xvEPA
United States
Environmental Protection
Agency
Air And Radiation
(6204J)
EPA430-R-95-001a
October 1995
Acid Deposition Standard
Feasibility Study
Report To Congress
PROPERTY Qf
OF
METRJROLOGV
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
REPORT TO CONGRESS
U.S. Environmental Protection Agency
Office of Air and Radiation
Acid Rain Division
October 1995
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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. Finally, EPA
appreciates public comments received and the
technical review by the Acid Deposition Effects
Subcommittee of the Ecological Processes and
Effects Committee of the EPA Science Advisory
Board. Both review processes provided important
insight and improved this report.
in
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TABLE OF CONTENTS
Acknowledgments iii
List of Exhibits vii
List of Acronyms and Abbreviations xi
Executive Summary xiii
1 Introduction 1
What acidic deposition levels are neces-
sary to protect sensitive regions'1 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? 4
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? 4
2 Environmental Goals 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
Surface Water Acidification 10
2.2.2 Episodic Acidification 14
2.2.3 Cumulative Loading Effects 15
2.2.4 Recovery of Acidified Ecosys-
tems 16
2.3 Characterizing Resources at Risk
from Acidic Deposition 17
2.3.1 Defining Sensitive Resources 1 8
2.3.2 Identifying Resources at Risk 20
2.4 Identification of Resource and Re-
gional Priorities 21
2.4.1 United States 21
2.4.2 Qualitative Assessment of
Sensitive Aquatic Resources in
Three Regions of the United
States 25
2.4.3 Qualitative Assessment of
Sensitive Aquatic Resources in
Canada 27
2.5 Assessing Protection Needs and Re-
source Responses in the Control of
Acidic Deposition 28
2.5.1 Model Selection and
Application 28
2.5.2 Direct/Delayed Response Pro-
ject
.30
2.5.3 Nitrogen Bounding Study 36
2.5.4 Overview of International and
State Acidic Deposition Crite-
ria and Standards 50
2 5.5 Spatial and Temporal Issues in
Development of a Standard 55
2.6 Controlling Sulfur and Nitrogen to
Reduce Surface Water Acidification 56
3 Source-Receptor Relationships and Depo-
sition Reductions under Various Emissions
Scenarios 59
3.1 Introduction 59
3.2 The Regional Acid Deposition Model
(RADM) 60
3.2.1 Emissions and Atmospheric
Chemistry 63
3.2.2 Modeling Source-Receptor Re-
lationships and Source Attribu-
tion 65
3.2.3 Transport, Chemistry, and
Source-Receptor Relationships 67
3.2.4 Confidence in Results 69
3.3 Source Attribution 70
3.3.1 Changes from 1985 to 2010 70
3.3.2 Regional Emissions Distribu-
tion in 2010 71
3.4 Emissions Reductions Scenarios 72
3.5 Deposition Reductions under Various
National Emissions Reductions Sce-
narios 76
3.5.1 Impact of SO2 Allowance
Trading on Sulfur Deposition 76
3.5.2 Effect of Additional SO2 Emis-
sions Reductions on Sulfur
Deposition 78
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
3.5.3 Decrease in Total Nitrogen
Deposition from Decreases in
NOX Emissions 79
3.6 Emissions Reductions Strategies to
Achieve Geographically Targeted
Sulfur Deposition Loads 82
4 Potential Benefits of an Acid Deposition
Standard on Visibility, Human Health, Ma-
terial, and Cultural Resources 89
4.1 Introduction 89
4.2 Relationship of Visibility to Acidic
Deposition 89
4.2.1 Visibility Impairment 89
4.2.2 Visibility Protection Laws and
Class I Areas 90
4.2.3 Visibility Metrics and the Pro-
jected Impact of CAAA on
Visibility 90
4.2.4 Potential Impact of Further
Sulfur Dioxide Reductions on
Visibility 93
4.3 Relationship of Human Health to
Acidic Deposition 93
4.4 Relationship of Materials Damage
and Cultural Resources to Acidic
Deposition 95
4.4.1 Acidic Deposition Effects on
Materials and Structures 95
4.4.2 Material Life-Cycle and Dam-
age Estimates 96
5 Implementation Issues 99
5.1 Introduction 99
5.2 Regional Targeted Approach 100
5.2.1 Description of Regional
Targeted Approach 100
5.2.2 Integration with Title IV 100
5.2.3 Impediments to Implementa-
tion .
.100
5.3 National Emissions-Based Approach 101
5.3.1 Description of National
Emissions-Based Approach 101
5.3.2 Integration with Title IV 101
5.3.3 Impediments to Implementa-
tion
.101
5.4 Economic Impacts 101
5.4.1 2010 CAAA Scenario (with
Trading) 102
5.4.2 50 Percent Utility SO2 Reduc-
tion Scenario 102
5.4.3 50 Percent Utility and Indus-
trial SO2 Reduction Scenario 105
5.4.4 Geographically Targeted Re-
duction Scenario 105
5.4.5 NOX Reductions—50 Percent
Utility and Industrial 105
5.4.6 Summary of Economic Im-
pacts
.107
5.5 Monitoring Program Effectiveness 108
5.6 Conclusions 109
6 Integration and Conclusions 111
6.1 Introduction 111
6.2 Determining Environmental Goals 111
6.3 Projected Environmental Conse-
quences of Acidic Deposition Reduc-
tion Scenarios 114
6.4 Selecting Deposition Goals 116
6.5 Feasibility of Establishing and Imple-
menting an Acid Deposition Standard 119
Appendices
A Summary of Selected NAPAP Reports A-1
B Plots from EPA's Nitrogen Bounding
Study B-1
C Range of Influence of Emissions from
RADM Tagged Subregions C-1
D Summary of Science Advisory Board
Review and Public Comments and
Responses 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 19
3. Critical pH for selected taxa in lakes
and streams 23
4. MAGIC 31
5. Study regions included in the Di-
rect/Delayed Response Project and the
Nitrogen Bounding Study 33
6. Target populations included in the
NSWS, DDRP, and NBS Studies 34
7. NBS model projections for year 2040
percentage of target population Adi-
rondack lakes with ANC<0 u.eq/1 39
8. NBS model projections for year 2040
percentage of target population
mid-Appalachian streams with ANC<
50u,eq/l
.40
9. NBS model projections for year 2040
percentage of target population South-
ern Blue Ridge streams with ANC<50
u.eq/1 41
10. Interpreting NBS plots 42
11. Time to watershed nitrogen saturation 43
12. Summary of NBS results: Range of
minimum (background deposition) to
maximum (implementation of CAAA)
percentages of acidic and sensitive tar-
get waters 45
13. Surface water responsiveness to reduc-
tions in deposition beyond the CAAA:
Detectible improvements in long-term
ANC by 2040
.47
14. Impact of CAAA on sensitive surface
waters: NBS model projections for Year
2040
.51
15. LRTAP 53
16. Physical and chemical processes con-
tributing to acidic deposition 61
17. The RADM modeling domain 62
18. Map of annual sulfur emissions density
in 1985 (tons/year) 65
19. Tagged RADM subregions 66
20. Proportion of annual sulfur deposition
contributed by RADM Subregion 15
(OH/WV/PA border region) 67
21. Percentage cumulative range of influ-
ence of RADM Subregion 15
(OH/WV/PA border region) 67
22a. Source-receptor relationships in the
Northeast: Cumulative percent sulfur
deposition 68
22b. Source-receptor relationships in the
lower Ohio Valley: Cumulative percent
sulfur deposition 68
22c. Source-receptor relationships in the
Southeast: Cumulative percent sulfur
deposition 68
23. Percent contribution to sulfur emis-
sions of 53 tagged RADM regions 70
24. Percent reduction in tagged regions
from 1985 to 2010 as a function of
relative contribution of each region to
all tagged emissions 70
25. Percentage of tagged emissions by
tagged regions for 1985 and 2010 71
26. Contribution of top-10 SO2 emitting
regions to sulfur deposition in sensitive
regions 71
27. Comparison of proximate and major
emitting regions to sulfur deposition in
sensitive areas in 2010 72
28. Estimated U.S. SO2 emissions with and
without Title IV from 1980 to 2015 73
29. Predicted SO2 utility emissions from
1990 to 2010 75
30. SO2 emissions in the U.S. RADM do-
main (eastern United States) 76
31. Annual average RADM total sulfur
deposition (kg-S/ha): 1980 76
32. Annual average RADM-predicted total
sulfur deposition (kg-S/ha): Post-2010
full CAAA implementation 77
33. Percentage reductions in sulfur deposi-
tion from CAAA implementation 77
VII
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
34. Impact of trading on sulfur deposition
in sensitive regions
35.
77
Annual average RADM total sulfur
deposition (kg-S/ha) by geographic
area in 2010: Ranges of differences in
deposition between post-2010 full
implementation 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 2010.
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.
49. Nitrogen deposition to sensitive re-
gions under base case and additional
utility and industrial NOX emissions
reduction scenario
.78
.80
.80
.80
.80
.81
.81
.81
.82
.82
.82
.83
.83
.83
.83
50. Example: Selection of maintenance
loads 85
51 a. Geographically targeted additional util-
ity SO2 reduction in contiguous RADM
subregions 86
54.
51 b. Geographically targeted additional util-
ity SO2 reduction in major RADM
subregions contributing to deposition
(not contiguous)
.86
52. Map of extent of contiguous geo-
graphic regions for achieving targeted
deposition loads equivalent to addi-
tional nationwide utility SO2 reduc-
tions 87
53a. Geographically targeted additional util-
ity and industrial SO2 reduction in con-
tiguous RADM subregions 88
53b. Geographically targeted additional util-
ity and industrial SO2 reduction in ma-
jor RADM subregions contributing to
deposition (not contiguous)
.88
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
.88
.88
.89
.91
.91
.91
.92
.93
.93
,.94
VIM
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LIST OF EXHIBITS
64. Relationship of acidic deposition proc-
esses to health effects
65. Percentage of metal corrosion attrib-
uted to atmospheric factors
66. 2010 annual costs and SO2 emissions
by Census region: CAAA scenario
67. 2010 annual costs and SO2 emissions
by Census region: CAAA scenario ver-
sus additional 50 percent utility emis-
sions reduction scenario
...94
...96
.103
.104
68. 2010 annual costs and SO2 emissions
by Census region: CAAA scenario ver-
sus additional 50 percent utility and
industrial emissions reduction scenario .
69a. Annual costs of geographically tar-
geted reductions equivalent to nation-
....106
wide 50% utility SO2 reduction
(contiguous RADM subregions) 107
69b. Annual costs of geographically tar-
geted reductions equivalent to nation-
wide 50% utility SO2 reduction: Major
RADM subregions contributing to
deposition (not contiguous) 107
70. Summary of costs of various emissions
reductions scenarios 108
71. Year 2040 NBS projections for Adiron-
dack lakes 115
72. Year 2040 NBS projections for mid-
Appalachian streams 115
73. Year 2040 NBS projections for South-
ern Blue Ridge Province streams 116
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
Ch 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
mi
MM4
N
NAAQS
NADB
NAPAP
MAS
NBS
NH3
NH4+
NO
NO2
NO3-
NOAA
NOX
NRC
NSPS
NSS
NSWS
NURF
NYSDEC
03
PAN
PM10
P043+
ppm
PSD
RADM
RIA
S
SBRP
SCR
SIP
SNCR
SO2
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
National Oceanic and Atmospheric
Administration
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
particulate matter smaller than 10
micrometers
phosphate
parts 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
XI
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
SO42' sulfate jieq/l microequivalents per liter
SOMA Sulfur Oxide Management Area u.g/1 micrograms per liter
SOS/T state-of-science/technology jim micrometer
SOX sulfur oxide 4DDA four-dimensional data assimilation
UNECE United Nations Economic Commission
for Europe
yr year
XII
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EXECUTIVE SUMMARY
Scientific evidence has shown that atmospheric
deposition of sulfur and nitrogen compounds can
harm ecosystems. Title IV of the Clean Air Act
Amendments of 1990 (CAAA or the Act) addresses
the problem of such effects by mandating reduc-
tions in emissions of sulfur and nitrogen oxides,
the major precursors of acidic deposition. Cou-
pled with Titles I and II of the Act, which address
new and existing stationary and mobile sources of
sulfur and nitrogen oxides, implementation of Title
IV is expected to provide significant benefits to the
United States and Canada. These benefits include
decreases in the acidity of lakes and streams, con-
comitant improvements in fish population diversity
and health, decreases in soil degradation and for-
est stress, improvements in visibility (especially to
scenic vistas), decreases in damage to materials
and cultural resources, and a reduction in human
health effects. Congress included Section 404 in
Title IV (Appendix B of the Act) requiring the Envi-
ronmental Protection Agency (EPA or the Agency)
to provide a report to Congress on the feasibility
and effectiveness of an acid deposition standard or
standards to protect sensitive and critically sensi-
tive aquatic and terrestrial resources. Specifically,
Congress listed six areas to be addressed in the re-
port:
* 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.
Congress also requires the National Acid
Precipitation Assessment Program (NAPAP) to
conduct a study similar to the technical portions of
this report, on the reduction in deposition rates
needed to prevent adverse ecological effects.
NAPAP is required to submit its report to Congress
in 1996 (Section 901 [j] of the Clean Air Act
Amendments).
DEVELOPING A STANDARD TO PROTECT
SENSITIVE RESOURCES
An acid deposition standard is a level of deposi-
tion (most likely in units of kilograms of pollutant
per hectare per year) that provides a predeter-
mined level of protection to specific ecological re-
sources. The natural resources most at risk from
acidic deposition and those most amenable to a
quantitative assessment are aquatic systems.
Therefore, a standard designed to protect against
the ecological effects of acidic deposition would
most likely be developed based on effects to
aquatic systems. Other ecological resources such
as high elevation red spruce forests in the eastern
United States and Canada may also be at risk, but
less is known about the effects process, and the
rate and extent of impacts on those resources. Re-
search conducted under the auspices of the Na-
tional Acid Precipitation Assessment Program
(NAPAP) concluded that regions in the United
States most at risk from continued acidic deposi-
tion are located along the Appalachian Mountain
chain stretching from the Adirondacks to the
XIII
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
Southern Blue Ridge. Although many surface wa-
ters in western North America are as sensitive as,
or more sensitive than, aquatic systems in the East,
deposition levels in the West are sufficiently low
that the risk of chronic (long-term) acidification to
resources in most of the West is low at present and
is expected to remain low in the foreseeable fu-
ture. Episodic acidification (from spring snow-
melts) adversely affects some eastern surface wa-
ters. It may be affecting high-elevation western
surface waters, as well.
An acid deposition standard or standards could be
designed to achieve a variety of environmental
protection goals. For example, the goal of a stan-
dard may be to (a) maintain specific conditions as
observed at a particular point in time (e.g., condi-
tions observed in 1984 during the National Surface
Water Survey); (b) protect all systems from any
harmful anthropogenic effects (i.e., return to
pre-industrial conditions); or (c) balance effects,
costs, and other societal values. A standard can be
designed to address chronic or episodic acidifica-
tion and could vary by region based on the re-
gional variability of ecological sensitivity. No
guidance is provided in the statutory language re-
garding the degree of protection desired by a stan-
dard or standards.
Target populations of Adirondack lakes, Mid-
Appalachian streams, and Southern Blue Ridge
streams were selected for detailed analysis in this
study because they represent areas that receive
fairly high levels of acidic deposition, are sensitive
to acidic deposition, have the best historical data,
and have been extensively studied by scientists.
Potential future impacts of acidic deposition are
estimated by modeling the response ol a target
population of aquatic systems (lakes and streams)
in these areas to various levels of deposition. Tar-
get populations are selected to represent sensitive
surface waters over broad geographic regions. In-
dividual target populations of sensitive surface
waters used for various acidic deposition studies
have become progressively smaller over time as
investigators have refined their work to study more
intensively the acidification processes influencing
the most sensitive surface waters. For example, of
the total population of lakes in the eastern United
States, 18,156 of these lakes potentially most sen-
sitive to acidification were included within the
target population of the National Surface Water
Survey (NSWS, 1984). In turn, further refinement
of sensitivity characteristics for the NSWS lakes led
to targeting a total population of 3,227 lakes in the
Northeast during the Direct/Delayed Response
Project (DDRP, 1988) and 703 lakes in the Adi-
rondack Mountains during the Nitrogen Bounding
Study (NBS, 1994). Similar refinements also oc-
curred for lakes and stream reaches in other re-
gions. This approach is illustrated diagrammati-
cally in Exhibit I (not to scale). In this report,
analyses of risks from acidic deposition to sensitive
lakes and streams focuses extensively on extrapo-
lations regarding the highly sensitive, but limited,
target populations used during the NBS studies.
EXHIBIT I. TARGET SURFACE WATER POPULATIONS
INVESTIGATED BY NSWS, DDRP, AND NBS
SELECTING AN ACID DEPOSITION STANDARD OR
STANDARDS
The analyses presented in this study provide model
projections of direction and magnitude responses
for modeled target watersheds under various sulfur
and nitrogen deposition scenarios. Based on the
remaining scientific uncertainties, particularly re-
garding the effects of nitrogen and the rate of those
effects on the watershed, selection of an appropri-
ate level or levels for a standard to achieve any
particular environmental goal is very difficult.
Therefore, this study does not recommend an acid
deposition standard or standards at this time. The
modeling analyses in this report however, do indi-
cate important watershed responses to emissions
reductions in the Clean Air Act and provide esti-
mates of deposition reductions that would be
needed to achieve a range of environmental goals
within the uncertainty of the modeling results.
IMPACT OF THE CAAA ON SENSITIVE SURFACE
WATERS
Model projections from the Nitrogen Bounding
Study (NBS) indicate that sulfur deposition reduc-
tions mandated by Title IV of the Clean Air Act
Amendments would benefit sensitive surface wa-
ters by the year 2040. Exhibits II-IV show ranges
XIV
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EXECUTIVE SUMMARY
EXHIBIT II. PERCENT OF NBS TARGETED ADIRONDACK LAKES PROJECTED
TO BE CHRONICALLY ACIDIC (ANC<0 (IEQ/L) IN 2040
PERCENT OF TARGET POPULATION ADIRONDACK
LAKES OBSERVED ACIDIC IN 1 984 = 1 9%
TARGET POPULATION = 700 LAKES
Time to Watershed
Nitrogen Saturation
Never
250 years
1 00 years
50 years
Percent of Targeted Waters
Projected to be Acidic in 2040
Without CAAA
25%
23%
36%
50%
With CAAA
11%
15%
26%
43%
EXHIBIT III. PERCENT OF NBS TARGETED MID-APPALACHIAN STREAMS
PROJECTED TO BE CHRONICALLY ACIDIC (ANC<0 I1EQ/L) IN 2040
PERCENT OF TARGET POPULATION MID-APPALACHIAN
STREAMS OBSERVED ACIDIC IN 1 985 = 4%
TARGET POPULATION = 4,300 STREAMS
Time to Watershed
Nitrogen Saturation
Never
250 years
1 00 years
50 years
Percent of Targeted Waters
Projected to be Acidic in 2040
Without CAAA
8%
21%
23%
33%
With CAAA
0%
4%
5%
9%
EXHIBIT IV. PERCENT OF NBS TARGETED SOUTHERN BLUE RIDGE PROVINCE
STREAMS PROJECTED TO BE CHRONICALLY ACIDIC (ANC<0 LIEQ/L) IN 2040
PERCENT OF TARGET POPULATION SOUTHERN BLUE RIDGE
PROVINCE STREAMS OBSERVED ACIDIC IN 1985 = 0%
TARGET POPULATION = 1,300 STREAMS
Time to Watershed
Nitrogen Saturation
Never
250 years
1 00 years
50 years
Percent of Targeted Waters
Projected to be Acidic in 2040
Without CAAA
0%
1%
2%
13%
With CAAA
0%
0%
0%
4%
of percent target population lakes or streams in
each sensitive region projected to be chronically
acidic (acid neutralizing capacity [ANC] < 0 Lieq/l)
by 2040 with and without implementation of the
Clean Air Act. The scenarios are described ac-
cording to the extent and rate of nitrogen deposi-
tion effects on watersheds (e.g., percent target
waters in Adirondacks projected to be acidic in
2040 with implementation of the CAAA range
from 11 to 43 percent). Nitrogen saturation may
be defined as the condition reached at which the
supply of nitrogenous compounds to a
watershed exceeds the ability of
biogeochemical processes within the
watershed to retain those compounds on
a net basis. That is, the point at which
the supply of nitrogen exceeds demand.
When this capacity is reached, nitrogen
losses from watersheds increase,
principally in the form of nitrate
leaching. The time to nitrogen
saturation varies among and within
regions due to differences in tempera-
ture, moisture, length of growing season,
soil fertility, forest age, and historic
nitrogen deposition. There is significant
uncertainty regarding times to watershed
nitrogen saturation in each sensitive
region. As reflected in the exhibits, in
each modeled region, the proportion of
targeted acidic surface waters would
have been higher, in some cases dis-
tinctly, without the sulfur dioxide (SO2)
reductions in the CAAA.
ENVIRONMENTAL ANALYSIS OF
EMISSIONS TRADING
Projections of sulfur deposition in 2010,
when the SO2 emissions trading pro-
gram is fully implemented and utilized,
is part of the analysis of the impact of
the CAAA on sensitive regions. Atmos-
pheric modeling projects no more than
a 10 percent difference in sulfur deposi-
tion at any location in 2010 with and
without trading. Over most of the east-
ern United States, the difference in
deposition is less than 5 percent, and
there is no difference in eastern Canada.
Exhibit V is a map that shows the pro-
jected annual average difference in sul-
fur deposition between trading and no
trading over the eastern United States
and Canada. Differences in deposition
of less than 10 percent are projected not
to measurably change ecological effects.
Furthermore, a recently released Gen-
eral 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.*
Therefore, while the allowance trading program is
expected to reduce costs of control, it is not
* U.S. General Accounting Office. December 1994. Air
Pollution Allowance Trading Offers an Opportunity
to Reduce Emissions at Less Cost. Washington, DC.
xv
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT V. ANNUAL AVERAGE RADM TOTAL SULFUR DEPOSITION (KG-S/HA) BY GEOGRAPHIC AREA IN 2010:
RANGES OF DIFFERENCES IN DEPOSITION BETWEEN IMPLEMENTATION OF THE CAAA WITH AND WITHOUT TRADING
-0.80 TO -1.16
H -0.50 TO -0.80
H -0.20 TO -0.50
CH -0.20 TO 0.20
0 0.20 TO 0.50
^ 0.50 TO 0.90
• 0.90 TO 1.53
projected to have a measurable negative environ-
mental impact.
IMPLICATIONS FOR FURTHER DEPOSITION
REDUCTIONS
Scientific analysis indicates that nitrogen as well as
sulfur deposition are important contributors to
chronic and episodic acidification of surface waters.
Further reductions in nitrogen as well as sulfur
deposition may be necessary in order to realize
protection of target sensitive systems. Model pro-
jections indicate that, if the time to nitrogen satu-
ration in the Adirondacks is 100 years or less,
maintaining the proportion of chronically acidic
target surface waters in the Adirondacks in the
year 2040 near proportions observed in 1984 may
require reducing anthropogenic sulfur and nitrogen
deposition by 40 to 50 percent or more below
levels achieved by the CAAA. In the mid-
Appalachians, implementation of the CAAA
should maintain 1985 proportions of chronically
acidic target streams in the year 2040 if the time to
nitrogen saturation is 250 years or longer; more
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 in the year 2040. In
XVI
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EXECUTIVE SUMMARY
addition to acidity (ANC<0 u.eq/1), another useful
measure, particularly for streams at risk of episodic
acidification, is the sensitivity of a lake or stream
to becoming acidic (i.e., very low ANC such as 50
u.eq/1). Modeling projections using this measure
(as described in Chapter 2) also indicate that
further deposition reductions may be necessary for
full protection of target sensitive surface waters.
Exhibit VI compares deposition levels produced by
several sulfur dioxide emissions scenarios. The
additional reduction scenarios were chosen to il-
lustrate the effect of further emissions reductions
and to serve as examples for cost and implementa-
tion analyses; they do not represent a reduction
necessary to meet any particular level of protec-
tion. In comparison with 1980 deposition levels,
implementation of the CAAA is projected to re-
duce deposition by 30 to 40 percent by 2010,
standards may consider reduction of effects
resulting from episodic acidification a key
environmental endpoint.
NATIONAL OR TARGETED EMISSIONS
REDUCTIONS
To achieve an acid deposition standard or stan-
dards for particular sensitive regions, some have
suggested targeting emissions reductions rather
than reducing national emissions. By 2010, Title
IV will produce the largest emissions reductions in
the highest emitting regions (i.e., Ohio, Indiana,
West Virginia, and western Pennsylvania). An
analysis of geographically targeted emissions re-
ductions using the Regional Acid Deposition
Model (RADM) shows that to achieve deposition
reductions beyond the CAAA (equivalent to those
achieved by an additional national 44 percent SO2
EXHIBIT VI. SULFUR DEPOSITION TO SENSITIVE REGIONS UNDER VARIOUS SO2 EMISSIONS SCENARIOS
Emissions Scenario
1980
1985
2010 after CAAA implementation
CAAA plus additional 50% utility^SO2 reduction
CAAA plus additional 50% utility and industrial SO2 reduction
Annual Average Deposition Level (kg-S/ha)
Adirondacks
11
9.8
6.9
5.5
4.7
Mid-Appalachians
19
17
11
8.1
6.9
Southern
Blue Ridge
14
13
9.7
6.8
5.5
Exhibit VII 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.
EPISODIC ACIDIFICATION
Episodic acidification occurs when pulses of acidic
waters enter lakes and streams with stormwater
runoff and spring snowmelt. Both nitrates and sul-
fates originating from atmospheric deposition can
contribute significantly to episodic acidification
events. Rapid, acutely toxic changes in surface
water chemistry can result from such episodic
acidification. Such events often occur at the most
biologically significant time of year (i.e., during
spawning). Significantly more lakes and streams
become episodically acidic than are chronically
acidic. Lower levels of acidic deposition would
decrease the number and severity of acidic and
toxic episodes driven by sulfate and nitrate.
Development of an acid deposition standard or
EXHIBIT VII. PERCENTAGE REDUCTIONS IN
SULFUR DEPOSITION FROM 1980 TO 2010
FROM IMPLEMENTATION OF THE CAAA
AND CANADIAN ACID RAIN PROGRAM
LEQgMQ;
0-23
-*25 - 30
W30 - 35
»35 - 40
• > 40
XVII
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
emissions reduction) in sensitive receptor regions,
zones targeted for emissions reductions may po-
tentially include 6 to 11 states and require source-
specific, sulfur dioxide reductions of about
95 percent. To achieve deposition loadings in all
three sensitive receptor regions (equivalent to
those achieved by an additional national
44 percent SO2 emissions reduction), both geo-
graphically targeted and national emissions reduc-
tions strategies would require about the same total
emissions reductions at approximately the same
total cost. It may be appropriate to assume that
some level of cost savings associated with an un-
restricted national trading program (as assessed for
implementation of the Acid Rain Program under
Title IV) could also result in reduced costs of
compliance with broad emission reductions be-
yond the current program. This could widen the
cost difference between a geographically targeted
and national emissions reduction strategies. Thus,
for this level of additional reductions, there does
not appear to be an economic or environmental
advantage to geographically targeting regions for
emissions reductions.
IMPLEMENTING AN ACID DEPOSITION
STANDARD
In order to determine the effectiveness of an acid
deposition standard or standards for protecting
sensitive resources, it is necessary to describe how
the standard might be implemented. This study de-
scribes two basic approaches to using an acid
deposition standard. Under the first approach (a
regional target approach), EPA would set a stan-
dard or standards, either using existing authority (if
adequate) or seeking further authority from Con-
gress to set such standards and provide deadlines
for their attainment. Then, similar to Title I, states
would determine source-specific limits using
source-receptor models and technical and cost
analyses, incorporate those limits in State Imple-
mentation 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 (a national emissions-
based approach), Congress would direct EPA to set
a deposition standard or standards and to deter-
mine the national (or regional) emissions levels for
sulfur dioxide and nitrogen oxides that would meet
those standards. Congress would then set an
emissions cap and allowance allocations for nitro-
gen 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. The costs of further emis-
sions reductions characterized in this report could
lead to costs that are more than double those of
the current acid rain control program. However,
the emissions reductions achieved under either
implementation scenario may also provide ancil-
lary benefits in multiple effect areas, such as visi-
bility, human health, and material resources. The
level of potential benefits in these areas will de-
pend on the level and type of standard developed
and cannot be determined until such information
is available. Development of an acid deposition
standard or standards would require further analy-
sis of costs and benefits. This is necessary to de-
termine the level of incremental benefits in a range
of effect areas as compared to the deposition re-
ductions necessary to meet a range of standard
levels and the costs associated with these levels.
Environmental resources have ranges of sensitivi-
ties and risks to potential effects caused by acidic
deposition. Resources having equivalent sensitivi-
ties have different risk potentials for harmful effects
that depend on how much acidic deposition they
receive. Aquatic modeling results presented in this
report indicate that additional reductions in sulfur
and/or nitrogen would reduce regional proportions
of chronically acidic surface waters and propor-
tions of surface waters most sensitive to episodic
effects. The magnitude of benefits varies by re-
gion, and modeling uncertainty is too significant to
indicate more than direction and magnitude at this
time.
Scientific uncertainties make setting an acid
deposition standard or standards at a particular
level difficult. However, even when the uncer-
tainties have been resolved or reduced, setting a
single, uniform standard may be an inappropriate
approach in view of the differing sensitivities and
risks associated with resources in different regions
of the country. Many have suggested that acidic
deposition goals (rather than standards) established
through consideration and analysis of resource
sensitivity and risk would provide useful informa-
XVIII
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EXECUTIVE SUMMARY
tion and guidance. Such goals could provide ref-
erence points to assess the effectiveness of pollu-
tion control efforts and would serve as guides for
environmental policymakers at various levels until
scientific uncertainties that inhibit setting a stan-
dard or standards are better understood.
FEASIBILITY AND EFFECTIVENESS OF AN ACID
DEPOSITION STANDARD
This report responds to the request by Congress to
assess the feasibility and effectiveness of an acid
deposition standard or standards for protection of
sensitive aquatic and terrestrial resources. The re-
port concludes that establishing such standards for
sulfur and nitrogen deposition are technically fea-
sible, but two critical areas of uncertainty advise
against completion of the task at this time. First,
policy decisions regarding appropriate or desired
goals for protecting sensitive aquatic and terrestrial
resources are needed to help guide the Agency in
continued analyses and decisions regarding possi-
ble establishment of acidic deposition standards.
Such policy decisions would be based on Con-
gressional guidance and continued efforts to ad-
dress social science uncertainties related to the
level of protection desired by the public and the
costs and benefits associated with incremental or
significant changes. Second, key scientific un-
knowns, particularly regarding watershed proc-
esses leading to nitrogen acidification and remain-
ing times to watershed saturation with nitrogen,
limit the ability to recommend a specific standard
for any region of North America at this time. With
this level of scientific uncertainty, a policy goal of
protecting all ecosystems could only be assured
through standards reflecting pre-industrial deposi-
tion levels. This scientific uncertainty can only be
reduced through additional research and environ-
mental monitoring.
Determining specific requirements for an appro-
priate deposition standard or standards calls for
additional research to understand the simultaneous
effects of sulfur and nitrogen deposition in produc-
ing acidifying effects on sensitive aquatic and ter-
restrial receptors. Establishing specific needs and
appropriate limits for deposition standards requires
advancing the scientific understanding of factors
key to (1) defining times to watershed nitrogen
saturation and (2) the interaction of sulfur and ni-
trogen in acidifying watersheds and surface waters.
Additional scientific studies to determine the po-
tential impact of acidic deposition on climate
change may also be reliant to fully characterize
ecological effects.
If a deposition standard or standards were devel-
oped, an extensive monitoring program would be
necessary to (1) ensure compliance with the stan-
dard^), (2) determine the effectiveness of the stan-
dard(s), and (3) verify whether standards were cor-
rectly defined or an alternative standard may be
more appropriate.
EPA's Science Advisory Board (SAB) review of this
report emphasized that policy concerns will influ-
ence which science questions will drive research
priorities regarding the need and feasibility of set-
ting acid deposition standards. Results from new
research addressing the issues described above are
needed to provide the scientific confidence and
assurance to develop and implement effective acid
deposition standard or standards. The SAB em-
phasized the importance of environmental moni-
toring of deposition, ecological indicators, and
ecological endpoints as a parallel and comple-
mentary strategy to modeling in order to assess
ecological resource issues.f Furthermore, guidance
from Congress and the public is critical to defining
the extent of protection desired for acid- sensitive
aquatic and terrestrial resources.
Finally, the effectiveness of an acid deposition
standard or standards depends heavily on the ap-
proach used to implement it. Although the two
basic approaches discussed in this report could
have similar compliance costs and effects on
aquatic resources, the national market-based
emissions reduction approach may be less admin-
istratively cumbersome and more compatible with
the existing Title IV and is more likely to be imple-
mented. Further analysis is necessary to determine
what level of ancillary benefits to human health,
visibility, and material and cultural resources
either approach would provide and the extent to
which one approach provides greater benefits than
the other. Title IV was designed to address re-
gional air pollution problems, especially those in-
volving long-range transport of pollutants and their
transformation products. The likelihood of
achieving deposition reductions is viewed as a
critical factor in judging effectiveness.
t An SAB Report: Review of the Acid Deposition
Standard Feasibility Study Report to Congress. Acid
Deposition Effects Subcommittee of the Ecological
Processes and Effects Committee, United
States Environmental Protection Agency Science
Advisory Board, EPA-SAB-EPEC-95-019, July, 1995.
XIX
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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
harmful 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 materials and cultural resources, and a
reduction in human health effects. Congress
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;
4 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.
Congress also requires the National Acid Precipi-
tation Assessment Program (NAPAP) to conduct a
study similar to the technical portions of this re-
port, on the reduction in deposition rates needed
to prevent adverse ecological effects. NAPAP is
required to submit its report to Congress in 1996
(Section 901 [j] of the Clean Air Act Amendments).
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
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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-
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 achievement of the
Act's goal of reducing the effects of acidic
deposition in sensitive areas, and that emissions
levels designed to provide a level of protection are
necessary.
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 the multiple impacts of sulfur and nitrogen
acidic deposition affect ecosystems. In addition,
meteorological variability, uncertainties in
emissions inventories, and the complexity of at-
mospheric chemistry limit the ability to relate spe-
cific ecosystem 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 when considering 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 theme is
uncertainty in (1)data and models, (2) future
ecosystem behavior, and (3) future economic and
policy decisions that may influence decisions
regarding feasibility.
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. An acid
deposition standard or standards can be designed
to achieve a variety of environmental goals. For
example, the goal of a standard may be (a) mainte-
nance of specific conditions as observed at a par-
ticular point in time (i.e. conditions observed in
1984 during the National Surface Water Survey),
(b) return to pre-industrial conditions, or (c) a level
which balances effects, costs, and other societal
values. A standard can be designed to address
chronic or episodic acidification and could vary
by region based on the regional variability of
ecological sensitivity. No guidance is provided in
the statutory language regarding the desirable
degree of protection afforded by a standard or
standards.
Chapter 2 of this feasibility study brings together
the most current scientific understanding regarding
the relationship between acidic deposition 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
-------
CHAPTER 1: INTRODUCTION
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
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, although
fewer data are currently available in these areas.
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, including: nitrogen, as well as
sulfur, produces episodic surface water acidifica-
tion effects, especially during spring snowmelts;
and some watersheds in the Northeast may be
approaching the limit of their ability to sequester
nitrogen, leading to increased acidification from
nitrogen deposition. The importance of consider-
ing the effects of nitrogen deposition on eu-
trophication of estuarine bodies is identified in this
report with fuller discussion provided in other EPA
reports.1
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. (Note that an effects-based analysis and
development of an acid deposition standard or
standards does not necessarily imply emissions
reductions associated with implementation 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 affect visibility, human
health, and materials. Decreasing acidic deposi-
tion can also provide benefits in these areas, de-
pending on what size emissions reductions may be
involved. 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 169A 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, particulate
matter (including sulfates and nitrates), and O3.
Nevertheless, any control program or standard es-
tablished to reduce acidic deposition could also
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 1 993. 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
further impact these other related areas. Chapter 4
of this report highlights potential impacts a stan-
dard may have on visibility, human health, and
materials in a primarily qualitative manner. How-
ever, additional research is required to determine
the extent to which a standard would provide an-
cillary benefits in these areas.
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?
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
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 protect ecosystems from
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 inherent
flexibility in choosing compliance strategies and
the market-based allowance trading program pro-
mote the most cost-effective strategy for achieving
SO2 reductions, rather than requiring a specific
type of control on certain sources. Variation in
sensitivity to acidic deposition among geographic
regions raises the question of whether targeted or
regional standards are needed to protect sensitive
resources. Before such a question can be an-
swered, determining the level of protection that
will be provided 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 scenarios chosen were limited by the
availability of emissions inventories at the time this
report was developed.
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,
the mid-Appalachians, and the Southern Blue
Ridge Province.
HOW WOULD AN ACID DEPOSITION STAN-
DARD^) 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:
4 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 Add Deposition
Model and Engineering Model. NAPAP SOS/T
Report 4. In: Acidic Deposition: State of Science and
Technology. National Acid Precipitation Assessment
Program.
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CHAPTER 1: INTRODUCTION
* STATUTORY AUTHORITY: Is existing authority
adequate, or would Congress need to pro-
vide additional authority to implement 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-
dress emissions from these sources. What
effect would implementation of an acid
deposition standard have on these and
other environmental programs?
* Economic Impacts: What would be the
costs and economic impacts of an acid
deposition standard to the regulated com-
munity, as well as the national and local
economies?
Chapter 6 integrates analyses of environmental
goals, emissions reductions, and implementation
issues and provides conclusions concerning the
feasibility of developing and implementing a stan-
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 effects of acidic
deposition on environmental resources by mandat-
ing 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 specific acid
deposition standards may be appropriate. Under
Section 404 (Appendix B) of the CAAA, EPA must
assess the feasibility and effectiveness of establish-
ing an acid deposition standard, or standards, 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 biological species,
also provide useful information on resource health.
Atmospheric deposition of sulfur and nitrogen
compounds that form acids is the principal cause
of surface water acidification. Most recent atten-
tion has focused on the effects and control of sul-
fur deposition (Section 2.2.1). Although many
studies have focused primarily on long-term acidi-
fication processes, recent EPA research supports
the contention that short-term acidification caused
by rainstorms and snowmelt may often be the in-
itial cause of many of the most severe acidification
effects in surface waters. Consideration of acid
deposition standards may take into account impli-
cations to both the long- and short-term acidifica-
tion processes (Section 2.2.2). Recent research also
indicates that acidification effects caused by nitro-
gen deposition are increasingly important in some
areas. The increasing degree of nitrogen saturation
in some watersheds may be 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 al-
though 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), restoration of ecological systems to
their predisturbance conditions may not be possi-
ble.
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 in italics. 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 Canada, roughly south of James
Bay and east of the Manitoba-Ontario border, re-
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
source concerns similar to concerns 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 has limitations, however, as results carry
considerable uncertainty (Section 2.5.1). EPA has
completed two major effects modeling studies. The
first study, conducted under the National Acid
Precipitation Assessment Program (NAPAP), pro-
jected the impact of sulfur deposition on long-term
soil and surface water acidification, with conse-
quent loss of aquatic habitat for sensitive fish spe-
cies, in three broad geographical regions of the
eastern United States. Adverse effects were pro-
jected to continue unless sulfur deposition was re-
duced, and sufficient reductions in sulfur deposi-
tion 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 importance was projected as
likely to increase in future years unless deposition
rates decreased (Section 2.5.3). This study also
projected that the 1990 CAAA would provide clear
benefits to surface water in three sensitive regions
of the eastern United States. The accuracy of the
model projections is highly uncertain, however,
largely because researchers and available predic-
tive models lack the ability to precisely estimate
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 may have begun, how exten-
sive the effects are and may become, and the criti-
cal periods when these effects may be most severe.
