appendix presents a synopsis of major conclu-
sions from 10 of the NAPAP State of Science and
Technology Reviews. In a few of these summa-
ries, where knowing how the study was designed
and conducted can help the reader better appreci-
ate the study's key results, background information
on the study's methods is presented. Most infor-
mation summarized in this appendix came from
NAPAP's Summary Report of the U.S. National
Acid Precipitation Assessment Program.1 Major
conclusions from these NAPAP studies that influ-
ence identifying key relationships between acidic
deposition and receptor effects are presented in
the order that they were presented by the individ-
ual SOS/T report summaries. Also, not all conclu-
sions from those studies are presented in the fol-
lowing sections. In some case, similar key conclu-
sions appear in more than one report, conse-
quently, they are also repeated in more than one
of the case summaries below.
SOS/T REPORT 9: CURRENT STATUS OF
SURFACE WATER ACID-BASE CHEMISTRY
Phase I of the National Surface Water Survey
(NSWS) consisted of three major surveys. The
1984 Eastern Lake Survey (ELS) sampled 1,592
lake of 4 ha and larger that represented an esti-
mated population of 17,953 lakes in the Northeast,
Upper Midwest, the Southern Blue Ridge region,
and Florida. The 1985 Western Lake Survey (WLS)
sampled 719 lakes of 1 ha and larger that repre-
sented an estimated population of 10,393 lake
throughout the Sierra Nevada, Cascade, and Rocky
Mountain ranges. Finally, the National Stream
Survey (NSS) sampled 500 stream reaches in 1986
that represented an estimated population of 56,000
stream reaches (200,000 km) throughout much of
the Mid-Atlantic Coastal Plain, mid-Appalachian,
Poconos/Catskills, Interior Southeast, and Florida
Regions. A Phase II lake survey (ELS-II) was con-
Irving, P.M. 1991. Acidic Deposition: State of
Science and Technology. Summary Report of the
National Acid Precipitation Assessment Program.
Office of the Director, National Acid Precipitation
Assessment Program, Washington, DC.
ducted in 1986 to evaluate seasonal chemical
variability in Northeast Lakes. Thus, seven major
subregions were sampled during the NSWS:
* Northeast Subregion
* Mid-Appalachian Subregion
* Mid-Atlantic Coastal Plain Subregion
* Interior Southeast Subregion
* Florida Subregion
* Upper Midwest Subregion (northern Michi-
gan, Wisconsin, and Minnesota)
* West Subregion
Several key findings from the NSWS include:
1. An estimated 4.2% (1,181) of the National
Surface Water Survey (NSWS) lakes were
acidic, defined as having ANC<0 /veq//;
nearly all were in the East. These lakes
had pH levels in the range 5.0 to 5.5.
2. Acid lakes tend to be smaller than non-
acidic lakes. In the East acidic lakes aver-
age 12 ha versus 17 ha for non-acidic
lakes.
3. Nearly 20% of the NSWS lakes had an
ANC of 50 ueq/l or less.
4. About 3.0% of the NSWS lakes had total
inorganic monomeric aluminum (Alim)
concentrations of less than 50 ug/l.
5. No sampled lakes were acidic in the Inte-
rior Southeast or Minnesota, and only one,
a geothermal spring, was acidic in the
West. These results lead to the conclusion
that there are virtually no acidic lakes in
these subregions, within the subpopulation
of surface waters sampled.
6. Based on total stream length with the NSS
target population, 2.7% (5,506 km) were
acidic (ANC<0 ueq/l) and 12% (23,595
km) had ANC<50 ueq/l, excluding reaches
affected by acid mine drainage.
A-1
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APPENDIX A: SUMMARY OF NAPAP REPORTS
7. For 8% (16,780 km) of the NSS stream
length, the pH was 5.5 of less and for 18%
(35,771 km) the pH was 6.0 or less.
8. A greater percentage of streams reaches
were acidic in their upstream reaches
(6.1%) compared to their downstream
reaches (2.3%)
9. Most of the acidic stream reaches in the
NSWS target population occurred in the
mid-Appalachian and Mid-Atlantic
Coastal Plain regions, with smaller length
totals in the Poconos/Catskill subregion
and Florida.
10. Acidic and low pH streams in the mid-
Appalachian region and Poconos/Catskill
subregions were restricted to watersheds
smaller than 30 km^ and were generally
found at elevations higher than 300 m.
These were clearwater systems with a me-
dian DOC of 1.1 mg/l and Alim of 202
MB/"-
Based on results from this survey six "high-interest
regions" were identified that contained most of the
acidic lakes and streams identified in the NSWS.
While the combined high-interest lakes popula-
tions included only 26% of the all NSWS lakes,
they included 95% of all inorganic acidic lakes.
Similarly, 37% of all NSS upstream reaches were
in the high-interest population, but 84% of all in-
organic acidic NSS upstream reach ends were in-
cluded. The six areas targeted were
* SOUTHWEST ADIRONDACK MOUNTAINS:
Within this subpopulation, 38% of the
lakes were acidic (ANC<0 ueq/l) and 51%
had closed-system pH of 5.5 or less. These
acidic lakes are typically rapidly flushed
drainage lakes in which SO42~ is the domi-
nant anion and DOC concentrations
ranged 3 to 4 mg/l. Many had high
concentrations of inorganic monomeric Al,
with 36% having Alim greater than 100
MB/'-
* NEW ENGLAND: Within this subpopulation
4.7% of the NSWS lakes had ANC<0 ueq/l
and 6.8% had pH<5.5. In the Seaboard
Lowlands area 7.7% of the NSWS lakes
were acidic (ANC^O ueq/l). The majority
of the acidic lakes in this region were
dominated by inorganic acids, but about
one-fifth, mostly in the Seaboard Lowlands,
were dominated by organic acids.
FORESTED MID-ATLANTIC HIGHLANDS (A.K.A.,
MID-APPALACHIAN REGION): Within the
stream populations, 11.5% were acidic and
16.7% had closed-system pH<5.5. Among
lakes of this subpopulation, 10% had ANC
<0 ueq/l and 9% had pH<5.5. Acidic
surface waters of this subpopulation typi-
cally had low DOC (stream mean of 1.5
mg/l, lake mean of 2.6 mg/l) and high
SO42- concentrations (stream mean of 148
ug/l, lake mean of 122 ug/l). All acidic
lakes and streams were dominated by inor-
ganic acids, with SO42~ being the dominant
anion with relatively low NO3' concentra-
tions (mean < 10 ueq/l). Acid streams had
the highest concentrations of inorganic
monomeric Al of all high interest areas,
with a mean Aljm of 202 ug/l. Acid lakes
had a mean Alim of 77 ug/l.
ATLANTIC COASTAL PLAIN: At their upstream
ends, 14% of the NSS stream reaches were
acidic (ANC<0 ueq/l) and 17% had pH of
5.5 or less. Both mineral and organic acids
provided important contributions in the
acid-base status in these streams. Among
the acidic streams, 65% were dominated
by organic acids, while 32% were
dominated by inorganic acids. Acidic
streams in the New Jersey Pine Barrens
were dominated by inorganic acids, but
with considerable influence by organic
acids, and were similar to organic acid
dominated streams. In the rest of the
region, NSS streams were dominated by
organic acids. Twenty acidic lakes
identified on Cape Code were dominated
by chloride and had salt water SO42" con-
centrations of 110 to 175 ueq/l.
NORTHERN FLORIDA HIGHLANDS: This subre-
gion includes the Central Lake District
(Trail Ridge) north of 29°N latitude and the
Panhandle. For this subregion, 63% of the
lakes were acidic (ANC<0 ueq/l) and
52.6% had closed system pH of 5.5 or less.
Most (90%) of the acidic lakes were
seepage lakes. Of the acidic lakes, 80%
were dominated by inorganic acids, with
median SO42~ concentrations of 83 ueq/l,
median DOC of 2.4 mg/l, and mean Alim
concentrations of 39 ug/l. Acid streams
had low ionic concentrations (mean base
cation of 21 ug/l, SO4" of 16 ueq/l). Three-
quarters of the acid streams were
A-2
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APPENDIX A: SUMMARY OF N APAP REPORTS
dominated by inorganic acids, with DOC
of less that 2 mg/l.
« LOW-SILICA LAKES IN THE EASTERN UPPER
MIDWEST: This subregion includes lakes in
northeastern Wisconsin and the Upper
Peninsula of Michigan having silica
concentrations of less than 1 mg/l. Of
these lakes, 15.9% were acidic (ANC<0
ueq/l) and 19.3% had closed-system pH of
5.5 or less. Of these acidic lakes, 80%
were seepage lakes, with low silica
concentrations reflecting the lack of well-
buffered ground water inputs. Inorganics
dominated the acidity in 87% of these
lakes. SO42- was generally the dominant
anion, with a mean concentration of 69
ug/l. DOC concentrations were relatively
high (mean of 3.9 mg/l), indicating
substantial influences by organic acids. Al
concentrations were low.
SOS/T REPORT 10: WATERSHED AND LAKE
PROCESSES AFFECTING SURFACE WATER ACID-
BASE CHEMISTRY
1. Atmospheric deposition is often an impor-
tant, yet highly uncertain, component of
the acid-base budget in many watersheds
and in lakes and streams having low ANC.
Greatest uncertainty is in regions holding
high elevation or rough terrain and for the
processes of particulate and gaseous depo-
sition. The uncertainty in nitrogen deposi-
tion is less important to acid-base budgets
compared with internal fluxes. The uncer-
tainty in base cation deposition may cause
large uncertainty in acid-base budgets in
some watersheds.
2. Watersheds having a greater proportion of
their water flowing through shallow, more
acidic soils tend to have more acidic and
lower ANC surface waters than water-
sheds in which a large proportion of the
water flows through deeper, more
weatherable materials. This conclusion is
generally valid for baseflow chemistry and
for episodic changes in acid-base chemis-
try for surface waters. The major path-
ways for movement of water through wa-
tersheds are subsurface flows, even during
most extreme flow events. Generally,
overland flow is rarely observed.
3. Even without acidic deposition, the natu-
ral sequence in watershed development is
one of soil acidification associated with
base cation accumulation in biomass (i.e.,
in vegetation and humus), an increase in
soil cation exchange capacity, and in-
creased leaching of soluble or weather-
able materials from upper soil horizons.
4. Naturally acidic lakes and streams domi-
nated by organic anions occur predomi-
nantly in areas where bedrock or uncon-
solidated sediments are highly resistant to
weathering. Most are in regions with
large buildups of organic matter (e.g., ar-
eas with spodic soils and in wetlands),
often in more northern and coastal plain
regions having low-relief terrain and rela-
tively poor drainage.
5. Adsorption-desorption properties of soils
in the watershed regulate export of at-
mospherically deposited SO42" into surface
waters. Vegetation has only a limited
capacity to immobilize SO42~, even under
optimal conditions. Great uncertainty still
exists regarding the reversibility of SO42~
adsorption and critical factors controlling
desorption. But, at least one documented
study that examined adsorption-desorption
across a variety of soils showed that most
soils exhibited some degree of irreversible
adsorption.
6. The extent to which atmospheric nitrogen
deposition affects the export of NO3- from
watersheds depends on biological rather
than geochemical process in the water-
shed. Most forest soils (including those
that are not currently affected by nitrogen
deposition leading to increased nitrogen
accumulation and possible saturation)
have the potential to produce and leach
NO3- to surface waters.
7. Sulfate deposition often causes an anion
shift from drainage water compositions
dominated by HCO3~ and organic anions
to one dominated by SO42~.
8. The relative mix of cations in soil solution
depends on the mix of cations on soil ex-
change sites (i.e., soil base saturation) and
cation selectivities of the exchange sites.
In extremely acid soils (i.e., less than 10%
base saturation), Al and H+ dominate the
cations in solution, resulting in reduced
ANC concentrations in drainage waters
from the watershed; in less acid soils (i.e.,
A-3
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APPENDIX A: SUMMARY OF N APAP REPORTS
greater than 20% base saturation), base
cations dominate and ANC remains unal-
tered. Data on soil chemistry and miner-
alogy, bedrock mineralogy, and water
flow paths are all necessary to predict the
chemistry of waters draining terrestrial sys-
tems.
9. Water quality changes that occur when
soil water rich in CO2 flows into surface
waters depend on ANC in both waters. If
SO42" and NO3" from deposition produce
negative ANC in the soil solution, the pH
effects observed within the soil solution
may be minimal but the consequential
change produced by discharge of this low
pH and high Al soil water to surface water
may be substantial.
10. Many seepage lakes (lakes having no
stream input) that receive even 5% of their
total water input from groundwater are
seldom likely to be affected by acidic
deposition to their surfaces. That is, ANC
supplied by a small proportional volume
of ground water is often sufficient to neu-
tralize incident acidic precipitation. The
chemistry of seepage lakes that derive
greater than 95% of their input from pre-
cipitation are greatly influenced by incom-
ing precipitation.
11. Numerous processes modify the acid-base
chemistry of water entering lakes and
streams, including base cation production
and SO42- and NO3" retention. These
process occur in all systems, but rates for
these processes vary among systems. For
example, those systems having long resi-
dence times and relatively deep mean soil
depths tend to have the greatest influence
on acid-base chemistry in their drainage
waters. Further, in some lakes, more ANC
is derived from in-lake production than
that generated within the terrestrial water-
shed. The most important in-lake proc-
esses generating ANC are likely to occur
in the sediment rather than the water-col-
umn of acid-sensitive lakes.
SOS/T REPORT 11: HISTORICAL CHANGES IN
SURFACE WATER ACID-BASE CHEMISTRY IN
RESPONSE To ACIDIC DEPOSITION
1. The role of acidic deposition as a cause of
acidic surface waters is supported by nu-
merous lines of evidence, including the
current chemical composition of acidic
surface waters, extensive paleolimnologi-
cal analyses of bottom sediment deposits,
the worldwide distribution of acidic sur-
face waters, experimental studies, moni-
toring and re-survey data, and principles
of geochemistry.
2. Acidification of low pH and low ANC (<
50 ueq/l) Adirondack lakes has been less
than previously believed because of con-
siderable watershed and in-lake neutrali-
zation of acidic inputs via enhanced base
cation mobilization.
3. Outside the Adirondack Mountains, the
chemistry of drainage lakes and streams in
several areas is consistent with the hy-
pothesis of acidification of sensitive sys-
tems by acidic deposition. This is most
notable for the Poconos/Catskill subre-
gion, the Mid-Atlantic Highlands (i.e.,
mid-Appalachians), the eastern portion of
the Upper Midwest, and the New Jersey
Pine Barrens.
4. The chronic acid-base character of lakes
in Maine has been generally unaffected by
acidic deposition. Chemistry of acidic
streams in the Mid-Atlantic Coastal Plain,
outside the New Jersey Pine Barrens, in
most cases, suggest acidification due to
organic acidity and not acid deposition ef-
fects. Chronic acidification of western
lakes from acidic deposition has not ap-
parently occurred.
5. The acid-base character of acidic streams
in the Florida Panhandle can be ascribed
to a combination of organic acidity, ma-
rine cation retention, and near zero
weathering inputs to some systems. For
some waters, acidic deposition may also
have provided minor contributions.
6. In the Upper Midwest, the chemistry of
sensitive (ANC<50 ueq/l, SiO2<1 mg/l)
seepage lakes exhibit both increasing sen-
sitivity (lower base cation concentrations)
and increasing effects from acidic deposi-
tion across a longitudinal gradient from
west to east. In the Upper Peninsula of
Michigan, 15% of the lakes are of this
type and many of these are currently
acidic because of high SO42- relative to
base cation concentrations. These lakes
have probably been acidified by acidic
A-4
-------
APPENDIX A: SUMMARY OF NAPAP REPORTS
deposition. Throughout most of the Upper
Midwest, however, substantial regional
acidification of lake water from acidic
deposition has not occurred.