This section briefly reviews the process of surface
water acidification, important considerations in
identifying at-risk resources, episodic acidification,
and useful information for setting regional and re-
source priorities for acidic deposition controls. The
section 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 toward acidic conditions is
countered in most surface waters by the dissolu-
tion of common, alkaline minerals such as lime-
stone that dissolve into them. This dissolution neu-
tralizes the acidity, often producing slightly alka-
line conditions (alkalinization). The dissolution of
-------
CHAPTER 2: ENVIRONMENTAL GOALS
many minerals not only neutralizes acidic condi-
tions in waters, it produces a buffering capacity
that enables these waters to minimize pH changes,
while allowing their mass of dissolved acids to
vary within certain limits. Additional buffering ca-
pacity can be produced by solutions of weak ac-
ids, including carbonic acid and many organic ac-
ids. The extent of acid-base buffering within any
watershed in the environment is determined 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, especially below pH 6.5, 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).
significantly moderated potential for pH fluctua-
tions below 6.0. Also, they generally have minimal
development of acidic water qualities that can be
stressful, or even toxic, to aquatic organisms. In
turn, waters with ANC 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 can
be characterized (e.g., as within 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 exhibit shows, for example, that an ANC of
50 ueq/l correlates to a pH of approximately 6.5
across these regions.
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 carbonate and
bicarbonate, which generally dissolve into
the water from calcium carbonate (the
predominant chemical constituent of
limestone, for example). Concentrations of
borates, phosphates, silicates, sulfides, and organic
anions can also contribute to total ANC in surface
water. In earlier literature, the term alkalinity was
often used in place of ANC.5 In most recent
literature, however, alkalinity is used primarily in
discussing total dissolved concentrations of
bicarbonates, carbonates, and hydroxides in water.
The total capacity of a surface water to neutralize
acidity can include other chemical and biological
processes—the most important of which are the
biologically mediated processes of sulfate (SO42~)
and nitrate (NO3-) reduction.
Surface waters with higher ANC are generally
more resistant to acidification and have higher pH
levels. That is, lakes and streams with ANC greater
than 200 microequivalents per liter (ueq/l) have
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- Appalachi-
ans
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
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 long-term,
chronically toxic, and lethal environmental effects.
Second, acutely acidic conditions can rapidly
develop during periods leading to, accompanying,
or following episodic events, which primarily
accompany discharges of storm and snowmelt
water runoff. Pulses of highly acidic water flushing
into and through soils, streams, and lakes often
expose soil and aquatic biota to short-term,
acutely toxic, lethal chemical conditions.
When considering acidification effects, it is impor-
tant to recognize that the earliest effects on bio-
logical components of an aquatic ecosystem
commonly accompany early episodic acidification
events. For acid-sensitive fish species in some
lakes and streams, for example, these events can
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
cause complete spawning or recruitment failures.
As chronic acidification becomes more pro-
nounced, such effects can become more frequent
and may result in further harmful effects to overall
species richness. In contrast, for surface waters in
regions recovering from acidification, this se-
quence reverses as occasions of episodic effects
become less and less frequent, until acidification
effects apparently end. In some recovering re-
gions, however, proportions of lakes and streams
affected by episodic acidification may increase for
a time before decreasing. This temporary increase
would likely result wherever recovery of lakes and
streams affected predominantly by long-term
chronic acidification progressed more rapidly then
recovery of those waters affected predominantly
by short-term episodic acidification.
As more fully discussed in several of the following
sections, available information indicates that sur-
face waters with ANC<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 load 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, and through biological
uptake processes adjacent to plant roots and soil
microbes. Watershed ions exchanged for depos-
ited ions enter soil water solutions and can subse-
quently 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
in time due to internal watershed processes (i.e.,
"delayed" acidification).
6 National Academy of Sciences. 1984. Acid Deposition:
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, Washington, DC. 11 pp.
10 .
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CHAPTER 2: ENVIRONMENTAL GOALS
Watershed processes regulating base cation ex-
change and retention of atmospherically deposited
sulfur are two primary controls on surface water
acidification rates that must be understood to al-
low projection of potential acidic deposition ef-
fects. In addition, as discussed later in this report,
an expanding body of research shows that nitrogen
deposition often also can be an important compo-
nent and potentially a primary cause of present
and future soil and surface water acidification ef-
fects within some regions of North America.
Other important research further reveals that
chronic nitrogen loading and ozone are the two
most important air pollutants adversely affecting
forest ecosystems in North America.7
Past assessments minimized considering the effects
of nitrogen deposition as a potential cause of sur-
face water acidification, largely because nitrogen
is an essential nutrient for many biological proc-
esses. The frequent scarcity of nitrogen in many
environments, relative to other required nutrients,
often limits plant growth and production of other
organisms. Thus, because nitrogen is commonly
thought of as a fertilizer that often quickly incorpo-
rates into organisms, many researchers held the
view, and some continue to do so, that an insuffi-
ciently small mass of atmospherically deposited ni-
trogen would typically remain, after the fertiliza-
tion effects of nitrogen were maximized, to acidify
soils and surface waters or to cause adverse effects
to resident plants or animals. In fact, nitrogen
deposition to some areas, particularly parts of the
West, occurs primarily as the non-acid, "buffered"
compound ammonium nitrate. Second, biological
demand for nitrogen is also highly dependent on
such factors as tree species composition, forest
age, disease, fire, and land management practices.
This produces high geographical variability among
watersheds and high seasonal variability within
watersheds and makes it difficult to understand
and model potential watershed acidification proc-
esses associated with nitrogen deposition.
Two lines of evidence now suggest that we need to
more thoroughly address acidification problems
caused by nitrogen, regardless of the chemical
form of its deposition from atmosphere. First, in-
creasing evidence reveals that dry deposition is
usually a significant, and sometimes the dominant
portion of total atmospheric deposition of both sul-
fur and nitrogen. For example, across all sites in-
cluded in a recent review, dry deposition ranged
from 9 to 59 percent of total deposition
(wet+dry+cloud) for S, 25 to 70 percent for NO3,
and 2 to 33 percent for NH3.8 Thus, in many areas
nitrogen deposition especially attaches to foliage
primarily in dry chemical form (e.g., as nitric acid
vapor or ammonium nitrate), rather than with
deposition in precipitation. Most affected vegeta-
tion can not readily derive nutrient benefits from
such deposits of dry nitrogen. Because of this,
Taylor et al. (see footnote 7) suggested that the
analogies equating atmospheric nitrogen deposi-
tion to nitrogen additions accompanying broadcast
fertilization are often inappropriate. This is espe-
cially the case when considering adverse acidifi-
cation effects from dry deposition of nitric acids at-
tached to forest and other terrestrial vegetative sur-
faces.
Second, both seasonal conditions and physiologi-
cal processes produce limits on potential maxi-
mum nitrogen use by terrestrial organisms in many
low productivity watersheds. These limits restrict
the total mass of deposited nitrogen that can be in-
corporated into organic matter by the combined
plant and microbial growth needs within water-
sheds. When these needs are met, i.e., when ni-
trogen is no longer the limiting nutrient for bio-
logical production and growth, nitrogen com-
pounds can increasingly accumulate through a wa-
tershed process termed nitrogen saturation.9 In
other words, when nitrogen inputs are non-zero,
many low productivity forests that are not affected
by widespread management or natural distur-
bances have the potential to reach nitrogen satura-
tion at some time.10 Moreover, increasing rates of
nitrogen loadings (e.g., through acidic deposition)
would tend to shorten times required to achieve
watershed nitrogen saturation and increase the ni-
trogen leaching losses expected after saturation
occurs (see footnote 10).
7 Taylor, G.E., D.W. Johnson, and C.P. Andersen. 1994. Air
pollution and forest ecosystems: a regional to global
perspective. Ecological Applications 4:662-689.
8 Lovett, C.M. 1994 Atmospheric deposition of nutrients and
pollutants in North America: an ecological perspective.
Ecological Applications 4:629-650.
9 Aber, J.D., K.J. Nadelhoffer, P. Steudler, and J.M. Melillo.
1989. Nitrogen saturation in northern forest ecosystems.
Bioscience 39:378-386.
10 Aber, J.D., J.M. Melillo, K.J. Nadelhoffer, J. Pastor, and R.D.
Boone. 1991. Factors controlling nitrogen cycling and
nitrogen saturation in norther temperate forest ecosystems.
Ecological Applications 1:303-315.
11
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
Beyond the biological growth processes, potential
for nitrogen saturation can also be affected through
physical processes that incorporate and store ni-
trogen within watersheds. These processes are
poorly understood and generally thought to be a
relatively minor component of the nitrogen cycle
in watersheds. Potential for saturation also can be
extended due to microbial denitrification. This
process, which emits nitrogen gases from water-
sheds, most commonly occurs in .anaerobic
(lacking oxygen) water-saturated soils and deep
aquatic sediments, and denitrification rates tend to
increase with increases in environmental tempera-
tures.11 Disturbances produced by forest man-
agement and natural events, e.g., timber windfalls,
insect infestations, or disease infections, can effec-
tively either shorten or lengthen times to potential
watershed nitrogen saturation, depending on the
nature of the disturbance.
As suggested in the literature cited above, initial
symptoms indicating the potential onset of water-
shed nitrogen saturation can include higher nitro-
gen concentrations in plant foliage and forest litter,
faster litter decay rates, enhanced nitrification
rates, increased soil acidity, and increased loss of
nitrogen gases from watershed soils. With con-
tinuing saturation, nitrogen compounds may in-
creasingly leach to layers below the root zones
and from watersheds through surface waters drain-
ages, principally as NO3~. Further, excess avail-
ability of NO3- in watersheds can lead to depletion
of base cations and surface water acidification
through the same processes as those involving ex-
cess SO42-. Overall, other environmental factors
equal, times to nitrogen saturation tend to be
shorter for forested watersheds with lower water
availability and higher nitrogen deposition rates.
Forest watersheds in diverse regions of North
America show clear symptoms of developing wa-
tershed nitrogen saturation.12 In particular, early
stages of saturation have been noted for surface
waters and/or watersheds in New Hampshire,
Vermont, the Adirondack and Catskill mountains
of New York, and West Virginia; advance stages of
saturation have appeared in several high elevation
streams within the Great Smoky Mountains in
Tennessee and North Carolina.13 Symptoms of
ongoing watershed nitrogen saturation have also
be reported for areas of the Blue Ridge Mountains
in North Carolina,14 Colorado Rocky Mountains,15
California mountain ranges down wind from Los
Angeles,16 and in the Algoma district of northern
Ontario.17 Other data suggest that many European
forests apparently are becoming nitrogen satu-
rated; additional emission controls options to pro-
tect European forests and surface waters from the
detrimental effects of excessive nitrogen deposition
are being evaluated.18 Some analyses suggest that
to reverse trends of adverse effects in some regions
of Europe, nitrogen deposition may need to be re-
duced by 85 percent.19 With considering the
European situation, however, it is important to
note that nitrogen deposition rates of concern
there are many times greater than occurs to areas
containing sensitive resources in North America.
11 Campbell, R. 1977. Microbial Ecology. John Wiley and
Sons, New York.
12 Johnson, D.W. and S.E. Lindberg, editors. 1992.
Atmospheric Deposition and Forest Nutrient Cycling: a
Synthesis of the Integrated Forest Study. Ecological Series
91. Springer-Verlag. New York, USA.
13 Stoddard, J.L. 1994. Long-term changes in watershed
retention of nitrogen: its causes and aquatic consequences.
Pages 223-284 in L.A. Baker, editor. Environmental
Chemistry of Lakes and Reservoirs. Advances in chemistry
series number 237, American Chemical Society,
Washington, DC.
14 Aneja, V.P., and A.B. Murthy. 1994. Monitoring deposition
of nitrogen-containing compounds in a high-elevation forest
canopy. Journal of Air & Waste Management Association
44:1109-1115.
15 Baron, J.S., D.S. Ojima, E.A. Holland, and W.j. Parton.
1994. Analysis of nitrogen saturation potential in Rocky
Mountain tundra and forest. implications for aquatic
systems. Biogeochemistry 27:61-82.
16 Fenn, M.E., and A. Bytnerowicz. 1993. Dry deposition of
nitrogen and sulfur to ponderosa and Jeffrey pine in the San
Bernardino National Forest in southern California.
Environmental Pollution 81:277-285.
Fenn, M.E., and M.A. Poth. 1994. Preliminary evidence of
nitrogen saturation in the San Bernardino Mountains in
southern California. Presented at the 16th International
Meeting for Specialists in Air Pollution Effects on Forest
Ecosystems, Sept. 7-9, 1994, New Brunswick, Canada.
Riggan, P.J., R.N. Lockwood, and E.N. Lopez. 1985.
Deposition and processing of airborne nitrogen pollutants in
Mediterranean-type ecosystems of southern California.
Environment Science and Technology 781-789.
17 Mitchell, M.J., N.W. Foster, J.P. Shepard, and I.K. Morrison.
1992. Nutrient cycling in Huntington Forest and Turkey
Lakes deciduous stands: nitrogen and sulfur. Canadian
Journal of Forest Research 22:457-464.
18 Sullivan, T.J. 1993. Whole-ecosystem nitrogen effects
research in Europe. Environmental Science and Technology
27(8):1482-1486.
19 Freemantle, M. 1995. The acid test for Europe. Chemical
and Engineering News. May 1, 1995:10-17.
12
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CHAPTER 2: ENVIRONMENTAL GOALS
Shifting roles between nitrogen and sulfur deposi-
tion appear both in regions of North America and
in Europe. Studies from Europe first showed that
increased deposition of nitrogen compounds ap-
pear to offset potential benefits from reduced
deposition of sulfur compounds.20 Similarly in
North America, for example, lakes included in the
Adirondack Long-Term Monitoring program have
shown long-term temporal declines in concentra-
tion of SO42 "consistent with declining SO2 emis-
sions and declining SO42' concentrations in pre-
cipitation falling across the eastern United States.21
The decreased lake SO42~ concentrations have not,
however, coincided with a recovery in lake levels
of ANC. Rather, ANC values have continued to
decline in some lakes. This decrease accompanies
increasing concentrations of NO3~ in most of the
lakes monitored, and decreasing atmospheric
deposition of base cations. To illustrate further,
soil NO3~ in the Woods Lake watershed within the
Adirondacks was positively and SO42~ was nega-
tively correlated with calcium and aluminum con-
centrations. Thus, from recent data, NO3~ concen-
trations appear to exert greater relationship to spa-
tial and seasonal temporal patterns of calcium and
aluminum in the assessed surface waters. Further,
NO3" concentrations appear to have increased in-
fluence on soil acidification in this watershed than
does SO42" concentrations.22 The effect that de-
creasing deposition of base cations has on water-
shed acidification processes remains poorly under-
stood and requires additional research.
Additional research is needed to evaluate more
completely the extent that soil nitrogen accumula-
tions are now and may potentially become a pri-
mary cause of acidification in watersheds, not only
in the Adirondack Mountains, but in other regions
of the North America. Recent research finds that
major uncertainties regarding the capacities of wa-
tersheds to assimilate nitrogen deposition limit
abilities to project directly surface water acidifica-
20 Grennfelt, P. and H. Hultberg. 1986. Effects of nitrogen
deposition on the acidification of terrestrial and aquatic
ecosystems. Water, Air, and Soil Pollution 30:945-963.
Henriksen, A. and D.F. Brakke 1988. Increasing
contributions of nitrogen to the acidity of surface waters in
Norway. Water, Air, and Soil Pollution 42:183-201
21 Driscoll, C.T., and R. Van Dreason. 1993. Seasonal and
long-term temporal patterns in the chemistry of Adirondack
lakes. Water, Air and Soil Pollution 67:319-344.
22 Geary, R.J., and C.T. Driscoll. 1995. Forest soil solution:
acid/base chemistry and response to calcite treatment.
Biogeochemistry (in press).
tion rates from acidic deposition, and that com-
puter modeling will be vital to assess concerns re-
garding watershed nitrogen saturation.23 Addi-
tionally, short-term and seasonal changes in depo-
sition and processing patterns often tend to mask
true long-term trends in both atmospheric and wa-
tershed processes, including nitrogen saturation.
Therefore, true long-term trends are best deter-
mined through consistently implemented and
regularly sampled monitoring programs.24
Of additional concern are episodes of storm flow
or snowmelt runoff that can expose organisms to
short-term, acutely lethal, acidic water.25 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.26 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
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 sig-
nificant seasonal cause of episodic acidification in
surface waters in some regions, often occurring at
the most biologically significant time of year (i.e.,
during fish spawning and reproduction).
23 Taylor, G.E., D.W. Johnson, and C.P. Andersen. 1994. Air
pollution and forest ecosystems: a regional to global
perspective. Ecological Applications 4:662-689.
24 Likens, G.E. 1992. The Ecosystem Approach: Its Use and
Abuse. Volume 3, O. Kinne (editor), Excellence in Ecology.
Ecological Institute, Germany.
25 Wigington, P.J., Jr., J.P. Baker, D.R. DeWalle, W.A. Kretser, P.S.
Murdoch, H.A. Simonm, 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
Biological Results of the Episodic Response Project. EPA/600/R-
93/190. Office of Research and Development, U.S.
Environmental Protection Agency, Washington, DC.
26 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 - Regional Case Studies. Springer-Verlag, New
York, NY.
13
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
2.2.2 Episodic Acidification
Most remaining sections of this chapter primarily
focus on chronic effects to surface waters associ-
ated with long-term exposure to acidic deposition,
because much past research has emphasized proc-
esses leading to long-term chronic acidification. In
surface waters that have not completed processes
leading to chronic acidification or are in the proc-
ess of recovering from chronic acidification, the
largest impacts of acidic deposition most com-
monly 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 result of rainstorms or
snowmelt. Acid anions (i.e., sulfate and nitrate)
that reach surface waters during 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 sys-
tem during the high storm flows. Acids or toxic
aluminum compounds leached from soils may ac-
company the acid anions during leaching. Both
sulfates and nitrates originating from atmospheric
deposition can contribute significantly to episodic
acidification events.27 Episodic acidification can
cause lakes and streams that have positive ANC
during most of the year to become acidic (ANC<0
ueq/l) and have high toxic aluminum concentra-
tions for periods of hours to days.
Preliminary results (pending technical peer review)
of a recent modeling study compared projections
for the minimum observed ANC during the worst
annual episodes to pre-episode ANC for lakes and
streams in the Northeast and mid-Appalachian re-
gions.28 Data on which this study was based were
collected between the early-mid 1980s to Spring
1990. (Appropriate data were not available to
conduct a similar analysis for streams of the
Southern Blue Ridge Province.) Because the sever-
ity of episodes is influenced markedly by deposi-
tion loadings that occurred during the study pe-
riod, empirical relationships reported in the fol-
27 O'Brien, A.K., 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(9):3029-3039.
28 Church, M.R., ). VanSickle, P.J. Wigmgton, Jr., and B. J.
Cosby, Jr. 1994. Combining process and empirical models
to predict future episodic acidification of streams and lakes.
Presented at NATO Advanced Research Workshop on
"Ecosystem Modeling: Delineating the Possible from the
Impossible", Bishofsgrun, Germany, February 20-25, 1994.
lowing comparisons should be considered appli-
cable only for the conditions (e.g., deposition
loadings) during which the data were collected.
That is, these calculated empirical relationships
should not be extrapolated to different deposi-
tional loadings occurring at other times.
Across the sample of lakes and streams studied and
over the time period of this modeling study, ap-
proximately 70 percent of lakes within the target
population were projected to be affected during
the worst annual episode in the Adirondacks. This
was about 3.5 times the number of lakes in this
target population observed to be chronically acidic
during the NSWS survey. Similarly, for streams in
the mid-Appalachian, approximately 30 percent of
the target population was projected as likely to
become acidic during the worst annual episode
occurring over the study period. This projection
was approximately 7 times the number of chroni-
cally acidic stream reaches found for the target
population in this region during spring index
sampling conducted by the National Stream Sur-
vey. Further decreases in levels of acidic deposi-
tion in these regions would likely lead to decreases
in the number and severity of acidic episodes.
EPA recently completed its Episodic Response Pro-
ject.29 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
29 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
Biological Results of the Episodic Response Project. EPA/600/R-
93/190. Office of Research and Development, U.S.
Environmental Protection Agency, Washington, DC.
14
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CHAPTER 2: ENVIRONMENTAL GOALS
* 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 Alim levels than
would have occurred due to natural
processes alone.
* Even when SO42~ and NC>3~ 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
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, the authors concluded that
stream assessments based solely on chemi-
cal measures during low flow do not accu-
rately predict 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. Rela-
tive to streams that did not episodically
acidify, both density and biomass of brook
trout were significantly lower in stream
reaches that became episodically acidic.
This last point supports the hypothesis that epi-
sodic acidification can be a 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 ecological
effects from episodic events often blend with and
become indistinguishable from all other effects ac-
companying chronic acidification. The findings
from this study and from other analyses clearly
point to the importance of considering potential
effects of both long-term chronic and short-term
episodic acidification when considering the effec-
tiveness of an acid deposition standard or stan-
dards.
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. Nevertheless, existing evidence
clearly show that both SO42~ and NO3~ deposition
can have major influences on both long-term and
short-term (episodic) surface water acidification
processes. Further, the relative importance be-
tween these two chemicals in producing acidifica-
tion effects often varies among regions and seasons
within regions. Evaluating the effectiveness of and
options for acid deposition standards should in-
clude simultaneous consideration of both acidifi-
cation 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.
15
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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. Whereas most
watersheds can assimilate considerable quantities
of both chemicals without significant adverse ef-
fects, their assimilative capacities are finite. This is
especially the case in watersheds holding acid-
sensitive aquatic resources, a primary focus of
concern in this report. Watershed assimilative ca-
pacities vary with how rapidly deposited chemi-
cals are assimilated and the time over which re-
peated deposition events impair these abilities. In
other words, there are varying deposition fre-
quencies, rates, and durations when watershed as-
similative capacities reach saturation. This is
sometimes called steady state, the point when the
output (loss) of a substance from a watershed (e.g.,
sulfur leaving a watershed in the form of SO42~ in
stream flows) equals its input (e.g., as sulfur-con-
taining compounds in deposition) 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 regrowlh, fire, in-
sect infestation, disease, as well as natural vegeta-
tive succession. Despite these factors, nitrogen
saturation does appear to occur in some systems,
as discussed above in Section 2.2.2. Conse-
quently, assessing the history of both sulfur and ni-
trogen deposition is important in assessing
long-term regional effects attributable to cumula-
tive loadings by acidic deposition. Similarly, a re-
source's or region's current sensitivity to acidic
deposition also needs to be evaluated with respect
to the historical deposition patterns and responses.
Many regions with ample buffering capacity and
remaining capabilities to retain inputs of sulfur and
nitrogen may benefit little from future decreases in
acidic deposition. Other regions facing imminent
depletion of their buffering or adsorption capaci-
ties, however, would likely be highly responsive to
decreasing deposition rates. The DDRP, discussed
in Section 2.5.2, provides a useful beginning for
understanding underlying relationships and defin-
ing remaining uncertainty about dynamics of at-
mospheric sulfur deposition in watersheds within
three regions of the eastern United States. The Ni-
trogen Bounding Study (NBS), discussed in Sec-
tion 2.5.3, provides additional useful results to im-
prove our understanding of the influence of nitro-
gen 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.30 Degrees of ecological recovery varied
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.31
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-
30 Baker, J.P., D.P. Bernard, S.W. Christensen, M.J. Sale, J. Freda,
K. Heltcher, D. Marmorek, L. Rowe, P. Scanlon, G. Suter, W.
Warren-Hicks, and P. Welbourn. 1990. Biological Effects of
Changes in Surface Water Acid-base Chemistry. NAPAP Report
13. In: Volume II, National Acid Precipitation Assessment
Program, Acidic Deposition: State of Science and Technology.
Superintendent of Documents, Washington, DC.
31 Keller, W., J.R. Piblado, and ). Carbone 1992. Chemical
responses of acidic lakes in the Sudbury, Ontario area to
reduced smelter emissions, 1981-1989. Canadian Journal of
Fisheries and Aquatic Sciences 49 (Suppl. 1 ):25-32.
16
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CHAPTER 2: ENVIRONMENTAL GOALS
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.32 Alterna-
tively, rehabilitating or rejuvenating selected at-
tributes or functions may be all that is required for
restoration to be deemed successful. For example,
some have argued that short-term mitigation
measures such as liming a specific number of lakes
may be an appropriate, albeit temporary, step.
A recent work group review and assessment con-
cluded that uncertainty remains concerning the
definition of appropriate measures of reversibility
and recovery for acidified ecosystems.33 Differ-
ences exist particularly between setting goals
based on human-centered objectives (e.g., fish
production for human use) versus more intangible
ecological and conservation purposes. Further, as-
sessment of ecosystem recovery following deposi-
tion reductions can be obscured by other envi-
ronmental perturbations such as climate change
and modified land-use practices.
Importantly, beyond this uncertainty, this work
group also concluded that changes in abiotic (i.e.,
chemical and physical) responses of ecosystems to
acidic deposition are reversible when acidic depo-
sition is decreased. As abiotic environmental con-
ditions improve, many biological components of
the ecosystem also will progress, sometimes rap-
idly, toward recovery. But notable delays in this
biotic (i.e., biological) recovery can occur, often
depending on the rates at which emissions are re-
duced and on watersheds factors affecting internal
reposes to the remaining deposition. Further,
some irreversible changes may not recover.
32 Cairns, j., Jr. 1989. Restoring damaged ecosystems- Is
predisturbance condition a viable option? Environmental
Professional 11:152-159.
33 Dise, N, W. Ahlf, C. Brahmer, B.J. Cosby, J. Fott, M. Hauhs, I.
Juttner, K. Kreutzer, C.C. Raddum, and R.F. Wright 1994.
Croup Report: Are Chemical and Biological Changes
Reversible? Pages 275-381 in C E.W. Steinberg and R.F. Wright
(editors) Acidification of Freshwater Ecosystems: Implications
for the Future. J. Wiley and Sons, New York, NY.
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 potential
produced by acidic deposition over discrete geo-
graphic regions. This approach helps to define re-
gional need for and effectiveness of acid deposi-
tion standards.
In this assessment, sensitivity is an inherent attrib-
ute of an individual resource that increases its sus-
ceptibility to likely adverse effects due to acidic
deposition. Exposure is determined by the deposi-
tion intensity, frequency, duration, and specific
times that acidic deposition falls into an area. Risk
is the probability that exposure to potentially haz-
ardous environmental conditions produced by
acidic deposition will exceed the tolerance level
for a sensitive resource and cause an adverse ef-
fect. For any sensitive resource to be at high risk
from any hazardous substance or environmental
condition, it must have a high probability of being
sufficiently exposed to the substance or condition,
such that its inherent ability to tolerate the change
will be exceeded and harmful effects will likely re-
sult.34 Because environmental resources have
ranges of sensitivities and risks to potential effects
caused by acidic deposition, resources having
equivalent sensitivities can have different risk po-
tential for effects depending on where they are lo-
cated.
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 ap-
propriate primary focus of protection,
* Appropriate environmental assessment in-
dicators, and
* The extent of protection afforded.
34 This discussion of sensitivity and risk is consistent with the
concepts presented by the Risk Assessment Forum's
Framework for Ecological Risk Assessment. EPA/630/R-
92/001 U.S. Environmental Protection Agency, Washington,
DC
17
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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, watersheds in a region with highly alkaline
surface waters may begin to leak nitrogen into its
surface waters. This could lead to no change in
surface water acidity, but could lead to significant
increases in eutrophication downstream. Conse-
quently, 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 groupings), different de-
grees of resource sensitivities, and different time-
scales at which resources experience effects. Also,
each use has different scientific and policy impli-
cations. Such differences affect each potential cri-
terion used in determining acid deposition stan-
dards. Thus, when considering potential resource
effects and risks, it is important to carefully define
the specific ecosystem components within the re-
gion and specific concerns regarding the sensitivity
being addressed.
An early MAS report indicated that lakes and
streams with alkalinity of 200 ueq/l or less are
sensitive and subject to damage at moderate acidic
deposition rates, whereas surface waters with al-
kalinity of 40 ueq/l or less are critically sensitive to
such effects.35 Although alkalinity or ANC (as
noted in Section 2.2, these terms are essentially
synonyms) is an important response indicator of
potential surface water sensitivity, it is not the only
relevant response indicator of sensitivity. For ex-
ample, the presence or absence of acid-sensitive
fish, invertebrates, algae, and higher plant species
are other relevant indicators of potential sensitivity
and acidification problems in lakes and streams.
Further, knowing the ANC of surface or ground
waters provides little indication of the actual sensi-
tivity of neighboring terrestrial resources. For ex-
ample, injury to red spruce foliage attributable to
acidic deposition typically has little direct relation-
ship to the ANC of neighboring soils or waters.
Consequently, when there is a need to assess po-
tential effects of acidic deposition on terrestrial re-
sources or ecosystems, assessments often need to
consider other parameters or indicators of sensitiv-
ity 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 management
and resource use practices (e.g., changes in fishing
pressure, point and nonpoint nutrient discharges,
mining runoff, and other watershed activities) also
potentially confound 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, watersheds, and the natural
resources they contain. These interactions cause
differences in sensitivity and responses to acidic
deposition among resources within individual
watersheds and among adjacent watersheds. Many
factors summarized in the exhibit are discussed in
greater detail in subsequent sections.
Most factors presented in Exhibit 2 that can in-
crease the sensitivity of watersheds to acidifica-
tion, also usually correlate with increasing poten-
tial for watersheds to saturate with nitrogen, which
may lead to concurrent increases in surface water
acidification effects caused by nitrogen deposition.
For example, older forests are often less efficient in
using deposited nitrogen and, consequently, can
leach greater quantities of potentially acidifying ni-
trogen compounds into surface drainage waters.36
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
environmental changes in soils, lakes, and streams.
35 National Academy of Sciences. 1983. Acid Deposition:
Atmospheric Processes in Eastern North America. National
Academy Press.
36 Mitchell, M.J., N.W. Foster, J.P. Shepard, and I.K. Morrison.
1992. Nutrient cycling in Huntington Forest and Turkey
Lakes deciduous stands: nitrogen and sulfur. Canadian
Journal of Forest Research 22:457-464.
18
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CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBIT 2. PRINCIPAL WATERSHED AND SURFACE WATER CHARACTERISTICS
THAT INFLUENCE RESOURCE SENSITIVITY TO ACIDIFICATIONa
Category
Bedrock geology
Soils:
Buffering capacity
Depth
SO42~ adsorption
SO42~ reduction
NO3~ retention
Topography
Elevation
Watershed to surface water area ratio
Lake size
Lake flow characteristics
Flushing rate for drainage lakes
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
Higher
Lower
Smaller
Seepage
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
Lower
Higher
Larger
Drainage
Lower
Deciduous
Agriculture, municipal
Deforestation
Ffigher (> 200 ueq/l)
Higher potential
Less oligotrophic to eutrophic
Higher concentrations
Absent
Lower
Lower
Longer
Higher
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.
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;
19
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
* 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 and most acid sen-
sitivity species in sensitive surface waters, ANC
often is a useful response indicator. If concern is
broadened to include the potential sensitivity of all
natural resources (e.g., forest tree species), the ap-
proach 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
sensitive" 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
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 ANC
of 50 ueq/l or less, a value that approximates the
alkalinity value of 40 ueq/l considered by NAS 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 ANC of
50 ueq/l or less can generally be interpreted as ap-
plying 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 effect of concern.37 Sensitive resources
are at low risk when located where acidic deposi-
tion loads are currently below and are projected to
remain below thresholds likely to cause significant
effects. 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 acidic deposition than are similar
resources in eastern North America. Because cur-
rent 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,
chronic acidification effects in the West might ex-
ceed 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 significant 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
37 Risk Assessment Forum. 1992. Framework for Ecological Risk
Assessment. EPA/630/R-92/001. U.S. Environmental Protection
Agency, Washington, DC.
20
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CHAPTER 2: ENVIRONMENTAL GOALS
possible relationships? The DDRP and NBS, as
discussed in Sections 2.5.2 and 2.5.3, begin to an-
swer these questions. Another question, outside
the scope of this report, is whether a risk of im-
paired productivity exists for resources in regions
not sensitive to harmful effects from acidification
(such as forests managed for timber production
and agricultural crops) due to fertilization effects
of high sulfur and nitrogen deposition?
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.38 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 States39 and Canada,40 as well as from
This review is primarily drawn from conclusions presented by
P.M. Irving (editor). 1991. Acidic Deposition: State of the
Science and Technology - Summary Report of the U.S. National
Acid Precipitation Assessment Program. National Acid
Precipitation Assessment Program, Washington, DC.
39 The primary source for this additional summary information is
NAPAP. 1992. Report to Congress. National Acid Precipitation
Assessment Program, Washington, DC.
40 Brydges, T.G. 1991. Critical loads, reversibility and
irreversibility of damage to ecosystems. Pages 245-260 in
Electricity and the Environment, International Atomic Energy
Agency, Vienna, Austria.
more recent research assessing the sensitivity of
individual resources.
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. FOREST SOIL CHEMISTRY EFFECTS: In the east-
ern United States, concentrations of sulfur
in forest soils generally follow trends in sul-
fur deposition. In some regions, soil con-
centrations of calcium and magnesium are
inversely related to sulfur deposition loads
resulting in soil nutrient depletion. Further,
a recent review reports that most calcium
and magnesium in the soil of the spruce-fir
ecosystem in the Northeast was lost 20-40
years ago due to acidic deposition, when
deposition rates were increasing rapidly.41
This review also reports that, while ex-
perimental 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. It concludes further, that within
forested regions, acidic deposition primar-
ily exerts its stress on nutrient cycling and
aluminum mobilization. No systems pres-
ently appear to be deficient in necessary
nutrients as a result of deposition and it is
unknown if they ever will become defi-
cient, even with accelerated leaching
rates.42 Many important studies are con-
tinuing, however. The most apparent influ-
ence of soil chemistry responses attribut-
41 Brandt, C.J. 1994 Acidic Deposition and forest Soils:
Potential Changes in Nutrient Cycles and Effects on Tree
Growth. Report to Watershed Response Program,
Environmental Research Laboratory, U.S. Environmental
Protection Agency, Corvallis, OR
42 Johnson, D.W. 1992. Relationships among atmospheric
deposition, forest nutrient status, and forest decline. Pages
577-580 in D.W. Johnson and S.E. Linberg (editors).
Atmospheric deposition and forest nutrient cycling: A
synthesis of the Integrated Forest Study. Ecological Studies
Volume 91. Springer-Verlag, New York.
21
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
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 the exception of
observed decline in health of the Ontario
maple.43 The vast majority of forests in the
United States and Canada have not de-
clined. Some evidence suggests that lichen
communities and chemistries may be use-
ful early indicators of forest health effects.
Ambient acidic deposition levels have not
been shown to be responsible for agricul-
tural crop yield reductions.
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 ANC of 50 ^ieq/l or
more—probably have not experienced re-
cent chronic declines in pH or ANC asso-
ciated with acidic deposition.
4. REGIONS CONTAINING ACIDIFICATION AF-
FECTED SURFACE WATERS: The National Sur-
face Water Survey (NSWS) conducted un-
der the auspices of NAPAP in 1934-85,
identified six "high-interest areas" contain-
ing most of the surface waters surveyed
43 U.S. EPA. 1994. U.S. Canada Air Quality Agreement Progress
Report.
(95 percent of the lakes and 84 percent of
the stream reaches) that were chronically
acidified as indicated by concentrations of
inorganic anions, predominately SO42',
NO3-, and CK These areas include the
southwest Adirondack Mountains, New
England, mid-Appalachian 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
subregion, mid-Appalachians, eastern por-
tion 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
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
below about 6.0-6.5 (Exhibit 3) and as inor-
22
-------
CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBIT 3. CRITICAL pH FOR SELECTED TAXA IN LAKES AND STREAMS'* b
6.5
Critical pH Levels for Selected Aquatic Organisms
6.0 5.5 5.0
4.5
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. 1990 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
44
ganic monomeric aluminum concentra-
tions increase above 30-50 ^jg/l, especially
at concentrations of dissolved calcium less
than 2 to 3 mg/l. These NAPAP conclu-
sions generally followed those from a
slightly earlier review of more than 300
peer-reviewed publications that reported
results from field and laboratory studies of
acidic effects.44 That review concluded,
across all aquatic taxa and systems studied,
the overall number and severity of reported
adverse effects tend to increase as pH de-
creases below about 6.5 to 6.0 and as total
Marcus, M.D., B.R. Parkhurst, J.P. Baker, C.S. Creager, T.S.
Fannin, C.G. Ingersoll, D.R. Mount, and F ). Rahel 1986.
An Evaluation and Compilation of the Reported Effects of
Acidification on Aquatic Biota. Volume!: Compiled Data.
EPRI EA-4825. Final Report. Electric Power Research
Institute, Palo Alto, CA.
aluminum concentrations increase above
about 10 ug/l. Further, adverse effects tend
to be reduced at given adverse pH and
aluminum levels as dissolved calcium con-
centrations increase above 1 to 2 mg/l; for
increases above 6 to 8 mg/l, calcium ap-
pears to have little additional benefit. Both
reviews suggested that changes in water
quality produced by increased acidity tend
to affect aquatic species first by decreasing
their ability to survive, reproduce, or com-
pete in acidic surface waters. Such re-
sponses can eliminate affected species and
reduce species richness (i.e., the number of
species living within a surface water). Such
changes typically occur first in affected sur-
face 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 wa-
ters). System-level processes such as com-
23
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
position, nutrient cycling, oxygen usage,
and photosynthetic rate are fairly robust
and are affected only at relatively high lev-
els of acidity (e.g., chronic pH less than
5.0-5.5).45
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
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 should likely first
focus on recovery and protection of sensi-
tive aquatic resources in eastern North
America (including the region from Florida
north and east into southern Ontario). A
second focus may include red spruce for-
ests in the northern Appalachians and high-
elevation regions of the Northeast. These
regions hold most of the natural resources
showing the greatest magnitude of past and
ongoing impacts and have the highest on-
going rates of acidic deposition. Both fac-
tors indicate the potential need for possible
additional deposition control. Protection
of sensitive aquatic resources should par-
ticularly focus on lakes and streams located
where watersheds are smaller, have shal-
low acidic soils with rapid, shallow subsur-
face 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,
45 Schindler, D.W. 1987. Detecting Ecosystem Responses to
Anthropogenic Stress. Canadian journal of Fisheries and
Aquatic Sciences 44(Suppl.):6-25.
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. Additional monitoring and as-
sessment to evaluate whether continuing
acidic deposition will affect sensitive re-
sources would be necessary at all areas
listed above, as well as in parts of Maine,
New Hampshire, Vermont, Massachusetts,
Connecticut, and Rhode Island; northern
Minnesota; parts of the Ozark Mountains,
Ouachitas Mountains, the Carolina Pied-
mont, and the Atlantic Coastal Plain; and
parts of the Rocky Mountains, Sierra Ne-
vada 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 reported by NAPAP and other
sources cited above, to protect aquatic re-
sources in sensitive watersheds from the ef-
fects of long-term, chronic acidification, a
general goal is to maintain the pH of sensi-
tive lakes above pH 6.0-6.5 and inorganic
monomeric aluminum below 30-50 ug/l.
To protect these resources from the poten-
tial effects of episodic, acute acidification,
surface water ANC should be maintained
at or above 50 ueq/l. No single water qual-
ity goal, however, addresses all needs to
protect sensitive watershed resources.
Goals to protect resources may also ad-
dress site-specific needs to maintain sensi-
tive species, species of special concern
(e.g., listed threatened or endangered spe-
cies), and species richness in these sensi-
tive watersheds. When establishing protec-
tion goals and objectives for sensitive
aquatic resources, this effort certainly must
include recognition and allowances that
pH levels less than 6.0 and ANC less than
50 ueq/l occur in some naturally acidic
(e.g., organically acidic) surface waters,
and that levels of pH less than 6.0 can oc-
cur naturally in some locations accompa-
nying periods of episodic stormwater and
snowmelt runoff from relatively unpolluted
24
-------
CHAPTER 2: ENVIRONMENTAL GOALS
deposition. That is, the specific environ-
mental objectives of any acid deposition
standard should accommodate the natural
ranges of chemical qualities occurring in
waters in the environment. Furthermore,
they may be designed to protect those
special biological communities evolved to
inhabit naturally acidic surface waters.
2.4.2 Qualitative Assessment of Sensitive
Aquatic Resources in Three Regions
of the United States
Most of the rest of this chapter discusses results
from EPA model analyses for three case study re-
gions: 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 time and primarily
data limitations. Instead, and in addition to the re-
sults of the NAPAP National Surface Water Survey,
EPA supported a recent review that assessed the
responses 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, northern Florida,
and eastern Canada.46 That review addressed ef-
fects from acidic deposition, specifically sensitive
aquatic resources in each region. The approach
incorporated key results available from past re-
search and assessment efforts in North America
and Europe. The major conclusions derived during
this review that specifically related to the three
regions assessed in the United States are presented
below. Conclusions from this study regarding
sensitive Canadian resources are included in the
next section. (Some conclusions from this review
regarding general deposition and response rela-
tionships duplicate the findings of other studies re-
ported above and are not repeated in this section.)
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-
46 Sullivan, T.J., and ).M. Eilers. 1994. Assessment of Deposition
Levels of Sulfur and Nitrogen Required to Protect Aquatic
Resources in Selected Sensitive Regions of North America.
Final Report. Environmental Research Laboratory-Corvallis,
U.S. Environmental Protection Agency, Corvallis, OR.
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, should atmospheric
deposition loadings increase.
* 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
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
25
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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 eflect 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
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.47
* 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
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
ANC near zero in this region may 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.
47 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. 212pp.
26
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CHAPTER 2: ENVIRONMENTAL GOALS
* 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
chemistries of these systems than was pre-
viously estimated. Therefore, the extent of
possible water quality changes due to
acidic deposition 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
due to acidic deposition within the sensi-
tive regions studies in northern Florida.
2.4.3 Qualitative Assessment of Sensitive
Aquatic Resources in 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).48
(Implications to Canadian forests , as summarized
by NAPAP, were reviewed earlier in this chapter.)
This area contains more than 700,000 lakes cover-
ing about 160,000km2 (excluding the Great
Lakes). Extrapolation of survey information indi-
cates that 14,000 lakes are presently acidic. Mod-
eling projections for eastern Canada indicate that
at least an additional 10,000 to 40,000 lakes
would become chronically acidic at 1985 deposi-
tion levels, as watershed input-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
100cm/yr or greater), nearly all lakes in
eastern Canada can be hydrologically
characterized as drainage lakes.49 (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
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. 50
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 many 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.
48 Information presented in this section regarding sensitive 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.
49 Drainage lakes are lakes with permanent surface water inlets
and, usually, outlets. Seepage lakes are lakes with no
permanent surface water inlets or outlets.
50 Sullivan, T.). 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.
27
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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.51
Concern also continues regarding probable epi-
sodic influence of acidic deposition on Atlantic
salmon in tributary streams along the Atlantic
coast from Maine northward.52
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-
51 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.
1V.3-7.
52 Lacroix, G.L., and D.R. Townsend. 1987. Responses of
juvenile Atlantic salmon (Salmon salar) to episodic increases in
acidity in Nova Scotia rivers. Canadian Journal of Fisheries and
Aquatic Sciences 44:1475-1484.
Lacroix, G.L. 1989. Ecological and physiological responses of
Atlantic salmon in acidic organic rivers of Nova Scotia, Canada.
Water, Air, and Soil Pollution 46:375-386.
Lacroix, G.L. 1989. Production of juvenile Atlantic Salmon
(Salmon salar) in two acidic rivers of Nova Scotia. Canadian
Journal of Fisheries and Aquatic Sciences 46:2003-2018.
Lacroix, G.L., D.J. Hood, and J.A. Smith. 1995. Stability and
microhabitat use by brook trout and juvenile Atlantic salmon
after stream acidification. Transactions of the American
Fisheries Society 124:588-598.
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. Special concerns regarding spatial and
temporal issues of importance in developing a po-
tential deposition standard or standards are also in-
troduced.
2.5.1 Model Selection and Application
A major goal established for the Acid Deposition
Standard Feasibility Study is to assess how
changes in acidic deposition loading could affect
the chemistry of surface waters:
* In a dynamic fashion;
* Over regional scales;
* For combinations of sulfur and nitrogen
deposition;
» In areas containing potentially vulnerable
lakes and streams; and
* Where high levels of both sulfur and nitro-
gen deposition exist.
The geographic region selected by the Agency for
this study was the eastern United States, an area
containing lakes and streams with low ANC and
receiving the highest combined levels of sulfur and
nitrogen deposition in the United States.
To address this goal, the assessment must be com-
pleted over the scale of regions. Modeling and as-
sessment of possible effects from acidic deposition
on waters draining from a single watershed (or a
few watersheds) has very limited usefulness, be-
cause extrapolation from such model projections
to regional-scale implications is not reliable. That
is, knowing what such a modeling result would
mean for the distribution of surface waters over a
region is not possible. Thus, a regional modeling
approach is required.
Active work on modeling simulations for the Acid
Deposition Standard Feasibility Study began in
early-mid 1992. At that time there had been pub-
lished only a single watershed-scale model that
combined above-ground forest processes with be-
low-ground biogeochemistry—the Nutrient Cy-
cling Model (NuCM).53 The model includes both
53 Liu, S., R. Munson, D.W. Johnson, S. Gherini, K. Summers,
R. Hudson, K. Wilkinson and L.F. Pitelka. 1992. The
(continued)
28
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CHAPTER 2: ENVIRONMENTAL GOALS
sulfur and nitrogen cycling and was developed as
an extension of the ILWAS model and as a part of
the activities of the Integrated Forest Study.54 At
that time, published reports of model applications
were limited to a few intensively studied sites and
then to estimating effects on leaching rates of base
cations of deposition of sulfur.
As indicated above, the Acid Deposition Standard
Feasibility Study calls for regional assessments on
surface water chemistry of the leaching of nitrate.
To complete such an assessment requires both a
model capable of such simulation and the data
necessary to run the model. The NuCM model re-
quires a variety of specific data inputs to simulate
the complex cycling of forest nutrients (e.g., nitro-
gen). Needed inputs include information, for ex-
ample, on vegetation growth rates, rates of foliar
leaching and exudation, rates of organic matter
decay, and rates of nitrification. In early-mid
1992, high-quality data on such rates existed at
only a very few intensively studied watershed sites
within the United States. Sufficient necessary data
did not exist to run the NuCM model in a reliable
regional application. Therefore, at that time, the
goal of running a process-based watershed acidifi-
cation model that could account for the effects of
both nitrogen and sulfur inputs and cycling and
that could be credibly used at regional scales for
estimating future effects on lake and stream
chemistry was not attainable. The possibility re-
mained, however, to investigate the potential
bounds of nitrogen deposition effects on surface
water chemistry in combination with process
modeling of effects of sulfur deposition. This was
accomplished through the Nitrogen Bounding
Study.55
The Nitrogen Bounding Study (NBS) was built on
model approaches and field sampling information
compiled during the Direct/Delayed Response Pro-
ject (DORP),56-57 a regional study that used three
Nutrient Cycling Model (NuCM): overview and application.
Pages 583-609 in D.W. Johnson and S.E. Lindberg (editors).
Atmospheric Deposition and Forest Nutrient Cycling,
Ecological Studies 91, Springer-Verlag, New York. 707 pp.
54 Johnson, D.W., and S.E. Lindberg (editors). Atmospheric
Deposition and Forest Nutrient Cycling, Ecological Studies
91, Springer-Verlag, New York. 707 pp.
55 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. Environmental Research
Laboratory, Corvallis, OR.
56 Church, M.R., K.W. Thornton, P.W. Shaffer, D.L. Stevens,
B.P. Rochelle, G.R. Holdren, M.G. Johnson, J.J. Lee, R.S.
(continued)
watershed models to assess effects produced by
sulfur deposition on lake and stream chemistry in
the eastern United States.58 In the DDRP, the
Agency gathered watershed and soil data at hun-
dreds of eastern U.S. watersheds to run multiple
models of watershed acidification on the future ef-
fects of sulfur deposition only in the Northeast,
mid-Appalachians, and Southern Blue Ridge Prov-
ince of the United States. Nitrogen deposition ef-
fects could not be modeled at that time. Water-
sheds modeled during the DDRP were selected as
a statistical subset of those surveyed by the Na-
tional Surface Water Survey.59 Because of this sta-
tistical basis, each watershed modeled during the
DDRP held a statistical sampling weight represent-
ing a definable proportion of the watersheds
within each of the three study regions. In turn,
this allows results from this modeling to be ex-
trapolated over regional scales to project responses
by the target population of interest (low-ANC wa-
tersheds). Not all of the three models used during
the DDRP could be successfully calibrated for all
of the sensitive watersheds in the sample. By far,
the greatest number of successfully calibrations
occurred for the Model of Acidification of
Groundwater in Catchments (MAGIC).60 Signifi-
cantly fewer successful calibrations occurred for
Turner, D.L. Cassell, D.A. Lammers, W.C. Campbell, C.I.
Liff, C.C. Brandt, L.H. Liegel, G.D. Bishop, D.C. Mortenson,
S.M. Pierson, and D.D. Schmoyer. 1989. Direct/Delayed
Response Project: Future Effects of Long-term Sulfur
Deposition on Surface Water Chemistry in the Northeast and
Southern Blue Ridge Province. EPA/600/3-89/026a-d. U.S.
Environmental Protection Agency, Washington, DC. 887 pp.
57 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. Pierson, D.L. Stevens, B.P. Rochelle, and
R.S. Turner. 1992. Direct/Delayed Response Project: Future
Effects 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. Environmental Protection Agency,
Washington, DC. 384 pp.
58 Section 2.5.2 presents provides additional detailed discussion
and reference to publications resulting from the DDRP.
59 Linthurst, R.A., D.H. Landers, J.M. Eilers, D.F. Brakke, W.S.
Overton, E.P. Meier, and R.E. Crowe. 1986. Characteristics
of lakes in the eastern United States, volume I: Population
description and physio-chemical relationships. EPA/600/4-
86/007a. U.S. Environmental Protection Agency,
Washington, DC. 136 pages.
60 Cosby, B.J., G.M. Hornberger, J.N. Galloway, and R.F.
Wright. 1985. Modeling the effects of acid deposition:
Assessment of a lumped parameter model of soil water and
streamwater chemistry. Water Resources Research
21:51-63.
29
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
the Enhanced Trickle Down Model61 and the Inte-
grated Lake Watershed Acidification Study
(ILWAS) Model62 (predecessor of the NuCM
model).
For lake watersheds in the Northeast that were
successfully calibrated by all three models during
the DDRP, the results showed that each model
produced similar regional-scale watershed projec-
tions for the target population of future effects from
sulfur deposition on lake chemistry (see footnote
56). That is, the three models projected similar
trends of regional-scale change in the distribution
of ANC values in the target population of lakes
over the various modeled changes in sulfur load-
ing. For the Southern Blue Ridge Province only
two of the watershed models (MAGIC and ILWAS)
were used. Again, significantly more v/atersheds
were successfully calibrated for the MAGIC model
than the ILWAS model. In simulations of stream
watersheds in the Southern Blue Ridge Province,
MAGIC projected greater adverse effects of sulfur
deposition than did ILWAS.
A second phase of the DDRP repeated the process
of modeling future effects of sulfur deposition for
stream watersheds in the mid-Appalachian region.
In this latter case, for purposes of time and cost ef-
ficiency, modeling was performed using only the
MAGIC model. Comparison of model projections
of ANC by MAGIC to those made by the ILWAS
model for two of the same mid-Appalachian wa-
tersheds, indicated only slight differences between
these projections at 20 or 50 years (see footnote
57).
In addition to providing the primary analytical ba-
sis for the DDRP and the NBS, as described in the
next two subsections, an increasing diversity of
other studies have effectively applied MAGIC to
assess many watershed processes associated with
acidic deposition (Exhibit 4). MAGIC has been
tested more than any other acidic deposition ef-
fects model. Results from these tests (including
some still underway) indicate that MAGIC cor-
rectly projects the direction of change of water-
shed responses and accurately projects the magni-
tudes of rates of change for surface water ANC and
pH. MAGIC reasonably represents sulfur retention
61 Nikolaidis, N.P., H. Rajaram, J.L. Schnoor, and K.P.
Georgakakos. 1988. A generalized soft water acidification
model. Water Resources Research 24:1983-1996.
62 Gherini, S.A., L. Mok, R.J. Hudson, G.F. Davis, C.W. Chen,
and R.A. Goldstein. 1985. The ILWAS model: Formulation
and application. Water, Air, and Soil Pollution 26:425-459.
within watersheds and the generation and leaching
of cations from watersheds, two functions gener-
ally 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. It is important to recognize
also, as discussed in Appendix B, that all models
often are difficult to test. That is, most models
may remain largely "unverified," "unconfirmed," or
"unvalidated." In fact, it can be strongly argued
that a model can never be confirmed to be true, it
can only be falsified by failing to project accu-
rately some outcome. Further, when a model does
accurately predict an outcome, its validity is not
proven, because the "right" result may have been
projected for the wrong reason. Therefore, in re-
viewing model projections from the DDRP and the
NBS studies on potential effects attributable to fu-
ture sulfur and nitrogen deposition, it remains im-
portant to keep in mind the associated uncertain-
ties that are highlighted in the following 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). Depending on rates of in-
puts, the capabilities of watersheds to perform
these processes can decrease over time. Conse-
quently, based largely on the NAS conclusions,
defining needs to protect various aquatic and ter-
restrial resources from acidic deposition depended
on whether acidification is immediately propor-
tional to the intensity of the deposition (i.e.,
"direct") or lags in time (i.e., "delayed") through
such watershed processes.
30
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CHAPTER 2: ENVIRONMENTAL GOALS
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 acidic deposition on surface water chemistry. The
model uses a minimum number of critical chemical ana 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: (1) Time scales of catchment acidification: A quantitative model for
estimating freshwater acidification. Environmental Science and Technology 19:1144-1149; and (2) Modeling
the effects of acidic deposition: Assessment of a lumped parameter model of soil water and stream water
chemistry. Water Resources Research 21:51-63.
Church et al. (see footnote 56) summarize various studies using MAGIC. Recent modifications of the model
are summarized by T.J. Sullivan, 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. De-
partment 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: (1) individual process formulations in
the model have been tested against laboratory experiments with soils; (2) 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; and (3) predictions of the effects from whole-watershed manipulations
have been compared to observed effects.
Two very recent studies examined varying formulations and calibration approaches for the application of
MAGIC to Adirondack lakes: (1) T.J. Sullivan and B.J. Cosby. In press. Testing, improvement, and confirma-
tion of a watershed model of acid-base chemistry. Water, Air, and Soil Pullution; and (2) T.J. Sullivan, B.J.
Cosby, C.T. Driscoll, D.F. Charles, and H.F. Hemond. Influence of organic acids on model projections of lake
acidification. Water, Air, and Soil Pollution. The authors found that increasing the calibration value of the
background (i.e., pre-industrial) levels of lake sulfate concentration from sea salt contributions to 13% of cur-
rent levels together with calibrating Adirondack lakes using soil sulfate adsorption isotherms from only Adi-
rondack soils (rather than from all Northeast soils) increased the percentage of target population of lakes pro-
jected to have ANC<0 u,eq/l at 50 years under a 30% deposition decrease scenario from 6% (using MAGIC as
formulated and calibrated within the DDRP) to 14%. Changes in either organic acid representation or formu-
lation of the aluminum algorithm had no further effect on projections for ANC<0 ueq/l. None of the changes
had any effect on model projections for ANC<50 u.eq/1. The model calibration and application tests resulted
in somewhat more striking changes in the estimates of percentages of target populations below various pH
thresholds or above aluminum concentration thresholds, thus indicating the sensitivity of MAGIC to the se-
lection of calibration approaches for these variables. This sensitivity was one reason that the Nitrogen
Bounding Study (Section 2.5.3) chose to compute pH empirically (rather than internal to MAGIC) and chose
to forego the use of output chemistry (i.e., pH and aluminum) in any predictive models of fish response.
MAGIC illustrates, as do all models, problems associated with uncertainty, parameterization, and validation.
For example, as discussed in the text, MAGIC currently does not explicitly represent detailed cycling or proc-
esses 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 leading to surface water acidification are
poorly known. Better nitrogen models to address the questions are being developed. The Nitrogen Bounding
Study developed for this report used a series of four scenarios to "bound" the possibilities for time-to-nitrogen-
saturation.
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. The purpose of the NBS was to evaluate the
bounds of effects of possible scenarios of deposition and time to watershed nitrogen saturation.
The DDRP was designed to begin assessing the
state and influence of these processes to support
analyses conducted by the National Acid Precipi-
tation Assessment Program (NAPAP). 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. Its data and model calibrations formed
the basis of subsequent NAPAP modeling using a
large number of alternative scenarios of atmos-
pheric sulfur deposition.
31
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
Two principal project reports produced by the
DDRP assessed potential long-term effects of sulfur
deposition on lake and stream water chemistry in
the eastern United States:
* Church, M.R., K.W. Thornton, P.W. Shaffer,
D.L. Stevens, B.P. Rochelle, G.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, LH. Liegel, G.D.
Bishop, D.C. Mortenson, S.M. Pierson, and
D.D. Schmoyer. 1989. Direct/Delayed Re-
sponse Project: Future Effects of Long-Term
Sulfur Deposition on Surface Water
Chemistry in the Northeast and Southern
Blue Ridge Province. EPA/600/3-89/026a-
d. U.S. Environmental Protection Agency,
Washington, DC. 887 pp.
* 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. Pierson, D.L. Stevens, B.P.
Rochelle, and R.S. Turner. 1992. Di-
rect/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. Environmental
Protection Agency, Washington, DC. 384
pp.
The first report focused on analysis of lake re-
sources in the Northeast and stream resources in
the Southern Blue Ridge Province (SBRP). The se-
cond report addressed potential stream chemistry
effects in the mid-Appalachian Region, and sum-
marized and integrated conclusions from I he three
regional analyses. Additional documenlation of
the DDRP and subsequent NAPAP modeling ac-
tivities based upon this project is included in the
following publications:
* Models Planned for Use in the NAPAP Inte-
grated Assessment. 1989. National Acid
Precipitation Assessment Program, Wash-
ington, DC.
* Methods for Projecting Future Changes in
Surface Water Acid-Base Chemistry. 1990.
Acidic Deposition: State of Science and
Technology: Report 14. National Acid
Precipitation Assessment Program., Wash-
ington, DC.
* 7990 Integrated Assessment Report. 1990.
National Acid Precipitation Assessment Pro-
gram, Washington, DC.
»Turner, R.S., P.F Ryan, D.R. Marmorek,
K.W. Thornton, T.J. Sullivan, J.P. Baker,
S.W. Christensen, and M.J. Sale. 1992.
Sensitivity to change for low-ANC eastern
US lakes and streams and brook trout
populations under alternative sulfate depo-
sition scenarios. Environmental Pollution
77:269-277.
Exhibit 5 shows the locations of three study re-
gions used during the DDRP. General characteris-
tics and sizes of target surface water populations
for all regions included as part of the NSWS,
DDRP, and NBS studies are presented for com-
parison in Exhibit 6. Specific characteristics of the
three DDRP study areas and their surface waters
are summarized in the following.
Northeast63
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. Both pH and ANC tended to decrease as the
63 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. Characteristics of Lakes in the Eastern
United States, Volume I: Population Descriptions and Physico-
chemical Relationships. EPA/600/4-86/007a. U.S. Environ-
mental Protection Agency, Las Vegas, NV.
32
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CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBIT 5. STUDY REGIONS INCLUDED IN THE DIRECT/DELAYED
RESPONSE PROJECT AND THE NITROGEN BOUNDING STUDY
Northeast
Adi rondacks
Mi d-Appal aclti an
i on
Sout hern Bl ue Ri
Provi nee
33
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
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 extrapolated. Studied target popula-
tions of surface waters generally became refined and smaller in
each of these successive studies, allowing acidic deposition re-
search efforts to focus increasingly on relationships in more
sensitive surface waters (see figure—not drawn to scale). As a
consequence of narrowing research efforts, proportions of
sensitive surface waters and the magnitude of the potential re-
sponse to acidic deposition by these respective target popula-
tions tend to increase through subsequent studies. General
characteristics of these target populations are presented below.
As part of the NSWS, the Eastern Lake Survey (ELS) includes lakes between 4 ha (10 acres, the lower
limit of resolution of the 1:250,000-scale maps used to design this study) 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 estuar-
ies); broad waters with apparent flows (reservoirs were included, however); marshes or swamps; and
waterbodies surrounded by urban, industrial, or agricultural activities (i.e., lakes with extensive cul-
tural disturbance in their watersheds). The target lake population in the Northeast was estimated to in-
clude 326 lakes (4.6%) with ANC<0 ueq/l; 1,364 lakes (19%) with ANC<50 ueq/l; 4,258 lakes (59%)
with ANC<200 ueq/l; 240 lakes (3.4%) with pH<5.0; and 916 (13%) lakes with pH<6.0. 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 <155 km2 (<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 po-
tential effects of acidic deposition. At least 50 percent of the stream reach had to be within the desig-
nated 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 de-
velopment, or tidal influence. Of the approximate 72,000 km (44,600 miles) of streams included in
the target population in the mid-Appalachian Region, acidic streams (ANC<0 ueq/l) accounted for
2,330 km (1,450 miles) (i.e., 3 percenl of the target streams' length); 18 percent of the upstream ends
and 7 percent of the downstream ends had ANC of 50 (j.eq/1 or less. The 9,036 km (5,615 mi) of tar-
get streams in the Southern Blue Ridge Province (SBRP) included some of the lowest concentration of
dissolved solids of any region sampled in the United States (median conductivity of less than 40 u.
S/cm) and had among the highest deposition rates for H+, SO42-, and NCy. Although no acidic
streams were found in the SBRP during the NSS, acidic streams are known to occur in this region;
7.8 percent of the target streams had ANC of 50 u.eq/1 or less.
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. During
the DDRP, 123 lakes in the Northeast, 29 stream reaches in the mid-Appalachians, and 30 stream
reaches in the Southern Blue Ridge were modeled.
The NBS target lakes included a subset of about 700 lakes from the DDRP target lakes within Adiron-
dack Subregion 1A having ANC<200 ueq/l (approximately 45 percent of the region's lakes meeting
the ELS sampling requirements). The NBS target stream reaches included the same subsets of target
reaches for both the M-APP and SBRP study areas as used during the DDRP studies. For the mid-Ap-
palachians, this included about 4,300 stream reaches (approximately 17 percent of the total included
in the NSS); for the SBRP, it included about 1,300 stream reaches (approximately 65 percent of those
included in NSS).
34
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CHAPTER 2: ENVIRONMENTAL GOALS
lake elevation increased in the Adirondacks, a re-
lationship not observed in other NSWS subregions
of the Northeast. Drainage lakes were the most
common type of lake (77 percent of the target
population). A higher proportion of lakes with ar-
eas of less than 4 ha in the Adirondacks tended to
be more boglike and more strongly influenced by
organic acidity, compared to the larger lakes in
the Adirondacks. Approximately half of the Adi-
rondack lakes having pH 5.0 were organic acidic
dark-water lakes, while the remainder were clear
water acidic lakes. Inorganic ions, including min-
eral acids, were likely the primary cause of acidity
in these low pH clear water lakes.
Mid-Appalachian Region64
This region included most stream reaches poten-
tially 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
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).65 The SBRP includes
a steep mountainous region characterized by high
rainfall, highly weathered base-poor soils, and
relatively unreactive bedrock. Target surface wa-
ters of this region contain some of the lowest con-
centrations of dissolved solids of any region sam-
pled in the United States, and among the highest
deposition rates for H+, SO42-, and NO3'. This area
includes the Great Smoky Mountains National
Park. Although no acidic streams (ANC<0 u.eq/1)
were found during the NSS, statistical analysis 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. Also, a sepa-
rate non-random 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.66
Watershed retention of SO42- and NO3~ is the ma-
jor process generating ANC in surface waters
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
with the exception of certain high-elevation forests
affected by insects. The primary cause of ANC
differences in these streams appears to be different
rates of acidic cation mobilization from the re-
gion's watersheds. Dissolved organic carbon con-
centrations are typically low and do not appear to
provide significant contributions to stream acidity.
64 This summary is primarily drawn from Herlihy, A.T., P.R.
Kaufmann, M.R. Church, P.). 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.
65 This summary for the Southern Blue Ridge Province is primarily
drawn from Elwood, J.W., M.J. Sale, P.R. Kaufman, and C.F.
Cada. 1991. The Southern Blue Ridge Province. Pages 319-
364 in D. F. Charles (editor). Acidic Deposition and Aquatic
Ecosystems Regional Case Studies. Springer-Verlag, New York,
NY.
66 Winger, P.V., P.J. Lasier, M. Hudy, D.L. Fowler, and M.J. Van
Den Avyle, 1987. Sensitiviiy of high-elevation streams in the
Southern Blue Ridge Province to acidic deposition. Water
Resources Bulletin 23:379-386.
35
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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 significant biological effects from
acidic deposition on streams and lakes have been
conclusively demonstrated for the SBRP.
Overview of DDRP Results
The DDRP projected changes in target surface
water chemistry for several sulfur deposition sce-
narios, while holding nitrogen deposition and re-
tention constant, using up to three watershed
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 potential appears 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.67 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.
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 Magic model projected that in 50
years the proportion of target acidic (ANC<0 ueq/l)
stream reaches would increase from 3 percent and
11 percent. This same deposition 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) (see footnote 67).
2.5.3 Nitrogen Bounding Study
Modeling Approach
When the Nitrogen Bounding Study began in mid-
1992 to assess the combined effects of nitrogen
and sulfur deposition over regional scales, no
combination of dynamic watershed model and sta-
tistically based regional watershed data existed
sufficient for the regional modeling of effects of ni-
67 National Acid Precipitation Assessment Program. 1990.
Integrated Assessment Report. Washington, DC.
36
-------
CHAPTER 2: ENVIRONMENTAL GOALS
trogen deposition on watershed nitrogen retention
and surface water chemistry. Furthermore, it was
clear that such a goal, desirable as it is, could not
be achieved during the time frame required for this
assessment. Therefore, the NBS focused on com-
bining existing modeling approaches for dynamic
sulfur effects on watersheds at regional scales with
estimations of potential bounds of nitrogen ef-
fects.68 This modeling expanded upon MAGIC
model runs for sulfur deposition effects made dur-
ing the DDRP.
When the NBS started in 1992 the MAGIC model
did not have a full representation (e.g., set of equa-
tions) for nitrogen dynamics. It did, however, con-
tain a representation of watershed nitrogen input
and output (the latter as nitrate in surface water).
That is, based upon observed surface water chem-
istry and estimated levels of deposition, MAGIC
contained an estimation of net annual nitrogen re-
tention within each modeled watershed. The man-
ner in which this retention might change (i.e., the
magnitude and direction of change) over different
loadings modeled provides the assessment basis
used during the NBS.
From a functional view, the effects of nitrate
leaching on base cation depletion and surface
water chemistry is analogous to those of sulfate
leaching. Both ions act as mobile anions. In-
creased leaching of sulfate, nitrate, or both these
anions would lead to increases in base cation and
hydrogen ion leaching on a net annual basis. Un-
less leached base cations equaled or exceeded the
leached anions, increased leaching of either nitrate
or sulfate would lead to decreased surface water
ANC.
A representation within the MAGIC model of such
effects allows direct computation of effects from
changes in nitrate leaching. The NBS used alter-
native scenarios of change in net annual water-
shed nitrogen retention together with potential fu-
ture alternative loading rates of atmospheric nitro-
gen and sulfur to simulate effects on surface water
chemistry. These simulations projected potential
effects in target populations of sensitive surface
waters as watershed retention of nitrogen de-
creased from their estimated current states to
where only 5 percent or less of the atmospheric
68 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. Environmental Research
Laboratory, Corvallis, OR.
nitrogen deposited was retained in the watershed,
on a net annual basis. (Retention of nitrogen pres-
ently is fairly high for most target watersheds
studied.) The shift in the percent of nitrogen re-
tained by the watersheds was assumed to have the
shape of inverse logistic curves (roughly, a left-
tilted S-shaped curve, with flattened bottom and
top portions). These shifts, from nearly no water-
shed nitrogen leaching to 95 percent or more
leaching, were separately modeled to occur over
periods of 50 years, 100 years, and 250 years. The
NBS also simulated the effects of no changes in
net annual retention for watershed nitrogen.
Thus, the NBS did not directly simulate how nitro-
gen deposition might alter watershed retention of
nitrogen; no combination of model and regional
data available in early-mid 1992 could do that at
the regional scales required for this report. Rather,
the NBS illustrated what would be the result on
surface water chemistry at regional scales //certain
scenarios of changes in watershed nitrogen reten-
tion were to come to pass. In so doing, the NBS
effectively bounded all reasonable possibilities of
such effects. To mimic important biological ef-
fects on watershed nitrogen cycling, NBS included
curves of declining watershed nitrogen retention.
That is, at the start, modeled retention was high
(e.g., because of biological uptake), then the net
annual retention was modeled to decline over time
(e.g., due to biological uptake and sequestration)
until the net annual retention was very low (e.g.,
nitrogen no longer limited productivity in the wa-
tershed). This approach mimics, in a simplified
sense, watershed systems moving towards nitrogen
saturation (i.e., principally biological saturation)
through the processes described in various pub-
lished research reports discussed in Section 2.2.1.
A continuing research need remains to develop
both (1) dynamic watershed process models that
can be used to model nitrogen deposition effects at
regional scales and (2) the necessary regional data
sets to run such models. Since 1992, EPA has
funded two parallel projects (one at the University
of New Hampshire and the second at the Univer-
sity of Virginia) to develop the necessary dynamic
watershed models of combined sulfur and nitrogen
cycling and effects.
EPA designed the NBS to begin providing a quanti-
tative estimation of potential effects attributable to
nitrogen deposition during surface water acidifica-
tion. This study examined the combined effects on
surface water chemistry due to potential changes
in the deposition rates of total sulfur and total ni-
37
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
trogen, and due to possible alternative rates of ni-
trogen 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 10 million tons of SO2 and the 2 million tons
of NOX) were fully implemented.
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 Chapters. The NBS results
represent responses for proportions of NBS target
surface waters; they do not represent responses for
either all surface waters or for all NSWS sampled
surface waters in the modeled regions.
The NBS study has important implications regard-
ing the feasibility of aquatics-based acid deposi-
tion standards in the United States. Therefore, it is
valuable to understand clearly the nature of the re-
sults produced by this study. Exhibits 7 to 9 present
3 of over 60 similar appearing sets of four plots
presenting NBS model results. These sets of plots
show modeled responses for percentages of the
target population of Adirondack Region lakes pro-
jected to meet the criterion 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 10 intro-
duces guidance for interpretation of the NBS plots
presented in Exhibits 7 to 9 and in Appendix B.
More detailed interpretative guidance is presented
also 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, whereas other results from
the experimentally manipulated watersheds of
Bear Brook in Maine and Fernow in West Virginia
indicate shorter response times to increased nitro-
gen additions. Also, surface water nitrate concen-
trations have noticeably varied recently at regional
scales in the Catskills and Adirondacks, confound-
ing the question as to whether 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
possible regional difference in times to nitrogen
saturation are presented in Exhibit 11.
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 other scenarios mod-
eled deposition rates that decreased to back-
ground deposition rates over the period from 2010
to 2020. Rates for still other scenarios reduced to
levels between these extremes from 2010 to 2020.
(Background deposition rates include only those
due to airborne natural, agricultural fertilizer, and
domestic livestock sources.) Each modeled depo-
sition rate was then assumed to remain constant at
the specific modeled 2020 rate until the year
2040, the end of the model projection period.
Sets of plots similar to those shown in Exhibits 7
through 9 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: ANC<
0 ueq/l, ANC<50 ueq/l, pH<5.0, pH<5.5, and pH<
6.0. Similar plots for all four water chemistry cri-
teria 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 both chemical and biological sensitivity to
acidification (see Section 2.3.1). Projected water
quality changes are likely to be highly transient in
nature for the year 2015, largely because potential
benefits from implementation of 1990 CAAA re-
quirements will still be accruing at that time.
Therefore, this section focuses primarily on pro-
jected ANC changes in the year 2040. For those
38
-------
CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBIT 7. NBS MODEL PROJECTIONS FOR YEAR 2040 PERCENTAGE
OF TARGET POPULATION ADIRONDACK LAKES WITH ANC<0 U.EQ/L
12 -
10 -
*• 8H
6 -
4 -
2 -
0 -
&
o
8°
6%
i
6
8
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
I
o
Q
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT 8. NBS MODEL PROJECTIONS FOR YEAR 2040 PERCENTAGE OF TAR-
GET POPULATION MID-APPALACHIAN STREAMS WITH ANC< 50 JIEQ/L
14 -
f ,2-
Q
c
I
10 -
8 -
6 -
4 -
2 -
0 -
5.3%
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
14 -
1? 12 -i
.c
z
» 10 -
i an
o
Q
ID
z
I 2"
o -
4.8%
i
0
i
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
8
Q.
O
I
14 -
12 -
10 -
8 -
6 -
4 -
2 -
0 -
4.6%
T
6
8
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
14 -
•£ 12 H
.c
o
4)
Q
10 -
8 -
6 -
2 4 -
Z
S 2 -
\-
0 -
T
6
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
40
-------
CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBIT 9. NBS MODEL PROJECTIONS FOR YEAR 2040 PERCENTAGE OF TAR-
GET POPULATION SOUTHERN BLUE RIDGE STREAMS WITH ANC<50
g
.
Z
"3
12 -
10 -
* 8
6 -
4 -
2 -
0 -
4.4%
T
4
I
6
8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 50 yr)
I
f
.2
8-
Q
0>
12 -
10 -
6 -
4 -
2 -
0 -
3.8%
i
0
8
I
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 100 yr)
I
¥
§
8"
12 -
10 -
8 -
6 -
4 -
2 -
0 -
3.4%
I
0
i
4
1
6
i i
8 10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen saturation @ 250 yr)
12 -
10 -
r 8-
_o
I R _,
o_ 6 -<
4 -
i
8
i
10
Total Sulfur Deposition (kg S/ha/yr)
(Assumes nitrogen uptake constant)
41
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT 10. INTERPRETING NBS PLOTS
To illustrate interpretation of the NBS plots, the four individual plots
in Exhibit 7 show projected percentages 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 ni-
trogen 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
modeled ranges of deposition. These ranges begin at projected back-
ground deposition rates for sulfur and nitrogen (1 kg S/ha/yr and 4 kg
N/ha/yr, as shown by the respective axes) and extend to their maxi-
mum modeled concentrations for 2040 (7.5 kg S/ha/yr and 11.3 kg
N/ha/yr), i.e., the rates projected to accompany fully implementing
the 1990 CAAA (see Chapter 3). Thus, for the upper right plot of Ex-
hibit 7, 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. (Appendix B
provides additional guidance on interpreting these plots.)
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 re-
sponse relationships. Nearly vertically plotted response con-
tours indicate that the projected ANC response (i.e., change)
is attributable primarily to sulfur deposition. Nearly horizon-
tal plotted response contours indicate the plotted ANC re-
sponse is attributable primarily to nitrogen deposition. A
forty-five degree diagonal contour indicates equal contribu-
tions by both sulfur and nitrogen deposition.
« Changes in the spacing between individual response contours
within each plot appears to be dependent on patterns in
sample weighting during model projections, rather than due
to some intrinsic character of the deposition-response rela-
tionships.
» The density of response contours across the modeled deposi-
tion ranges for each plot directly relates to the potential aver-
age responsiveness by target waterbodies to potential
changes in sulfur and nitrogen deposition 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 changes in
deposition.
42
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CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBIT 11. TIME TO WATERSHED NITROGEN SATURATION
Available information suggests that some individual watersheds in diverse regions of North America
show symptoms of various states of nitrogen saturation. Yet, for each of the three regions modeled
during the NBS, existing scientific knowledge does not allow us to quantify the likely times to
nitrogen saturation for specific proportions of the target watershed populations. Further, for
watersheds within any of these regions or in any region, no scientific consensus exists regarding the
probable times to nitrogen saturation. Indeed, NBS investigators and its technical reviewers deem it
scientifically premature to define specific times to saturation for any region.
Suggesting that potential times to nitrogen saturation would tend to vary among regions for those wa-
tersheds where that process would most likely occur is, nevertheless, reasonable. The basis for such
variations would include differences in seasonal temperatures, moisture, soil fertility, primary produc-
tion rates, decomposition rates, forest age, and the accumulation of plant biomass, different histories
of nitrogen deposition among the regions, and other factors shown in Exhibit 2. In addition, given
historic and current nitrogen deposition rates in these three regions, we can reasonably assume that
various of the more sensitive forested watersheds included in the target population for each region
would eventually reach nitrogen saturation (barring major disturbances in these watersheds such as
extensive logging, fires, blow downs, and insect infestations).
As a speculative example, watersheds in the Adirondacks have cooler annual temperatures, shorter
growing seasons, lower inherent productivity potential, 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, barring other severe watershed disturbances, watersheds in these areas
are likely to include those having the shortest remaining times to nitrogen saturation. Some research-
ers suggest that saturation times in this region may average less than 50 or 100 years. Recalling that
various Adirondack watersheds now show initial symptoms of nitrogen saturation, some suggest that
it may even be as short as 25 years; other recent data suggest the nitrogen trends in the earlier data
may be reversing, so a time of 100 to 250 years may be more appropriate. In comparison, more
southerly watersheds in the mid-Appalachians have generally warmer annual temperatures, longer
growing seasons, less restrictive forestry practices, and greater inherent productivity potential. These
watersheds also have greater recent nitrogen deposition rates, although historic rates may have been
lower. Such circumstances suggest that target watersheds in this region could have longer remaining
durations to nitrogen saturation.
Watersheds even farther south in the Southern Blue Ridge Province have even warmer annual tem-
peratures, longer average growing seasons, the greatest productivity potential, the fastest decomposi-
tion rates, historically lower nitrogen deposition rates, higher recent deposition rates, smaller pools of
soil nitrogen, and generally very low stream nitrogen concentrations. Here, the remaining time to
watershed nitrogen saturation may likely be greater still. Yet, in recognizing that watersheds across
any single region generally include a widely diverse continuum of times to potential watershed nitro-
gen saturation, some watersheds in the Great Smoky Mountains National Park contain mature old-
growth forests; times to nitrogen saturation would likely be shorter for these areas and, as cited in the
text of this report, some of these watersheds have been reported to show symptoms of advancing ni-
trogen saturation. Thus, considerable uncertainty exists for durations remaining for modeled propor-
tions of target watersheds within each region to reach advanced stages of nitrogen saturation.
also interested in projected changes in pH, the ANC<0 ueq/l and ANC<50 ueq/l approximate pH<
ANC changes discussed can be related to corre- 5.3 and pH<6.5, respectively. Also, NBS plot pro-
sponding pH changes using the empirical relation- jection for pH changes using empirical pH/ANC
ships between these variables presented in Ex- relationships are presented in Appendix B.
hibit 1: on average, across the three NBS regions,
43
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
Peer Review of the Nitrogen Bounding Study
The Nitrogen bounding Study (NBS) report was
primarily a methods report (see footnote 68).
Thus, peer review of the NBS focused on methods
used in the study. Peer reviewers were asked to
comment on any part of the report that they
wished and to answer the following specific
questions. Questions pertaining to the regional
modeling approach included:
* Is the regional approach to the modeling
appropriate for the purposes of the work?
» Is there a better regional approach?
* Is the target population appropriate?
» If there are alternative suggestions for a re-
gional approach, what data are available
for implementation?