7. Most lakes and streams in the United
States, especially those that have current
ANC greater than about 50 ueq/l, have
probably not had declines in pH or ANC
within their recent histories.
SOS/T REPORT 12: EPISODIC ACIDIFICATION
OF SURFACE WATER ACID-BASE CHEMISTRY
1. Episodic acidification is the process by
which lakes and streams develop short-
term decreases in ANC, usually during
hydrological events and over time scales
of hours to weeks. Episodes are stochastic
or probabilistic in nature, in terms of oc-
currence, frequency, intensity, duration,
and, to some extent, composition.
2. Episodic acidification is practically a
ubiquitous process in streams and drain-
age lakes. Presently, data are not avail-
able that allow rigorous population esti-
mates of episodic acidification in the
United States to be completed. Most
states plus southeastern Canada, however,
where monitoring data have been col-
lected, display characteristics of episodic
acidification.
3. Episodic acidification is controlled by a
combination of natural and anthropogenic
factors. The relative importance of these
factors varies among regions and among
watersheds within regions. There are
three primary natural process that can
produce episodes: (1) dilution, (2) nitrifi-
cation, and (3) organic acid production.
4. Episodic acidification is not symptomatic
of human caused chronic acidification.
5. The severity (minimum ANC or highest
dissolved aluminum) of episodes is in-
creased by acidic deposition in some ar-
eas.
6. While improvements in water chemistry
during episodes in some lakes and streams
would be expected, especially in the
Northeast and Mid-Atlantic, this issue has
not been addressed by scientific investiga-
tions. The roles of nitrogen and sulfur
deposition and of organic acids in causing
episodic acidification all need to be exam-
ined.
7. Modeling episodic acidification in surface
waters has been only moderately success-
ful, primarily because of a lack of data
and a lack of understanding of important
hydrological flow and biogeochemical
process.
SOS/T REPORT 13: BIOLOGICAL EFFECTS OF
CHANCES IN SURFACE WATER ACID-BASE
CHEMISTRY
1. The most important chemical properties of
surface waters influencing biological re-
sponses to acid-base chemistry are pH,
inorganic monomeric aluminum, and cal-
cium. Decreases in pH (particularly be-
low 6.0-6.5) and increases in the concen-
tration of inorganic monomeric aluminum
(above 30-50 ug/l for the most sensitive
organisms) can increasingly cause adverse
biological effects. Small changes in cal-
cium are particularly important at low
calcium concentrations (< 100-150 ueq/l).
2. A number of the species that commonly
occur in surface waters susceptible to
acidic deposition cannot survive, repro-
duce, or compete in acidic waters. Thus,
with increasing acidity, these "acid-sensi-
tive" species are eliminated and species
richness (the number of species living in a
given lake or stream) declines. These
changes in aquatic community structure
are found to begin in many surface water
systems as chronic pH levels drop below
the range of about 6.0 to 6.5. Acid-sensi-
tive species occur in all major groups of
aquatic organisms. Both chronic and epi-
sodic acidification can affect aquatic or-
ganisms, with chronic acidification per-
haps the primary cause of continuing ef-
fects in acidified lakes and episodic acidi-
fication being particularly important case
of effects in streams.
3. System level processes, such as decompo-
sition, nutrient cycling, and productivity,
are fairly robust and affected only at rela-
tively high levels of acidity (e.g., chronic
pH<5.0-5.5).
4. Relatively few studies have been con-
ducted on the recovery of biological
communities following reduction of acid
A-5
-------
APPENDIX A: SUMMARY OF NAPAP REPORTS
inputs. Based on our current understand-
ing of the processes of biological re-
sponse, decreases in indicateity would be
predicted to likely allow acid sensitive
species and species richness both to
increase in acid affected surface waters.
5. Laboratory toxicity experiments and field
surveys provide and adequate basis for
quantifying the relationship, on a regional
scale, between changes in pH, aluminum,
and calcium and acidity-induced stress on
fish populations. Thus, toxicity-based
models, field based models, and models
that combine laboratory and field data can
be used to evaluate the biological signifi-
cance of projected changes in acid-base
chemistry given alternative deposition and
emissions scenarios.
6. The loss of fish populations and/or ab-
sence of fish species as a result of acid-
base chemistry changes has been docu-
mented for some lakes and streams in sev-
eral regions of the United States. Applica-
tion of fish response models suggest that
the percentage of NSWS waters with acid-
base chemistry unsuitable for the survival
of acid-sensitive fish species range from
less than 5% in areas such as the Upper
Midwest to near 60% for upper stream
reaches in the Mid-Atlantic Coastal Plain.
An estimated 23% of the Adirondack lakes
and 18% of the mid-Appalachian streams
classified as potential brook trout habitat
currently have acid-base chemistry unsuit-
able for brook trout survival.
SOS/T REPORT 14: METHODS FOR PROJECTING
FUTURE CHANCES IN SURFACE WATER ACID-
BASE CHEMISTRY
1. The Direct/Delayed Response Project
(DDRP) approach utilized the best avail-
able procedures for projecting the effects
of sulfur deposition on future changes in
surface water acid-base chemistry for tar-
get populations of lakes in the Northeast,
Upper Peninsula of Michigan, and Florida
and streams in the mid-Appalachians and
Southern Blue Ridge Province.
2. Several models provide credible projec-
tions of selected subpopulations of target
lakes.
3. Measurement, parameter, input, and
population extrapolation error can be
quantitatively estimated for model projec-
tions, but aggregation and model assump-
tion error can be estimated only qualita-
tively. Results from individual watershed
projects can be extrapolated through the
probability sampling frame for regional es-
timates of population attributes.
4. Although there are remaining uncertainties
with respect to structural error, aggre-
gation, and long-term projection confirma-
tion, model projections are the only
feasible approach for comparing the
effects of different illustrative emissions
control scenarios on future changes in sur-
face water acid-base chemistry.
SOS/T REPORT 15: LIMING ACIDIC SURFACE
WATERS
1. Liming can effectively mitigate many of
the adverse ecological effects of surface
water acidification independent of reduc-
tion of acidifying emissions.
2. Conventional whole-lake liming is a more
established mitigation alternative than lim-
ing running waters and watersheds.
3. Liming surface waters commonly results in
significant positive and predictable
physiochemical changes in aquatic eco-
systems.
4. Liming generally increases nutrient cy-
cling, decomposition, and primary pro-
ductivity and results in positive responses
in fish and other aquatic biota.
SOS/T REPORT 16: CHANGES IN FOREST
HEALTH AND PRODUCTIVITY IN THE UNITED
STATES AN DC AN AD A
1. The vast majority of forests in the United
States are not affected by decline.
2. There is experimental evidence that acid
deposition and associated pollutants can
alter the resistance of red spruce to winter
injury; through this mechanism, acidic
deposition may have contributed to red
spruce decline at high elevations in the
northern Appalachians. Evidence of red
spruce decline and pollutant involvement
in the southern Appalachians is less sub-
stantial.
A-6
-------
APPENDIX A: SUMMARY OF NAPAP REPORTS
3. Most sugar maple trees and stands in the
United States and Canada are not affected
by decline. Sugar maple declines are sig-
nificant problems, however, in Quebec
and in some parts of Ontario, Vermont,
and Massachusetts. Natural stresses in-
cluding nutrient deficiencies and defolia-
tion by insects are important factors in
these declines. Involvement of acidic
deposition and/or ozone as significant
contribution or predisposing factors have
not been demonstrated, but such involve-
ment cannot be ruled out on the basis of
available information.
4. A regional decline of southern pines has
not been demonstrated. The reported
growth rate reductions in certain classes of
trees and stands are not fully understood,
however, and require further investigation.
The occurrence of reduction in tree
growth rates in natural pine stands is an
expected consequence of historical land
use patterns, increases in stand age and
competition, and other non-pollutant fac-
tors. Available information is not ade-
quate to determine whether the magnitude
of reported growth reduction is greater or
less than would be expected in the ab-
sence of acid deposition and associated
pollutants. Results of exposure-response
experiments indicate that ozone at ambi-
ent concentrations can alter growth and
physiology of southern pines seedlings and
justify concern about adverse effects of
ozone on the health and productivity of
southern pine forests.