Questions pertaining to the sulfur modeling in-
cluded:
* With regard to sulfur and associated base
cation dynamics, is the model selected
(MAGIC) appropriate for the analyses?
* Is there a better model? If so, identify it
and describe how it might be preferable
and how it would be applied in a regional
context.
Questions pertaining to the nitrogen modeling in-
cluded:
* Is the approach pursued for mimicking the
potential interactions of N appropriate? If
not, what other approach would be prefer-
able (be as specific as possible)?
* Is the shape of the transition of watershed
retention of N-to-N saturation reasonable?
If not, what other shape would be prefer-
able and how mathematically would it be
represented and parameterized?
* Are the starting points for watershed nitro-
gen retention reasonable? If not, what
method of computation and what data
would be preferable? Do the scenarios of
time to N saturation span the possibilities
in a useful way? Of the scenarios (50, 100,
250 years, never), which do you think is
most appropriate for each of the regions?
NBS reviewers were asked whether results of the
work were presented in a manner useful to policy
makers and whether other graphical presentation
formats or additional information would be prefer-
able. Finally, reviewers were asked if there were
other ways in which the study and reporting might
be improved.
Peer review comments were very positive. For ex-
ample, given the existing limitations on dynamic
modeling of nitrogen cycling in watersheds, all re-
viewers endorsed the general regional modeling
approach, the specific use of the MAGIC model for
sulfur dynamics, and the representation of scenar-
ios of potential changes in watershed nitrogen re-
tention used. There were no strongly negative re-
view comments on the study. One reviewer rec-
ommended that the effects of declining base cation
deposition be evaluated, and such an analysis was
added to the study. The first version of the NBS
analyses included a model analysis of potential ef-
fects of surface water chemistry changes on fish
populations. Upon further consideration of uncer-
tainties associated with these analyses (especially
regarding model projections of pH and aluminum
species), this modeling was dropped. This deci-
sion was endorsed in subsequent reviewing. All
reviewers recommended publication of the NBS
report as a peer-reviewed EPA Research Report.
Summary of NBS Results
Exhibit 12 summarizes the observed and modeled
percentages of surface waters in each NBS region
target population for both ANC criteria. The ob-
served values were those measured during the
1984 NSWS studies in the Adirondacks and the
1985 studies in the other two regions. For exam-
ple, 19 percent of the target lakes in the Adirond-
acks used during the NBS were observed to be
acidic (ANC<0 ueq/l) during the 1984 NSWS.
Note again, however, that the target population of
the NBS modeling included generally more sensi-
tive subsets of target population surface waters
than were included in the NSWS (see Exhibit 6).
Exhibit 12 shows proportions of surface waters in
the two ANC categories projected by the NBS for
the year 2040 under the assumed times of 50
years, 100 years, 250 years, and never for water-
shed nitrogen saturation for each region. This
range brackets the modeled times for watershed
nitrogen saturation occurring across the three NBS
regions for proportions of waters within each ANC
category. The percentages presented encompass
the range of NBS results for modeled minimum
(background) and modeled maximum deposition
rates for both total sulfur and nitrogen. For exam-
ple, with an assumed time to watershed nitrogen
44
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CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBIT 12. 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)
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
Sensitive (ANC<50 peq/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
6^3
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
saturation of 100 years, the lower left plot of Ex-
hibit 7 shows that background total sulfur and ni-
trogen deposition 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 modeled deposition rates 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, as
shown in the upper right corner of that plot. Also,
across 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 7 shows that the model projects that be-
tween 0 percent and 15 percent of these same tar-
get Adirondack lakes will have ANC of 0 ueq/l or
less. Exhibit 12 shows these two ranges and sum-
marizes 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 12 provide one 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 ANC 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,
many sources of variability and uncertainty affect
the overall uncertainty of these model projections.
If these sources were included in an overall evalu-
ation of uncertainty, the associated uncertainty
could be greater, with projections of future re-
sponses by target waterbody populations potential-
ly falling beyond either end of all modeled ranges
presented in Exhibit 12.
The uncertainty in any computation or modeling
analysis is the estimation of the potential differ-
ence between the calculated value (under a set of
conditions) and the "true" value. It is impossible to
quantify levels of total uncertainty in model pro-
jections such as those made within the Nitrogen
Bounding Study (or for any watershed acidification
model used in a predictive sense) in any absolute
objective manner.69 This is because there exist im-
portant categories of uncertainty that defy quanti-
fication. For regional watershed modeling exer-
cises such as those performed in the DDRP or
NBS, certain components of model uncertainty
69 Thornton, K.W., D.R. Marmorek, and P.P. Ryan. 1990.
Methods for forecasting future changes in surface water acid-
base chemistry, NAPAP Report 14. Acid Deposition: State of
the Science and Technology, National Acid Precipitation
Assessment Program, Washington, DC.
Turner, R.S., P.F Ryan, D.R. Marmorek, K.W. Thornton, T.J.
Sullivan, J.P. Baker, S.W. Christensen, and M.J. Sale. 1992.
Sensitivity to change for low-ANC eastern US lakes and
streams and brook trout populations under alternative sulfate
deposition scenarios. Environmental Pollution 77:269-277.
45
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
(e.g., sample uncertainty, input uncertainty) may
be estimated quantitatively. Previous studies have
addressed, for example, the quantitative uncertain-
ties associated with sampling and input uncer-
tainty in earlier modeling of effects of sulfur depo-
sition and have found that, given the scope and in-
tent of the analyses (regional scenario testing for
policy analysis), these uncertainties appear rea-
sonable.70 the difficulty in estimating total uncer-
tainty, however, is that other components of un-
certainty (e.g., aggregation uncertainty, structural
uncertainty) likely overwhelm the former and
these latter components cannot be estimated quan-
titatively.71
A question remains regarding the reliability of the
modeling results from the NBS relative to their in-
tended use. The purpose of the model runs per-
formed for the Acid Deposition Standard Feasibil-
ity Study was to test, at regional scales, the sensi-
tivity of potential watershed responses to varying
scenarios of (1) nitrogen deposition, and (2) water-
shed transition to nitrogen saturation, in relation to
projected effects of sulfur deposition. Further,
rather than to focus on explicit quantitative esti-
mates of percentages of target populations, the
utility of the watershed simulations is to examine
direction and magnitude of projected relative
changes. Prior modeling assessments and model
evaluations have established the credibility of the
MAGIC model and its basic structure for estimat-
ing the direction and magnitude of future effects of
sulfur deposition on surface water chemistry in
that model projections are consistent with theory
and observational data,72 including recent water-
shed manipulations.73 In that it allows variable
(increased) nitrate leaching within the model (with
For detailed discussion of these studies, see the two DDRP
principal project reports by Church and others published in
1989 and 1992, as cited in Section 2.4.2; see also the two
studies by Thorton, Turner and others cited in foolnote 68
For additional consideration of these issues see the reports
by Thornton, Turner, and others as cited in footnote 68.
72 This has been shown by the utility of the DDRP results.
Other examples are discussed by Church and others the
1992 DDRP principal project report. See also, Wright, R.F.,
B.J. Cosby, R.C. Ferrier, A. Jenkins, A.J. Bulger, and R.
Harriman. 1994. Changes in acidification of lochs in
Galloway, southwestern Scotland, 1979-1988: The MAGIC
model used to evaluate the role of afforestation, calculate
critical loads, and predict fish status. Journal of Hydrology
161:257-285
73 Cosby, B.J., S.A. Norton, and J.S. Kahl. In press. Using a
paired-watershed manipulation experiment to evaluate a
catchment-scale biogeochemical model. Science of the
Total Environment.
its consequent effects on base cation exchange and
equilibrium chemistry), the combined sulfur and
nitrogen effects modeling exercised in this study
makes no changes to the basic structure of
MAGIC.
The purpose of the watershed modeling performed
in support of this Feasibility Study was to examine
at regional scales how important might be the ef-
fects of nitrogen deposition as compared to those
of sulfur deposition. Results indicate clearly, that
under some scenarios, the effects might be compa-
rable in direction and magnitude.
As noted in Exhibit 10, the density of contours
across the modeled deposition ranges in NBS plots
for ANC, including those in Exhibits 7-9, appears
to relate to the potential average responsiveness of
target waterbodies across regional scales to poten-
tial changes in deposition rates. (Vertical and hori-
zontal contours indicate a strong role of sulfur or
nitrogen, respectively.) 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 sensitivities. These categories provide a
basis for evaluating the relative confidence that
reducing sulfur or nitrogen deposition below levels
projected to accompany the 1990 CAAA would
produce detectable improvements in ANC within
the NBS target surface waters across the analyzed
regions. Exhibit 13 presents the results of the sur-
face water responsiveness categorization for the
three modeled regions. The following summary of
regional relationships to acidic deposition rates is
drawn from Exhibits 12 and 13 and from the indi-
vidual plots for all three NBS study regions.
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 an illustrative model
for protective goals used in this report.
Regional Summaries74
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
74 Most of this discussion was developed from evaluations of
results from the National Surface Water Survey and model
projections from the Nitrogen Bounding Study.
46
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CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBIT 13. SURFACE WATER RESPONSIVENESS TO REDUCTIONS IN DEPOSITION
BEYOND 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)
<0u.eq/l
<50 u.eq/1
<0|ieq/l
<50u,eq/l
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-
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 reflects
the fact that this region has a high proportion of
lakes which naturally have ANC 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 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
47
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
equals or exceeds 250 years, the model projects a
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 the overriding consideration.
EPISODIC ACIDIFICATION:75 Because episodes are
driven principally by deposition acidity, reductions
in acidic deposition rates for either sulfur, nitro-
gen, or both can be expected to significantly re-
duce the occurrence of acidic episodes in the tar-
get population of Adirondack lakes. This would be
expected to occur at a more rapid rate than the re-
duction in proportions of chronically acidic lakes
because deposition 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 Adi-
rondack 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.
Although considerable uncertainty regarding time
to watershed nitrogen saturation exists, if the aver-
age time for Adirondack watersheds to reach ni-
trogen saturation is close to 100 years or less, the
model predicts that maintaining the proportion of
chronically acidic (ANC<0 ueq/l) target population
Adirondack lakes near their 1984 proportions in
2040 may require reducing anthropogenic sulfur
and nitrogen deposition by 40-50 percent or more
below 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, may be necessary to
maintain proportions of sensitive lakes IANC<50
ueq/l) near their 1984 levels (i.e, 55 percemt) /Ythe
time to watershed nitrogen saturation approaches
50 years or less. If the time to saturation actually is
100 years or longer, the model projects that depo-
sition reductions accompanying the 1990 CAAA
75 Also see Section 2.2.2 on episodic acidification.
will allow proportions of Adirondack lakes with
ANC<50 ueq/l to maintain their approximate 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 potential future surface water acidifica-
tion for this region (Exhibit 8).
CHRONIC ACIDIFICATION: As progressively shorter
times to watershed nitrogen saturation are as-
sumed, effects associated with nitrogen deposition
are projected to increase, essentially offsetting re-
duced proportions of affected streams resulting
from implementation of the 1990 CAAA sulfur
reductions in mid-Appalachians target streams.
Under assumptions of 250 years or less as the time
to watershed 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: Reducing deposition of
sulfur, nitrogen, or both would be expected to re-
duce 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 con-
cluded 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 ANC of 50 ueq/l or less, re-
gardless of the time to nitrogen saturation. The na-
ture 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 ANC of 50 ueq/l or less. NBS projections indi-
cate that if the average time to watershed nitrogen
saturation approximates 250 years or greater, im-
plementation of the 1990 CAAA would likely re-
sult in target stream reaches maintaining their
1985 proportions of chronically acidic (ANC<0
48
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CHAPTER 2: ENVIRONMENTAL GOALS
ueq/l) as well as sensitive (ANC<50 ueq/l) 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 potential 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 ANC 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: As the number of stream
reaches in the SBRP target population with chronic
ANC of 50 ueq/l or less increases, the possibility of
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 (decreasing ANC) 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 ANC 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
ANC 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 response
times 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 as
opposed to a single national standard.
The acidifying effects of nitrogen deposition
should be considered when evaluating options and
potential need 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,
given the reductions taking place under Title IV.
49
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
Exhibit 14 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
after the year 2000); (2) without the 1990 CAAA
sulfur 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
uncertainties described previously. As such, they
indicate approximate proportions of the surface
water target populations projected to have ANC<0
or <50 fieq/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 benefits in improving ANC and
reducing acidic stress in the lakes and streams of
the three regions that hold a major proportion of
sensitive aquatic resources in the eastern United
States.
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 tend to lengthen
the actual times to watershed nitrogen saturation
in some watersheds sensitive regions. This process
is similar to the ongoing process whereby reduc-
tions in sulfur deposition due to the 1990 CAAA
are likely extending times for 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
for precipitation gave way to using uniform
maximum allowable mass deposition rates, with
20 kg-wet SO42Vha/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 research efforts in
the late 1960s.76
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
77
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.
76 Nilsson, J. and P. Grennfelt (editors). 1988. Critical Loads for
Sulphur and Nitrogen Report from a Workshop Held at
Skokoster, Sweden, 19-24 March 1988, UN/ECE and Nordic
Council of Ministers.
77 Strickland, T.C., C.R. Holdren, Jr., P.L. Ringold, D. Bernard, K.
Smythe, and W. Fallen. 1993. A National Critical Loads
Framework for Atmospheric Deposition Effects Assessment: I.
Method Summary. Environmental Management 1 7:329-324.
50
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CHAPTER 2: ENVIRONMENTAL GOALS
EXHIBIT 14. 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
ANQSO |jeq/l
ANC<50 ueq/l
ANC<0 ueq/l
ANC<50 ueo/l
ANC<0 ueq/l
ANC<50 peq/l
Deposition Scenario
w/o CAAAb
Constant S and Nc
CAAAd
w/o CAAAb
Constant S and Nc
CAAAd
w/o CAAAb
Constant S and Nc
CAAAd
w/o CAAAb
Constant S and Nc
CAAAd
w/o CAAAb
Constant S and Nc
CAAAd
w/o CAAAb
Constant S and Nc
CAAAd
Observed
Proportion3
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
1 00 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
3 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 decreases in sulfur deposition from implementation of Title IV; nitrogen deposition held constant at
1985 levels.
Assessment endpoints are formal expressions of the
environmental value(s) to be protected. They can
include thresholds for "deleterious conditions"
(commonly some ecological condition of concern)
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
51
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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 UNIECE 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.78 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,
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,
Norway, Sweden, Switzerland, and several of the
newly Independent States reflect forests and sur-
face waters.79
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
78 Task Force on Mapping. 1993. Manual on Mapping Critical
Levels/Loads. Coordination Center for Effects, U.N. Economic
Commission for Europe. Berlin, Germany.
79 Coordination Center for Effects, National Institute of Public
Health and Environmental Protection. 1991. Mapping Critical
Loads for Europe. CCE Technical Report No. 1. U.N.
Economic Commission for Europe, Bilthoven, Netherlands.
52
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CHAPTER 2: ENVIRONMENTAL GOALS
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. (The United
States did not sign the Second Sulfur
Protocol because it lacks statutory authority
to reduce emissions to meet critical loads.)
A Nitrogen Oxide Protocol was signed in
1988 by most countries, including the
United States. The NOX Protocol outlined
steps to reduce national annual NOX
emissions. It also initiated research and
cooperative efforts on critical loads for
nitrogen.
which individual critical load values are selected
among the different cells. Qualitative considera-
tions, which often stem from political agreements,
also have a role in this process.80Despite 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
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 2000 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:81
. 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 each. Taken
together, these four European industrialized
countries represent a population very close to that
of the United States. By 2010, their emissions of
sulfur dioxide will be less than 5 million tons,
while the United States is projected to have
emissions of around 15 million tons. Canada
committed to reducing its emissions by 46 percent
within a Sulfur Oxide Management Area (SOMA),
which represents a targeted approach to the acidif-
ication problem in Eastern Canada. Canada, with a
population of about 10 percent that of the United
States, is committed to a national cap of 3.2 mil-
lion metric tons (about 3.5 million 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
80 Henriksen, A., and D.F. Brakke. 1988. Sulfate deposition to
surface waters. Environmental Science and Technology
22(1):8-14.
81 United Nations Economic Commission for Europe. 1994.
Protocol to the 1979 Convention on Long-Range Transboundary
Air Pollution on Further Reduction of Sulphur Emissions.
ECE/EB,AIR/40. Geneva.
53
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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.82 Included in this
approach were allowances for maintaining cond-
itions 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 ANC 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.83 This level
was not viewed, however, as adequate to protect
the most sensitive resources within these regions.
Maryland developed critical loads based on the
sensitivity of individual streams to acidification.84
This effort included as its overall goal an assess-
82 Federal/Provincial Research and Monitoring Coordination
Committee. 1990. The 1990 Canadian Long-Range Transport
of Air Pollutants and Acid Deposition Assessment Report. 8
parts. Research and Monitoring Coordination Committee,
Canada.
83 New England Governor's Conference. 1985. History and the
Development of the New England Position on Acid Rain. New
England Governor's Conference, Inc.
New York State Department of Environmental Conservation
(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.
84 Sverdrup, H., P. Warfvinge, M. Rabenhorst, A. Janicki, R.
Morgan, and M. Bowman. 1992. Critical Loads and Steady-
State Chemistry for Streams in Maryland. Environmental
Pollution 77:195-203.
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.
Wisconsin has a precipitation pH goal of 4.7, cor-
responding to a wet sulfate deposition rate of ap-
proximately 11 kg/ha/year.85 Wet deposition is
used as the evaluation base rather than total depo-
sition because of existing uncertainties regarding
dry deposition rates and the effects of dry inputs
on biological systems. A primary intent of this
goal is to assist decision makers in determining
whether adequate environmental protection is oc-
curring. When properly developed, such goals
can help provide adequate protection of sensitive
biological systems. Studies in Wisconsin indicate
the potential for acute acidification of sensitive
aquatic ecosystems by atmospheric inputs of sulfur
have been reduced by state mandated emission
reductions intended to address this goal.
Minnesota is the only state with an established
deposition standard for sensitive areas.86 Sensitive
areas are defined based on lake ANC, with the
state's deposition standard of 3.7 kg-S/ha/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
85 Based on a March 31, 1995, letter from Donald Theiler,
Director, Bureau of Air Management, State of Wisconsin, to
Rona Birnbaum, Acid Rain Division, U.S. Environmental
Protection Agency.
86 Minnesota Pollution Control Agency. 1985. Statement of Need
and Reasonableness: Proposed Acid Deposition Standard and
Control Plan. State of Minnesota Pollution Control Agency, St.
Paul, MN.
54
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CHAPTER 2: ENVIRONMENTAL GOALS
of sensitive Minnesota lakes (ANC<40 (jeq/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 po-
tentially 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 sulfur
in much of the western United States. For exam-
ple, NO3'approximately equals SO42~ deposition in
some areas of California. Consequently, critical
loads for nitrogen deposition have been estimated
for California,87 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 45kg-
N/ha/yr, depending on the region. Other state ef-
forts are also 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
87 Takemoto, B.K., M. Bergen, N. Motallebi, M. Mueller, H,
Margolis, and S. Prasad. 1992. The Atmospheric Acidity
Protection Program: Annual Report to the Governor and
Legislature. Draft report. State of California Air Resources
Board, Research Division, Sacramento, CA.
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 effec-
ts monitoring (further described in the following
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 cond-
itions is often highest during the spring due to the
mobilization of the winter accumulations of de-
posited 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
55
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
spring meltwaters and their potential adverse eff-
ects 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 poten-
tially 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 re-
main 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. Model projections indicate that at
such times and under the deposition scenarios
tested, 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 imminent, the
greatest potential benefits will come primarily
from control of sulfur emissions and deposition.
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.88 In these areas, as
effects accompanying chronic acidification due to
sulfur deposition are reduced, overall effects due
to episodic acidification would likely continue to
impair the water quality in many of these surface
waters, but the extent of these effects would likely
be reduced because reducing the chronic sulfur
effects also decreases potential episodic 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 ni-
trogen saturation. For these regions, nitrogen
deposition is now or would likely become a more
direct cause of chronically acidic conditions in
sensitive waters, with the potential effects of acidic
sulfur and nitrogen deposition becoming approxi-
mately equal and directly additive. In fact, addi-
tional limits on nitrogen deposition would likely
produce a two-fold potential benefit by both re-
ducing acidic deposition rates and lengthening av-
erage times to watershed nitrogen saturation.
These benefits would effectively allow a greater
mass of NO3~ to be deposited 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 significant
impact. Available information does indicate, how-
ever, that additional deposition reduction through-
out the range of potential reductions in sulfur
and/or nitrogen deposition 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 potentia-
lly 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
88 This does not imply that sulfur deposition is not often a key
component of episodic acidification, because, as discussed
Section 2.2.2, sulfur has often been found to be the primary
cause of episodic acidification in areas both within and outside
the Northeast.
56
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CHAPTER 2: ENVIRONMENTAL GOALS
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
percentage points may mean many lakes or miles
of 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 quanti-
fying regional differences in their remaining times
to watershed nitrogen saturation.
57
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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.
59
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ACID DEPOSITION STANDARD 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-
89,90,91 RADM continues to undergo periodic
peer reviews, evaluations, and improve-
ments.92-93'94 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
89 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.
90 Chang, J.S., P.B. Middletori, 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.
91 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.
92 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.
93 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.
94 External Review Panel report on RADM evaluation
for the Eulerian Model Evaluation Field Study Pro-
gram.
60
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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.95 An ag-
gregation technique, described fully by Dennis et
EXHIBIT 16. PHYSICAL AND CHEMICAL PROCESSES CONTRIBUTING TO ACIDIC DEPOSITION
SOURCES
s~
\ '
Gaseous
Pollutants
in
Atmosphere
|W- Dry D'-xJsition
^
Depo
't h
\
\/
Pollutants
Cloud Wat
and
Precipitatu
Wet
sition v
M 1 A .
\ '
Paniculate
Pollutants
in
Atmosphere
/ e
o
VI
o
in ex
er S
>~
3n £
i
. — • —
Natural
RECEPTORS
Anthropogenic
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., developed during NAPAP is used to develop
annual estimates of acidic deposition (see footnote
91). Meteorological 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 sam-
pling. A total of 30 cases were used in the current
aggregation approach. Deposition results for these
cases were weighted according to the strata sam-
pling frequencies to form annual averages.
95 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.
61
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ACID DEPOSITION STANDARD 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
62
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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, H202, NH3, PAN, HCHO, CO,
aerosol SO42~
* WET DEPOSITION: SO42-, NO3- as HNO3,
NH3, H+
* 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.96 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 (see footnote
90).
Inputs to RADM include hourly emissions of SO2,
sulfate, nitric oxide (NO), nitrogen dioxide (NO2),
96 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.
ammonia (NH3), carbon monoxide (CO), particu-
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 1,97
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.98
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
97 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.
98 Novak, J.H., and T.E. Pierce. 1993. Natural
Emissions of Oxidant Precursors. In Water, Air, and
Soil Pollution. Vol. 67, pp. 57-77.
63
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
about 6 percent of anthropogenic emissions and
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-
ule" 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 cell 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.100
* S02
» 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
99 Walters, R., and M. Saeger. 1990. The NAPAP Emis-
sions Inventory: Development of Species Allocation
Factors. EPA-600/7-89-010f. U.S. Environmental
Protection Agency, Research Triangle Park, NC.
100Office 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.
64
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
EPA101 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 EMIS-
SIONS DENSITY IN 1985 (TONS/YEAR)
45000 - 160000 |
Day- and hour-specific gridded emissions are cre-
ated for each of the 30 RADM aggregation cases.
Temporal allocation factors were developed for
NAPAP102 that provide day-specific estimates
based on tabulation of representative relative diur-
nal emissions patterns by day of the week and by
season for each source. Where emissions strongly
respond 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
101Modica, 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. <
102 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 Model103 was developed un-
der NAPAP to study such relationships. The
Tagged Species Model gives the Eulerian RADM
modeling system the capability to identify, for as-
sessment purposes, the concentration and deposi-
tion fields attributable to specified SO2 emissions
source regions in the presence of the full concen-
tration fields. The Tagged Model preserves the
oxidant competition across space and time. A tag-
ging concept is applied in which additional, iden-
tical mass conservation equations are solved for a
portion of the sulfur concentration field that origi-
nates from specific geographical locations within
the full modeling domain. This allows tagged con-
centration fields and tagged wet and dry deposi-
tion to be identified and tracked in the model
separate from, yet as portions of, the total sulfur
chemical environment that is nonlinear and that
produces the complete concentration and deposi-
tion fields. Exhibit 19 shows the tagged RADM
regions created for the Engineering Model and
103McHenry, 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.
65
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ACID DEPOSITION STANDARD 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 Northwest 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/CA 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 CA 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
Northern FL Peninsula
FL Panhandle
Southern MS
66
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
their geographical description. The use of the
Tagged Model in this study represents the first ex-
tensive use of a Eulerian model to study source-re-
ceptor 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 1,200km. 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 500km. 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 SUL-
FUR DEPOSITION CONTRIBUTED BY RADM SUBRE-
GION 15 (OH/WV/PA BORDER REGION)
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 pro-
duces lighter winds and more convective con-
ditions, including a typically large proportion of
67
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
corrective 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.104
An optimization model for acidic deposition could
simultaneously minimize SO2 removal cost and
average exceedance of target deposition rates over
the receptor model domain.105 Such a model
could calculate costs and emissions reductions
necessary to achieve a regionally averaged target
load comparable to the average annual deposition
level calculated by RADM. This model was inves-
tigated for this report but not used because of the
EXHIBIT 22A. SOURCE-RECEPTOR RELATION-
SHIPS IN THE NORTHEAST: CUMULA-
TIVE PERCENT SULFUR DEPOSITION
104Streets, D.C., 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.
105 Ellis, J.H. 1988. Multiobjective mathematical pro-
gramming models for acid rain control. European
Journal of Operational Research 35(3):365-377.
EXHIBIT 22e. SOURCE-RECEPTOR RELATION-
SHIPS IN THE LOWER OHIO VALLEY: CUMULA-
TIVE PERCENT SULFUR DEPOSITION
EXHIBIT 22c. SOURCE-RECEPTOR RELATIONSHIPS IN THE
SOUTHEAST: CUMULATIVE PERCENT SULFUR DEPOSITION
computational difficulty in combining the non-
linear RADM transfer coefficients into a linear pro-
gramming optimization model. Although an opti-
mization model could have been employed using
linear transfer coefficients, at the time this report
was being developed, no linear transfer coeffi-
cients that approximated the RADM transfer coef-
ficients were available. Optimization models are,
however, used extensively when important critical
68
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
or target load decisions have already been made
(e.g., in European countries). Development of lin-
ear 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.106
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.
106 pacjfjc 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. IAC
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.107
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 (see footnote 91).
The bounding analysis showed that errors that
could contribute to scatter in comparisons with
measurements do not result in the same level of
uncertainty in the relative change predictions. The
uncertainty regarding the predicted change is
much less. The effect of known model errors that
could contribute most to biasing or changing the
model's sensitivity to emissions change was ex-
plored. The results were that the bounded range
of RADM predictions of relative change is roughly
10 percent around the best estimate of deposition
change. That is, a model estimate of change in
sulfur deposition of 40 percent has an uncertainty
the order of ±4 percent. There is greater confi-
dence in the upper bound, but less in the lower
bound because it is affected by our lack of com-
plete understanding of the nonlinear processing
affecting sulfur deposition. The narrow range
would suggest that there is little risk that the model
will misguide users regarding the predicted change
107 U.S. Environmental Protection Agency. 1994. Pro-
gress Report for the U.S.-Canada Air Quality
Agreement.
69
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
in sulfur deposition, despite shortcomings uncov-
ered 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
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
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.
EXHIBIT 24. PERCENT REDUCTION IN TAGGED RE-
GIONS FROM 1985 TO 2010 AS A FUNCTION
OF RELATIVE CONTRIBUTION OF EACH RE-
GION TO ALL TAGGED EMISSIONS
V)
•5
1
00
«-
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•£
o
1
1
1 •
1
1
1 •
1
1 ,
1 %
1
1
L • ^ » -
•\ * •
, ) .%_-_ — - — •
-100 -SO 0 50 100
Percent Reduction from 1985 to 2010
• 53 Emissions Regions
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 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
1 5,420,000 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
1 2,245,000 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
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-
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
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-Appa-
lachians, and the Southern Blue Ridge, respec-
tively. 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.
70
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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
g
'1
E
"D
0)
0)
BJO
(0
4_<
0)
u
8.0
0.0
3 5 7 9 11 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
2010
1985
EXHIBIT 26. CONTRIBUTION OFTOP-IO SO2 EMITTING RE-
GIONS TO SULFUR DEPOSITION IN SENSITIVE REGIONS
Year
1985
2010
Top-10 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- Appalachi-
ans
49.4%
37.3%
Southern
Blue Ridge
30.8%
16.1%
a See Exhibit 19 for geographical descriptions of RADM subregions.
3.3.2 Regional Emissions Distribution in
2010
With greater emissions reductions coming from the
heavier-polluting regions, the relative importance of
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
71
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT 27. COMPARISON OF PROXIMATE AND MAJOR EMITTING RE-
GIONS 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 re-
gions
C. Sources from top-10 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- Appalachi-
ans
24.0%
28.0%
37.0%
46.0%
Southern
Blue Ridge
12.0%
26.0%
34.0%
41.0%
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
row B). In the mid-Appalachians and th€' Southern
Blue Ridge the top emitting regions contribute
only 60 percent more deposition than (Jo 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
allowances. The second represents additional SO2
emissions 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 reductions 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.108 As with the NADB, the 1990 Interim
Inventory was developed by updating the 1985
NAPAP Emissions Inventory. The 1985 inventory
108U.S. Environmental Protection Agency. June 1992.
Regional Oxidant Modeling—Emissions Inventory
Development and [mission Control Scenarios.
U.S. Environmental Protection Agency, May 1989.
Regional Ozone Modeling for Northeast Trans-
port—Development of Base Year Anthropogenic
Emissions Inventory.
72
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
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
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,109 which was
developed to support rulemaking under Title IV.
Existing and planned electric
utility boilers identified in NADB
Version 3.11 plus generic plants
required 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 emissions
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 trad-
ing in order to minimize the
overall cost of reducing SO2
emissions by 10 million tons
below 1980 levels).
(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
EXHIBIT 28. ESTIMATED U.S. SO2 EMISSIONS
WITH AND WITHOUT TITLE IV FROM 1980 TO 2015
Projections of 2010 non-utility
SO2 emissions from the 1990 In-
terim Inventory were based on a
straightforward approach devel-
oped by EPA.110 First, emissions
from the 1990 Interim Inventory
were grown according to the Bu-
reau of Economic Affairs (BEA)
industrial earnings growth factor
20
18 •
16
14
1980 1985 1990
109 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.
110U.S. Environmental Protection Agency, May 1993.
Regional Interim Emissions Inventories (1987-
1991). Volume I: Development of Methodologies.
1995 2000
Year
2005 2010 2015
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.
73
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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-
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 I 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-201 0 FULL IMPLE-
74
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT 30. SO2 EMISSIONS IN THE U.S.
RADM DOMAIN (EASTERN UNITED STATES)
Scenario
1980
1985 NAPAP
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 repre-
sents 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 TO-
TAL 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 reduc-
76
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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-
2010 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
2010 (with trading)
Post-2010 full implementation
2010 (without trading)
Annual Average Deposition Level (kg-S/ha)
Adirondacks
11.0
9.8
7.1
6.9
6.8
Mid- Appalachi-
ans
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
77
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT 35. ANNUAL AVERAGE RADM TOTAL SULFUR DEPOSITION (KG-S/HA) BY GEOGRAPHIC AREA IN 2010:
RANGES OF DIFFERENCES IN DEPOSITION BETWEEN POST 2010 FULL IMPLEMENTATION AND NO TRADING SCENARIOS
-0.80 TO -1.16
-0.50 TO -0.80
EH -0.20 TO -0.50
D -0.20 TO 0.20
0 0.20 TO 0.50
il 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-Appa-
lachians between post 2010 full implementation
and no-trading scenarios listed in Exhibit 34 is due
to increases in one area being offset by decreases
in another. Nonetheless, the modeling estimates of
the differences in sulfur deposition between the
post 2010 full implementation and no-trading sce-
narios 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 apparent
from Exhibit 35 that trading is projected to have
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
78
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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.111 Therefore, the scenario ana-
1 Van Sickle, ]., 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-^5 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 STANDARD FEASIBILITY STUDY
EXHIBIT 36. RADM-PREDICTED ANNUAL AVERAGE
TOTAL SULFUR DEPOSITION (KG-S/HA) IN
2010 UNDER ADDITIONAL UTILITY SO2 EMIS-
SIONS REDUCTION SCENARIO
EXHIBIT 37. RADM-PREDICTED ANNUAL AVERAGE
TOTAL SULFUR DEPOSITION (KG-S/HA) IN
2010 UNDER ADDITIONAL UTILITY AND INDUS-
TRIAL 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 SUL-
FUR DEPOSITION FROM POST-201 0 FULL IMPLE-
MENTATION 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 NAP AP
Post-2010 full implementation
CAAA implementation plus additional
utility SC>2 reduction
CAAA implementation plus additional
utility and industrial SO2 reduction
Annual Average Deposition Level (kg-S/ha)
Adirondacks
11.0
9.8
6.9
5.5
4.7
Mid-Appalachi-
ans
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 RE-
GIONS 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 SO2 reduction
Percent Reduction
Adirondacks
39
51
58
Mid- Appalachi-
ans
41
56
63
Southern
Blue Ridge
31
52
60
EXHIBIT 42. ESTIMATED U.S. NOX EMISSIONS
WITH AND WITHOUT TITLE IV FROM 1980 TO 201 0
26 T
24 -•
a 22 -.
20 ••
o
18 •
16 ••
14
1980
2005 2010
81
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ACID DEPOSITION STANDARD 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
82
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
EXHIBIT 46. ANNUAL AVERAGE RADM TOTAL NI-
TROGEN DEPOSITION (KG-N/HA) IN 1990
EXHIBIT 47. RADM-PREDICTED ANNUAL AVER-
AGE TOTAL NITROGEN DEPOSITION
(KG-N/HA) UNDER UTILITY AND INDUSTRIAL
NOy EMISSIONS REDUCTIONS SCENARIO
EXHIBIT 48. PERCENTAGE REDUCTIONS IN NITROGEN
DEPOSITION UNDER UTILITY AND INDUSTRIAL
NOy EMISSIONS REDUCTIONS SCENARIO
EXHIBIT 49. NITROGEN DEPOSITION TO SENSITIVE REGIONS UNDER BASE CASE AND ADDI-
TIONAL UTILITY AND INDUSTRIAL NOX EMISSIONS REDUCTION SCENARIO
Emissions Scenario
1990 Base Case
Additional utility and industrial NOX reduc-
tions from 1 990 base case
(% reduction from base case in parenthesis)
Annual Average Deposition Level (kg-N/ha)
Adirondacks
9.5
8.1
(14%)
Mid- Appalachi-
ans
14.3
11.3
(21%)
Southern
Blue Ridge
11.9
9.9
(16%)
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ACID DEPOSITION STANDARD 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 ihe 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.
84
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CHAPTER 3: SOURCE-RECEPTOR RELATIONSHIPS AND DEPOSI-
TION REDUCTIONS UNDER VARIOUS EMISSIONS SCENARIOS
EXHIBIT 50. EXAMPLE: 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 exam-
ple 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 propor-
tions 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 include,
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 2.5.3). 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 STANDARD FEASIBILITY STUDY
EXHIBIT 51 A. GEOGRAPHICALLY TARGETED ADDITIONAL UTIL-
ITY SO2 REDUCTION IN CONTIGUOUS RADM SUBREGIONS
Deposition
Subregions
SO2 Emissions
Reduction (tons)
c
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
ensitive Region
Mid-Appalachi-
ans
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 Re-
ceptor Re-
gions
—
—
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)
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-Appalachi-
ans
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 Re-
ceptor Re-
gions
—
_
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
beween 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 (see Exhibit 50) developed
for illustrative purposes and loads defined by the
deposition levels achieved by the nationwide
50 percent utility plus industrial SO2 emissions re-
duction 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
86
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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
utility emissions in the targeted regions. Thus,
targeting scenarios that included industrial emis-
sions 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 the utility reduction targeted
scenario (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 re-
gions at the same time. Using a maintenance load
chosen for illustrative purposes in this report, it
appears not to be very advantageous to geographi-
cally target regions individually to achieve a par-
ticular load.
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT S3A. 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 INDUS-
TRIAL SO2 REDUCTION IN MAJOR RADM SUBREGIONS CONTRIBUT-
ING TO DEPOSITION (NOT CONTIGUOUS)
Deposition
Subregions
Sensitive Region
Adirondacks
4.7 kg-S/ha
15,13,5,14,7,20,
22,44,10,12,45,
17,9,25,26,3
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 MAINTE-
NANCE LOAD OF 5 KG-S/HA IN MAJOR RADM SUBREGIONS CONTRIBUT-
ING 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
88
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CHAPTER 4
POTENTIAL BENEFITS OF AN ACID 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. Acidic deposition and its precursors
affect a broad range of resources, including human
health, visibility, and materials. Actions that may
influence sulfur dioxide and nitrogen oxide emis-
sions levels might also affect these other resources.
This chapter discusses the type of ancillary benefits
that further reductions in sulfur dioxide and nitro-
gen oxide emissions might have on visibility, hu-
man health, and materials. However, the level of
potential benefits depends heavily on the types
and level of any such reduction and cannot easily
be estimated without knowing such information.
4.2 RELATIONSHIP OF VISIBILITY TO ACIDIC
DEPOSITION
In certain areas of the United States, visibility is a
significant environmental indicator of air qualiiy.
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).112 Exhibit 56 summarizes findings for rural
regions in each area based on conditions prior to
the implementation of the Clean Air Act Amend-
ments. NAS also calculated that anthropogenic
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
CONTRIBUTIONS TO VISIBILITY IMPAIRMENT112
Contaminant
Sulfates
Organics
Elemental Carbon
Suspended Dust
Nitrates
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-
112Committee 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
89
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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
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 pollutanl 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
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CHAPTER 4: POTENTIAL BENEFITS OF A STANDARD ON VISIBILITY,
HUMAN HEALTH, MATERIAL, AND CULTURAL RESOURCES
economic benefits of improvements in visibility.113
The visual range maps (Exhibits 57 and 58) illus-
trate impressive changes in visibility associated
with Title IV. Although results are preliminary, the
economic analysis indicates potentially significant
monetary benefits to residential areas of 31 eastern
states in the United States and to national parks in
the southeastern United States. However, there
has been no analysis of whether there would be
any monetary benefits for deeper and more ex-
pensive emissions reductions beyond the CAAA.
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.
A 1993 EPA Report to Congress presented visibility
improvements to Class I areas that could be ex-
pected to accompany implementation of the 1990
EXHIBIT 57: ANNUAL AVERAGE VISUAL
RANGE (KM) PROJECTED FOR 2010 WITHOUT
TITLE IV: SOTH-PERCENTILE VISIBILITY
113 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
Atmospheric Optics: Radiation Balance and Visual
Air Quality, Air & Waste Management Association
International Specialty Conference, Snowbird,
Utah, September 30.
EXHIBIT 58. ANNUAL AVERAGE VISUAL RANGE (KM)
PROJECTED FOR 2010 WITH TlTLE IV, INCLUDING
TRADING: 50TH-PERCENTILE VISIBILITY
EXHIBIT 59. ANNUAL AVERAGE IMPROVEMENT IN
50TH-PERCENTILE VISIBILITY (DV) FROM 1980 TO
2010 WITH TITLE IV, INCLUDING TRADING
CAAA.114 The analysis evaluated impacts of con-
trol provisions for NOX, SO2, and paniculate mat-
ter by assuming implementation of key provisions
of Titles I, II, and IV of the CAAA. Exhibit 60 lists
specific provisions of each title. The analysis did
114 Office of Air Quality Planning and Standards.
October 1993. Effects of the 1990 Clean Air Act
Amendments 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
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
not assume attainment of the particulate matter
(PM) standard in all areas which will also have an
impact on visibility in some regions, but rather as-
sumed across the board PM reductions in nonat-
tainment areas and estimated PM levels using size
range distributions for each source category. Emis-
sions inventories based on other Clean Air Act
controls such as NOX controls that nonattainment
areas might impose to achieve the ozone standard
were unavailable at the time of development of
the 1993 Report to Congress.
Because sulfates dominate visibility impairment in
the East, and no single chemical species dominates
in the Southwest, EPA modeled each region sepa-
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 emis-
sions scenario depicting trading was unavailable
at that time. (Note, however, that the analysis of
the environmental impact of trading allowances,
described in Section 3.5.1, found only minimal
differences in deposition due to trading.) Reduc-
tions in Canadian SO2 emissions predicted by En-
vironment Canada as part of the 1990 NAPAP In-
tegrated 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.115 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 inven-
tories 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.)
115 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 significantly affect qualitative conclusions,
however.
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CHAPTER 4: POTENTIAL BENEFITS OF A STANDARD ON VISIBILITY,
HUMAN HEALTH, MATERIAL, AND CULTURAL RESOURCES
Comparing emissions for SO2, NOX, and
fine participates revealed only minor dif-
ferences 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 emissions
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.
Three-hour average visual range estimates were
developed for representative Class I areas in six
geographic regions: Central Coast (California), Si-
erra, 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 av-
erages.
Exhibit 61 indicates that neither the growth in
emissions between 1988 and 2005 nor implemen-
tation of the 1990 CAAA at sources in the South-
west will have an appreciable effect on visual
range in Class I areas. The insensitivity of pre-
dicted 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 par-
ticulates during this period.
4.2.4 Potential Impact of Further Sulfur
Dioxide Reductions on Visibility
While EPA cannot estimate the impact of an acid
deposition standard without first defining the type
and level of the standard, EPA has studied the im-
pact of further SO2 reductions on visibility in the
East. For this study, visibility in the East was calcu-
lated using the RADM EM-VIS model for two SO2
emissions scenarios described in Chapter 3. Visi-
bility ranges for the post-2010 full implementation
scenario (reflecting the 8.95-million ton SO2 cap)
and for the additional utility and industrial SO2 re-
duction scenario (approximately 44 percent de-
crease in SO2 emissions beyond CAAA reductions)
were calculated for 90th-percentile worst days.
Maps in Exhibits 62 and 63 show percentage
changes in annual average visibility for these two
EXHIBIT 61. AVERAGE ANNUAL VISUAL RANGE ESTIMATES
FOR REPRESENTATIVE CLASS I AREAS IN THE SOUTHWEST
Geographic Region
Central Coast
Sierra
Southern California
Desert Southwest
Golden Circle
Rockies
Representative
Class I Areas(s)
Pinnacles
Yosemite
San Gorgonio
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
EXHIBIT 62. PERCENT INCREASE IN VISUAL RANGE FROM
1985 TO 2010 WITH FULL CAAA IMPLEMENTATION
EXHIBIT 63. PERCENT INCREASE IN VISUAL RANGE
FROM 1 985 TO 2010 WITH ADDITIONAL
SO2 REDUCTION BEYOND CAAA
93
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
scenarios between 1985 and 2010. To
assess the impact of changes in visibility
due only to decreases in ambient sulfate
concentration, visibility impairment from
other ambient species remained constant
in the models.
The greatest improvements in visual range
between 1985 and the baseline scenario
in 2010 lie in a band from northern
Mississippi to southwestern New York
State. Improvements in visibility for
Class I areas in the mid-Atlantic region,
which includes the Great Smoky Moun-
tains 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 England.
The study conducted by EPA for Class I areas in
the Southwest, the region of the country with the
largest concentration of Class I areas, demonstrates
that no single pollutant or source category is
responsible for most of the visibility impairment in
that region. Thus, 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
sulfate and nitrate aerosols, some of which are
acidic), NO2, and ozone (O3) in ambient air can
cause adverse health effects. (Ozone is a related
concern for acidic emissions and deposition stan-
dards because NOX is a major precursor in O3
formation.) Possible health effects related to acidic
deposition and its precursors are quite complex
because of the variety of pollutants, possible routes
of exposure, and mechanisms involved
(Exhibit 64).
Current applicable standards include National
Ambient Air Quality Standards (NAAQS) for SO2,
NO2, and O3. These standards are designed to
protect human health from all significant known
health effects due to these pollutants. The
standards are subject to revision as new scientific
EXHIBIT 64. RELATIONSHIP OF ACIDIC DEPOSI-
TION PROCESSES TO HEALTH EFFECTS
Nitrogen Oxides
VOCs
information becomes available. Further, the
CAAA, through the State Implementation Plan
process, has the mechanisms in place to bring all
areas into attainment. Several studies show that
the 10-million ton SO2 emissions reduction from
1980 levels under Title IV is expected to result in
human health benefits associated with reduced
SO2 and fine particulate exposures. Analyses are
ongoing to quantify these benefits.
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 or ag-
gravated by ambient air concentrations of SO2,
particulate matter (including sulfate and nitrate
aerosols), NO2, and O3 (separately and in combi-
nation). Effects include aggravation of existing
cases, as well as new cases, of chronic bronchitis,
bronchoconstriction, other pulmonary function im-
pairments, chest discomfort, cough, lung inflam-
mation, increased incidence of infectious respira-
tory disease, and increased mortality rates. The
elderly, the very young, and individuals with pre-
existing respiratory diseases, such as asthma, are at
greatest risk and would benefit most from reduc-
tions in the atmospheric concentrations of these
pollutants.
Under Sections 108 and 109 of the CAAA, EPA
establishes primary NAAQS, which protect the
most sensitive segments of the population, with an
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CHAPTER 4: POTENTIAL BENEFITS OF A STANDARD ON VISIBILITY,
HUMAN HEALTH, MATERIAL, AND CULTURAL RESOURCES
adequate margin of safety. Under current statutory
deadlines, all areas are required to be in
attainment with all health-based standards. If an
acid deposition standard were adopted and to the
extent such deadlines slip, however, the additional
emissions reductions achieved by an acid
deposition standard could facilitate the attainment
and maintenance of the primary NAAQS es-
tablished under Sections 108 and 109 of the Act.
A number of recent epidemiological studies have
associated particulate pollution with excess mor-
tality 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, in-
cluding sulfates, and mortality in humans and also
indicated that acidic aerosol concentrations were
directly associated with increased prevalence of
bronchitis in children.115 Clinical studies suggest
that asthmatics may exhibit sensitivities to short-
term exposures to acidic aerosols. As a result, 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.117
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
jim) than coarse particles (2.5 to 10 ^.m). If it is de-
termined, after completion of the ongoing review,
that a new fine particulate standard(s) is appropri-
ate, the associated control strategies will focus on
the control of the precursors (e.g., SO2, NOX, am-
monia, and condensible hydrocarbons) to secon-
dary fine particulates. While still speculative, if the
science eventually bears out this scenario, the
116 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.
117 L.G. Chestnut. September 1994. Human Health
Effects Benefits Assessment of the Acid Rain
Provisions of the 1990 Clean Air Act Amendments.
(Draft final report.) Prepared for the Acid Rain
Division, U.S. Environmental Protection Agency,
Washington, DC. (Final report to be released
October 1995.)
reductions needed to attain a revised primary
particulate standard would overlap and reduce the
need for an acid deposition standard.
Insofar as acidic SO42~ trends roughly parallel total
SO42-trends, 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.118 The upper Midwest (Michigan and Wis-
consin) and the upper Northeast (Maine and New
Hampshire), which had lower ambient 1-hour
sulfate levels, are estimated to have only slightly
improved atmospheric concentrations in the years
2000 and 2020 under this scenario. Several ongo-
ing benefit assessments will address the extent of
monetary health benefits associated with imple-
mentation of Title IV.119
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 and to nitrate aerosols,
which are a common component of fine particu-
lates. Consequently, reductions in NO2 or NOX
emissions are key components of the particulate
and 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
118 This section is drawn primarily from the National
Acid Precipitation Assessment Program, 1991
(7990 Integrated Assessment Report. NAPAP Office
of the Director, Washington, DC).
119 See footnote 117.
95
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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
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 material!;, includ-
ing sandstone, granite, and brick. European studies
show that SO42" and NO3~ concentration? 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
EXHIBIT 65. PERCENTAGE OF METAL CORROSION ATTRIBUTED TO ATMOSPHERIC FACTORS^
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. 1 993. 1992 Report to Congress. National Acid Precipitation Assessment Program, Washington,
DC.
b Corrosion rates are mean measurements from NAPAP field sites.
96
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CHAPTER 4: POTENTIAL BENEFITS OF A STANDARD ON VISIBILITY,
HUMAN HEALTH, MATERIAL, AND CULTURAL RESOURCES
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-
tures) potentially represent an overall cost to
society. Acidic deposition control can therefore be
linked to potential monetary benefits. Relating
acidic deposition-induced physical damage to the
shortened usefulness of materials remains an
important area of research. Quantifying changes in
maintenance and replacement cycles attributable
to changes in acidic deposition is necessary 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 in-
creases 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
potential benefits of the Acid Rain Program.120 Es-
timates 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. While additional
benefits can be expected for SO2 reductions be-
yond the CAAA, there have been no studies to
suggest what those marginal benefits might be. It
is likely that the marginal cost of additional reduc-
tions will increase.
Additional materials damage and pollution reduc-
tion benefit efforts are also underway to determine
120 ICF Incorporated. September 30, 1994. Acid Rain
Program Evaluation: Valuing Potential Reductions
in Automobile Finish Damages-Scoping Study.
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
the benefits of acidic deposition control on a func- value placed on one-of-a-kind resources. These
tional item such as steel bridges as well as the po- and other costs associated with acidic deposition-
tential benefits of control to preserve cultural re- induced damage would likely decrease with im-
sources of historical importance. Damage to cul- plementation of an acid deposition standard, but
tural resources can result in potentially high repair EPA has no current analyses designed to determine
and maintenance costs, replacement costs, and the whether these costs are significant.
98
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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 established, 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 [acid
deposition] standard or standards with the
control program required by Title IV of the
Clean Air Act; and
» Description of the impediments to imple-
mentation of such control program [based
on acid deposition standard or standards]
and the cost-effectiveness of deposition
standards compared to other control
strategies including ambient air quality
standards, new source performance stan-
dards and the requirements of Title IV of
the Clean Air Act.
This chapter describes two basic approaches to
implementing an acid deposition standard. Under
the first approach (referred to here as a regional
targeted approach), EPA would set a standard or
standards, either using existing authority (if ade-
quate) or seeking further authority from Congress
to set such standards and provide deadlines for
their attainment. Then, similar to Title I, states
would determine source-specific limits using
source-receptor models and cost analyses, incor-
porate 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 (referred to here as a
national emissions-based approach), Congress
would direct EPA to set a deposition standard or
standards and to determine the national (or re-
gional) emissions levels for sulfur dioxide and ni-
trogen oxides that would meet those standards.
Congress would then set an emissions cap and al-
lowance allocations 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 imple-
ment the emissions programs.
For these two basic approaches, this chapter will:
* Describe how each would be carried out,
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.
Finally, both illustrative approaches would require
monitoring for program effectiveness. This chapter
summarizes current monitoring efforts and identi-
fies deposition and effects (e.g., surface water)
monitoring as critical components to development
and implementation of an acid deposition standard
or standards as well as to efforts to assess the im-
pact of the CAAA.
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
5.2 REGIONAL TARGETED APPROACH
5.2.1 Description of Regional Targeted
Approach
Under new or existing authority (see Section
5.2.3), EPA would either set national standards for
sulfur and nitrogen deposition, or set different re-
gional standards for sulfur and nitrogen based on
the different sensitivities of different regions to
sulfur and nitrogen. EPA would also establish
deadlines for the attainment of such standards,
unless such deadlines were established through
new statutory authority.
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 standard or standards
would not, by itself, directly affect other
environmental programs unless it required
emissions reductions. The specific sources and
level of emissions reductions would determine the
direct impact on other programs and the usefulness
of coordination and integration.
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 may be dif-
ficult for sources to conduct efficient interstate
trading of NOX emissions without federal legisla-
tion. It would also be useful if Congress specified
the degree of protection desired.
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 are deter-
100
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CHAPTER 5: IMPLEMENTATION ISSUES
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
emissions in another state. The newly created
Northeast Ozone Transport Region extends from
Northern Virginia to Maine. The Northeast Ozone
Transport Commission is currently developing
plans to achieve attainment of the ozone standard
by determining both local and transport-region
wide limits on nitrogen oxides emissions. The
Commission is also considering market-based ap-
proaches (e.g., NOX trading within the transport
region) to achieve maximum protection at least
cost. This effort represents a possible variation on
the regional targeted 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 NATIONAL EMISSIONS-BASED APPROACH
5.3.1 Description of National Emissions-
Based Approach
Congress would direct EPA to provide (1) a range
of target loads (standards) and emissions levels of
sulfur and nitrogen designed to provide a range of
ecosystem protection (and other benefits), (2) lev-
els of national and regional sulfur and nitrogen
emissions that met those target loads, and (3) esti-
mates of the benefits and costs of meeting those
emissions levels.
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 the standards
through conforming 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.
The Title IV implementation process was designed
to address regional air pollution problems, espe-
cially those involving long-range transport of pol-
lutants and their transformation products. Congress
developed Title IV as a comprehensive program
aimed at reducing sulfur dioxide emissions across
broad regions to achieve increased protection for
sensitive receptor areas both local to and hundreds
of miles downwind of major point sources of SO2
emissions. Enhanced effects monitoring (i.e., sur-
face water monitoring) would be desirable to track
the effectiveness 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-
101
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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 and
time constraints 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:
* 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.
* 201OCAAA 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 Analysis121 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.
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
Scenario
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
121 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.
102
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CHAPTER 5: IMPLEMENTATION ISSUES
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CHAPTERS: IMPLEMENTATION ISSUES
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). All such units would
not necessarily scrub (e.g., units may fuel switch,
conserve energy, or use other technologies), but
this analysis provides an estimate of the costs units
could face. 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.
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 reductions.
Costs were estimated by applying the EPA retrofit
scrubbing cost assumptions to achieve 95 percent
SO2 removal from utility boilers identified in each
subregion. 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 $1,000 per ton removed, are es-
sentially identical for the contiguous and non-con-
tiguous cases. Thus, no increase in efficiency
would be gained by choosing the more complex
non-contiguous approach over sequential contigu-
ous geographical targeting.
Costs were not estimated for the geographically
targeted utility and industrial scenario because
specific cost functions for SO2 removal from indi-
vidual industrial sources were not available. De-
veloping cost functions for individual industrial
sources would require an extensive cost develop-
ment effort beyond this scoping study.
5.4.5 NOX Reductions
and Industrial
-50 Percent Utility
EPA is currently developing regulations for the
control of NOX emissions from electric utilities af-
fected under Title IV of the CAAA. Regulations for
Group 1 boilers in Phase I and Phase II were
promulgated on April 13, 1995. Regulations for
Group 2 boilers are under development and the
costs and emissions reductions expected from
these regulations were not available for this report.
Preliminary information on the cost of controlling
NOX emissions from various types of electric utility
boilers is available, however, from a recent EPA
report.122 These costs vary significantly depending
}22 Alternative Control Techniques Document—NOX
Emissions from Utility Boilers, March 1994, U.S.
EPA, Office of Air Quality Planning and Standards,
EPA-453/R-94-023.
105
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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106
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CHAPTER 5: IMPLEMENTATION ISSUES
EXHIBIT 69A: ANNUAL COSTS OF GEOGRAPHICALLY TARGETED REDUCTIONS EQUIVALENT TO
NATIONWIDE 50% UTILITY SO2 REDUCTION (CONTIGUOUS RADM SUBREGIONS)
Emissions Reduction (tons)
Cost ($ million)
Cost-Effectiveness ($/ton)
Sensitive Region
Adirondacks
3,018,000
3,131
1,037
Mid- Appalachi-
ans
1,952,000
2,127
1,089
Southern
Blue Ridge
1,508,000
1,610
1,068
All Three
Receptor Regions
4,526,000
4,741
1,048
EXHIBIT 69s: ANNUAL COSTS OF GEOGRAPHICALLY TARGETED REDUCTIONS EQUIVALENT TO NATIONWIDE 50%
UTILITY SO2 REDUCTION: MAJOR RADM SUBRECIONS CONTRIBUTING TO DEPOSITION (NOT CONTIGUOUS)
Emissions Reduction (tons)
Cost ($ 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
on the type of technology applied, NOX control
efficiency, 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 1.5 million tons
annually at an average cost of about $200 per ton
NOX removed.123 The RIA considers a variety of
NOX control technologies. Applied to pre-New
Source Performance Standard (NSPS) coal-fired
electric utility boilers, these technologies are ca-
pable of achieving NOX emissions reductions of
about 10 to 60 percent. For 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 such as
123 Regulatory Impact Analysis of NOX Regulations,
February 1994, U.S. EPA Office of Atmospheric and
Indoor Air Programs, Acid Rain Division.
selective catalytic reduction (SCR) or selective
non-catalytic reduction (SNCR). SCR can achieve
NOX removal efficiencies ranging from 75 to 85
percent. In combination with LNB technologies,
SCR 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 re-
moved at a coal-fired unit operating in baseload.
In combination with LNB technology, SCR appli-
cation could cost about $1,300 to $2,490 per ton
NOX removed. These costs would be expected to
decline with the wide-scale application of SCR
throughout the electric utility industry, based on
economies of scale.
As with utility boilers, a wide variety of NOX con-
trol technologies are applicable to industrial boil-
ers. These include LNB, SNCR, and SCR. It is
usually more cost-effective to apply these tech-
nologies to electric utility boilers than to industrial
boilers because electric utility boilers generally
provide more suitable operating conditions and
economies of scale.
5.4.6 Summary of Economic Impacts
Exhibit 70 summarizes the total costs and costs per
ton of SO2 removed for each reduction strategy for
which costs were developed. The additional
50 percent nationwide SO2 utility reduction
107
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT 70. SUMMARY OF COSTS OF VARIOUS EMISSIONS REDUCTIONS SCENARIOS
Scenario
CAAA baseline
50% utilitySO2 removal
Targeted utility SO2 removal
(contiguous)
Targeted utility SO2 removal
(not contiguous)
50% utility and industry SO2
removal
SO2 Removed
(Tons x 1000)
9,166
5,047
4,526
4,658
7,047
Annual Cost
($ 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
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 maximum cost-effectiveness, and, thus,
utility sulfur dioxide reductions beyond that
required by the Act would necessarily be less
economically attractive. Additional emissions
reductions would likely impact the emissions
trading program and may limit the compliance
flexibility inherent in the current program. From
Exhibit 70 it is apparent that the difference in 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. It may be
appropriate to assume that some level of cost
savings associated with an unrestricted national
trading program (as assessed for implementation of
the Acid Rain Program under Title IV) could also
result in reduced costs of compliance with broad
emission reductions beyond the current program.
This could widen the cost difference between a
geographically targeted and national emissions
reduction strategies. Interestingly, the 50 percent
utility and industrial reduction scenario is about
equal in cost-effectiveness to the 50 percent utility
reduction scenario. This indicates that emissions
reductions from major industrial sources would be
as cost-effective as additional 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.
Furthermore, development of an acid deposition
standard or standards would require additional
analysis of costs and benefits to determine the
level of incremental benefits in a range of effect
areas as compared to the deposition reductions
necessary to meet a range of standard levels and
the costs associated with these levels.
5.5 MONITORING PROGRAM EFFECTIVENESS
To assess the adequacy and effectiveness of an
acid deposition standard or standards, a process
for measuring the impact of the program must be
created. Attainment of ambient air quality stan-
dards is determined by measuring ambient levels
of criteria pollutants at sites which can reasonably
be expected to demonstrate violations of the stan-
dard. An acid deposition standard or standards
could be structured similarly. Two types of
monitoring programs would be needed: a deposi-
tion monitoring program to measure the effect on
deposition and to assess progress towards the goal,
and an ecosystem monitoring program to deter-
mine the environmental impacts of the standards.
Depending on the intensity of the monitoring and
the geographic coverage, the costs of such moni-
toring may be in the range of $10-20 million per
year. (Some of the monitoring already exists as a
108
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CHAPTER 5: IMPLEMENTATION ISSUES
result of programs begun under NAPAP in the
1980s and in response to the CAAA).
To implement an acid deposition standard, depo-
sition monitoring would have to match the re-
quirements of the standard. The principal species
of interest for deposition monitoring are wet and
dry sulfate and nitrate. Wet and dry ammonia is a
significant contributor to total nitrogen deposition;
however, few data are available on deposition of
this pollutant. In the 1980s a number of federal,
state, and private sector monitoring efforts were
initiated to gather information on acidic deposition
and trends. The monitoring networks were
designed to collect information on acidic
deposition and ozone.
Currently, the largest continuing wet deposition
monitoring network is the National Atmospheric
Deposition Program (NADP)/National Trends
Network which operates about 200 sites in the
United States. Data from this network, which was
initiated in 1978, have been used extensively by
NAPAP for assessment and to produce maps of wet
sulfate and nitrate. Measurement of dry deposition
is resource intensive, and thus the number of dry
deposition sites is limited. The National Oceanic
and Atmospheric Administration (NOAA) operates
a small network of intensive dry deposition sites
and a larger number of indirect measurement sites.
EPA operates a 50-station National Dry Deposition
Network (NDDN). Both networks use models to
infer dry deposition from meteorological and at-
mospheric measurements.
EPA established the Clean Air Status and Trends
Network (CASTNET) which is a multi-agency ap-
proach to monitoring and developing assessment
tools. Participants include federal and state agen-
cies and universities. Data from the NADP, the
NDDN, and the NOAA wet deposition network
provides weekly sulfur and nitrogen deposition
data on the coarse spatial scale of deposition
across the country.
Deposition monitoring as part of the implementa-
tion of an acid deposition standard or standards
could become part of CASTNET. The purposes of
a monitoring program to accommodate monitoring
for an acid deposition standard would need to be
established and data quality objectives would have
to be developed. The data quality objectives
would depend on whether deposition values
would be used in a regulatory sense to determine
specific violations or to assess trends and progress
toward environmental goals. Program require-
ments would need to identify pollutants to be
monitored, standard procedures for determining
wet and dry deposition, spatial resolution, tempo-
ral requirements, and approaches to establish ref-
erence standards.
Implementation of an acid deposition standard
would require effects monitoring (i.e., surface
water) to determine the effectiveness of the
standard. Key aquatic monitoring indicators for
surface water chemistry include changes in ANC,
pH, inorganic monomeric aluminum, and acid-
sensitive aquatic species, especially fish. Several
ongoing monitoring and assessment programs
provide useful monitoring approaches.
Continuation of these programs would likely
address many important monitoring concerns.
Two examples of these program are highlighted
here. First, the Long Term Monitoring (LTM)
Project collects data from 80 lakes and streams
located in mostly acid sensitive portions of Maine,
Vermont, New York, Pennsylvania, Michigan,
Wisconsin, and Colorado. This project provides
the best available data to date on patterns and
trends in surface water acidification at individual
study sites sensitive to acidic deposition.
Assessing continuing benefits from the CAAA on
aquatic ecosystems is the goal of both the LTM
and the Temporally Integrated Monitoring of Eco-
systems (TIME) projects. The TIME project began
monitoring a statistically representative sample of
lakes in the Northeast in 1991 and a similar selec-
tion of streams in the mid-Appalachians in 1993.
Central to the objectives of this project is the abil-
ity to detect statistically significant changes in
chronic acidification trends across the regional
populations of lakes and streams. These sites are
selected to represent regional subpopulations.
Continuation of effects monitoring would be a
necessary tool to evaluate the environmental ef-
fectiveness of a standard or standards to protect
sensitive ecosystems.
5.6 CONCLUSIONS
In this chapter, the discussion has been focused on
implementation issues associated with an acid
deposition standard or standards. 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 waters and/or the multiple effects
associated these pollutants, it is feasible to
implement such an approach. There are different
approaches that could be taken and various factors
109
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
to be considered (administrative complexity,
resource demands on the government and regu-
lated industry, costs, interactions with other pro-
grams). Based on the multiple effects of acidic
deposition and its sulfur and nitrogen precursors, it
is recommended that if further emissions
reductions are pursued, they be as broad as
possible; either a national approach or a regional
approach that incorporates a large area of the
country (e.g., east of the Mississippi River). Fur-
thermore, Title IV is an administratively efficient
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.
Further emissions reductions characterized 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. Compliance costs
could be significantly lessened by the timing of
any further emissions reductions. Reductions
required later rather than earlier may cost less
based on cost-saving technologies demonstrated
through clean coal technology and pollution
prevention efforts and based on the replacement of
existing sources by new, lower emitting sources.
The benefits would likely be in multiple effects
areas.
110
-------
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 poten-
tial benefits to visibility, human health, material,
and cultural resources accompanying additional
reductions in acidic deposition. Chapter 5 ad-
dressed feasibility and effectiveness of implement-
ing 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.
Section 6.4 highlights the merits of understanding
the inherent ecological processes of sensitive re-
gions and the usefulness of developing resource-
specific goals to provide a basis upon which to
measure program effectiveness. Section 6.5 sum-
marizes the range of feasibility and effectiveness
issues regarding developing and implementing an
acid deposition standard or standards.
6.2 DETERMINING ENVIRONMENTAL GOALS
Determining environmental goals for protecting
ecosystems from acidic deposition requires charac-
terizing potential environmental effects and bene-
fits over a range of sulfur and nitrogen deposition
loadings and then deciding on the desired level of
protection for the ecosystem. Resource-specific
goals can be used to determine what emissions
and deposition reductions would likely be needed.
A regionally specific acid deposition standard can
be used to achieve effective and efficient envi-
ronmental protection of those resources and eco-
systems most sensitive to adverse effects and most
likely to benefit from acidic deposition control.
Establishing environmental goals requires selection
of appropriate ecological endpoint criteria and
indicator measures. Such measures must provide
information to accurately judge how successfully
the key ecosystems and resources of concern are
being protected and at what time benefits may be
anticipated (e.g., rate of recovery). The applicabil-
ity of these measures varies among regions and, in
some cases, among individual systems (e.g., water-
sheds, lakes, or streams). Although the analysis
presented in this chapter focuses on changes in
surface water quality reflected by two ANC meas-
ures within these waters, other endpoints may be
equally or more appropriate for protecting sensi-
tive 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 in a region or some defined in-
dex of ecological structure. An index can also be
designed to address a region as a whole and re-
fined to address natural acidity issues.
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 determin-
ing appropriate environmental goals and setting an
effective acid deposition standard or standards.
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 and therefore affect the loca-
tion and form in which sulfur and nitrogen
are deposited. In total, production of at-
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
mospheric acids is a complex process, in-
volving 140 known reactions among 60
chemical species, 40 of which are organic
compounds.
* NATURAL EMISSIONS SOURCES: Natural emis-
sions of acid precursor species, organic
matter, and alkaline materials (dust) are
generated by vegetative matter, soil and
saltwater microbes, geochemical activity,
lightning, and natural combustion (e.g.,
forest fires). Emissions of SO2, sulfates, and
nitrogen oxides are significantly smaller
from natural than from anthropogenic
sources by a factor of ten or more on an
annual basis.
* 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 an-
thropogenic emissions of SO2, electric util-
ity fuel combustion contributes 70 percent,
industrial fuel combustion contributes 14
percent, and the balance comes from other
sources. For anthropogenic emissions of
NOX, both electric utility fuel combustion
and highway vehicles each are estimated
to contribute 32 percent, industrial fuel
combustion provides 15 percent, and off-
highway vehicles produce 12 percent, with
the balance coming from other sources.
The potential need to effectively and effi-
ciently further reduce anthropogenic
source emissions and deposition would
likely focus primarily on limiting emissions
from these major sources.
* CAUSES OF ACIDIC DEPOSITION EFFECTS: Ac-
cumulating scientific evidence verifies that
deposition of acid-forming sulfur and nitro-
gen compounds, both independently and
synergistically, can be significant causes of
surface water acidification effects. Al-
though past research and control efforts
have primarily focused on the control of
sulfur emissions and deposition, recent re-
search indicates that nitrogen deposition
often may be an equally and sometimes
more important cause of some surface
water acidification effects. For example,
considerable evidence indicates that nitro-
gen deposition is generally a greater acidi-
fication concern in the western United
States and that nitrogen deposition as well
as sulfur deposition can be a significant
contributor to episodic acidification of
surface waters in the East.
* WATERSHED NITROGEN SATURATION: There
are limits to the amount of nitrogen that
can be sequestered (e.g., 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 and within
regions. Nitrogen saturation is a potentially
significant concern that contributes to the
acidification process, even if total satura-
tion 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 acidic [i.e.,
having an acid neutralization capacity
(ANC) of 0 ueq/l or less] due to inorganic
ions, predominantly SO42~, NO3~, and CK
These areas include the southwest Adiron-
dack Mountains in New York, New Eng-
land, mid-Appalachian Region, Atlantic
Coastal Plain, northern Florida Highlands,
and low-silica lakes in the upper Midwest.
Compiled evidence indicates that acidic
deposition most likely caused significant
acidification of surface waters in the Adi-
rondacks, 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 the greatest
consideration when determining the need
for protection from future acidic deposition
loadings.
* MOST SENSITIVE RESOURCES AT RISK: An acid
deposition standard or standards could
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
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CHAPTER 6: INTEGRATION AND CONCLUSIONS
potential risk from acidic deposition are
aquatic systems and, potentially, certain
high-elevation red spruce forests.
NATURALLY ACIDIC SURFACE WATERS: Evalu-
ation of acidic deposition should include
the realization that many regions hold
naturally acidic surface waters or surface
waters with low ANC. For example, about
40 to 50 percent of the target population
surface waters in the Adirondacks with
ANC of 50 ueq/l or less (i.e., sensitive) are
likely to persist even with complete elimi-
nation of anthropogenic acidic deposition.
Certain biota evolve to live in naturally
acidic systems. Management and policy
decisions should recognize the existence of
these systems and consider protecting
populations and communities that have
naturally evolved as part of these ecosys-
tems.
ENVIRONMENTAL GOALS TO PROTECT SENSITIVE
AQUATIC RESOURCES: The assessment end-
points of an environmental goal are formal
expressions of the environmental value(s)
thresholds for harmful conditions
(commonly some ecological condition of
concern) that a standard would attempt to
prevent. Assessment endpoints should be
biologically relevant, operationally defin-
able, scientifically predictable and measur-
able, and sensitive to the pollutants of con-
cern. From a policy perspective, assess-
ment endpoints should also be socially
relevant (mutually understood and valued
by the public and decision makers). The
biological effects associated with acidic
deposition are minimized as the level of
acidic deposition is decreased and pH and
ANC levels in sensitive waters are kept
relatively high. For example, based on
laboratory and field studies of sensitive
aquatic species, one general goal may be
to maintain surface water pH above 6.0.
Greatest protection of sensitive aquatic re-
sources occurs in surface waters where
ANC is generally maintained above 50
ueq/l. Another way to state this goal is that
there would be no deposition-driven
chronically acidic lakes or streams and no
episodically acidic lakes or streams. Other
assessment endpoints may also be appro-
priate.
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 being as-
sessed and in certain cases utilized in
European countries and Canada. A critical
and/or target load approach is conceptually
similar to the deposition 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 or deposition standards. It
does, however, provide the foundation
upon which to determine critical loads or
deposition standards. The scientific uncer-
tainty regarding watershed nitrogen satura-
tion makes determining critical or target
loads or a standard difficult at this time.
* MONITORING TO ASSESS EFFECTIVENESS AND
BENEFITS OF CONTROLS: Although the analy-
ses presented in this report focused on
deposition standards appropriate for reduc-
ing regional proportions of surface waters
with ANC below 0 ueq/l and maintaining
surface water ANC above 50 ueq/l, moni-
toring to assess the actual effectiveness of
any emissions or deposition controls should
assess not only the potential benefits of
controls 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 in National
Parks, and degradation of materials 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. Other research efforts have focused
on the role of atmospheric nitrogen depo-
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ACID DEPOSITION STANDARD FEASIBILITY STL DY
sition in producing nutrient enrichment of
surface waters (e.g. estuarine and near
coastal) leading to nuisance algae blooms
low oxygen conditions, and other problems
associated with the widely recognized
water quality problem of eutrophication.
For example, EPA has reported that from
25 to 40 percent of the total nitrogen
loading entering the Chesapeake Bay
comes from atmospheric deposition.124
Additional scientific studies to determine
the potential impact of acidic deposition
on climate change may also be relevant to
fully characterize ecological effects
6.3 PROJECTED ENVIRONMENTAL CONSE-
QUENCES OF ACIDIC DEPOSITION
REDUCTION 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 pri-
marily focused on understanding acidic deposition
relationships within three representative case-study
regions along this area: lakes in New York's Adir-
ondack Mountains and acid-sensitive stream
reaches in the mid-Appalachian Region (portions
of New York, New Jersey, Pennsylvania, Mary-
land, West Virginia, and Virginia) and in the
Southern Blue Ridge Province (portions of North
Carolina, South Carolina, Tennessee, and Geor-
gia). These representative regions receive fairly
high levels of acidic deposition, have the best his-
torical data, and are best understood 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
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
beyond those achieved by the CAAA and a 24
percent decrease in projected NOX emissions from
1990 levels. 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
124 U.S. Environmental Protection Agency. May 1994.
Deposition of Air Pollutants to the Great Waters.
First Report to Congress. EPA-453/R-93-055.
deposition levels to probable proportions of sur-
face waters in the two ANC groupings. Because
times to watershed nitrogen saturation are not
known for these three (or any other) regions, pro-
jections using four possible times for watersheds to
reach nitrogen saturation were modeled by NBS.
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 48).
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 modeling projects that the 1990 CAAA
would reduce the proportions of surface
waters projected to be acidic (i.e., ANC<
0 ueq/l) by 2040 in all three regions, rela-
tive 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 as-
sessed time to watershed nitrogen satura-
tion.
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CHAPTER 6: INTEGRATION AND CONCLUSIONS
EXHIBIT 71. YEAR 2040 NBS PROJECTIONS FOR ADIRONDACK LAKES
50 Yr 100Yr 250 Yr Never 50 Yr 100Yr
Time to Watershed Nitrogen Saturation
250 Yr
Never
• 1 984 Base
I I w/o CAAA
• 1990 CAAA
• CAAA-additional S
[~~l CAAA-additional N
• CAAA-additional S+N
EXHIBIT 72. YEAR 2040 NBS PROJECTIONS FOR MID-APPALACHIAN STREAMS
50 Yr 100Yr 250 Yr Never 50 Yr 100Yr 250 Yr
Time to Watershed Nitrogen Saturation
Never
115
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT 73. YEAR 2040 NBS PROJECTIONS FOR SOUTHERN BLUE RIDGE PROVINCE STREAMS
30
_c
u
ra
o
4)
oc
0)
a
01
a.
20
15
10
5
Note: Maximum range is 30%
ANC<0
n
ANC<50 ueq/l
1
50 Yr 100Yr 250 Yr Never 50 Yr lOOYr 250 Yr
Time to Watershed Nitrogen Saturation
Never
• 1 985 Base
I I w/o CAAA
• 1 990 CAAA
WM CAAA-additional S
I I CAAA-additional N
• CAAA-additional S+N
* The modeling projects that the 1990 CAAA
would reduce the proportions of stream
reaches in the mid-Appalachians projected
to become increasingly sensitive to poten-
tial effects from episodic acidification (i.e.,
ANC<50 ueq/l) by 2040. This benefit is
projected to be lesser in magnitude for the
lakes in the Adirondacks and stream
reaches in the Southern Blue Ridge.
* The modeling indicates that sensitivities of
target aquatic resources and their potential
responses to changes in acidic deposition
clearly differ among the modeled regions.
* The modeling indicates that the benefits to
sensitive surface waters from sulfur deposi-
tion reductions mandated by the 1990
CAAA may be lessened due to future in-
creases in nitrogen leaching caused by
continuing nitrogen deposition and satura-
tion 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 wa-
tershed 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 project-
ing the impact of additional reductions in
sulfur and nitrogen deposition.
* 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 SELECTING DEPOSITION GOALS
Data and analyses presented in this report indicate
that the sulfur reductions in the Clean Air Act will
provide significant benefits to sensitive ecosys-
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CHAPTER 6: INTEGRATION AND CONCLUSIONS
terns. The data also suggest that adverse impact of
nitrogen deposition on watersheds can be signifi-
cant. Available research indicates that nitrogen
plays a significant role in episodic (short-term
acutely toxic) acidification. The most recent re-
search recognizes the importance of nitrogen in
long-term chronic acidification as well. Reducing
atmospheric loading rates for nitrogen deposition
to watersheds would lead to three different but
related potential environmental benefits. First, for
many of the most sensitive watersheds, such red-
uctions would markedly lessen the potential direct
influence of nitrogen in causing gradual long-term
chronic effects. Second, for many sensitive water-
sheds, such reductions would also reduce the po-
tential for harmful episodic short-term acute ef-
fects. Third, for these watersheds, such reductions
would reduce the frequency and lengthen times
necessary to reach nitrogen saturation. With re-
gional times to watershed nitrogen saturation
lengthened, the potential for adverse effects pro-
duced by the remaining sulfur and nitrogen depo-
sition together would significantly decrease.
(Nitrogen saturation occurs when an affected wa-
tershed accumulates deposited nitrogen com-
pounds in excess of the growth requirements for its
resident plant and animal populations. This excess
nitrogen then becomes available to acidify surface
and ground waters draining from the watershed.)
Environmental resources have ranges of sensitivi-
ties and risks to potential effects caused by acidic
deposition. Resources having equivalent sensitivi-
ties have different risk potentials for harmful effects
that depend on how much acidic deposition they
receive. For lakes and streams, biological effects of
acidic deposition are minimized as the level of
deposition is decreased and pH and ANC levels in
sensitive waters are kept relatively high. A general
goal and one used to guide numerous acid deposi-
tion standard and critical load efforts in the United
States and in other countries may be to maintain
pH in sensitive lakes above 6.0 (i.e., the goal
strives for no deposition-driven chronically acidic
lakes or streams). To minimize effects to aquatic
resources from acute episodic acidification,
surface water ANC should be maintained above
50 ueq/l (i.e., goal strives for no, or reduced,
deposition-driven episodic events). This report
identifies several scientifically credible resource
sensitivity endpoints but no policy decision is
made to choose among the range of options at this
time. Future efforts could focus on characterizing
resource-specific sensitivities and appropriate
environmental indicators on additional sensitive
areas in the United States. Further effects
research, particularly concerning the extent and
rate of nitrogen impacts on watersheds and moni-
toring of appropriate environmental indicators
would facilitate this effort.
Aquatic modeling results presented in this report
indicate that additional reductions in sulfur and/or
nitrogen would reduce regional proportions of
chronically acidic surface waters and proportions
of surface waters most sensitive to episodic effects.
The magnitude of benefits varies by region. Al-
though the model results presented are the best
now obtainable, considerable uncertainty accom-
panies these projections. Despite these uncertain-
ties, the model results presented in this report indi-
cate, as consistent with theory, that reducing
deposition loadings for both sulfur and nitrogen
compounds would lessen the number of surface
waters adversely affected by deposition of both
compounds within all three regions modeled.
Scientific uncertainties make setting an acid
deposition standard or standards at a particular
deposition level difficult at this time. Even when
the uncertainties have been resolved or reduced,
setting a single, uniform national standard may be
an inappropriate approach in view of the differing
sensitivities and risks associated with resources in
different regions of the country. Some have ar-
gued, however, that acidic deposition goals (rather
than standards) established through consideration
and analysis of resource sensitivity and risk would
provide useful information and guidance. Such
goals would provide reference points to assess the
effectiveness of pollution control efforts and would
serve as guides for environmental policy makers
until such time as the scientific uncertainties that
inhibit setting an acid deposition standard or stan-
dards are better understood. Because such depo-
sition goals are intended only as reference points,
they should not be construed as standards within
the meaning of the CAA.