5. Compared to ozone and many non-pollut-
ant stress factors, acidic deposition ap-
pears to be a relatively minor factor affect-
ing the current health and productivity of
most forest in the United States and Can-
ada. Most of these forests are receiving
acidic depositions at doses that have not
had a serious impact on health and pro-
ductivity. The possibility of long-term
(several decades) adverse effects on some
soils appears realistic. Sulfate deposition
increases leaching losses of nutrient cat-
ions from many different forest soils and
over the long term may reduce the fertility
of soils with low buffering capacities or
low mineral weathering rates.
SOS/T REPORT 17: DEVELOPMENT AND USE OF
TREE AND FOREST RESPONSE MODELS
1. The models presented in this report are
preliminary, and they emphasize our lack
of knowledge about fundamental tree and
forest processes. Nonetheless, considera-
tion of the dynamics of implied by what
we do know of the processes indicate that
considerable caution is needed in project-
ing long-term effects from acidic deposi-
tion and ozone. In particular, long-term
dynamics generated by synergies, and
compensations between mechanisms, can
produce threshold effects. The possible ex-
istence of these threshold effects implies
that simple projections will not be ade-
quate to capture long-term effects of acid
deposition. Therefore, the null hypotheses
of no long-term effect should not be ac-
cepted without caution, even if it appears
warranted by the current data and theory.
SOS/T REPORT 18: RESPONSE OF VEGETATION
To ATMOSPHERIC DEPOSITION AND AIR
POLLUTION
1. Based on crop-effects research conducted
by NAPAP and other research programs,
acidic precipitation at ambient levels in
the United States has not been shown to
be responsible for regional crop yield re-
duction.
2. Ambient fog acidity concentrations are not
great enough to reduce the yield of agri-
cultural crops, but under certain localized
conditions may occasionally be high
enough to cause visible injury to plant tis-
sue and thereby reduce the marketability
of sensitive crops.
3. Ambient SO2 concentrations by them-
selves are not responsible for regional-
scale crop yield reductions in the United
States.
4. Nitrogen dioxide at ambient concentra-
tions is not a direct source of regional-
scale growth or yield reduction in U.S. ag-
ricultural crops.
5. Although pollutant mixtures (e.g., SO2 +
O3, or SO2 + NO2) are of undetermined
important on a national scale, at least in
some regions (e.g., Ohio River Valley),
ambient air quality monitoring suggests
A-7
-------
APPENDIX A: SUMMARY OF N APAP REPORTS
the potential for effects from mixed expo-
sures.
6. There is evidence that acidic cloud water
in combination with other stresses affects
some high elevation spruce forests in the
eastern United States.
7. Long-term changes in the chemistry of
some sensitive soils is expected from
acidic deposition, but it is uncertain
whether this will result in reduced forest
health, how effects will be manifest, how
much of the forest resources will be im-
pacted, or how long it will take for such
effects to occur.
8. There is no conclusive evidence that
acidic precipitation is a major causal fac-
9.
tor in sugar maple decline, but in limited
areas where nutrient deficiency symptoms
are currently evident, acidic deposition
could further exacerbate their expression.
Ambient SO2 concentrations are not re-
sponsible for regional-scale growth reduc-
tions in the United States.
10. Nitrogen dioxide at ambient concentra-
tions is not a direct source of regional-
scale growth reduction in forests of the
United States.
11. The same concern about possible effects
from pollutant mixtures discussed for
crops (Conclusion 5, above) applies to
forests.
A-8
-------
APPENDIX B
SELECTED PLOTS FROM EPA's
NITROGEN BOUNDING STUDY
-------
-------
APPENDIX B
SELECTED PLOTS FROM EPA'S NITROGEN BOUNDING STUDY
INTRODUCTION
The Nitrogen Bounding Study (NBS)1 is one of
several recent and ongoing studies conducted by
the U.S. Environmental Protection Agency (EPA) to
investigate aquatic and terrestrial effects of acidic
deposition. This study was initiated to address a
major issue that arose during EPA's completed Di-
rect/Delayed Response Project (DDRP), namely to
investigate the role of nitrogen compounds in the
soil water and surface water acidification within
forested watersheds. Models and analyses used
during the DDRP focused on sulfur deposition and
its effects on water chemistry, but effects due to ni-
trogen cycling received much less attention. That
difference in focus was due to the general lack of
quantitative knowledge regarding nitrogen trans-
formation processes in soils. In addition, evidence
available when the DDRP was designed and initi-
ated primarily indicated that most deposited at-
mospheric nitrogen is taken up and held by biota,
thus making little contribution to acidification.
More recent evidence suggests, however, that
some forest catchments in the eastern United
States, for example, can become nitrogen saturated
and that nitrogen leaching from these systems can
contribute substantially to lake and stream acidifi-
cation, particular during runoff episodes. While
several long-term studies intended to address the
role of nitrogen deposition in surface water acidi-
fication are under way, the NBS was intended to
provide near-term information on which to
"bound" likely relationships for nitrogen and sulfur
deposition on surface-water acidification re-
sponses.
The NBS evaluated target populations of surface
waters in three sensitive geographic regions: lakes
in the Adirondack Region and stream reaches in
the Mid Appalachian Region and the Southern
Blue Ridge Province. Model projections com-
pleted during the NBS used a modification of the
Model of Acidification of Groundwater in Catch-
ments (MAGIC), the model of current choice for
1 Van Sickle, J., M.R. Church. 1995. Methods for
Estimating the Relative Effects of Sulfur and Nitrogen
Deposition on Surface Water Chemistry. U.S.
Environmental Research Laboratory, Corvallis, OR.
assessing many watershed processes associated
with acidic deposition. It includes a minimum
number of critical chemical and hydrological
processes occurring in watersheds to simulate soil
solution and surface water chemistry, and to pro-
ject average monthly or annual concentrations of
acid-base chemistry in surface water. Primary
input data for its use in NBS came from the Na-
tional Surface Water Survey (NSWS), the DDRP,
and updated deposition information from EPA at-
mospheric modeling studies discussed in
Chapter 3 of this report.
Nitrogen uptake parameters in the model were
used to provide simple surrogates for complex
processes within the nitrogen cycle. That is, these
parameters were adjusted to yield "best case" (i.e.,
maximum nitrogen retention in the biota within a
watershed) and "worst case" (i.e., complete nitro-
gen saturation in the biota within a watershed) ap-
proximations to estimate the resulting combined
effects by nitrogen and sulfur deposition on lake
and stream acidity. Thus, model results provide
upper and lower bounds on the levels of acidifica-
tion that more realistic models (currently under
development) would likely project.
The NBS projected surface water chemistry for two
target years (years 2015 and 2040) using the as-
sumption that emissions reductions of 10 million
tons SO2 and 2 million tons NOX mandated by the
1990 Clean Air Act Amendments of (CAAA) were
fully implemented. As such, deposition rates for
sulfur and nitrogen were assumed to equal those
projected by atmospheric models to accompany
emissions reductions with full CAAA
implementation at the year 2010 and that these
rates would be attained by the year 2020.
After the year 2010, the NBS defined different
deposition scenario projections for years between
the years 2010 and 2020 using SO42' and NO3-
deposition rates that decline linearly from the
common year 2010 rates to a selection of different
modeled year 2020 deposition rates for each sce-
nario modeled. For example, some modeled sce-
narios maintained the 2010 deposition rates
through the year 2020, while some alternative
B-1
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
modeled scenarios decreased the year 2010 rate to
background deposition rates only by the year
2020. Rates for still other modeled scenarios
decreased to levels between these extremes.
(Background deposition rates are those materials
that originate from only natural, agricultural fertil-
izer, and domestic livestock sources.) Each mod-
eled deposition rate was then assumed to remain
constant at the specific modeled 2020 rate until
the year 2040, the end of the model projection pe-
riod. The selected plots from the NBS contained
in this appendix include model projections for the
years 2015 and 2040. The 2040 plots were
primarily depended upon for the review and
conclusions from the NBS presented in Chapter 2
of this report.
In examining the NBS plots it is first important to
recognize that the plotted response surfaces repre-
sent only projected proportions for the subpopula-
tions of sensitive surface waters modeled by the
NBS; they do not represent responses for either all
surface waters or for all NSWS sampled surface
waters in the modeled regions. Each page holds
four NBS plots displaying projected response sur-
faces over ranges of possible sulfur and nitrogen
deposition rates at the year 2040. Each of the four
plots represent one of the four possible durations
used to bound likely alterative times to watershed
nitrogen saturation: 50 years, 100 years, 250
years, and never (i.e., assumes nitrogen uptake
remains constant into the future at recently esti-
mated rates).
Sets of pages are grouped within the three mod-
eled regions, (1) Adirondacks Region, (2) mid-Ap-
palachian Region, and (3) Southern Blue Ridge
Province. Within the 10 pages for each region,
one each is presented for projections of relative
proportions of lakes with ANC<0 ueq/l, ANGS50
ueq/l, pH<5.0, pH<5.5, and pH<6.0 over the
ranges of sulfur and nitrogen depositions assessed
for each region and for each of the four bounding
times to watershed nitrogen saturation, for the
years 2015 and 2040.
INTERPRETING THE NBS PLOTS
To illustrate interpretation of the NBS plots, the
first page of year 2040 plots shows projected
percentages of NBS target population lakes in the
Adirondack Mountains having ANC of 0 ueq/l or
less, under assumptions of four bounding times to
watershed nitrogen saturation equal 50, 100, and
250 years, and never (i.e., constant). Response
contours for each plot show how percentages of
target waters are projected to relate to possible
differences in total sulfur and nitrogen deposition
loading rates throughout the modeled ranges for
these depositions. For these plots, deposition
ranges for the year 2040 begin at projected
background deposition rates for sulfur and nitrogen
(1 kg S/ha/yr and 4 kg N/ha/yr) and extend to their
maximum modeled concentrations (7.5 kg S/ha/yr
and 11.3 kg N/ha/yr), which are the rates pro-
jected to accompany full implementation of the
1990 CAM.
Thus, the plots on page B-10 shows NBS
projections for proportions of Adirondack lakes
having ANC<0 ueq/l. The upper right plot shows
projections for an assumed 100 years to watershed
nitrogen saturation. Here, in the upper right
corner of this upper right plot, model projections
estimate that approximately 26 percent of the
target Adirondack lakes may be acidic (ANC<0
ueq/l) in the year 2040 for modeled sulfur and
nitrogen deposition rates projected to accompany
implementation of the 1990 CAAA. With only
"background" deposition of sulfur and nitrogen, as
shown in the lower left corner of the upper right
plot, 3.4 percent of these target lakes are projected
to be acidic in 2040.
In reviewing these plots, it is helpful to recognize
that several relationships generally apply to all of
them:
* The slopes of contour lines in each of the
NBS response plots reflect the relative im-
portance of sulfur and nitrogen in causing
the projected response relationships.
Nearly vertically plotted response contours
indicate that the projected ANC response is
attributable primarily to sulfur deposition.
Nearly horizontal plotted response
contours indicate the plotted ANC response
is attributable primarily to nitrogen
deposition. A forty-five degree diagonal
contour indicates equal contributions by
both sulfur and nitrogen depositions.
* Changes in the spacing between individual
response contours within each plot appears
to be attributable to patterns in sample
weighting during model projections, rather
than due to some intrinsic character of the
deposition-response relationships.
* The density of response contours across the
modeled deposition ranges for each plot di-
rectly relates to the potential average re-
sponsiveness by target waterbodies to po-
B-2
-------
APPENDIX 8: SELECTED NBS PLOTS
tential changes in sulfur and nitrogen depo-
sition rates on the specified water quality
classification variable modeled (e.g., ANC<
0 ueq/l). Therefore, plots with a high
density of contour lines depict a high level
of responsiveness to future possible
changes in deposition rates.
* In general, modeled ANC responses to re-
ductions in sulfur deposition found during
the NBS appear to be linear and propor-
tionally equivalent across the ranges of
modeled sulfur reductions. Additional in-
vestigation may help to determine whether
this relationship is due to actual environ-
mental functions or to some artifact
inherent in the model's application.
In considering possible individual extrapolation of
the results presented with these plots, beyond that
presented in the preceding chapters, care must be
taken to ensure that these results are not over ex-
trapolated. That is, in applying the NBS model re-
sults, as is the case when applying any simulation
modeling results, it is important that the assump-
tions underlying the modeling be understood and
carefully considered relative to additional condi-
tions or systems to which they are to be applied.
In doing this, the modeled processes should be de-
scribed and evaluated to determine how well they
correspond to the system (e.g., watershed) for
which the application is intended. In general,
models should only be applied to (i.e., constrained
to) systems, conditions, and assumptions that fall
within or very near the boundaries of those used to
develop the model. Whenever models are applied
outside these boundaries, the consequences of
knowingly violating the model's constraints should
be assessed as part of the model analysis. Unfortu-
nately, violation of model assumptions cannot al-
ways be readily known or easily assessed. Never-
theless, when model constraints are not met by the
natural processes modeled or by the data collected
for model input, model projections typically will
deviate from reality. The magnitude of such
deviations contribute markedly to what is gener-
ally called model uncertainty. Reasons why wa-
tershed models, including MAGIC, are particular
difficult to design and test include the following.
* Processes controlling watershed functions
are very difficult to observe either in nature
or in any laboratory experiment. Thus,
these processes may be either virtually un-
known or inaccurately represented in the
model, i.e., the model might not be a "true"
model.
* Actual conditions within individual water-
sheds that determine processing or transfer
rates may be unknown or poorly
quantified. Consequently, the model might
be poorly parameterized, i.e., the modeled
parameters may be poorly adjusted or
calibrated to approximate parameters
actually occurring within the modeled
system.
« Inputs to the models can be poorly known
or unable to be accurately predicted (e.g.,
dry deposition loads to a watershed).
* Models often are difficult to test. That is,
models may be largely "unverified,"
"unconfirmed," or "unvalidated." In fact, it
is often argued that a model can never be
confirmed to be true, it can only be falsi-
fied by failing to accurately project some
outcome. Further, when a model does ac-
curately predict an outcome, its validity is
not proven, because the "right" result may
have been projected for the wrong reason.
Further, it is useful to remember that environ-
mental monitoring and simulation modeling have
complementary environmental assessment roles.
Effects monitoring (i.e., surface water chemistry) is
necessary to determine the actual effects of acidic
deposition rates on environmental resources and to
provide data needed to develop and test models.
In turn, simulation modeling is useful to project
potential differences among future deposition or
environmental scenarios. This is particularly true
for projections of acidic deposition effects because
no set of monitoring records of sufficient length
exist that allow establishing a clear statistical
relationship linking changing historic ecological
responses to changing acidic deposition input
rates. Without this historic record it is not possible
statistically to project future changes, i.e., we
cannot predict the future directly from the past
because we do not know the past.
Simulation models, such as the MAGIC model,
generally do not require well quantified historical
relationships to provide potentially useful
projections of future conditions. But watershed
simulation modeling, including the NBS, often
continues to include significant uncertainties, as
noted in the above paragraphs and in Chapter 2.
This is especially the case regarding the modeling
of nitrogen cycling within watersheds and the po-
B-3
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
tential acidifying effects of nitrogen deposition on
soils, watersheds, and associated surface waters.
Additional uncertainties also remain regarding ex-
actly which processes to include in such models
and about how such processes should be linked
within these models. Further, there is almost a
complete dearth of monitoring and survey
information on the regional distributions of water-
shed characteristics that would allow such models
to be calibrated and applied to project future
effects. Improving capabilities to model these rela-
tionships will lead to better projections of potential
future environmental effects from both sulfur and
nitrogen deposition. Present knowledge of nitro-
gen cycling and early steps toward nitrogen mod-
eling provides a solid foundation for more impor-
tant and productive advances in this field. These
advances, particularly, would lead to significantly
reduced uncertainty in potential future effects of
nitrogen.