In developing deposition goals, it is important to
note that the available scientific data indicate that
individual watersheds in certain regions of North
America are progressing toward and, in a few in-
stances, have reached nitrogen saturation. Re-
maining times to nitrogen saturation for water-
sheds, however, can only be roughly approxi-
mated. This is true on both national and regional
scales. Thus, establishing interim deposition goals
requires making expert assumptions regarding not
only existing times to nitrogen saturation for wa-
tersheds over regional scales, but also the effect
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
that reducing nitrogen deposition would have on
future times to nitrogen saturation.
Reducing both sulfur and nitrogen deposition
would lessen proportions of surface waters ad-
versely affected by acidification for the most ex-
tensively impacted region studied, the Adirond-
acks. Reducing nitrogen deposition is especially
critical, if the time to nitrogen saturation is as short
as some reasonably contend (e.g., 50-100 years or
less). If, however, these times are much longer
(e.g., 100-250 years or more), as others also rea-
sonably contend, then efforts to reduce sulfur
deposition would be a higher priority than nitro-
gen deposition reduction. The modeling results
presented in this report can provide rough esti-
mates of deposition reductions that would be
needed to achieve a range of environmental goals.
It is important to recognize that these estimates are
highly dependent on assumptions made with re-
spect to time to nitrogen saturation. Furthermore,
no guidance has been provided by Congress re-
garding the degree of protection desired by a stan-
dard or standards. An acid deposition standard
can be designed to achieve a variety of environ-
mental goals such as (1) maintenance ol specific
conditions as observed at a particular point in
time; (2) return to pre-industrial conditions; or (3) a
level that balances effects, costs, and other societal
values.
Within the limitations of significant uncertainty
associated with modeling projections and times to
watershed nitrogen saturation, several illustrateve
examples are provided. If the time to nitrogen
saturation in the Adirondacks is assumed to be 50
years, model projections suggest that significant or
even complete elimination of sulfur deposition,
without a significant reduction in nitrogen
deposition, would provide at most a few
percentage points of change in the affected
sensitive lakes at year 2040. To maintain the
proportion of Adirondack lakes with ANC<0 ueq/l
at levels approximating those found during the
NSWS in 1984 (19 percent), model projections
indicate that nitrogen deposition may need to be
reduced to approximately 6.5 to 7 kg
N/hectare/year from the current level of 9.5 kg
N/hectare/yr. Such a reduction likely would also
extend the projected time for nitrogen saturation
over the Adirondacks. If this reduction results in
an increase in the regional time to saturation to
100 years, then the model projects that sulfur
deposition may need to be reduced to ap-
proximately 5.5 to 6 kg S/hectare/year. Deposition
values of 6.5 to 7 kg N/hectare/year and 5.5 to 6
kg S/hectare/year in the Adirondacks correspond
approximately to a 50 percent reduction in nitro-
gen emissions from utility, industrial, and mobile
sources from 1990 levels and an additional 50
percent reduction in utility sulfur emissions be-
yond those mandated by the CAAA.
If instead, the time to nitrogen saturation in the
Adirondacks is assumed to be 250 years, deposi-
tion values of 9.5 kg N/hectare/year and 6.9 kg
S/hectare/year (reflecting 1990 NOX emissions
and the SO2 emissions that are projected in this
report to result from full implementation of the
CAAA) are projected to reduce the proportion of
Adirondack lakes with ANC<0 ueq/l below 1984
levels. If all anthropogenic NOX and SO2 emis-
sions were eliminated, background deposition
levels may approximate 4 kg N/hectare/year and 1
kg S/hectare/year, and the model projects that in
this case, only naturally acidic lakes would
remain.
Moving to more southerly regions, watersheds tend
to have longer times to nitrogen saturation due to
warmer temperatures and differences in forestry
practices. For the Mid-Appalachians, if the time to
nitrogen saturation is 100 years or longer, reduc-
tion of deposition levels to approximately 8.3 kg
N/hectare/year and 8.1 kg S/hectare/year
(reflecting approximately a 50 percent reduction in
nitrogen emissions from utility, industrial, and
mobile sources from 1990 levels and an additional
50 percent reduction in utility sulfur emissions be-
yond those mandated by the CAAA) are projected
to reduce the percent of streams with ANC<
50 ueq/l below the 27 percent observed in the
1985 NSWS. If all anthropogenic NOX and SO2
emissions were eliminated, the model projects that
the number of sensitive mid-Appalachian streams
could be reduced to less than 5 percent.
For the Southern Blue Ridge, if the time to nitrogen
saturation is 250 years or longer, reduction of
deposition levels to approximately 7.9 kg
N/hectare/year and 6.8 kg S/hectare/year
(reflecting the 50 percent NOX and SO2 emissions
reduction scenarios described above) is projected
to reduce the percent of streams with ANC<
50 ueq/l to slightly more than the 6 percent
observed in the 1985 NSWS. If all anthropogenic
NOX and SO2 emissions were eliminated, the
model projects that the number of sensitive
Southern Blue Ridge streams could be reduced to
less than 5 percent. Approximate ranges of
environmental goals in each case study region are
subject to high levels of uncertainty.
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CHAPTER 6: INTEGRATION AND CONCLUSIONS
Actual environmental effects should continue to be
assessed through ongoing and new research efforts
to evaluate the reliability of these and other pro-
jections presented in the report. This research is
also needed to support development and refine-
ment of models for projecting watershed responses
to the simultaneous deposition of sulfur and nitro-
gen with increased certainty, and to determine
whether future restrictions on deposition of either
is warranted.
Several ongoing and potential regulatory efforts are
likely to result in significant reductions in acidic
deposition levels. The Northeast Ozone Transport
Commission under Title I has efforts underway to
reduce NOX emissions in order to facilitate attain-
ment of the existing national ambient air quality
standards (NAAQS) for ozone. EPA is also under-
taking reviews of the ozone and particulate matter
NAAQS. Adoption of revised ozone NAAQS and
a new fine particle NAAQS could likely result in
further NOX and SO2 emissions reduction as part of
the associated control strategies. In assessing pro-
gress toward the goals described above, it will be
important to integrate the nitrogen and sulfur red-
uctions that are likely to be achieved by these
other programs.
This report recognizes that there is merit and im-
portance to understanding the inherent ecological
processes of sensitive regions and monitoring the
health and changes of those ecosystems. Develop-
ing resource-specific goals would provide a guide
to assessing whether existing programs are effec-
tively protecting the environment. Much uncer-
tainty remains to be addressed if a formal standard
were to be pursued.
6.5 FEASIBILITY OF ESTABLISHING AND IM-
PLEMENTING 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 would 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
geographically targeted reductions in
emissions of SO2 and/or NOX. EPA would
either set a standard or standards using ex-
isting authority (if adequate) 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. The regional approach would be
similar to the SIP program used to imple-
ment Title I of the Act regarding attainment
of National Ambient Air Quality Standards
(NAAQS).
* Second, a national, emissions-based ap-
proach which would involve congressional
direction to EPA to set an acid deposition
standard or standards and to determine
emissions levels for SO2 and NOX needed
to meet the standards within a specified
time frame. An emissions cap and allow-
ance allocations would have to be set for
NOX and, as appropriate, adjusted for SO2.
The national approach would be 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
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 re-
ductions), and impairment to the cost sav-
ings of the current Acid Rain Program due
to regional restrictions on allowance trad-
ing.
* 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-
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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). Based on the mul-
tiple effects of acidic deposition and its
sulfur and nitrogen precursors, it is recom-
mended that if further 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).
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 quan-
tified here, would be in multiple effects ar-
eas such as human health, visibility, and
materials, as well as aquatic systems.
Finally, additional research as well as con-
tinued and enhanced environmental
monitoring) (i.e., deposition and effects
monitoring) would be necessary to evalu-
ate the effectiveness of current emission
reduction efforts, to determine the appro-
priate level of a standard or standards, and
to assess the adequacy of that standard.
Further research on the long-term effects of
nitrogen on watersheds, including the ex-
tent and rate of impact, is necessary to
more fully understand watershed dynamics
and the simultaneous impacts of sulfur and
nitrogen. Refinement of the available at-
mospheric and watershed models is critical
for adequate nitrogen deposition and nitro-
gen retention/cycling dynamics to provide
regional-scale reformation across the
country.
This report concludes that establishing acid depo-
sition standards for sulfur and nitrogen deposition
may at some point in the future be technically
feasible although appropriate deposition loads for
these acidifying chemicals cannot now be defined
with reasonable certainty. Major scientific un-
knowns, particularly regarding watershed process
leading to nitrogen acidification and remaining
times to watershed saturation with nitrogen, limit
the current ability to recommend an appropriate
standard for any region. Furthermore, policy de-
cisions regarding appropriate or desired goals for
protecting sensitive aquatic and terrestrial re-
sources are needed to help guide the Agency in
continued analyses and decisions regarding possi-
ble establishment of acid deposition standards.
This includes Congressional guidance and
continuing efforts to address social science
uncertainties related to the public's desired level
of protection of standards and the costs and
benefits associated with meeting such standards.
Therefore, an acid deposition standard is not rec-
ommended at this time.
120
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APPENDIX A
SUMMARY OF SELECTED NAPAP REPORTS
-------
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 peq/l;
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 ueo/l or less.
4. About 3.0% of the NSWS lakes had total
inorganic monomeric aluminum (Al,m)
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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ACID DEPOSITION STANDARD FEASIBILITY STUDY
7. For 8% (16,780 km) of the NSS stream
length, the pH was 5.5 of less and for 1 8%
(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/Catski 1 1 subregion
and Florida.
10. Acidic and low pH streams in the mid-
Appalachian region and Poconos/Catski 1 1
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 Al,m of 202
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
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<0 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 Alim 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. Most acidic
streams in the New Jersey Pine Barrens
were dominated by inorganic acids, but
with considerable influence by organic
acids. 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' concentrations 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
dominated by inorganic acids, with DOC
of less that 2 mg/l.
A-2
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APPENDIX A: SUMMARY OF NAPAP REPORTS
* 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 paniculate 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 composition
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.,
greater than 20% base saturation), base
cations dominate and ANC remains unal-
A-3
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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 CHANCES 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 paleolirnnologi-
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
deposition. Throughout most of the Upper
Midwest, however, substantial regional
A-4
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APPENDIX A: SUMMARY OF NAPAP REPORTS
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 and pH, usually
during hydrological events and over time
scales of hours to weeks. Episodes are
stochastic or probabilistic in nature, in
terms of occurrence, 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 and pH or
highest dissolved aluminum) of episodes is
increased by acidic deposition in some ar-
eas.
6. While improvements in water chemistry
during episodes in some lakes and streams
would be expected with reduced loading
by acidic deposition, 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
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
inputs. Based on our current understand-
ing of the processes of biological re-
sponse, decreases in acidity 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 an 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 AND CAN ADA
1. The vast majority of forests in Jhe 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
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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 the growth
and physiological responses of southern
pines seedlings. This justifies 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 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
importance on a national scale, at least in
some regions (e.g., Ohio River Valley),
ambient air quality monitoring suggests
A-7
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
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
PLOTS FROM EPA's
NITROGEN BOUNDING STUDY
-------
APPENDIX B
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 implementa-
tion at the year 2010 and that any further reduc-
tion 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 S and N deposition
rates that decline linearly from the common year
2010 rates to a selection of different modeled year
2020 deposition rates for each scenario modeled.
For example, some modeled scenarios maintained
the 2010 deposition rates through the year 2020,
while some alternative modeled scenarios de-
B-1
-------
ACID DEPOSITION STANDARD FEASIBILITY STUDY
creased the year 2010 rate to background deposi-
tion rates only by the year 2020. Rates for still
other modeled scenarios decreased to levels be-
tween these extremes. (Background deposition
rates are those materials that originate from only
natural, agricultural fertilizer, and domestic live-
stock sources.) Each modeled deposition rate was
then assumed to remain constant at the specific
modeled 2020 rate until the year 2040, the end of
the model projection period. 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 pre-
sented 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 target sensi-
tive surface waters of the NBS; they do not repre-
sent responses for either all surface waters or for
all NSWS sampled surface waters in the modeled
regions. Each page holds four NBS plots display-
ing projected response surfaces over ranges of pos-
sible 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 al-
terative times to watershed nitrogen saturation: 50
years, 100 years, 250 years, and never (i.e., as-
sumes nitrogen uptake remains constant into the
future at recently estimated 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, ANC<50
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
1990CAAA.
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-
tential changes in sulfur and nitrogen depo-
sition rates on the specified water quality
B-2
-------
APPENDIX B: NBS PLOTS
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 de-
viations contribute markedly to what is generally
called model error. Estimates of the possible
magnitude of model error are termed model uncer-
tainty. Reasons why watershed 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: NBS PLOTS
Net annual N uptake <= 5% in 50 yr Net annual N uptake <= 5% in 100 yr
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ACID DEPOSITION FEASIBILITY STUDY
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Exhibit B2. Percent of target population lakes with ANC <> 50 meq/L for the
Adirondacks region at Year 2015. Deposition equals median deposition
at Year 2015.
B-6
-------
APPENDIX B: NBS PLOTS
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Total Sulfur Deposition (kg S/ha/yr)
Exhibit B3. Percent of target population lakes with pH < 5.0 for the Adirondacks
region at Year 2015. Deposition equals median deposition at year 2015.
pH is estimated from the empirical pH-ANC model.
-------
ACID DEPOSITION FEASIBILITY STUDY
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region at Year 2015. Deposition equals median deposition at year 2015.
pH is estimated from the empirical pH-ANC model.
B-8
-------
APPENDIX B: NBS PLOTS
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region at Year 2015. Deposition equals median deposition at year 2015.
pH is estimated from the empirical pH-ANC model.
B-9
-------
ACID DEPOSITION FEASIBILITY STUDY
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Adirondacks region at Year 2040. Deposition equals median deposition
at Year 2020.
B-10
-------
APPENDIX B: NBS PLOTS
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at Year 2020.
B-ll
-------
ACID DEPOSITION FEASIBILITY STUDY
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Exhibit B8. Percent of target population lakes with pH < 5.0 for the Adirondacks
region at Year 2040. Deposition equals median deposition at year 2020.
pH is estimated from the empirical pH-ANC model.
B-12
-------
APPENDIX B: NBS PLOTS
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8
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
Net annual N uptake <= 5% in 250 yr
Net annual N uptake unchanging
12-
10-
- 8H
g
'55
a e-
CD
Q
C
CD 4-
O) ^
O
I *
0-
10%
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
12-
10-
0)
c
g
8-
a e-
CD 4-
0) H
2
I 2-
0-
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
Exhibit BIO. Percent of target population lakes with pH < 6.0 for the Adirondack^
region at Year 2040. Deposition equals median deposition at year 2020.
pH is estimated from the empirical pH-ANC model.
B-14
-------
APPENDIX B: NBS PLOTS
Net annual N uptake <= 5% in 50 yr
14-
u.
| 12-
2
Deposition (kg
O> CD O
i i i
c
1 4-
2
"Jo 2-
o
0-
il
(
i i i
024
"^
X,
te *-•
)%
i i i
6 8 10
Total Sulfur Deposition (kg S/ha/yr)
1
14-
2 10-
0
1 8-
s.
S 6-
c
0>
O) .
o 4-
z
S 2-
0-
I
14-
"w*
c8 1 9—
i
g
~ 8—
o
Q.
8 6-
1
Q 4-
2
"S 2-
£
0-
Met annual N uptake <= 5% in 100 yr
A
(
i i i
024
K X
)%
I I I
6 8 10
Total Sulfur Deposition (kg S/ha/yr)
Met annual N uptake <= 5% in 250 yr
A
(
v/\^
^
3%
14-
^ 12-
Deposition (kg N
o> co o
i i i
0)
I5 4-
2
« 2-
0-
Net annual N uptake unchanging
(
}%
I
4
\
6
i
8
2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
Total Sulfur Deposition (kg S/ha/yr)
Exhibit Bll. Percent of target population streams with ANC < 0 meq/L for the
Mid-Appalachian region at Year 2015. Deposition equals median
deposition at Year 2015.
B-15
-------
ACID DEPOSITION FEASIBILITY STUDY
Net annual N uptake <= 5% in 50 yr Net annual N uptake <= 5% in 100 yr
14-
£
| 12-
gio-
^
0
i s-
a
& 6-
c
g> 4_
£
^p
« 2-
£
0-
^SsN^3^-'
^XX^4-
^^*'
r
20.7 %
14-
^.
>,
^ 12-
5 10-
c
o
1 8-
a
S 6-
c
o>
? 4-
k.
3 2~
J2
0-
\ \,
^^
\
Q
O
i
17.2 %
1 1 i i i i ill
02468 10 024
I I I
6 8 10
Total Sulfur Deposition (kg S/ha/yr) Total Sulfur Deposition (kg S/ha/yr)
Net annual N uptake <= 5% in 250 yr Net annual N uptake unchanging
14-
k_
>.
|i 12-
z
U 10-
^
1 8^
a
a e-
c
f 4-
Z
3 2-
o
H-
0-
>d "2ii
\.
\. \v
\V N.
"fl Tl/lX X.
\ ~
17.1 %
14-
>,
^ 12J
^
5 10-
c
o
I 8-
o
a
& 6-
c
CD
a> .
o 4—
Z
3 2~
Q
f—
0-
ie
1 1 1 1 1 1 III
02468 10 024
N\XS\XN?
^v\
N^Sc
>€
\
5.9 %
i i i
6 8 10
p
Total Sulfur Deposition (kg S/ha/yr)
Total Sulfur Deposition (kg S/ha/yr)
Exhibit B12. Percent of target population streams with ANC < 50 weq/L for the
Mid-Appalachian region at Year 2015. Deposition equals median
deposition at Year 2015.
B-16
-------
APPENDIX B: NBS PLOTS
Net annual N uptake <= 5% in 50 yr Net annual N uptake <= 5% in 100 yr
14-
£
| 12-
§
!io-
£2
O
1 8-
1
Q 6—
0)
o 4-
2
3 2-
,2
o-
^1
0%
i i i i i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
14-
>.
J 12-
2
O) An
^ 10-
--^'
C
1 8-
§.
& 6-
§
S 4~
2
2 2-
(2
o-
X
A
0%
1 i 1 1 1 I
0 2 4 6 8 10
Total Sulfur Depos tion (kg S/ha/yr)
Net annual N uptake <= 5% in 250 yr Net annual N uptake unchanging
14-
u.
>>
|j 12-
JMo-
^
o
! 8~
o
Q.
Q Q~
c
.
| 12-
C" m
J£_ 10-
c
o
1 8-
o
QL
® o
Q 6-
C
0)
1 4-
2
* 2-
i°
0-
\
0%
I
0
I
2
i
4
i
6
\
8
10
I
0
8
10
Total Sulfur Deposition (kg S/ha/yr)
Total Sulfur Deposition (kg S/ha/yr)
Exhibit B13. Percent of target population streams with pH < 5.0 for the
Mid-Appalachian region at Year 2015. Deposition equals median
deposition at year 2015. pH is estimated from the empirical pH-ANC
model.
B-17
-------
ACID DEPOSITION FEASIBILITY STUDY
Net annual N uptake <= 5% in 50 yr Net annual N uptake <= 5% in 100 yr
14-
]j| 12-
Deposition (kg t
en CD o
t i i
§
I1 4-
Z
ro 2-
^
0-
XN^
X^x
3.8 %
i i i i i
0 2 4 6 8 10
14-
| 12-
^
Deposition (kg I
en co o
i - i I
0)
O) .,
g 4-
15 2-
o
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 Depos tion (kg S/ha/yr)
t
14-
| 12-
Deposition (kg 1
co co o
I i I
CD
I ^
Z
15 2-
0-
Met annual N uptake <= 5% in 250 yr
^^
\^ N
0%
14-
>.
« 12-
^
Deposition (kg I
»^
CO CD O
I 1 1
CD
O) .
o 4-
« 2-
0-
Net annual N uptake unchanging
^
A
0%
2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
i
2
i
6
\
8
\
10
Total Suliur Deposition (kg S/ha/yr)
Exhibit B14. Percent of target population streams with pH < 5.5 for the
Mid-Appalachian region at Year 2015. Deposition equals median
deposition at year 2015. pH is estimated from the empirical pH-ANC
model.
B-18
-------
APPENDIX B: NBS PLOTS
Net annual N uptake <= 5% in 50 yr
14-
]j 12-
^
Deposition (kg I
en oo o
I I I
CD
§> 4-
Z
15 2-
jO
0-
i.
5
i i i
024
^§^i
"^^?
6%
i i i
6 8 10
>
14-
>»
J 12-
Deposition (kg
en oo o
_L L 1
C
0)
8> 4-
Z
To 2-
o
0-
Met annual N uptake <= 5% in 100 yr
^|^,
\^^>
\
4.4 %
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)
I
14-
^ 12-
Deposition (kg N
en co o
i i i
0)
O) .,
Q 4-
z
15 2-
£
0-
^Jet annual N uptake <= 5% in 250 yr
4
4
"^^^
^^
3%
14-
1 12-
Deposition (kg ^
en CD o
i i i
0)
en ,
Q 4-
Z
75 2-
o
0-
Net annual N uptake unchanging
—v
\^
3.5 %
I
10
8
i
10
Total Sulfur Deposition (kg S/ha/yr)
Total Sulfur Deposition (kg S/ha/yr)
Exhibit B15. Percent of target population streams with pH < 6.0 for the
Mid-Appalachian region at Year 2015. Deposition equals median
deposition at year 2015. pH is estimated from the empirical pH-ANC
model.
B-19
-------
ACID DEPOSITION FEASIBILITY STUDY
Total Nitrogen Deposition (kg N/ha/yr)
£ oMAcnoooro*.
1 _ 1 1 ! I 1 1 1 1
Total Nitrogen Deposition (kg N/ha/yr)
o to *>. en co o fo
1 1 1 ! 1 I I
Net annual N uptake <= 5% in 50 yr
4
(
\ XX
N^"^
)%
Total Nitrogen Deposition (kg N/ha/yr)
oro^cnoooroA
i i i i i i i 1 _,.
I I I I I I
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
^Jet annual N uptake <= 5% in 250 yr
\ ^
D%
Total Nitrogen Deposition (kg N/ha/yr)
oioAcnoooMA
i i i i i i i i
slet annual N uptake <= 5% in 100 yr
(
X
}%
i i i i i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
Net annual N uptake unchanging
*
3%
I
2
i
4
i
6
8 10
Total Sulfur Deposition (kg S/ha/yr)
\
6
i
8
i
10
2 4
Total Sulfur Deposition (kg S/ha/yr)
Exhibit B16. Percent of target population streams with ANC < 0 weq/L for the
Mid-Appalachian region at Year 2040. Deposition equals median
deposition at Year 2020.
B-20
-------
APPENDIX B: NBS PLOTS
Net annual N uptake <= 5% in 50 yr
14-
12-
o
:f 8-
o
Q.
I 6"
/I
I 4~
iz
— O-_
H
0-
I
10
02468
Total Sulfur Deposition (kg S/ha/yr)
14-
12H
c
o
o
o.
3 6H
0)
O)
o
4-
2-
0-
3.9 %
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
Exhibit B17. Percent of target population streams with ANC < 50 meq/L for the
Mid-Appalachian region at Year 2040. Deposition equals median
deposition at Year 2020.
B-21
-------
ACID DEPOSITION FEASIBILITY STUDY
Net annual N uptake <= 5% in 50 yr Net annual N uptake <= 5% in 1 00 yr
14-
>,
1 12-
Z
5 10-
c
o
'« 8~
o
Q.
Q 6"~
«
o* 4-
Z'~~
-3; /5
CO c.
J2
0-
" — ^\ ^X
NV ^
N.
>v
^2^
X
\
0%
14-
^| 12-
^10-
c
o
1 8~
a
3 6-
c
O) .*
o 4-
L_
Z._
— — o«.
t°
0-
^1
X
\
0%
1 1 I I I I 1 1 1 1 ! I
02468 10 02468 10
Total Sulfur Deposition (kg S/ha/yr) Total Sulfur Deposition (kg S/ha/yr)
Net annual N uptake <= 5% in 250 yr Net annual N uptake unchanging
14-
1 12-
z
I! 10-
c
o
1 8~
a
3 6-
c
CD
ITl
? 4-
i_
Z
nj 2~"
"Q
I —
0-
L
\
0%
14-
"C"
j| 12-
.§10-
c
o
1 8-
o
Q.
3 eJ
c
CD
O) A
O 4-
u.
iz
"co 2—
o
V-
0-
*
\
0%
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
i
0
i
2
r
4
r
6
8
i
10
Total Sulfur Deposition (kg S/ha/yr)
Exhibit B18. Percent of target population streams with pH < 5.0 for the
Mid-Appalachian region at Year 2040. Deposition equals median
deposition at year 2020. pH is estimated from the empirical pH-ANC
model.
B-22
-------
APPENDIX B: NBS PLOTS
Net annual N uptake <= 5% in 50 yr
O)
^
.i
'w
8.
Q)
8s
I
14-
12-
10-
8-
6-
4-
2-
0-
\
4
i
8
i
10
0246
Total Sulfur Deposition (kg S/ha/yr)
Net annual N uptake <= 5% in 100 yr
14-
12-
g
'55
o
Q.
0) -
Q 6-
0)
o> *
o 4-
« 2H
0-
o%
T
0
I
2
i
4
I
8
I
10
Total Sulfur Deposition (kg S/ha/yr)
Net annual N uptake <= 5% in 250 yr
14-
*CT*
^ 12-
gio-
.0
OT 8~
I
O 6"*
0)
I ^
2
—— rt^__
.2
0-
0%
i
10
2468
Total Sulfur Deposition (kg S/ha/yr)
Net annual N uptake unchanging
14-
12-
c
o
1
-------
ACID DEPOSITION FEASIBILITY STUDY
Net annual N uptake <= 5% in 50 yr
14-
12-
O>
o
10-
& 6H
I 4H
z
« 2H
0-
i
0
I I
4 6
_I |
8 10
Total Sulfur Deposition (kg S/ha/yr)
Net annual N uptake <= 5% in 100 yr
14-
12-
o
S 8-
o
CL
<§ 6-
c
o> .,
p 4-
2-
0-
1.6%
i
0
l
2
i
4
i
8
I
10
Total Sulfur Deposition (kg S/ha/yr)
Net annual N uptake <= 5% in 250 yr
Net annual N uptake unchanging
14-
12-
£.10-
o
1 8H
a
6-
0
? 4-^
— O
0-
1.2 %
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
I
2
O)
c
g
"w
a
0)
Q
(U
§>
14-
12-
10-
8-
6-
4-
2-
0-
0%
i i i i i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
Exhibit B20. Percent of target population streams with pH ^ 6.0 for the
Mid-Appalachian region at Year 2040. Deposition equals median
deposition at year 2020. pH is estimated from the empirical pH-ANC
model.
B-24
-------
APPENDIX B: NBS PLOTS
Net annual N uptake <= 5% in 50 yr Net annual N uptake <= 5% in 100 yr
12-
|p
^ 10-
z
en
^ 8-j
g
•'uj
O o
Q. 0
Q
c
o> 4-
cn
Z 2_
to
o
0-
a
(
*
\
~) °/o
12-
i"
^ 10-
z
O)
^ 8-
g
OJ
O c
CL 6
0)
Q
-------
ACID DEPOSITION FEASIBILITY STUDY
Net annual N uptake <= 5% in 50 yr
Net annual N uptake <= 5% in 1 00 yr
12-
>.
1 10-
y
o>
c" 8-
g
55
8. 6-
o>
O
1 4-
2
z
1 2"
o
0-
£
*
14.6%
i i i
024
I I
6 8
12-
!£
1 10-
z
O)
c" 8-
.g
"35
0 R
CL 0
0)
Q
I 4"
t_
iz
^~ rt
"<5
,2
0-
•4 ><
N.
X. N.
N.
\^ >\.
^so ^*w^
\
6.7 %
i 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)
Net annual N uptake <= 5% in 250 yr
Net annual N uptake unchanging
12-
|io-
z
O)
c" 8~
g
1 6-
0)
Q
0) 4-
cn H
9
—
^ 2-
!§
0-
J
N- • - -
b
\
5.9 %
1 1 1 1 1
02468
-
}
12-
|,0-
0)
c" 8~
g
'55
a e-
0)
Q
v
N ^
\
3.7 %
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)
Exhibit B22. Percent of target population streams with ANC <. 50 weq/L for the
Southern Blue Ridge region at Year 2015. Deposition equals median
deposition at Year 2015.
B-26
-------
APPENDIX B: NBS PLOTS
Net annual N uptake <= 5% in 50 yr Net annual N uptake <= 5% in 100 yr
12-
f,0-
O)
_^£
^ 8-
g
'tn
I 6-
o>
Q
Q
H 4-
O)
o
•^
1 2-
o
o-
L
>
\
0%
1 1 1 1 1 1 1 1 1
02468 10 024
i i
6 8
i
10
Total Sulfur Deposition (kg S/ha/yr) Total Sulfur Deposition (kg S/ha/yr)
Net annual N uptake <= 5% in 250 yr Net annual N uptake unchanging
12-
I
| 10-
2
O)
V*
^ 8~
l_
g
55
O c
Q. O~
0)
Q
QJ 4-
0)
2
1 2-
,2
0-
-
4*
\
0%
12-
1
=c 10-
2
O)
j<:
^ 8-
g
55
8. 6-
0)
Q
o> 4-
cn *
o
^»
1 2'
o
H-
o-
A
>
\
0%
2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
2468
Total Sulfur Deposition (kg S/ha/yr)
10
Exhibit B23. Percent of target population streams with pH < 5.0 for the Southern Blue
Ridge region at Year 2015. Deposition equals median deposition at year
2015. pH is estimated from the empirical pH-ANC model.
B-27
-------
ACID DEPOSITION FEASIBILITY STUDY
osition (kg N/ha/yr)
> co o ro
i i i
Total Nitrogen Deposition (kg N/ha/yr) Total Nitrogen Dep
oroAoioooro 01x3*^0
i i i i i i i _ i i i i
Net annual N uptake <= 5% in 50 yr
\
0%
12-
1,0-
Z
O)
c" 8~
g
'55
0 R
Q. o
I
2
I
4
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
i
8
4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
Exhibit B24. Percent of target population streams with pH < 5.5 for the Southern Blue
Ridge region at Year 2015. Deposition equals median deposition at year
2015. pH is estimated from the empirical pH-ANC model.
B-28
-------
APPENDIX B: NBS PLOTS
Net annual N uptake <= 5% in 50 yr Net annual N uptake <= 5% in 100 yr
12-
I
I 10-
O)
? 8~
g
5
O c
Q. °~
0>
O
o> 4-
s
E 2-
4-
o>
o
Ih.
1 2-
(0
,2
0-
A
V
o'%
12-
>%
73
| 10-
z
O)
j^;
^ 8-
g
'55
a e-
0)
Q
-------
ACID DEPOSITION FEASIBILITY STUDY
Total Nitrogen Deposition (kg N/ha/yr) Total Nitrogen Deposition (kg N/ha/yr)
ofu-p^oioooro o iv> A C5 03 o ro
i i i i i i i i i i i i i i
Net annual N uptake <= 5% in 50 yr
I
\. "4s
>
D%
12-
"S
=c 10-
2
CD
"c" 8-
0
55
O ft_
Q. O-1
CD
Q
o> 4-
0)
2
o
0-
i i i i i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
vJet annual N uptake <= 5% in 250 yr
A
(
V
3%
tiii i
0 2 4 6 8 10
Irogen Deposition (kg N/ha/yr)
*. O> 00 O IV3
i i i i i
2 2_
-------
APPENDIX B: NBS PLOTS
Net annual N uptake <= 5% in 50 yr
Net annual N uptake <= 5% in 100 yr
I
o
'55
o
ex
a>
Q
0)
81
CO
o
12-
10-
8-
6-
4-
2-
0-
4.4 %
~T
2
~T
4
6 8
Total Sulfur Deposition (kg S/ha/yr)
12-
10-
c
o
8-
8. 6-
CD
Q
c
Q>
O)
O
OJ
O
4-
2-
0-
\
3.8 %
i i I I
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
Net annual N uptake <= 5% in 250 yr
Net annual N uptake unchanging
12-
10-
8-
O /~_
0)
Q
I4
£
o
0-
T
3.4 %
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
o>
c
o
O
CL
0>
Q
c
CD
O)
O
12-
10-
8-
6-
4-
2-
0-
A
2%
2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
Exhibit B27. Percent of target population streams with ANC < 50 meq/L for the
Southern Blue Ridge region at Year 2040. Deposition equals median
deposition at Year 2020.
B-31
-------
ACID DEPOSITION FEASIBILITY STUDY
rogen Deposition (kg N/ha/yr) Total Nitrogen Deposition (kg N/ha/yr)
*> o> CD o N> o ro .&. o> oo o ro
i i i i i _ i i i i i i i
_ 2-
o
0-
Net annual N uptake <= 5% in 50 yr
t.
k
trogen Deposition (kg N/ha/yr)
— fc —A
*>• O> CO O IN3
CO
0
0-
i i i i i i
•0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
Met annual N uptake <= 5% in 250 yr
A
(
v.
12-
cr>
? 8~
g
o c
Q. 6-
0)
Q
c
IB 4-
o
I 2-
O
o-
Met annual N uptake <= 5% in 1 00 yr
L
h
i i i i i i
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
Net annual N uptake unchanging
£
(
:*
0 2 4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
i
2
i
6
4 6 8 10
Total Sulfur Deposition (kg S/ha/yr)
Exhibit B28. Percent of target population streams with pH < 5.0 for the Southern Blue
Ridge region at Year 2040. Deposition equals median deposition at year
2020. pH is estimated from the empirical pH-ANC model.
B-32
-------
APPENDIX B: NBS PLOTS
Net annual N uptake <= 5% in 50 yr Net annual N uptake <= 5% in 100 yr
rogen Deposition (kg N/ha/yr)
*>• o> co o ro
i i i i i
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2020. pH is estimated from the empirical pH-ANC model.
B-33
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ACID DEPOSITION FEASIBILITY STUDY
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model.
B-34
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APPENDIX C
RANGE OF INFLUENCE OF EMISSIONS
FROM RADM TAGGED SUBREGIONS
-------
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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ACID DEPOSITION STANDARD FEASIBILITY STUDY
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APPENDIX D
SUMMARY OF SCIENCE ADVISORY BOARD REVIEW
AND PUBLIC COMMENTS AND RESPONSES
-------
APPENDIX D
SUMMARY OF SCIENCE ADVISORY BOARD REVIEW
AND PUBLIC COMMENTS AND RESPONSES
TABLE OF CONTENTS
PART!: SCIENCE ADVISORY BOARD REVIEW D-3
PART 2: PUBLIC COMMENTS AND RESPONSES D-6
Process of Inviting Comments D-6
Response to Public Comment Notification D-6
General Comments on the Report D-6
Definition of Sensitivity and Risk and Selection of Targeted Populations D-6
Geographic Coverage D-10
Monitoring and Data Used in the Report D-10
Nitrogen Saturation D-11
Watershed Modeling D-13
Episodic Acidification D-15
Terrestrial Damage D-16
Uncertainty versus the Need for Standards D-17
Emissions Inventory D-18
Deposition Modeling D-20
Allowance Trading Program D-21
Benefits to Visibility, Human Health, Material, and Cultural Resources D-22
Costs D-23
D-1
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APPENDIX D: SUMMARY OF COMMENTS AND RESPONSES
PART 1: SCIENCE ADVISORY BOARD REVIEW
The draft Acid Deposition Standard Feasibility
Study Report to Congress was reviewed by the
Acid Deposition Effects Subcommittee of the Eco-
logical Processes and Effects Committee (EPEC) of
the Science Advisory Board (SAB) on April 12,
1995. The Subcommittee's review was followed
by EPEC review and finally by SAB Executive
Committee review on June 29, 1995. The SAB
concurred with the conclusion of the report that
the current state-of-the-science with regard to
acidic deposition effects on aquatic and terrestrial
systems does not support an acid deposition stan-
dard at this time.
The SAB made specific recommendations for im-
provement of the report and two broader recom-
mendations to the Agency. This section summa-
rizes the SAB's recommendations and describes
the revisions made to the report in response to
these recommendations.
• The Study should contain a clear state-
ment of the ecological resources and
resource endpoints to be protected by an
acid deposition standard and the level of
protection desired.
Congress requested information on the fea-
sibility and level of a standard; however,
no guidance was provided on the level of
protection desired by Congress or the pub-
lic. A goal could be to protect resources at
a level measured at a previous point in
time, to pristine conditions, or to another
level which balances ecological protection,
economics, and other societal values.
Different goals can also be developed for
episodic and chronic acidification. The
Executive Summary and Introduction have
been revised to explain these options and
how standards must be related to goals.
Chapter 6 has been expanded to present a
range of options for a standard or standards
as a function of potential goals considering
the inherent uncertainty in relating deposi-
tion levels to resource protection. Cur-
rently, an acid deposition standard level
would be driven by aquatic effects because
of the substantial scientific information
available relating deposition to aquatic ef-
fects. Much less is known about the rela-
tionship between deposition and terrestrial
effects (i.e., forests) and current knowledge
suggests these effects are less severe than
aquatic effects. Chapter 2 has been ex-
panded to more fully explain the relation-
ship between ANC and ecological vitality
(fish response) and why ANC was chosen
as the key modeled parameter.
The Study should better characterize and
quantify the uncertainties in model projec-
tions of acidic deposition effects.
Section 2.5, which describes the selection
and use of the MAGIC model, has been
expanded to more clearly address the ra-
tionale for selection of the model, uncer-
tainties associated with its application in
the Nitrogen Bounding Study (NBS), and
results from the Agency's external peer re-
view of the NBS. As discussed in that
section, the uncertainties related to that
study are presently best characterized using
a qualitative basis. It must be recognized
that, as with any projection model, most of
the critical factors contributing to uncer-
tainty cannot be quantified. The limited
quantitative error estimates related to
model projections that might be made
would suggest an unrealistic confidence of
the model's present capabilities. The text
emphasizes that the NBS model projections
should be used to provide estimates of the
direction and magnitudes of change to
different possible future scenarios of
deposition change. The bounding nature
of the NBS has been described in more
detail to address the SAB's request for
"confidence levels." Additional qualitative
discussion has also been added to clarify
that reductions in emissions of nitrogen
oxides from motor vehicle controls and
implementation of the ozone attainment
provisions of Title I, although not modeled
here because emission inventories were not
available at the time, may result in long-
term decreases in nitrogen deposition.
D-3
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
• The Agency should carefully review the
Executive Summary for two types of mis-
statements: (1) summary conclusions that
are inconsistent with the wording of the
body of the report, and (2) conclusions
that are not adequately supported by
available scientific evidence.
The Executive Summary has been rewritten
to more closely capture the conclusions
and supporting information presented in
the report. Additional uncertainty and tar-
get population language has been added to
the Executive Summary.
• The Study should clearly emphasize the
distinction between sensitive (at risk) eco-
logical resources (the focus of the study)
and the general populations of ecological
resources that are more resistant or fully
resistant to adverse impact via acidic
deposition.
Section 2.3 provides a detailed introduc-
tion to the concepts of risk and sensitivity.
It describes how these concepts apply to
problems of assessing and projecting po-
tential receptor responses in sensitive re-
sources caused by acidic deposition expo-
sure. Careful wording is used to convey to
the report's readers that most of the re-
sources having low sensitivity (i.e., high
resistance) to potential acidification effects,
and deposition levels lower than those
necessary to produce a response should be
viewed as being at low risk. Only those
resources having high sensitivities to
potential acidification effects that are ex-
posed to deposition rates sufficient to pro-
duce the adverse sensitive responses should
be considered as being at potentially high
risk. Section 2.3 also reviews different
resource scales that can be used to define
receptor resource sensitivity and that are
important for both scientific and policy
considerations. The section also provides
an overview of the most important site-
specific environmental factors (e.g., soil
type) that locally influence the sensitivities
of potential receptor resources.