-------
APPENDIX B: SELECTED NBS PLOTS
Percent of target, population lakes with ANC<=0,
AdirondacRs Region at Year 201 5
where depositjon=median regional @ year 2015.
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i i i i i I
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
B-5
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
Percent of target population lakes with ANC<=50,
Adirondacks Region at Year 2015
where deposition=median regional @ year 201 5.
12 •
-5
j 10 -
Deposition (kg N
O) 00
i i
c
is
~S 2 -
S
0 -
12 -
J 10 -
rogen Deposition (kg N)
A O> 00
i i i
;=:
3 2 -
0 -
^^
55.4%
i i i i i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
ei
53.5 %
i i i i i i
0 2 4 6 8 10
12 -
^?
g 10 -
f -
1 ••
O
Total Nitrogen
0 10 *
i i i
12 -
1,0-
2
O
i 8"
8 e -
8- 6
Q
o A
8° 4
ss
Z
3 2 r-
0 -
t
)
V
53.4%
i i i ii i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
~T
.A
53.5%"
i i 1 i ! '
024 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
Total Sutfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
-------
APPENDIX B: SELECTED NBS PLOTS
Percent of target population lakes with pH<=5.0,
Adirondacks Region at Year 201 5
where deposfton=median regional @ year 2015.
pH estimated from empirical pH-ANC model.
12 -
J 10 -
rogen Deposition (kg K
•*»• O) 00
. i i i
Z
"a 2 -
£
0 -
x^
^^VQ
16%
till
0246
i i
8 10
12 -
1 10 -
rogen Deposition (kg Is
4*
I
8%
6
j
8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
1
O9
Deposlllon (t
c
1
Z
12 -
10 -
8 -
6 -
4 -
2 -
0 -
\ VI
( i
Jl r n
3.3%
i i i i i i
0 24 6 3 10
*
.2
Si
O
O
o
8s
(0
"o
12 -
10 -
8 -
6 -
4 -
2 -
0 -
i
3.
\
- \
5%.
i i i i i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
B-7
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
Percent of target population lakes with pH<=5.5,
Adirondacks Region at Year 201 5
where deposition=median regional @ year 2015.
pH estimated from empirical pH-ANC model.
ogen Deposi
To
12 -
10 -
25.4%
i
0
i
2
i
4
i
6
i
8
a/yr
kg
io
Total Nitrogen Dep
10
i
0
i
2
i
4
i
6
i
8
i
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
12 -
J 10 -
~z
C3
^S a
_o
1 R
8- 6 "
a
i A
f 4 "
z
"5 2 -
"o
0 -
i
11
* •
) \\13
\ vl
|u As
.5%
i i i i i i
0 2 4 6 8 10
a/yr
otal Nitrogen Deposition (kg
0 -
*l4-
9.6 %
r^
0
!
4
i
6
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
Total Suifur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
B-8
-------
APPENDIX B: SELECTED NBS PLOTS
Percent of target population lakes with pH<=6.0,
Adirondacks Region at Year 2015
where deposition=median regional @ year 2015.
pH estimated from empirical pH-ANC model.
12 -
>.
^ 10 -
2 „
Total Nitrogen Deposition (kg N/ha/yr) Total Nitrogen Deposition
0 IU * 0> CO 0 • M OMATOO
r-J 1 1 1 1 II 1 I , 1 I
J ^ ?
38.1 %
i i i i i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
Hi
30.2%
i i i i i i
0 2 4 6 8 10
Total Nitrogen Deposition (kg N/ha/yr)
O M A O) 00 O N>
II 1 1 1 1 1
Jl
A. W«-
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
Percent of target population lakes with ANC<=0,
Adirondacks Region at Year 2040
where deposition=median regional @ year 2020.
o
f
a
o
12 -
10 -
a -
s -
4 -
2 -
0 -
6
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
12 -
10 -
3 s -I
§• SH
Q
I M
*_
Z
5 2 -
0 -
i
0
i
4
i^
6
i
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
12 -
10 -
* 8 -
-------
APPENDIX B: SELECTED NBS PLOTS
Percent of target population lakes with ANC<=50,
Adirondacks Region at Year 2040
where deposition=median regional @ year 2020.
12 -
10 -
I
o
8s
a
s
6 -
4 -
2 -
0 -
53.2%
i
0
B
i
8
i
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
t
s
Q.
O
o
O
0°
12 -
10 -
8 -
6 -
4 -
•a 2 -
0 -
50.9%
i
0
i^
4
i
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
12 -
j 10 -
f 8-
_o
6 -
4 -
<0
Q
c
o
I
-5 2 -
0 -
44.1 %
8
i
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
•^
I
o
Q
c
8"
a
2
12 -
10 -
8 -
6 -
4."
2 -
0 -
44%
i
2
8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
B-11
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
12 -
1 10 -
OJ
t 8-
£
I *-
a
i> * -
z
3 2 -
0 -
4.1 %
Percent of target population lakes with pH<=5.0,
Adirondacks Region at Year 2040
where deposition=median r „
pH estimated from empirical pH
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
12 -
10 -
* 8 -
6 -
1
o>
JC
^^
J
ID
O
O
8"
a
"3
0 -
i
0
iar 2020.
model.
0%
I
2
i
4
i
6
i
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
Q.
9
Q
a
"3
12 -
10 -
8 -
6 -
4 -
2 -
0 -
i
2
I
4
8
i
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
3
8-
a
8s
12 -
10 -
I 8
6 -
4 -
To 2
.o
0 -
0%
i
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
B-12
-------
APPENDIX B: SELECTED NBS PLOTS
Will i yH<=5.5,
percent or target
it or target popuiauon lanes wiui pn
AdirondacKS Region at Year 2040
wheredeposition=median regional ©year 2020.
pH estimated from empirical pH-ANC model.
CL
03
Q
o
f
a
"o
12 -
10 -
8 -
6 -
4 -
2 -
0 -
11.1
12 -
10 -
¥ 8-
I 6J
Q
I 4 H
a 2 -
0 -
6.1 %
8
10
i
0
i
8
i
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
12 -
8
Q.
O
Q
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
Percent of target population lakes with pH<=6.0,
AdirondacKS Region at Year 2040
wheredeposition=median regional ©year2020.
pH estimated from empirical pH-ANC model.
12 -
10 -
I 8H
* 8-
6 -
4 -
a
Q
S
»-•
o H
21.8%
\^
2
i^
4
I
6
i
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100yr)
-------
APPENDIX B: SELECTED NBS PLOTS
Percent of target population streams witn ANu<=u,
Mid-Appalachian Region at Year 2015
where deposition=median regional @ year 201 5.
a/yr)
n (kg
en Depos
? 4-.
otal Nhr
a/yr)
on (kg
ogen De
8
10
i
0
i
2
i
4
i
6
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ SO yr)
Total Sulfur Deposition (kg S/na/yr)
(Assumes nitrogen saturation @ 100 yr)
ion (kg N/h
«
Q
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
Percent of target population streams witn ANo<=50,
Mid-Appalachian Region at Year 201 5
where deposffion=median regional @ year 201 5.
14 -
•& 12 -
3 10 -
2 8 -
I
o —
0 6 -
e
o
? 4 -
a£
Z
1 2'
0 -
,
t
2C
II 1
~^
^
).7%
.
i i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
3
5
I
>
)
14 -
| ,2-
» 10 -
P*
§
S 8 -
8
a.
o
Q 6 -
c
0
? 4 -
^ *
Z
1 2-
0 -
4
17
i i i
\^
bs^ofta-— SM —
O o/
1 1 1
5
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr) (Assumes nitrogen saturation @ 1 00 yr)
14 -
I «-
^ 10 -
C
I 8-
8-
Q 6 -
c
o
0s 4 .
w ~
*s
Z
1 2"
0 -
5
i
17.1 %
1 1 1 1
i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
14 -
Ifr 12 -
S 10 -
* 8-
8
Q.