• The Agency should utilize acidic
deposition models that include the
biological processes controlling nitrogen
dynamics; MAGIC does not include these
processes.
As discussed in Section 2.5, the Agency
recognizes that biological processes con-
trolling nitrogen cycling in watersheds are
important. When the Nitrogen Bounding
Study began in mid-1992 to support this
Report to Congress, no adequate
combination of dynamic watershed model
(that included controls by biological
processes and surface water chemistry) and
statistically based regional watershed data
existed for the regional modeling of effects
of nitrogen deposition on watershed
nitrogen retention. The MAGIC model was
selected at that time as the best base from
which to implement a "bounding" analysis
on possible acidification effects in target
surface waters under various possible
scenarios of sulfur and nitrogen deposition
and over a selection of possible times to
watershed nitrogen saturation. The NBS
included curves of declining watershed
nitrogen retention to mimic, in a simplified
manner, the possible influence of changing
biological relationships to nitrogen
loadings over time. The peer reviewers of
the NBS concurred that the approach used
was likely the best available at the time.
The Agency presently funds two university
efforts to develop appropriate dynamic
watershed models of combined sulfur and
nitrogen cycling and effects. These model
include representations of specific
biological control processes that affect
nitrogen cycling.
The Study should more clearly character-
ize the scientific uncertainty regarding ter-
restrial ecosystem nitrogen saturation.
Both the draft and the revised report have
emphasized the considerable scientific un-
certainty pertaining to nitrogen saturation
at watershed and regional scales. This un-
certainty is the reason, for example, that
the Nitrogen Bounding Study included a
selection of modeled times to potential wa-
tershed saturation ranging from 50 years to
never. That later case likely applies to
most watersheds across North America, in-
cluding many forested watersheds poten-
tially sensitive to acidic deposition. The
D-4
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APPENDIX D: SUMMARY OF COMMENTS AND RESPONSES
revised report includes additional discus-
sion to emphasize this important point.
Perhaps more importantly, the revised re-
port also provides additional discussion,
including citations from a selection of re-
cently published research reports, to show
more clearly that nitrogen saturation can
occur in sensitive watersheds and that spe-
cific incidences of watershed nitrogen satu-
ration appear to be occurring now in di-
verse watersheds nationally. These
sensitive watersheds range from the
Adirondack and Catskill Mountains in New
York to the Middle Appalachian Mountains
to the Great Smoky Mountains National
Park in the Southeast to Rocky Mountain
National Park in Colorado to the San
Bernardino and San Gabriel Mountains in
California. Thus, watershed nitrogen
saturation cannot and should not be
characterized as a low probability, unlikely
phenomenon or as one lacking a
reasonable basis for concern.
» The Study should identify and emphasize
the importance of environmental monitor-
ing of deposition, ecological indicators,
and ecological endpoints as a parallel and
complementary strategy to modeling in or-
der to assess ecological resource risk from
acidic deposition.
A description of existing deposition and ef-
fects monitoring programs and discussion
regarding monitoring to assess the effec-
tiveness of the control program and en-
hanced monitoring needs to address a
standard or standards have been added to
Chapter 5 of the report.
The SAB also made two recommendations to the
Agency that are broad in scope. They are:
• The Agency should develop a conceptual
framework which identifies the relevant
science questions for the broader set of
acidic deposition effects (human health,
ecological resource health, visibility, ma-
terials erosion, atmospheric chemistry,
and other socioeconomic effects). While
Congress explicitly directed the Agency to
assess the implications of an acid deposi-
tion standard for ecological resources, a
more detailed evaluation of human health
and other possible benefits would be im-
portant were the Agency to develop an
acid deposition standard at some future
time.
This report identifies several categories of
scientific questions that would be
necessary to address in order to determine
and evaluate an acid deposition standard
or standards. Other efforts are underway in
the Agency to identify scientific questions
and the means to address them in areas
which cover effects of acidic deposition as
part of science policy issues related to, in
particular, effects of ground-level ozone
and fine particulate matter. Efforts are also
ongoing under the auspices of the National
Acid Precipitation Assessment Program to
gather current research information
pertaining to the causes and effects of
acidic deposition and to identify remaining
research gaps.
• Technical conclusions in EPA reports such
as this one should be based on references
from the peer-reviewed science-journal lit-
erature.
This report has been modified to include
numerous additional citations from peer-
reviewed journal literature, particularly on
the issues of ecological effects and envi-
ronmental monitoring.
D-5
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
PART 2: PUBLIC COMMENTS AND RESPONSES
PROCESS OF INVITING COMMENTS
EPA announced the availability of the draft Acid
Deposition Standard Feasibility Study Report to
Congress for public comment in the Federal Regis-
ter on February 10, 1995 (FR Vol. 60 No. 28 page
7965). Interested parties were notified that copies
of the report were available from the Office of Air
and Radiation Docket Information Center. Com-
ments were requested to be provided in writing by
April 1, 1995 and a phone number was provided
for those wishing additional information. In addi-
tion EPA mailed copies of the report to individuals
and organizations which had previously expressed
an interest in the Report.
A total of 490 copies of the draft report were dis-
tributed.
RESPONSE TO PUBLIC COMMENT NOTIFICATION
In response to its invitation for public comment,
EPA received 17 written submissions. A break-
down, by general category, of respondents is given
below.
EXHIBIT D-1. COMMENJERS BY€A TECORY
Federal Government 4
State Government 3
Industry Representatives 4
Environmental Groups 3
Academic Researchers 2
Canadian Government 1
The comments ranged from general reactions to
the report and its conclusions to specific section
by section critiques. A synthesis of comments on
particular aspects of the report is provided below
along with responses by EPA. The numbers in pa-
renthesis following comments refer to the source
of the comment as listed at the end of this appen-
dix.
GENERAL COMMENTS ON THE REPORT
Comments
A number of comments were received on the
overall presentation, content, and goals of the re-
port. One commenter stated that the report is well
written and the information presented is factual
and complete (17), another stated that the report is
a good, comprehensive technical coverage of the
topic (1), and a third stated that the report does a
good job in describing the factors involved in set-
ting and implementing acid deposition standards
(12). Two commenters criticized the report for
not adequately addressing or defining "feasibility"
(15,16). Several commenters stated that the report
did not adequately respond to the Congressional
directive in Section 404 of Title IV (Appendix B of
the Act) to describe and specify an acid deposition
standard (4, 5, 6, 7, 15, 16, 17).
Response
The feasibility of setting an acid deposition stan-
dard or standards is dependent on the scientific
basis for selecting a pollutant to achieve the goals
of the standard and on the statutory basis and ad-
ministrative complexity of establishing and enforc-
ing a limitation on emissions of that pollutant. The
report concludes that it may be feasible to set
deposition standards, but that uncertainty, particu-
larly regarding the scientific understanding of ni-
trogen, remains high making it difficult to choose
an appropriate level of a standard. Establishing a
standard or standards would require further guid-
ance from Congress and the public on the degree
of protection desired.
DEFINITION OF SENSITIVITY AND RISK AND
SELECTION OF TARGETED POPULATIONS
Comments
Three commenters (2,6,7) stated that the report did
not adequately characterize many of the most sen-
sitive aquatic and terrestrial ecosystems in North
America. Two commenters (2,3) suggested that
sulfur and nitrogen target deposition loads should
be established for Class I areas. One (2) added
that the report should divide the continent into
separate resource regions of interest before at-
tempting an analysis of target or critical loads.
D-6
-------
APPENDIX D: SUMMARY OF COMMENTS AND RESPONSES
Another (6) stated that the report should present to-
tal numbers rather than proportions of resources
affected. Another commenter (12) agreed that
aquatic systems are the natural resources most at
risk from acidic deposition, but requested more
specific information on the numbers, areas, and
proportions of the lakes, streams, and watersheds
affected or most at risk.
One commenter (2) stated that the report relies too
much on the assumption that already-acidified
aquatic systems (e.g., the Adirondacks) are the
most 'sensitive' (2). However, this same
commenter stated that ecosystems already altered
by air pollution should receive the most protection
by standards, as those systems have little available
buffering capacity and require special protection
from changes due to anthropogenic activities. This
commenter also suggested that sensitive systems
that might be affected by nitrogen deposition
should include tundra ecosystems of the moun-
tainous West, the boreal forests of the northern tier
States and Alaska, and other aquatic and terrestrial
ecosystems in other areas of the West.
Another commenter (4) agreed with the report's
conclusion that the Adirondacks should be in-
cluded among the regions most sensitive to acidic
deposition in the United States. This commenter
also suggested that any acid deposition standards
that may be established should consider the wide
range of sensitivities of New York's aquatic re-
sources and varying response times that are neces-
sary to evaluate the impacts of varying deposition
rates. This commenter stated that lakes in water-
sheds with thin till and mounded seepage, which
contain biological communities at risk, are the
most sensitive to acidification in the Adirondacks
and should be used as the basis for establishing
target sulfur and nitrogen deposition loads. Two
(6,16) commenters requested clarification of the
description of lake conditions included in the tar-
get population and the importance of naturally
acidic Adirondack lakes.
One commenter (15) expressed concern that be-
cause of the large number of small lakes in the
Adirondacks, the report's projections of surface
water populations of surface waters are unrepre-
sentative of the larger general population. This
commenter was also concerned that the report
does not use the Congressional terminology criti-
cally sensitive resources in the report. This
commenter added that the focus on ANC of 50
ueq/l in the report ignores other cutpoints for ANC
that could be used to help distinguish sensitive
from insensitive waters and added that the selected
goals include conditions that "Mother Nature
cannot meet and therefore [are] not a useful way
to make environmental policy recommendations."
Finally, this commenter suggested that upper and
lower bounds for risk should be presented and
other sources of acidification should be noted.
Several comments were received on the use of
ANC and pH to characterize aquatic resource sen-
sitivity. One commenter (6) urged EPA to consider
appropriate watershed-based ecological endpoints
or biological indicators of health rather than ANC.
Another commenter (10) stated that many studies
show that aquatic biological damage begins at pH
6.0, and thus allowing surface waters to become
acidified (ANC<0, pH<5.5) is not sufficiently pro-
tective. By contrast, another commenter (16)
stated that "selection of pH 6.0 appears somewhat
arbitrary". This commenter questioned how such a
standard based on pH 6.0 would apply to a region
that has many water bodies with naturally lower
pH and indigenous fish populations that live in
them and recommended that more emphasis be
placed on responses by dissolved aluminum,
which has been shown often to be the primary
cause of effects to fish exposed to acidic condi-
tions.
Response
Appendix A of this report summarizes the exten-
sive, peer reviewed findings from NAPAP studies,
which identify the regions of North America found
to hold natural resources having potentially high
sensitivities to effects from acidic deposition. The
appendix also characterizes the nature of the spe-
cific sensitive environmental resources within
these areas. These findings, and more recent pub-
lished data, are reviewed in Chapter 2. The dis-
cussion in this chapter clearly recognizes that a
continuous range exists of sensitivities by envi-
ronmental resources to potential acidification ef-
fects, ranging from extremely sensitive to insensi-
tive. Specific environmental characteristics that
tend to increase these sensitivities are tabulated
and described. Information provided in Chapter 2
describes which of these regions are at greatest
present and potential future risk from acidic depo-
sition. Indeed, ecosystems already altered by air
pollution have the greatest potential for benefiting
from additional reductions. Chapter 6 projects the
magnitude of future benefits of emissions reduc-
tions associated with the CAAA and benefits from
additional reductions beyond CAAA requirements.
D-7
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
The discussion of potentially affected resources in
Chapter 2 has also been expanded.
Class I areas can, by definition, be described as
sensitive areas for which air quality deterioration is
not acceptable. Given an adequate level of moni-
toring and assessment data, Class I areas could
serve as potential targets for standards setting ac-
tivities. The necessary monitoring and analyses do
not currently exist for surface waters in most Class
I areas. However, pursuant to the CAAA, EPA is
currently initiating development of regional haze
regulations for Class I areas. Effects in Class I ar-
eas are discussed further under "Geographic Cov-
erage".
The concepts of sensitivity and risk are sometimes
poorly distinguished. As used in the report, sensi-
tivity refers to the potential response relationship
that a receptor, such as a lake or a fish population,
has when affected by a stimulus such as environ-
mental stress produced by acidic deposition.
Highly sensitive lake and stream systems, for ex-
ample, have poor abilities to resist adverse chemi-
cal changes produced by acidic deposition. Alka-
linity or acid neutralizing capacity (ANC) is a
readily measured indicator of surface water
sensitivity, acidic status, the remaining ability to
buffer against future acidification, and the
likelihood that biological communities inhabiting
that aquatic system will respond to changes in
acidic addition. Measures of ANC, thus, provide
useful information on many aspects of aquatic
systems. In turn, risk is the probability that the
receptor of concern (e.g., a lake) will attain
sufficient exposure to a stimulus (e.g., a loading
rate of acidic deposition) such that its sensitivity
(e.g., its buffering capacity) is exceeded and an
adverse response is produced (e.g., ANC is
depleted over the short- or long-term).
Resources can have diverse combinations of both
sensitivity and risk. When a resource's sensitivity
is high, it does not necessarily follow that the risk
to the resource is high. A resource with high sen-
sitivity to acidification, for example, would have a
low risk of adverse effects whenever past, present,
and probable future acidic deposition is unlikely to
meet or exceed its sensitivity threshold. Therefore,
when assessing the need for acid deposition stan-
dards, it is necessary to consider both resource
sensitivity and risk.
An analysis of sensitivity over scales of nations and
continents requires a broadly applicable and
meaningful measure to help assure that the analy-
sis is both comprehensive and practical. ANC is
such a measure. As discussed in the report, it pro-
vides meaningful regional, national, continental
scale information on both the chemical and bio-
logical status of aquatic systems. Other measures,
such as growth and reproductive responses by sen-
sitive resident species, while very important for the
individual systems, provide information of progres-
sively shrinking significance as the scale of analy-
sis expand across regions, nations, and continents.
Further, potential sensitivities of most individual
species to changing surface water acidity are un-
known to science. Beyond that, the potential sen-
sitivities and the relative ranking for each species
also shifts between individual waters, over seasons
within the same water, and across the various geo-
graphic regions of concern. These shifts are due to
a great diversity of site-specific and life-history
considerations related to these species. Cataloging
the extensive matrix of these relationships for indi-
vidual species across the acid-sensitive regions of
North America is beyond the scope of this report.
This complexity of potential resource sensitivity
concerns again points to ANC as the appropriate
indicator measure to be used in analysis such as
presented in the report. First, ANC is the best
overall measure of the ability of surface water,
ground water, and watersheds to buffer against ad-
verse effects from deposited acidic compounds.
Second, ANC has a direct relationship to pH and
other acidity related water quality parameters in-
cluding aluminum. At lower pH levels, the bio-
logical availability of many toxic metals, including
aluminum, increase. As pH decreases, so does
ANC. As explained in Chapter 2, for the sensitive
regions considered, an ANC of 50 ueq/l approxi-
mates pH 6.5 and an ANC of 0 ueq/l approximates
pH 5.3.
Close examination of the Congressional history re-
lated to expectations for this Report, as discussed
in Chapter 2, reveals that Congress followed a
1983 National Academy of Science report in mak-
ing a distinction between 'critically sensitive' and
'sensitive' surface waters as those having alkalinity
< 40 ueq/l and < 200 ueq/l. Specifically, the Re-
port of the Committee on Environment and Public
Works, United States Senate (November 20, 1987,
pages 131 and 132) states:
... 40 units (microequivalents of alkalinity
per liter) of buffering capacity is considered
very low. resources with buffer capacity be-
low 40 ueq/l are extremely sensitive to acidic
deposition. Resources with buffering capacity
D-8
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APPENDIX D: SUMMARY OF COMMENTS AND RESPONSES
between 40 ueq/l and 200 ueq/l are consider-
ately moderately sensitive and subject to
damage at existing deposition rates. Alkalinity
is chosen from among other possible indi-
cators because it is a parameter that is now
routinely measured by surveyors; it can be
measured with reasonable accuracy by cur-
rent technologies; alkalinity is relatively in-
sensitive to contamination; and alkalinity is
less subject to daily and seasonal variability
than other measures of water quality such as
pH. . . . The report is to include an identifica-
tion of the sensitive (<200 ueq/l alkalinity) and
critically sensitive resources (< 40 ueq/l alka-
linity) ....
As discussed in Chapter 2, measurement aims for
alkalinity and ANC are the same, and many scien-
tists consider the terms to be synonymous. Recent
assessment of surface water response have focused
on ANC<0 and <50 ueq/l. Considering natural
environmental variability and projection times
used in the report, an alkalinity value of 40 ueq/l
and an ANC value of 50 ueq/l are essentially
equivalent. This is especially true as related to the
specific intent of Congress to address needs related
to "critically sensitive resources." Further, more
recent research has shown the earlier alkalinity
value of 200 ueq/l used by MAS to no longer be
the most applicable definition of surface water
sensitivity. For example, NAPAP's 1990 Integrated
Assessment reported (page 279):
In lakes with historically higher ANC
[greater than 50 ueq/l], ANC reductions
have not generally been observed except in
regions with very high levels of acidic
deposition such as southern Sweden and
near Sudbury in Ontario, Canada. Al-
though chronic decreases in ANC are less
likely to have occurred in waters with ANC
greater than 50 ueq/l, these waters may be
affected by episodic ANC and pH depres-
sions (sudden, short-term decreases in pH
and ANC related to rainstorms and snow-
melt) with consequent deleterious effects
on aquatic biota.
Therefore, the analysis in this report focused on
how changes in acidic deposition loadings could
alter acidified surface waters (ANC<0 ueq/l) and
those most critically sensitive to chronic effects
from continuing acidic deposition (ANCS50 ueq/l).
That is, the report uses both ANC<0 ueq/l and
ANC<50 ueq/l in considering potential acidic
deposition effects. The first of these is used primar-
ily to characterize those waters that have or may
become chronically acidic, causing serious long-
term adverse consequences to biological commu-
nities inhabiting these systems. The second is
used to characterize those waters most sensitive to
potential long-term acidification and those particu-
larly sensitive to acutely toxic impacts from epi-
sodic acidification, which can cause serious short-
term adverse consequences to resident aquatic
communities.
The report specified qualitatively that the numeric
values discussed are based on the specified target
populations of lakes and stream reaches and quan-
titatively identified the representativeness of target
populations to the total number of systems in each
case-study region. Additional quantification of the
target populations have been included in the re-
vised report. Further, it is important to emphasize
that the legislative language specifically indicated
that the report should focus on sensitive and criti-
cally sensitive resources. In doing this, the report
also tries not to encourage or misguide the reader
into using the information presented beyond the
defined target population.
Following an overview drawn from various NA-
PAP reports and other referenced information,
Chapter 2 presented three resulting conclusions.
The third of these is, "to protect aquatic resources
in sensitive watersheds from effects of long-term,
chronic acidification, a general goal is to maintain
the pH of sensitive lakes above pH 6.0-6.5 and in-
organic monomeric aluminum below 30-50 ug/l.
To protect these resources from potential effects of
episodic, acute acidification, surface water ANC
should be maintained above 50 ueq/l." This was
intended to provide guidance on how a general
goal to protect surface waters might appear. Other
considerations would have to be given for estab-
lishing possible watershed and water specific goals
related to protecting sensitive species and other
special biological considerations. Then, a caution
is provided that, [w]hen establishing protection
goals and objectives for sensitive aquatic re-
sources, this effort certainly must include recogni-
tion and allowances that pH levels less than 6.0
and ANC less than 50 ueq/l occur in some natu-
rally acidic surface waters, and that levels of pH
less than 6.0 can occur naturally in some locations
accompanying periods of episodic stormwater and
snowmelt runoff. That is, the specific environ-
mental objectives of any acid deposition standard
should accommodate the natural ranges of chemi-
cal qualities occurring in waters in the environ-
ment. Furthermore, they may be designed to pro-
D-9
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
tect those special biological communities evolved
to inhabit naturally acidic surface waters.
Regarding the comment on the need to bound risk,
the report strongly makes the point that time to
watershed nitrogen saturation includes consider-
able uncertainty for most individual watersheds
over both regional and nation scales. As such, the
report and the Nitrogen Bounding Study depict the
projections between the various times modeled to
potential nitrogen saturation as providing upper
and lower bounds of the proportion of resources at
risk. Thus, the name, "Nitrogen Bounding Study."
GEOGRAPHIC COVERAGE
Comments
Two commenters (2,3) suggested that the draft re-
port inadequately addresses areas having sensitive
Class I designations, especially in the West. One
(3) suggested that, while significant effects from
depositions are suspected in the Pacific Northwest,
monitoring to document effects to sensitive re-
sources are scarce.
Response
The report emphasizes discussion of resource sen-
sitivity in the Adirondacks, Mid Appalachians, and
the Southern Blue Ridge Province because these
three regions have been the most extensively
studied. Also, as noted in the report, existing
information supports the hypothesis that these are
the three regions holding the largest proportion of
the natural resources at greatest present risk of ad-
verse effects from acidic deposition. Support for
this hypothesis is based, in part, on the findings
from NAPAP's SOS/T Report 9 summarized in Ap-
pendix A of the report. No new regional-scale
analysis has appeared since to counter this
hypothesis. Instead, a 1994 review, supported by
EPA and summarized in the report, also supports
this hypothesis. However, the lack of observed
acidic deposition effects in other regions, particu-
larly the West, could be due to the scarcity of ap-
propriate monitoring data from this area. One
commenter (3) also recognized this data gap. Bet-
ter and more widespread regionally based moni-
toring data from the West are needed before this
hypothesis can be seriously challenged. To pre-
sent a more thorough picture of the status of po-
tential effects from acidic deposition in the West,
additional site-specific information has been in-
cluded in the report to illustrate apparent examples
of ongoing nitrogen saturation and acidification ef-
fects in select mountain regions of Colorado and
California.
The report provides little direct information or as-
sessment on potential effects to areas designated as
Class I under the CAAA, because relatively sparse
monitoring information is available for these areas.
Data are scarce for these areas primarily because
of their inaccessibility and because of restrictions
on monitoring activities stipulated both by the
CAAA and by the managers of these areas to
minimize human disruption. Special status is
given to Class I areas under the CAAA by Con-
gress. The intent of this report, however, is to pro-
vide regionally representative information. In
some cases, the data and conclusions presented in
the report regarding relationships of acid-sensitive
resources in non- Class I areas have similar
characteristics to Class I areas in terms of acid
impact potential. Therefore, while specific data
from most Class I areas are lacking, data from the
sensitive non-Class I areas discussed in the report
may implicitly provide surrogate information on
which to address concerns in certain Class I areas.
The use of information from "surrogate" areas to
address potential effects in Class I areas is a
practice previously established by the Forest Serv-
ice to assess potential deposition effects. This is il-
lustrated, for example, by the Forest Service de-
velopment and use of the Glacier Lakes Ecosystem
Experiments Site in Wyoming.
MONITORING AND DATA USED IN THE REPORT
Comments
One commenter (4) noted that the NBS study and
other parts of the report relied almost entirely on
data collected during EPA's National Surface
Water Survey, which did not include lakes smaller
than 4 hectares in area. The commenter's concern
was that these lakes, which represent about 40
percent of the more than 2,900 Adirondack lakes
and ponds, would be among the first aquatic
receptors to be affected by acidic deposition. This
commenter also suggested that long-term
monitoring data from Adirondack lakes showing
changes in sulfate, nitrate, and ANC should be
included in the report. A second commenter (7)
questioned the extensive use of NAPAP and NSWS
results in the report, which are suggested to have
limited value and questionable findings, rather
than presenting more recent research findings.
A third commenter (6) suggested that the report
should include more recent data on the Adirond-
acks available from the Adirondack Lake Survey
D-10
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APPENDIX D: SUMMARY OF COMMENTS AND RESPONSES
Corporation and the New York Department of En-
vironmental Conservation (6). This commenter
also suggested that EPA "propose and evaluate a
grid system for deposition loading and response
monitoring for the Adirondacks and the other sen-
sitive areas... Such a system is essential to tracking
emissions ...". Another commenter (2) questioned
why more of the references cited did not include
those from 1993-1995 to reflect more current in-
formation, especially regarding nitrogen deposition
effects.
Response
A large body of recent and older literature was re-
viewed during preparation of the report. Findings
presented in the more recent reports did not sig-
nificantly alter major regional scale findings pro-
vided in earlier reviews. The newer literature did
contribute useful details for refining the scientific
understanding regarding the effects of acidic depo-
sition, particularly regarding the effects from nitro-
gen. The report has been expanded to include
discussion of and citations for many newer
research efforts. This has not changed any of the
conclusions presented in the report.
Several questions were raised regarding target
populations for the modeling analyses. As noted
under in the under "Geographic Coverage", lower
limits on the area resolution for the maps used to
establish the peer-reviewed, statistically based
sampling design used by the NSWS caused only
lakes greater that 4 ha in size to be included in the
analysis. It is true that smaller lakes likely are
often more sensitive to the effects of acidic deposi-
tion. But necessary watershed data to model
acidic deposition effects on such lakes over re-
gional scales do not exist.
It is important to recognize that, for any group of
streams, lakes and watersheds sampled and mod-
eled, there will always exist extremes in character-
istics. Sampling and analysis could focus on wa-
tersheds at either end of the extreme. The disad-
vantage to such focusing is that the target popula-
tion may have marginal utility for policy purposes.
The target populations of lakes, streams and
watersheds sampled and analyzed by the NSWS,
NSS and DDRP have received extensive peer and
policy review both before and after these projects.
These populations have been found to be relevant
to concerns regarding potential adverse effects of
acidic deposition. Focusing modeling efforts on
systems of greater vulnerability would lead to
results showing that higher percentages of the
target population is at risk, although the same
number of streams would be affected. The water-
shed selection procedures are clearly described in
the description of the NBS and the explanation in
the report has been expanded.
Using lake or streams survey data collected in
years following the NSWS would provide limited
benefit to the modeling analysis. The model is
calibrated for a specific year (in this case, the year
of the NSWS sampling)—calibrating the model to
data collected in a later year or years, would
provide results that would not be substantially dif-
ferent for long-term future projections. In fact,
such a process undertaken for only a sub-sample
of lakes having survey data after the NSWS Phase I
survey could actually be counterproductive in that
it would introduce greater procedural uncertainty
into the modeling process.
To maintain a degree of scientific consistency
across the various regional-scale analyses com-
pleted by the Agency, the NSWS lake set contin-
ues to be used. Some regional analyses have fo-
cused on assessing information contained in sub-
sets of the original NSWS data that represent the
most sensitive resources. For example, the DDRP
and the NBS projects assessed potential responses
by the most sensitive lakes and streams extracted
from progressively refined subsets of the NSWS
data. That is, the environmental modeling per-
formed during these projects, as presented in this
report, were designed to project the effects of
changing potential deposition loads of sulfur, and
sulfur plus nitrogen, respectively, on the more sen-
sitive surface waters within each of the modeled
regions. In doing so, the responses studied by both
DDRP and NBS tend to include resources having
sensitivity characteristics more similar to those se-
nsitive resources (e.g., smaller lakes) that some
commenters considered to be ignored.
EPA agrees that effects and deposition monitoring
are necessary to assess future impacts and trends.
A discussion of these issues has been added to
Chapter 5.
NITROGEN SATURATION
Comments
Twelve commenters addressed, either directly or
indirectly, issues related to the watershed process
of nitrogen saturation. Indirect comments focused
on watershed modeling issues relating to nitrogen
saturation, will be discussed with the responses
addressing comments on that topic. The following
D-11
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
section responds to issues provided by seven
commenters specifically related to the nitrogen
saturation process.
One commenter (2) praised the report's emphasis
on the importance of nitrogen deposition effects
on ecosystem integrity. This commenter noted
that deposition of nitrogen in the West is princi-
pally "buffered" ammonium nitrate, which is not
"acidic deposition." This commenter suggested
that the report should discuss the interaction of ni-
trogen depositions with insects and disease in pro-
ducing forest effects and altering nitrogen reten-
tion. Further, this commenter requested that more
consideration be given to potential nitrogen depo-
sition effects in the tundra ecosystems of the
mountainous West, boreal forests of the northern
tier states, and recent data from California. This
commenter further suggested that the role of in-
sects and disease in exacerbating the effects of at-
mospherically deposited nitrogen on terrestrial and
aquatic systems, e.g., gypsy moth in Shenandoah
National Park and adelgid in Great Smoky Moun-
tains National Park appear to relate to the com-
bined effects of stress from excess nitrogen deposi-
tion and insect infestation. Insect stress in forests
reduces apparent times to nitrogen saturation by
impairing the ability of affected trees and forests to
incorporate available nitrogen.
Two commenters (13,16) suggested that nitrogen
saturation is a hypothesis without definitive proof,
especially in North America. They suggested that
since nitrogen saturation is unlikely to occur
where forests are disturbed, the potential for nitro-
gen saturation are low outside major parks and
wilderness areas. One (16) suggested that the re-
port incorrectly characterizes nitrogen saturation
as "when" not "if" events. This commenter also
suggested that the report asserts that the knowl-
edge of nitrogen cycling is poorly understood,
when considerable knowledge exists about the
process because of its use as a plant fertili2er. Both
commenters provided various suggestions on im-
proving the discussion regarding the nitrogen cy-
cle, especially a clear acknowledgment that deni-
trification can be an important mechanism for ni-
trogen loss from terrestrial systems.
In contrast, two commenters (4,6) suggested that
recent research shows that estimated time to wa-
tershed nitrogen saturation in the Adirondacks is
likely less than 50 years. These commenters noted
that some recent research results suggest that ni-
trate concentrations are increasing in Adirondack
waters, while sulfate concentrations are decreasing
and ANC levels appear largely unchanged. One
(4) acknowledged that while the extent of nitrogen
saturation in Adirondack watersheds or the time
scale for this phenomenon is not fully understood,
that considerable progress has been made in un-
derstanding the nitrogen saturation process.
Another commenter (8) suggested that additional
emphasis should be placed on the statement that
the time needed for nitrogen saturation is ". . . very
speculative. We do not really know the time
needed for nitrogen saturation to occur". This
commenter went on to note that historical evi-
dence of nitrogen saturation, as indicated by
drainage loss of nitrogen from Hubbard Brook Wa-
tershed, for example, shows that watershed loss of
nitrate can display unanticipated shifts in both di-
rections between high and low loss rates. This
commenter also cautioned that it should not be
implied that all watersheds will eventually become
nitrogen saturated. It is likely that many water-
sheds will not because land use changes can have
considerable influence on the potential for nitro-
gen saturation.
Another commenter (12) suggested that concluding
anything about the benefits from changes in ni-
trogen deposition is premature. This commenter
suggested that retention and loss of nitrogen in
ecosystems is regulated almost exclusively by bio-
logical processes. The commenter also expressed
the view that natural and man-caused changes on
the land can significantly affect these processes.
Noting that the report uses results from European
studies to help establish that nitrogen saturation
can occur, this commenter claimed that deposition
of nitrogen in Europe can be many times that
found in the United States.
One commenter (17) suggested that the time to
saturation scenarios used in the report should be
applicable to a significant component of the Ca-
nadian aquatic resources, and the results for the
Adirondacks probably reflect the Canadian situ-
ation quite well, particularly in southern Ontario
and Quebec. Further, this commenter noted that
the uncertainties raised with respect to the effect of
nitrogen deposition are similar to those raised by
the recently completed Nitrogen Assessment by
the Canadian Environmental Conservation Service.
Response
As conveyed in the diversity of comments re-
ceived, much uncertainty and little consensus exist
regarding either the process of nitrogen saturation
or the times required for watersheds to reach ni-
D-12
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APPENDIX D: SUMMARY OF COMMENTS AND RESPONSES
trogen saturation. This diversity underscores these
two sources of existing uncertainty, as stated in the
report, that prevent the Agency from recommend-
ing at this time either specific national or specific
regional deposition standards.
Despite this uncertainty and debate, the Agency
concludes that the bulk of the scientific evidence
suggests that the process of nitrogen saturation is
not a "hypothesis lacking proof," as suggested in
some public comments. Instead, watershed nitro-
gen saturation is a process that can occur at some
times in some watersheds. The report has been
revised to include additional evidence to support
this conclusion. Determining whether nitrogen
saturation had occurred or is occurring in any in-
dividual watershed requires site-specific monitor-
ing data.
Symptoms of nitrogen saturation and resulting sur-
face water acidification effects do not and likely
will not occur in most watersheds. Many forest
management actions, natural disturbances, and
low sensitivity to potential acidification impacts
will prevent these effects in most forest systems, as
suggested by several commenters. Yet, various
acid-sensitive watersheds, when subjected to high
loadings of nitrogen from the atmosphere, or from
other sources, can become nitrogen saturated and
acidify surface waters into which they drain. Ad-
ditional discussion of these important relationships
has been added to the report to clarify these
points.
Further, some comments reflect different defini-
tions used for nitrogen saturation within the scien-
tific community. As now further explained in the
Nitrogen Bounding Study (NBS) section of the re-
port, some investigators use the term nitrogen satu-
ration to indicate the first point where the supply
of nitrogen compounds from the atmosphere ex-
ceeds the demands for these compounds by water-
shed plants and soil microbes. In comparison, the
definition for watershed saturation used in the dis-
cussion of NBS follows that used by an early re-
searcher of nitrogen saturation and that used dur-
ing the Direct Delayed Response Project (DDRP)
regarding sulfur saturation. That is, the NBS uses
nitrogen saturation to mean the point when the
watershed retains less than 5 percent of all depos-
ited nitrogen on a net annual basis. Under this
definition, any year's deposition may be stored for
release during a later year, as long as at least 95
percent of the mass of nitrogen deposited within
any given year is released. The important differ-
ence in these two definitions is that the first refers
generally to a time occurring relatively earlier in
the nitrogen saturation process, when relatively lit-
tle nitrogen is being lost from the watershed. In
contrast, the definition used for the NBS refers to a
later time in the saturation process, when nearly
all of the annually deposited nitrogen is discharged
from the watershed.
Other specific issues regarding nitrogen saturation
raised by the public commenters are addressed in
the revised report.
WATERSHED MODELING
Comments
Twelve comments were received relating to water-
shed modeling issues including comments on the
dynamics of nitrogen in watersheds and potential
times to watershed saturation. Comments on the
latter two points are summarized and discussed in
the previous response. This section summarizes
and responds to comments relating directly to wa-
tershed modeling. Seven commenters (5, 8, 9, 12,
13, 15, 16) questioned the applicability of the
MAGIC model for assessment because of its per-
ceived limitations.
One commenter (8) stated that most versions of
MAGIC do not include biotic processing and that
this limitation should be identified, especially in
watersheds where nitrogen is important. This
commenter also noted that a new version of
MAGIC, which includes some nitrogen transforma-
tions, is under development. Another commenter
(12) claimed that the NBS is severely limited be-
cause the basic mechanisms of nitrogen cycling
and processes affecting the rate of nitrogen uptake
and release are not included in the model and that
the only use of MAGIC is to bound possible out-
comes. Two commenters (15,16) stated that the
NBS needs to be subjected to additional peer
review, beyond the EPA peer-review process.
Another commenter (13) suggested that, if the
process of nitrogen saturation is real, terrestrial sys-
tems will differ in the capacity to both assimilate
and lose nitrogen and that some portions of a sin-
gle watershed could reach saturation at one time
and other portions at another. This commenter
and another (16) questioned how differences in
time to nitrogen saturation over scales of individ-
ual watersheds would affect the results from NBS
and what implications this would have on poten-
tial standards development.
One of these commenters (16) requested that spe-
cific technical information be supplied to help in
D-13
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
evaluating the Nitrogen Bounding Study (NBS) for
use in evaluating the feasibility of an acid deposi-
tion standard:
* In relation to the total number of water-
sheds in the modeled regions, how may
watersheds were modeled in each region
by NBS, how were the modeled water-
sheds chosen, and what are the distribution
of watershed characteristics?
* In acknowledging that NBS included an at-
tempt to evaluate uncertainty in time to ni-
trogen saturation, better clarification was
requested on quantifying other uncertain-
ties inherent in the MAGIC model to allow
judging the significance of differences
among results presented within the various
NBS plots exhibited.
* Providing information regarding the impli-
cations of compounding the uncertainty
from the RADM projections when these re-
sults were used in the NBS model.
This commenter and another (15) also requested
more information on the model selection process.
Both claimed that the ILWAS model is the best
model available, because, unlike MAGIC, it con-
tains a lake component that can simulate episodic
acidification. One of these commenters (15) sug-
gested that the report should have discussed the
results of tests comparing MAGIC's predictions
with real data and with predictions using other
models. This commenter also questioned the
projections presented in the report from the Direct
Delayed Response Project (DDRP). The
commenter suggested that using projections of
surface waters in the Northeast studies by DDRP
to represent effects on broad classes of surface
waters produces incorrect conclusions, and that
the report lacks discussion of sensitivities for the
surface waters modeled.
Another commenter (9) claimed that both the NBS
and the DDRP investigations inappropriately com-
bined responses of watersheds in the Mid-Appala-
chians that have low sulfur-retention characteris-
tics with those that have high retention. This
commenter further claimed that the generaliza-
tions from this aggregating were misleading, and
that effects occurring in the most sensitive Class I
areas were effectively ignored in the model output.
This commenter suggested further that a more
focused assessment would have lead to different
conclusions regarding this region and that DDRP's
implementation of MAGIC could not be calibrated
successfully to streams of the region's Class I areas.
The commenter referred to recent model results
(not yet peer reviewed or published) that suggest
that reductions in sulfur deposition of 70 to 80
percent are needed to prevent additional stream
acidification in this region.
Response
Additional discussion has been added to Chapter 2
that explains that NBS was designed to address
one of the major goals of this study, to project the
dynamic regional-scale effects on surface water
chemistry from sulfur and nitrogen deposition into
areas containing potentially vulnerable lakes and
streams. The Chapter 2 discussion also notes that
when this effort began in 1992, limited options ex-
isted for model choice and regional sets available
for modeling. Because of these limitations, it was
possible only to investigate the potential bounds of
nitrogen deposition effects on surface water chem-
istry in combination with process modeling of ef-
fects of sulfur deposition.
In comparing the ILWAS and the MAGIC models
for regional-scale modeling, EPA determined that
although both models provide comparable re-
gional results, MAGIC could be successfully cali-
brated for significantly more watersheds than IL-
WAS. Thus, use of the MAGIC model in the NBS
was the chosen approach. Chapter 2 explains the
selection process in further detail.
The DDRP modeling and data, completed in sup-
port of the National Acid Precipitation Assessment
Program (NAPAP), formed the basis of subsequent
NAPAP modeling using many alternative scenarios
of atmospheric sulfur deposition. Documentation
of the DDRP and subsequent NAPAP modeling ac-
tivities based upon DDRP can be found in several
publications cited in Chapter 2 of the report.
As, noted in the report, the NBS did not simulate
how nitrogen deposition might alter watershed re-
tention of nitrogen; no combination of model and
regional data available in early-mid 1992 could do
that at the regional scales required for the Acid
Deposition Standard Feasibility Study. Rather, the
NBS illustrated what would be the result on
surface water chemistry at regional scales //"certain
scenarios of changes in watershed nitrogen
retention were to occur. In so doing, the NBS
effectively bounded all reasonable possibilities of
such effects. This is the first time this has been
accomplished.
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APPENDIX D: SUMMARY OF COMMENTS AND RESPONSES
As discussed under "Monitoring and Data Used in
the Report," the target populations of surface wa-
ters included in the NSWS, NSS, DDRP, and now
NBS have received extensive peer and policy re-
view both prior to and following those projects.
They have been found relevant for concerns re-
garding potential adverse effects of acidic deposi-
tion. In the Mid-Appalachians, for example, the
model was calibrated successfully to low ANC
streams in the region to represent adequately the
target population defined by the sampling frame.
If the analysis were to focus on lakes and streams
of potentially greater (or lesser) vulnerability, the
regional utility of the modeling would be lost.