Q 6 -
c
o
o° 4 -
£ 4
1 *-
0 -
4
^
3
)
3
16.9%
i i i
! !
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr) (Assumes nitrogen uptake constant)
B-16 :
-------
APPENDIX B: SELECTED NBS PLOTS
Percent of target population streams with pH<=5.0,
Mid-Appalachian Region at Year 2015
where deposition=median regional @ year 2015.
pH estimated from empirical pH-ANC model.
14 -
1 12-
z
Q? 10 -
*«
§.
Q 6 -
e
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
Percent of target population streams with pH<=5.5,
Mid-Appalachian Region at Year 2015
where deposition=median regional ©year 2015.
pH estimated from empirical pH-ANC model.
14 -
I 12-
Z
Deposition (kg
at oo o
§
1* 4 --
Z
I 2-
o -
X^'
^^
3.8%
i i i i i i
0 2 4 6 8 10
14 -
i" 12 -
2 10 -
i-
Q 6 -
c
C A —
^ "T
1.-
0 -
"X
\
0.5%
i i i i i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr) Total Sulfur Deposition (kg S/ha/yr)
14 -
z
2 10 -
1 '-
Q.
Q 6 -
o
O A .
1_ *T
Z
0 -
(Assumes nitrogen saturation @ 50 yr)
\
v >
i
0%
i i i i i i
0 2 4 6 8 10
14 -
1? 12 -
S 10 -
1 ,.
1
& e-
-------
APPENDIX B: SELECTED NBS PLOTS
Percent of target population streams with pH<=6.0,
Mid-Appalachian Region at Year 201 5
where deposition=median regional ©year 2015.
pH estimated from empirical pH-ANC model.
14 -
f 12-
o> 10 -
Q 6 -
o
I 2-
0 -
14 -
-& 12 -
€
Nitrogen DeposHion (kg N
* O) CO O
i i i i
1 2"
0 -
v \ 2J
^^^^1
5.6%
i i i i i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
N!i^tli'
^i
4.3%
i i i i i i
0 2 4 6 8 10
14 -
Ifr 12 -
Z
g 10 -
j
Q 6 -
o
O A. — «
^ "T
ss.
0 -
14 -
"I 12 -
^
Nitrogen Deposition (kg N
•It O) 00 O
i i i i
I 2-
0 -
^illr
x^lp
4.4%
i i i i i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
-\
x^
^ fc- ^
3.5-%
\\ii\i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
B-19
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
Percent of target population streams with ANC<=0,
Mid-Appalachian Region at Year 2040
where deposition=median. regional @ year 2020.
14 -
i .2-
o» 10 -
i-
& 6-
1 4"
l ^
0 -
14 -
•& 12 -
^
itrogen Deposition (kg N
A en oo o
i i i i
I 2-
0 -
J
(
\X^
\— -<
•
)%
i i i i i i
02 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
J
°
X
%
i i i i i i
0 2 4 6 8 10
14 -
•c-
• ^ 12-
S 10 -
Total Nitrogen Deposition
O CO 4k O) CO
ii ii i
14 -
i .2-
ilrogen Deposition (kg N
A 0> CO O
i i i i
z
1 2 *
0 -
4
(
\ X
N. N.
\
'
)%
i i i i ii
0246 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
4
-
0%
1 1 1 1 1 i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
B-20
-------
APPENDIX B: SELECTED NBS PLOTS
Percent of target population streams with ANC<=50,
Mid-Appalacnian Region at Year 2040
where deposition=median regional @ year 2020.
u -
I 12-
g 10 -
41
Q
s -
5.3%
0 2 4 6 3 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ SO yr)
14 -
I 12-
w 10 -
2
6 -
4 -
2 -
0 -
i
0
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
Q
«
I
I
14 -
12 -
10 -
a -
s -
4 -
2 -
0 -
4.6%
i
2
i
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
J
1
§•
Q
e
o
8s
a
o
14 -
12 -
10 -
8 -
S -
4 -
2 -
0 -
6.
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
B-21
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
Percent of target population streams with pH<=5.0,
Mid-Appalachian Region at Year 2040
where deposition=median regional ©year 2020.
pH estimated from empirical pH-ANC model.
14 -
| 12-
_§* 10 -
§
1 8 "
Total Nitrogen Depc
ro 4k o>
i ' t i
0 -
14 -
•J 12 -
j? 10 -
1 8'
Q.
! 6~
0
S 4-
Z
1 2 "
0 -
'
/ \\
NV
N.
.
*
0%
,
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
0
\
%
14 -
1 «-
S 10 -
1 ••
0
I 4-
Z
I 2-
0 -
14 -
£ 12 -
S 10 -
o
1 8'
0.
«
Q 6 -
3Ck/
79
1 1 1 I 1 1
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
0
"
\
4
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
B-22
-------
APPENDIX B: SELECTED NBS PLOTS
Percent of target population streams with pH<=5.5,
Mid-Appalachian Region at Year 2040
where deposition=median regional @ year 2020.
pH estimated from empirical pH-ANC model.
s
.2
i
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
Percent of target population streams with pH<=6.0,
Mid-Appalachian Region at Year 2040
where deposition=medan regional @ year 2020.
pH estimated from empirical pHrANC model.
O
0s
a
.o
12 H
10 H
8H
6 -
0 -
2.3%
14 -
I U-
10 -
8 -
Q 6 -
I ,
2 4 H
si
I 2~
0 -
8
10
i
0
8
I
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ SO yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
I
O>
3
CL
O
Q
o
f
Z
"3
"5
14 -
12 -
10 -
8 -
6 -
0 -
i
6
T
8
14 -
f 12-
w 10 -
J
Q
o
•5
o
8 -
0 -
10
i
4
i
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
8-24
-------
APPENDIX B: SELECTED NBS PLOTS
Percent of target population streams with ANC<=0,
SouthemBlue Ridge Region at Year 201 5
where deposition=meaian regional @ year 2015.
12 -
Total NHrogen DeposHlon (kg N/ha/yr)
ro i A o> oo o
•
i i i i i
0 -
12 -
Ion (kg N/ha/yr)
•A
00 O
1 1
I e-
O
Q
O A
£ "
Z
« 2 -
?
0 -
0%
1 . 1 1 1 1 1
0. 2 4 6 8 10
Total Sutfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
\
0%
i i i i i i
0 2 4 6 8 10
12 -
Total Nitrogen DeposHlon (kg N/ha/yr)
IO A O> 00 O
1 1 1 1 1
0 -
12 -
f 1Q-
o»
¥ 8"
1 s-
0
Q
1 «-
s:
Z
5 2 -
0 -
0%
i i i 1.1 i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
- -
V
0%
i i i i i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
Total Sutfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
B-25
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
Percent of target population streams with ANC<=50,
Southern Blue Ridge Region at Year 2015
where deposition=meaian regional @ year 2015.
osltlon (kg N/ha/yr)
» oo o ro
O
Q
o A
I ."
3 2 -
0 -
14.6%"
i i i i i
02468
i
10
Total Nitrogen Deposition (kg N/ha/yr)
o ro 4k o> a o ro
i i i i i i i
KJ. \v
>v T*
i: -a rO
6.7%
i i i i i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
o
Z
"3
12 -
10 -
8 -
4 -
2 -
0 -
-
A
5.9%
•
i i i i i i
0 24 6 8 10
>n (kg N/ha/yr)
1
a.
Q
c
Z
12 -
10 -
8 -
6 -
4 -
2 -
0 -
N\Sr
N ^
T
3.7%
i i i i i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
B-26
-------
APPENDIX B: SELECTED NBS PLOTS
Percent of target population streams with pH<=5.0,
Southern Blue Ridge Region at Year 201 5
where deposttion=median regional @ year 2015.
pH estimated from empirical pH-ANC model.
12 -
1 10 -
OJ
t 8 "
O
I 6-
01
a
e
« 4 _
Z
3 2 -
0 -
A
i i i
024
t»
.
0% •
1 1
6 8
12 -
j 10 -
51
* 8 -
§
! 6-
0
O
e
o A
.?
si
Z
5 2 -
0 -
^ h>
\
0%
1 i i i i i i
10 0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr) Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr) (Assumes nitrogen saturation @ 100 yr)
12 -
J 10 -
o
^ 8 -
c
1 6
Q. °
O
Q
c
1 4 "
Z
•5 2 -
o
1-
0 -
A
b
i
i
0%
i i i
024
• '
6 8
12 -
^| 10 -
a
* 8 -
c
8 s _
a. o
a
c
o ..
I
Z
5 2 -
'o
0 -
-
T -
\
\
0%
i i i i i i i
10 0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
B-27
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
Percent of target population streams with pH<=5.5,
Southern blue Ridge Region at Year 201 5
where deposjtjon=meaian regional @ year 2015.
pH estimated from empirical pH-ANC model.
12 -
^ 10 -
2
I
1 6-
4)
O
5? ^ ~
z
•S 2 -
0 -
\ -
0% '
i i i i i
02468
t
10
12 -
fotal Nitrogen Deposition (kg N/ha/yr)
to *. m oo o
i i i i i
0 -
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
12 -
g 10 -
i 8.
o
1 • -
-------
APPENDIX B: SELECTED NBS PLOTS
Percent of target population streams with pH<=6.0,
Southern Blue Ridge Region at Year 201 5
where deposition=median regional @ year 2015.
pH estimated from empirical pH-ANC model.
12 -
1 10 -
Z
O9
* a -
§
3S
1 6-
%
1 10 -
2
t 8 -
o
xs
I 6-
O
Q
c
1 '-
w
z
5 2 -
Q
*~
0 -
t
I
0%
i i i i i
0 24 6 8
12 -
5l
^ 10 -
O3
* 8 -
§
1 6-
a
a
c
1 «-
w
Z
•5 2 -
"o
l-
0 -
-
-
A
0%
i 1 1 1 1 I 1
10 0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
B-29
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
Percent of target population streams witn ANtx=u,
Southern Blue Ridge Region at Year 2040
where deposition=meaian regionai @ year 2020.
12 -
J 10 -
1 •-
2.
Total Nitrogen DepoaK
.6 10 4k 0)
i , ' . •
12 -
Itlon (kg N/ha/yr)
CO O
1 6-
-------
APPENDIX B: SELECTED NBS PLOTS
Percent of target population streams with ANC<=50,
Southern Blue Ridge Region at Year 2040
where deposffion=meaian regional @ year 2020.
§•
Q
8"
12 -
10 -
8 -
6 -
4 -
3 2 H
o
o H
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
ea
0.
o
a
a
I
12 -
10 -
s H
4 H
o H
3.8%
6
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
o
12 -
10 -
6 -
4 -
z
3 2 H
o H
i i ii r
a
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
OJ
x.
o
Q
o
12 -
10 -
8 -
6 -
4 -
2
4
~T^
6
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
B-31
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
Percent of target population streams with pH<=5.0,
Southern Blue Ridge Region at Year 2040
where deposltion=median regional @ year 2020.
pH estimated from empirical pH-ANC model.
*
3
8-
Q
4)
f
ffl
.0
12 -
10 -
8 -
6 -
4 -
2 -
0 -
t
0%
I
I
c
jo
I
o
Q
e
-------
APPENDIX B: SELECTED NBS PLOTS
Percent of target population streams with pH<=5.5,
Southern Blue Ridge Region at Year 2040
where deposition=median regional ©year 2020.
pH estimated from empirical pH-ANC model.
12 -
10 -
8 e -I
g- 6 "I
Q
0>
4 -
2 -
0 -
\
0%
I
6
i
8
12 -
10 -
I
I 8-
JO
1
Q.
0>
Q
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
Percent of target population streams with pH<=6.0,
Southern Blue Ridge Region at Year 2040
where deposition=meaian regional @ year 2020.
pH estimated from empirical pH-ANC model.
a/yr)
12 -
10 -
f 8H
_ 6 -
«
O
g . ,
£ 4 H
Z
« 2
0
a/yr
Total Nitrogen De
O
1
(kg
09
1
o>
1
O
1
6
8 10
i
0
i
2
6
i
8
i
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
12 -
j 10 -
1 "
l •-
Q
O *
? "
Z
CB 2 ™
0 -
^
c
)%
0 2 4 6 8 10
12 -
j 10 -
0)
* 8-
i
8 s -
o. o -
o
0
I 4"
Z
•« 2 -
s
0 -
i
C
"
)%
1 1 1 1 1 I
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
B-34
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APPENDIX C
RANGE OF INFLUENCE OF EMISSIONS
FROM RADM TAGGED SUBREGIONS
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-------
APPENDIX C
RANGE OF INFLUENCE OF EMISSIONS
FROM RADM TAGGED SUBREGIONS
This report includes the first extensive use of a
Eulerian model to study source-receptor relation-
ships. Source-receptor relationships are used in
this report to analyze changes in sources of deposi-
tion from implementation of Title IV of the Clean
Air Act Amendments of 1990 and to investigate
the effectiveness of several geographically targeted
emissions reductions strategies to achieving target
loads in sensitive receptor regions. The Tagged
Species Engineering Model1 was developed under
NAPAP to study such relationships. The Tagged
Species Model gives the Eulerian RADM modeling
system the capability to identify, for assessment
purposes, the concentration and deposition fields
attributable to specified SO2 emissions sources in
the presence of the full concentration fields. The
Tagged Model preserves the oxidant competition
across space and time. A tagging concept is ap-
plied in which additional, identical mass conserva-
tion equations are solved for a portion of the sulfur
concentration field that originates from specific
geographical locations within the full modeling
domain. This allows tagged concentration fields
and tagged wet and dry deposition to be identified
and tracked in the model separate from, yet as por-
tions of, the total sulfur chemical environment that
is nonlinear and that produces the complete con-
centration and deposition fields.
Calculations from the Tagged Species Model illus-
trate the distances over which an SO2 emissions
source can have an influence. The results from this
model permit the visualization of source attribu-
tion. Emissions from a subregion (see Exhibit 19 for
the 53 tagged RADM subregions) have a range of
influence is more than 1,000 kilometers. Typically,
the range of influence of a subregion extends out
to between 500 and 1,200 kilometers. The dif-
ference in scale of influence is primarily due to
meteorology. A number of meteorological factors
influence the existence of dominant transport di-
rections and determine how sources of SO2 emis-
sions influence nearby regions. Key factors are the
position of the jet stream, which moves storms
across the upper Mid-West; the influence of the
Appalachian Mountains on winds and rainfall pat-
terns; the Bermuda highs (stagnation) that move
Ohio River Valley emissions in a counter-clock-
wise direction; and the ocean and Gulf Coast
weather that produces lighter winds and more
convective conditions, including a typically large
proportion of convective clouds across the south-
ern states. Thus, the patterns and ranges of source
influence can vary. Models, such as those in the
RADM system, help to interpret and explain the
deposition at receptors of interest.
This appendix contains maps of which show the
proportion of total annual sulfur deposition con-
tributed by each of the 53 tagged RADM subre-
gions in 1985 and projected for 2010 with imple-
mentation of Title IV.
1 McHenry, J.N., F.S. Binkowski, R.L. Dennis, J.S.
Chang, and D. Hopkins. 1992. The tagged species
engineering model (TSEM). Atmospheric Environment
26A(8):1427-1443.
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
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RANGE OF INFLUENCE OF EMISSIONS
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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RANGE OF INFLUENCE OF EMISSIONS
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
C-21
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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„. ~vAppENDixC: RANGE OF INFLUENCE OF EMISSIONS
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
C-35
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
C-36
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APPENDIX C: RANGE oNr*
: J •/'(
OF EMISSIONS
C-37
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
C-38
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
C-39
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
*. a- ~R?::-.. ^safr.. '-s*
C-43
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
C-44
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: RANGE OF INFLUENCE OF EMISSIONS
C-45
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
C-46
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
C-47
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
C-48
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EMISSIONS
APPENDIX C: RANGE OF
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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APPENDIX C: RANGE OF INFLUENCE OF EMISSIONS
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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