The report now includes additional information
regarding uncertainty in the NBS modeling exer-
cise. The expanded discussion notes that uncer-
tainty in any computation or modeling analysis is
the potential difference between the calculated
value (under a set of conditions) and the "true"
value. Quantifying levels of actual error in model
projections, such as those made within the NBS (or
for any watershed acidification model used in a
predictive sense), is not possible because future
effects are not yet observable. Certain
components of model uncertainty (e.g., sample
uncertainty, input uncertainty) may be estimated
quantitatively, but other components (e.g.,
aggregation uncertainty, structural uncertainty)
cannot. These latter sources of uncertainty likely
overwhelm the former. The computation of
"confidence limits" around model projections
based only on estimations of those few sources of
uncertainty that can be quantified can be
misleading, because they underestimate the total
uncertainty involved.
For example, one public comment requested that
the effect of uncertainty in RADM projections be
propagated through the watershed modeling. In
fact, such an estimate of propagated uncertainty
would be very minor. RADM simulations were
used until the year 2010; after that, prescribed ex-
plicit scenarios of deposition were used. Any
computation of effects of uncertainties of the
RADM projections or the MAGIC projections for
the period up to year 2010 would not be meaning-
ful within the context of the study.
The purpose of the model runs performed for the
Acid Deposition Standard Feasibility Study was to
test the sensitivity of potential watershed responses
to varying scenarios of (1) nitrogen deposition, and
(2) watershed transition to nitrogen saturation in
relation to projected effects of sulfur deposition.
In this light, and as described in the report, the
utility of the watershed simulations is to examine
direction and magnitude of projected relative
changes, rather than to focus on explicit numerical
estimates of percentages of target populations. In
addition, the Agency is funding the development
of further nitrogen modeling capabilities in the
MAGIC model. The Agency is also funding
additional watershed nitrogen modeling
development as well as a watershed manipulation
project to allow comparing short term model
projections with effects of elevated deposition over
watershed scales.
The NBS methodologies were peer reviewed fol-
lowing standard EPA peer review procedures. The
number of peer reviews requested and received on
the report exceeded requirements. The revised re-
port describes the peer review process and its re-
sults.
EPISODIC ACIDIFICATION
Comments
Five commenters (2,4,6,13,16) addressed episodic
acidification. One (2) agreed that the report
places correct emphasis on the importance of
acidic episodes in low ANC aquatic systems. Two
commenters (4,6) reported that recent research has
shown that acidic episodes have great potential to
affect the survival of aquatic organisms. One (6)
emphasized that nitrogen plays a very significant
role in acidification episodes and suggested that
the report should increase its discussion of this re-
lationship. Both (4,6) recommended that a stronger
emphasis be placed on reductions in NOX emis-
sions because most snowmelt episodes are acidic
due to high nitrate levels. One (4) emphasized
that any acid deposition standard must account for
the episodic events resulting primarily from spring
snowmelt that affects the most sensitive Adirond-
ack water classes. This commenter claimed that,
based on information contained in the report (3.5
times the number of chronically acidic Adirondack
lakes could be affected by episodic acidification
and potentially 43 percent of the lakes in the Adi-
rondacks may be acidic in 2040 after CAAA im-
plementation if 50 years to nitrogen saturation is
assumed), then 100 percent of the Adirondack
lakes may be subject to episodic acidification in
2040.
A fourth (13) commenter suggested that most of
the work documenting effects related to episodic
acidification has occurred in small watersheds,
which may or may not reflect impacts at larger
D-15
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
landscape scales. This commenter additionally
suggested that there is no direct evidence linking
atmospheric deposition and episodic acidification.
A fifth commenter (16) questioned the source and
the basis for quantitative estimates oi episodic
acidification. This commenter also claimed that
reducing rates of "deposition can lead to an
increased number of systems that experience
episodic acidification if the number of systems
switching from chronic to episodic acidification
exceeds the number no longer being acidified epi-
sodically." The commenter suggested lhat more
detailed discussion is needed on other biogeo-
chemical sources, snowmelt dynamics, and in-
stream use of nitrogen that can potentially affect
receiving water effects produced during episodic
acidification events. The commenter further stated
that documented cases of episodic acidification
are limited to first order ephemeral streams, i.e.,
small streams with no tributaries that have flows
only during portions of most years.
Response
Several of these comments, particularly those re-
lated to the importance of nitrogen to episodic
acidification events, are discussed extensively in
the report. In response to these comments, this
discussion has been expanded to more clearly ex-
plain this phenomenon. The focus of much past
research, as noted in the report, regarding poten-
tial effects from episodic impacts occurring in
smaller, lower order streams is appropriate be-
cause these are the aquatic systems that tend to be
most sensitive to potential acidification effects.
Such streams often provide critical spawning and
rearing habitat for fish populations, including acid-
sensitive fish populations.
EPA agrees that in certain areas, as effects
accompanying chronic acidification due to sulfur
deposition are reduced, overall effects due to
episodic acidification would likely continue to
impair the water quality, but the extent of these
effects would likely be reduced because ieducing
the chronic sulfur effects also decreases potential
episodic effects as well. Such a hypothetical re-
sponse would need to be evaluated through sur-
face water monitoring. The potential for such a
response should be examined in detail through
appropriate modeling to project whether the need
for future deposition reductions should be acceler-
ated.
The comment that documentation of episodic
acidification is limited to observations in first order
ephemeral streams is not correct. All streams, for
example, included in the EPA study "Episodic
Acidification of Streams in the Northeastern
United States," as discussed in the Report, were
perennial, i.e., had continuous flows. Included in
this study were first and second order streams and
a pond outlet in the Adirondack Mountains; first,
second, and third order streams in the Catskill
Mountains; six second order streams in north cen-
tral Pennsylvania. All of the surface waters
showed characteristics of episodic acidification.
Similarly, in an article published in Water
Resources Research during 1993 by O'Brien et al.
and cited in the report, found episodic
acidification effects associated with increased
water concentrations of sulfur, nitrogen, or both in
first-, second-, and third-order perennial streams.
TERRESTRIAL DAMAGE
Comments
Three commenters (13,15,16) felt that the Execu-
tive Summary oversimplified the relationship be-
tween acidic deposition and terrestrial effects.
One commenter (13) suggested that the report in-
adequately explains that very limited data exist
that support the occurrence of degradation of soil
properties or stress to forest systems due to acidifi-
cation. This commenter also claimed that acidic
deposition appears to be a relatively minor factor
affecting the current health and productivity of
forests in the United States. Two commenters
(15,16) stated that the report did not document the
sensitivity of high elevation red spruce in the
Northern Appalachian acidic deposition. Both
stated that the report should acknowledge that a
serious and unresolved debate continues whether
acidic deposition causes any forest effects.
One commenter (16) claimed that the potential
productivity and economic benefits of acidic
deposition in producing fertilization effects in "90
percent" of the managed and non-sensitive forests
in the United States would be lost if sulfur and ni-
trogen deposition were reduced. This commenter
also claimed that the apparent trend of reducing
base cation deposition may be contributing to the
overall acidification response. This commenter
also suggested that discussion of acidification ef-
fects on soils should be revised.
A fourth commenter (17) was concerned that sus-
tainability of forest landscapes and effects on bio-
diversity were inadequately discussed in the re-
port. In noting that forests most at risk from acidic
deposition in eastern Canada are within southeast-
D-16
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APPENDIX D: SUMMARY OF COMMENTS AND RESPONSES
ern Ontario, southern Quebec, and the Maritimes,
this commenter stated that this was unfortunately
not recognized in the study, "especially since the
US NAPAP Review [SOS/T Report 18] identifies
acidic deposition as a possible cofactor in maple
decline in these forests. In addition, acidic fog is
increasingly being recognized as a factor in the
decline of coastal red spruce and white birches
along the portions of the Gulf of Maine and Bay of
Fundy coasts."
Response
The Executive Summary has been revised to clarify
the current state of knowledge regarding the rela-
tionship between acidic deposition and terrestrial
effects. Chapter 2 noted that, besides a few new
references cited therein, most technical informa-
tion on potential direct and indirect effects from
acidic deposition to soils, trees, and forests were
based on analyses completed and reported by the
NAPAP. In addition, Appendix A summarizes the
relevant NAPAP State of the Science and Technol-
ogy (SOS/T) reports.
The report does not claim widespread soil damage
or forest decline due to acidic deposition. Chapter
2 includes the statement "(w)hile control studies
quantitatively link changes in soil chemistries to
tree and other plant responses, similar studies link-
ing acidic deposition effects in nature remain
inconclusive." Further, Chapter 2 cites the
conclusion in NAPAP's SOS/T 16 report, "(t)he vast
majority of forests in the United States and Canada
have not declined." The report does note that
certain high elevation forests (i.e., high elevation
red spruce, primarily in northern Appalachians and
the Northeast) are sensitive to and may be affected
by acidic deposition, from particularly acidic
cloud water, interacting with other air pollutants.
This follows from the NAPAP 1990 Integrated
Assessment (page 45) finding, ". . . there is
evidence - from controlled experiments for
alteration of plant nutrition, cold hardening, and a
wide variety of physiological processes - of red
spruce being affected by high levels of acidic
deposition in cloud water." That report states
further, "Localized areas of forest decline (i.e., high
elevation red spruce) do occur, as a result of the
combined action of multiple stress factors, and in
those areas where high deposition amounts occur
in combination with other stress factors, acidic
deposition can increase the total stress level on the
forest system." The report does not make any
claims of further potential damage beyond what
was presented and discussed in the NAPAP
reports. Potential effects to Ontario maple are
among those noted in the body and Appendix A in
both the Draft Report and its successor.
UNCERTAINTY VERSUS THE NEED FOR STANDARDS
Comments
Eleven comments were received in this area rang-
ing from urging EPA to set standards immediately
to statements that there is no demonstrated need
for additional emissions reductions. Several
commenters (4,5,6,7) urged EPA to recommend
deposition standards for the Adirondacks in the re-
port. These commenters and one other (16) argued
that the report does not fully respond to the Con-
gressional directive to "describe the nature and nu-
merical value of a standard or standards that
would be sufficient to protect sensitive and criti-
cally aquatic and terrestrial resources". They add
that the Adirondacks is the region most strongly
impacted by acidic deposition and that further re-
ductions in sulfur and nitrogen deposition are
needed to protect sensitive aquatic resources in
this region. One commenter (7) suggests that "EPA
implement deposition standards in the range of 4
kilograms of sulfur per hectare and 7.5 kg of nitro-
gen per hectare on an annual basis" for the Adi-
rondacks. Another commenter (6) argued that a
deposition standard should account for episodic
acidification, define sensitive lake resources by in-
cluding physical and biological communities, and
include specifics on the interaction on nitrogen
and sulfur to protect sensitive resources in the Adi-
rondacks. This commenter and another (4) suggest
that a sulfur deposition standard of 3.5 - 3.7
kg/ha/yr would be required.
Two other commenters (10,11) supported develop-
ing acid deposition standards to protect resources
such as those in eastern Canada. Both
commenters felt that setting such standards is fea-
sible and that at a minimum critical loads should
be established. Another commenter (11) recom-
mended that EPA establish regional critical loading
goals for wet sulfur (sulfates) now and that critical
loading goals for nitrogen deposition be set once
ongoing nitrogen saturation studies are completed.
This commenter recommended that these regional
critical loads be set as goals not standards in order
to assist decision makers in determining whether
adequate environmental protection was occurring
and to foster a dialogue among the public, elected
officials, and scientists over the adequacy of the
reductions mandated by the Clean Air Act
Amendments. Further, these goals could serve as
D-17
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
an interim step in developing effects-based stan-
dards for sensitive regions. Another comrnenter (6)
stated that sufficient data are available to establish
critical loads for the Adirondacks.
Other commenters (12,13,15,16) felt that the
Agency does not have enough information or un-
certainties are too large at this time to recommend
deposition standards. One commenter (12) stated
that the report "should clearly delineate that a ni-
trogen deposition standard is not feasible at this
point" and that there is little risk in waiting to as-
sess the full impact of the Clean Air Act Amend-
ments of 1990 because sulfur and nitrogen deposi-
tion will be decreasing in response to mandates of
the Act. Another commenter (13) believes that
EPA does not have the authority to establish and
implement deposition standards under sections
108, 109, and 110 of the Act.
Response
In the report EPA concluded that an acid deposi-
tion standard or standards should provide adequate
protection for sensitive resources at the greatest
risk and that the most sensitive resources at the
greatest risk (i.e., receiving the highest levels of
deposition) are lakes and streams in the Eastern
United States. The subset of these resources at
greatest risk are Adirondack lakes. The report
concluded further that effects associated with
acidic deposition are minimized as pH and acid
neutralization capacity (ANC) are kept high. Gen-
eral goals set forth are to keep pH above 6.0 and
ANC above 50 ueq/l. The report does not set criti-
cal or target loads, but does provide the scientific
basis upon which critical loads can be developed.
Scientific uncertainties regarding times to water-
shed nitrogen saturation make choosing the ap-
propriate level of a standard or standards difficult
at this time. After reviewing this report, EPA's Sci-
ence Advisory Board unanimously concurred with
this conclusion. EPA has revised Chapter 6 of the
report to provide deposition levels for stlfur and
nitrogen which may be necessary to achieve a
range of environmental goals under different as-
sumed times to nitrogen saturation. The summary
states that under some assumed times to nitrogen
saturation, deposition reductions expected from
implementation of the CAAA would achieve envi-
ronmental goals; under other assumed times to ni-
trogen saturation additional reductions in both sul-
fur and nitrogen deposition would be necessary.
Further, scientific understanding of time to nitro-
gen saturation would provide needed insight.
Although some have suggested that geographically
targeted acid deposition standards could be
established under existing statutory authority, no
definitive determination has been made at this
time. The purpose of this report is to provide
Congress with the scientific information necessary
to determine the feasibility (i.e., scientific basis,
implementation and costs issues) and desirability
of setting a standard or standards. EPA would then
look to Congress for direction regarding
establishing a standards or standards, including the
environmental goals that such standards should
achieve.
EMISSIONS INVENTORIES
Comments
Several comments were received regarding the
completeness of the baseline emissions inventories
for NOX and SO2 and the assumptions made in de-
veloping the projection inventories. Four
commenters (4,12,15,16) noted that compliance
with the ozone provisions of Title I of the CAAA
will result in significant decreases in NOX
emissions from stationary and mobile sources in
ozone nonattainment areas and in the Northeast
Ozone Transport Region. Two of these
commenters also noted that Title II of the Act will
result in decreases in NOX emissions from mobile
sources nationwide. These commenters believe
that these reductions should have been factored in
to the NOX emissions reduction scenario
constructed for the report. Another commenter
(15) was concerned that the baseline emissions
inventory does not adequately characterize VOC,
anthropogenic SO2, and natural SO2 and NOX.
This commenter stated that VOC inventories
currently being prepared by States for ozone
nonattainment areas should have been included in
the baseline inventory. Another commenter (16)
stated that the 1 to 2 percent of total nitrogen
emissions estimated to be from natural sources
seems low in light of significant NO emissions
from soils.
One commenter (14) took issue with the statement
made in the report that utility SO2 emissions
should remain near the 8.95 million ton cap for
the years beyond 2010. This commenter estimates
that utility SO2 emissions will decline to less than
50 percent of 2010 levels by 2040 and that utility
NOX emissions will decline to around 50 percent
of 2010 levels by 2040. The SO2 estimates are
based on the assumption that all "SIP" units will be
over 60 years old by 2040 and will be retired or
D-18
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APPENDIX D: SUMMARY OF COMMENTS AND RESPONSES
will have undergone reconstruction or modifica-
tion. Such units or their replacements will be sub-
ject to New Source Review (NSR) which requires
the application of Best Available Control Technol-
ogy (BACT). BACT currently requires emissions
reductions of 95 percent which translates to emis-
sions rates of 0.1-0.3 lb/106 BTU. Thus SO2 emis-
sions from these plants will have dropped from the
cap rate of 1.2 lb/106 to less than a quarter of that
rate. Since electricity demand is expected to dou-
ble with new units having to achieve BACT the
combination of increased capacity and reduced
emissions rate will result in more than halving na-
tionwide utility SO2 emissions in 2040. For analo-
gous, but less definitive reasons, the commenter
projects that NOX emissions from utilities will fall
by 50 percent by the year 2040.
Regarding NOX emissions, the commenter believes
that given the long time horizon (2040), current
average electric utility NOX emission rates of 0.5
to 0.6 Ibs NOX per million Btu, and the availability
of NOX control technology capable of meeting a
rate of 0.1 to 0.2 Ibs NOX per million Btu, that it is
probable that a 50 percent NOX reduction relative
to 2010 will be achieved by 2040.
Response
Several ongoing sulfur and nitrogen related efforts
within EPA will affect acidic deposition loadings.
These include periodic review and analyses of
NO2 and SO2 national ambient air quality stan-
dards, and the efforts by states and Northeast
Ozone Transport Commission under Title I to re-
duce NOX emissions to facilitate compliance with
the ozone standard. Other ongoing efforts include
potential regulatory action towards a fine panicu-
late standard (PM25), which would likely result in
significant sulfate and nitrate reductions, and a re-
gional haze rulemaking effort under Title I. Com-
pliance with the provisions of Title I of the CAAA
will result in significant decreases in NOX emis-
sions from stationary sources in ozone nonattain-
ment areas and in the Northeast Ozone Transport
Region and compliance with the Title II mobile
source provisions will result in a decrease in na-
tionwide NOX emissions in the short term.
Deposition modeling requires that emissions be re-
solved spatially and temporally. Thus, source-spe-
cific emissions for point sources and area source
emissions by category and location must be devel-
oped. For SO2, the baseline inventory for 2010 is
based on a comprehensive utility model based on
full implementation of Title IV that incorporates
regional growth, energy availability, plant retire-
ments, control device requirements, and cost-ef-
fective compliance choices. EPA considers this
inventory to be a realistic projection of emissions
in 2010. On the other hand, implementation of
Title I by the states is ongoing and region-specific
inventories for NOX (as well as for VOCs) are not
yet available. Therefore, the 1990 interim inven-
tory was used as the baseline to estimate the im-
pact of reducing NOX emissions in future years.
As noted in the report, estimates of natural emis-
sions of SO2 and NOX are not as well character-
ized as anthropogenic emissions. These emissions
are, however, have been found to be less impor-
tant than man-made sources. Natural sulfur emis-
sions are estimated to be 6 percent of anthropo-
genic emissions and natural emissions of nitrogen
compounds are estimated to be 1 to 2 percent of
total nitrogen emissions. Further research is being
conducted by EPA and others to better character-
ize natural emissions.
A projection model to forecast electric utility SO2
emissions beyond 2010 at a sufficient level of dis-
aggregation necessary to support deposition mod-
eling does not exist. Even if such a model were
available, the uncertainty and subjectivity that
would be involved with specifying the geographic
location of unplanned new capacity needed to re-
place generation from retiring units and to meet
new electricity demand (i.e., between 2010 and
2040), would limit the usefulness of such a long-
term projection. In fact, any emissions projections
made that far into the future (2040) are based on
so many economic assumptions, that the confi-
dence in such projections would be very low.
With regard to the comment that all "SIP" units
will have retired or undergone significant modifi-
cation by 2040 and thus would have dramatically
lower SO2 emissions, the Inventory of Power
Plants in the United States 1993 published by
DOE's Energy Information Administration states
"Electric utilities have found it to be more eco-
nomically feasible to modify existing electric gen-
erating units than to construct new ones. ...unit
modifications are not subject to the new sources
laws, which would be applicable under the
...(CAAA) of 1990". For purposes of the report, it
was assumed conservatively that SO2 emissions
would remain constant beyond 2010 — the last
year sufficiently desegregated projections were
available. To the extent that SO2 emissions de-
cline beyond 2010, deposition levels will be cor-
respondingly reduced.
D-19
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
As noted in the report, electric utilities contribute
about 33 percent of NOX emissions, highway vehi-
cles about 33 percent, and other sources together
about 33 percent. EPA projects that total NOX
emissions will decrease between 1990 and 2000
and then gradually increase thereafter. The near-
term decrease in NOX emissions will be produced
by implementation of Titles I, II, and IV of the
CAAA. After 2000 the growth in highway mileage
driven will begin to overtake emissions reductions
and will control the rate of increase in nationwide
NOX emissions. Thus, any postulated decrease on
electric utility NOX emissions after 2010 may serve
to moderate the rate of increase of total NOX emis-
sions. Because reliably predicting regional NOX
emissions after 2010 would be subject to large un-
certainties, the modeling in the report is based on
the assumption that NOX emissions will remain
constant from 1990 through 2040.
DEPOSITION MODELING
Comments
Three organizations provided detailed comments
on the use of RADM for deposition and visibility
modeling. Two commenters questioned the ability
of RADM to accurately model nitrogen deposition
(13,15). They stated that the lack of plume treat-
ment in the model leads to instantaneous mixing
and too rapid oxidation of NO. One commenter
(16) stated that the ability of RADM to predict
deposition in future years is limited because the
model results have only been calibrated to meas-
ured deposition for the years 1985 through 1990.
This same commenter disagreed with the state-
ment made in the report that the uncertainties in
RADM lead to a spread of about 10 percent in
predicting changes in deposition due to changes in
emissions. This commenter stated that the variabi-
lity for wet deposition of sulfate alone is a factor of
2 and that the inability to measure dry deposition
of sulfate accurately leads to even greater uncer-
tainties. This commenter noted that significant
uncertainty is associated with the location of
"unplanned" sources in 2010 and stated that these
sources represent up to 50 percent of year 2010
emissions.
Response
Accurate Modeling of Nitrogen
It is correct that the instantaneous mixing in the
large RADM grids will cause NO to be converted
more quickly in the model than in the real world.
The instantaneous mixing likely means that more
nitric acid (HNO3) is formed in the model close to
strong NOX emissions sources than should be
formed. Comparison of a 20-km RADM version
with the 80-km version shows this effect, where
the high HNO3 levels spread more downwind in
the 20-km model. The result is a displacement of
maximum HNO3 formation by about an 80-km
grid cell, causing the maximum to be closer to the
emissions source. The difference between the two
models disappears by the time half of the source's
nitrogen is deposited, roughly 300-500 km. Thus,
RADM predictions are expected to show structure
in the deposition pattern associated with variations
in emissions density that is more enhanced than it
should be. However, such an enhancement of
deposition near strong NOX sources is not appar-
ent in comparisons against wet deposition meas-
urements. This is, in part, because nitric acid is a
secondary pollutant and transport distances are
large, 500-800 km, smoothing out the overall
deposition field for nitrogen. In other words, the
effect of the too rapid conversion is hard to see in
comparisons with large-scale regional fields.
Nonetheless, RADM is expected to somewhat ac-
centuate spatial differences due to spatial differ-
ences in emissions. The model can be used to ex-
plore differences in the spatial location of deposi-
tion associated with different emissions sectors if
the spatial separations, are large, as they are for the
eastern United States. Any quantitative conclu-
sions drawn from such an analysis would need to
be carefully caveated. However, in this report the
RADM results are used to draw attention to the
main qualitative result that the influence of utility
and mobile emissions on nitrogen deposition is
quite spatially distinct, and, hence, one will need
to be alert to this when one considers possible sen-
sitive areas that need l.o benefit from future reduc-
tions in NOX emissions. Based on the grid-size
sensitivity analysis, the errors that result from the
large grid-size of the RADM model do not pre-
clude the model from providing valuable insight at
the qualitative level. The RADM results show that
the dominance of certain emissions sectors is suffi-
ciently separated geographically that this knowl-
edge must be factored into any future analysis of
NOX emissions reductions. This is fully consistent
with the use of the RADM results in this report.
Ability to Predict Future Deposition
The criticism that the ability of RADM to predict
future years is limited because it is calibrated to
1990 misses the central point underlying the de-
velopment of RADM in the NAPAP research effort
D-20
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APPENDIX D: SUMMARY OF COMMENTS AND RESPONSES
and incorrectly associates RADM with a regression
model, which it most definitely is not. RADM was
developed to be a state-of-the-science model that
incorporates the basic physical and chemical
processes necessary to describe the transport,
transformation and fate of acidifying species.
RADM was developed as a result of the concern
that simple, parameterized (fitted) models, no
matter how advanced, might not be able to
reliably predict future year deposition resulting
from emissions changes (either increases or
decreases) due to the effects of oxidant limitation.
These effects cause nonlinearity in the system and
can cause the "effective" parameterizations of the
linear models to change as one extrapolates to
conditions quite different from those upon which
the parameterizations are based. Therefore,
advanced, comprehensive models, like RADM,
that can internally account for change and
dynamically compute the resultant effect on the
processes are the models with which one can
more reliably extrapolate to conditions quite
different from those of 1990. This can not be done
with the simple, parameterized (fitted) models.
Estimated Uncertainty of Prediction of
Deposition Change
The estimate for the uncertainty in the relative
change predictions from RADM are based on an
extensive bounding study that is documented as a
chapter in NAPAP State of Science and Technol-
ogy (SOS/T) Report No. 5 and is referenced in the
deposition modeling chapter of this report. The
bounding analysis showed that errors that could
contribute to scatter in comparisons with
measurements do not result in the same level of
uncertainty in the relative change predictions.
Several model errors were explored that, based on
our current understanding, would be expected to
contribute most to model biases and/or to chang-
ing the sensitivity to emissions changes. The re-
sults were that the relative change predictions dif-
fered on the order of 10 percent, that is, a baseline
predicted change in sulfur deposition of 40 percent
had an uncertainty the order of ±4 percent. These
results and conclusions were independently re-
viewed as part of the NAPAP SOS/T review proc-
ess. The analysis was deemed valid and the best
estimate at this time. The reviewer is incorrectly
associating scatter with accuracy of change predic-
tions. Much of the scatter relates to site represen-
tativeness and issues of comparing a volume aver-
age to a point. Such scatter could be inherent and,
as such, is not a reflection on the ability of the
model to represent a change in deposition. The
assumption here is that the scatter represents a sys-
tematic difference in pattern that is preserved
when moving to a different level of deposition.
Also, because scatter may be inherent, bias is the
more fundamental measure related to accuracy.
One must be careful, because scatter and bias can
be quite unrelated when dealing with an advanced
air quality model. RADM is not a regression
model and arguments and experience about pre-
dictability for regression models do not carry over
to advanced science models. The discussion of
uncertainty in Chapter 3 has been expanded to try
to reduce misinterpretation. Finally, it should be
noted that the uncertainty associated with the lo-
cation of "unplanned" sources in 2010 is minimal
since unplanned capacity is projected to represent
less than 2 percent of total emissions in that year.
ALLOWANCE TRADING PROGRAM
Comments
One commenter (12) concurred with the analysis
that emissions trading is cost-effective and main-
tains environmental benefits. Two commenters
(5,7) questioned the report's conclusion that the al-
lowance trading program would have less than a
10 percent impact on deposition of sulfur in the
Adirondacks in 2010. One of these commenters
(7) stated that EPA should evaluate the effects of
the allowance trading program by correlating
emissions monitoring data from Continuous Emis-
sions Monitoring Systems (CEMS) with actual
deposition data rather than relying on mathemati-
cal projections of trading and resulting deposition.
The other commenter (5) claimed that the allow-
ance trading completed in March 1995 will work
to the detriment of the Adirondacks because al-
lowance prices decreased from the previous two
years and because the allowances purchased will
be used in areas contributing to deposition in the
Adirondacks. Another commenter (16) was pleased
to see a comparison of acidic deposition re-
ductions from emissions trading and a realistic al-
ternative approach. This commenter suggested
that the conclusion that the current Title IV allow-
ance trading system is administratively efficient
should be included among the summary conclu-
sions.
Response
EPA agrees that emissions measured with CEMS
provide the most accurate and precise emissions
values. However, CEMS data are only now avail-
able for the first year of compliance for sources
D-21
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
subject to Phase I of the Acid Rain Program. The
analysis in this report is designed to evaluate the
impact of the allowance trading program in the
year 2010 when all affected electric utility sources
are in compliance.
EPA also agrees that deposition monitoring is a key
component in assessing the effectiveness of emis-
sions reductions programs on deposition. Deposi-
tion monitoring programs currently in place pro-
vide data on trends and spatial distributions of
acidic deposition species. The National Atmos-
pheric Deposition Program/National Trends Net-
work which operates at about 200 sites in the
United States provides data on deposition of wet
sulfate and nitrates. EPA also operates a 50-station
dry deposition network. As available, data from
these and other monitoring networks will continue
to be used to evaluate the effectiveness oif the Acid
Rain Program. Nevertheless, the projection of the
effectiveness of the Acid Deposition Program,
when it is fully implemented in 2010, can only be
estimated by modeling future emissions end depo-
sition scenarios. The models used in the report for
such projections have been extensively reviewed
and selected as the most realistically available
models.
EPA disagrees with the comments that the allow-
ance trading program and decreasing allowance
prices work to the detriment of deposition in the
Adirondacks. The fact that (1) allowance prices
were lower in the March 1995 auction than in the
previous year, and (2) prices have been much
lower than most analysts predicted prior to the first
EPA auction only means that the cost of reducing
SC>2 is not as high as previously expected. Many
analysts attribute the current low cost of allow-
ances to the availability of relatively inexpensive
fuel switching alternatives. Lower allowance costs
also result in the banking of allowances with con-
comitant early emissions reduction benefits. Fur-
ther, Duke Power and Virginia Electric Power pur-
chased 109,530 of the 176,400 (about 62 percent)
of the allowances sold at the March 1995 auction.
Emissions from plants owned by these utilities con-
tribute significantly less than 1 percent to acidic
deposition in the Adirondacks. Even if full
economic trading is assumed (i.e., electric; utilities
pursue trading to the fullest extent in order to
minimize the cost of compliance), EPA has dem-
onstrated in the study that allowance trading is ex-
pected to have less than a 2 percent impact on
acidic deposition in the Adirondacks.
BENEFITS TO VISIBILITY, HUMAN HEALTH,
MATERIAL, AND CULTURAL RESOURCES
Comments
One commenter (16) felt that the discussion of im-
provement in visibility due to reductions in ambi-
ent sulfate concentrations is oversimplified. As
sulfate concentrations decrease, the relative con-
tributions to visibility degradation of other light-
scattering species such as dust, water, nitrate, and
elemental carbon increase. Thus changes in con-
centrations and extinction coefficients of these
species will become increasingly important. A
second commenter (13) stated that RADM should
not be used in visibility assessment due to short-
comings relative to oxidant estimation and the in-
ability of the model to allow nitrate particles to be
formed.
Another commenter (15) stated that since the legis-
lative language does not specifically mention im-
pacts other than aquatic and terrestrial benefits,
the report should not include them. Furthermore,
the CAAA has specific sections dealing with
visibility which provide EPA regulatory authority
to impose restrictions on emissions that are precur-
sors to visibility impairment. This commenter also
stated that claims of additional health benefits
from reductions in precursors of acidic deposition
are inappropriate and are more properly addressed
by Title I of the CAAA. Finally, the commenter
argued that the materials damage section is biased
and misleading because the real issue is whether
cost savings that may result from reductions in
acidic deposition outweigh the costs to achieve
those savings.
Response
An acid deposition standard or standards would
most likely be derived using sensitive aquatic re-
sources as the environmental endpoint in selecting
the level of a standard. This report recognizes that
environmental and human health issues do not
function in isolation of one another. For example,
what may improve surface water conditions in the
Adirondacks may also benefit visibility in the mid-
Appalachian area (e.g., Shenandoah National
Park) and provide a further safeguard against the
health effects associated with fine paniculate mat-
ter (e.g., sulfates).
Anthropogenic visibility impairment in the eastern
United States is primarily caused by sulfates (65
percent), organics (14 percent), elemental carbon
(11 percent), nitrates (5 percent), nitrogen oxide (3
percent), and suspended dust (2 percent). The
D-22
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APPENDIX D: SUMMARY OF COMMENTS AND RESPONSES
contribution of sulfates to visibility impairment in
the Southwest and Northwest are 39 percent and
33 percent, respectively. The largest constituent
(sulfates) as well as others (nitrates and nitrogen
dioxide) share the same precursors as those re-
sponsible for acidic deposition. The visibility sec-
tion of this report is intended to identify visibility
improvement as another important benefit of SO2
and NOX emissions reductions, and to estimate the
visibility benefits that could be associated with an
acid deposition standard based on aquatic effects
in sensitive regions.
The shortcomings of current regional photo-
chemical models like RADM, as identified in the
peer-review process, has very little to do with the
prediction of sulfate air concentrations important
to visibility degradation. Earlier concerns about
RADM's predictions of hydrogen peroxide turned
out to be problems with the measurements. Sub-
sequent comparisons with measurements suggest
RADM is projecting somewhat too much hydrogen
peroxide, not too little as was earlier thought. This
could lead to too much conversion to sulfate in
cloud water in nonprecipitating clouds that are
later evaporated. Nevertheless, comparisons of
annual average concentrations of sulfates with
RADM annual average predictions are good, with
essentially no bias for higher levels of atmospheric
sulfate. This indicates that RADM can be used for
visibility assessments. It is true that RADM does
not allow nitrate particles to be formed. They ac-
count for 5 percent of the light extinction. EPA
agrees that as sulfate particles decrease in number
there ought to be an increase in the importance of
nitrate particles, assuming that ammonia concen-
trations do not change. However, organics
(associated with volatile organic compounds,
VOCs) account for 11 percent of the light extinc-
tion. They would be expected to decrease by at
least 25-30 percent due to oxidant controls
brought about by compliance with Title I of the
CAAA. Thus, there are offsetting trends that will
affect non-sulfate light extinction, causing it to
change much less that might be expected. Thus,
the simple approach used with RADM, that only
uses change in sulfate and ignores changes in ni-
trate- and organic-associated light extinction, is
expected to be a reasonable zeroeth approxima-
tion of a plausible change in overall light extinc-
tion.
The report's discussion of potential health benefits
associated with further reductions of acidic deposi-
tion precursors provides a brief summary of current
scientific information as well as regulatory-related
issues associated with Title I. This discussion is
qualitative in nature because there are several
regulatory and implementation issues under Title I
which are currently being explored, reviewed,
and/or developed by EPA and others. Based on
the current stage of review or development,
modeling inventories are simply not yet available.
The Agency is exploring whether reduced emis-
sions, particularly of sulfur dioxide, nitrogen oxide,
and the subsequent reduction in fine paniculate
matter (i.e., sulfates and nitrates), is likely to have
a beneficial impact on human health. Other
Agency efforts will quantify the relationship and
benefits.
The costs and cost savings of air pollution, specifi-
cally acidic deposition, on both materials of func-
tional as well as cultural or historical importance
is a growing field of research. Both wet and dry
acidic deposition are believed to contribute to the
overall decline in functionality and aesthetic well-
being of materials damaged by air pollution. The
monetary benefits associated with control of those
pollutants (i.e., sulfates) may be significant and are
currently being investigated by EPA, NAPAP, and
others.
COSTS
Comments
One commenter (15) noted that, although the total
costs of control are equal for the geographically
targeted and the regional [national] emissions red-
uctions approaches, the costs of the geographically
targeted approach will be absorbed by fewer
sources. Another commenter (6) questioned why
existing detailed economic models which consider
emissions variations and timing of achieving redu-
ctions were not used. This commenter also stated
that detailed costs for NOX control should have
been calculated. A third commenter (14) provided
"zeroeth order" cost-effectiveness comparison of
two alternative control strategies (based on appli-
cation of Best Available Control Technology after
a specified boiler life) with Title IV.
A fourth commenter (16) raised several points or
questions: although utility generating units are
already required to have continuous emissions
monitors (CEMs), the costs of monitoring emissions
from individual sources using CEMs could be an
impediment to implementing a control scenario
based on trading; did the costs calculated for the
additional national SO2 reductions scenarios
assume the use of allowance trading, and, if so,
how was trading accounted for in cost estimates;
D-23
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
average costs of control are inappropriate as the
basis for cost comparisons among reductions
scenarios (marginal costs would be more
appropriate); economic impacts to local
economies should be considered for the geo-
graphically targeted reductions scenarios; further
reduction requirements would provide greater im-
petus for technological innovation only under a
trading system as opposed to a targeted approach
that gives little flexibility to regulated sources; and
the 2010 CAAA Scenario (with trading) is not the
appropriate cost baseline because it incorporates
the allowance bank projected to be built up be-
tween 1995 and 2009 and thus, understates rela-
tive to post-2010 costs after the banked allow-
ances are used up.
Response
EPA agrees with the comment that even though the
total cost of an additional 50 percent SO2 emission
reduction from electric utilities is about the same
in the geographically targeted and non-targeted
scenarios, the costs are born by fewer sources in
the targeted reduction scenario than in the non-
targeted scenario. Cost-effectiveness ($/ton SO2
removed) is slightly higher in the geographically
targeted approach.
A detailed, comprehensive economic model was
used by EPA to determine costs of the Acid Rain
Program in 2010. As noted in the report, detailed
economic evaluation of the SO2 reduction scenar-
ios would require the implementation of at least
two economic sector models—one for ihe electric
utility sector and one for the industrial sector.
Using such models would be very costly and time
intensive, and was beyond the scope of this study.
Given the large uncertainty associated with
predicting economic behavior far into the future
(i.e., beyond 2010), EPA believes that little mean-
ingful additional information would be gained by
implementing such detailed models over the scop-
ing economic analysis conducted for this report.
In the report, EPA recommended that if Congress
decides to give further consideration to an acid
deposition standard, more detailed economic
analysis should be conducted at that time.
The NOX control scenario analyzed for this report
was developed to represent realistic future NOX
reductions. Because implementation of the sec-
tions of Title I and Title IV of the Art requiring
NOX reductions is still ongoing, no detailed inven-
tory of either baseline (full implementation of the
CAAA) nor additional control scenario exist. EPA
is currently developing regulations that will specify
the degree of control necessary for NOX emissions
and types of control technologies that will be re-
quired under Titles I and IV of the CAAA. Until fi-
nal regulation for Title I and IV have been devel-
oped, EPA does not believe that it can produce a
reasonable estimate of (1) NOX emissions far into
the future, and (2) the cost of reducing NOX emis-
sions as a result of Titles I and IV. Therefore, EPA
did not attempt to estimate NOX emissions or con-
trol costs post-implementation of the 1990 CAAA.
EPA agrees with the comment that the cost of
monitoring emissions from individual sources us-
ing CEMS should be considered in determining the
costs of control scenarios involving industrial
sources of SO2 which are not required to install
CEMS under Title IV. For a control program to be
integrated with the current allowance trading pro-
gram, industrial sources emitting SO2 must be able
to demonstrate that they can achieve equivalent
reliability in their reported emissions as utilities al-
ready in the program. Before any program such as
this would be implemented both the technical fea-
sibility and costs of achieving this requirement
would have to be analyzed.
The costs calculated for the additional SO2 reduc-
tion scenarios did not explicitly include allowance
trading, because EPA does not know how SO2
allowance allocations would be modified in order
to reduce the SO2 allowance cap by 50 percent.
Under EPA's scoping approach for estimating costs
under the additional SO2 reduction scenarios, EPA
assumed that units with the potential for significant
additional reductions in SO2 (i.e., those with emis-
sion rates greater than 0.8 Ibs SO2 per million Btu
in 2010) would be considered for additional cont-
rol. Using EPA's generic retrofit scrubbing model,
EPA ranked these units and their SO2 emission re-
duction potential in order of average cost-effec-
tiveness (i.e., measured as cost per ton SO2 re-
moved). EPA then assumed that units would apply
retrofit scrubber in order of cost-effectiveness until
the 50 percent additional SO2 reduction was
achieved. Note that this is consistent with an al-
lowance trading approach (i.e., the units with the
most cost-effective SO2 reductions make the addi-
tional reductions). It may be appropriate to
assume that some level of cost savings associated
with an unrestricted national trading program (as
assessed for implementation of the Acid Rain
Program under Title IV) could also result in
reduced costs of compliance with broad emission
reductions beyond the current program. This
could widen the cost difference between a geo-
D-24
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APPENDIX D: SUMMARY OF COMMENTS AND RESPONSES
graphically targeted and national emissions reduc-
tion strategies.
Next, EPA evaluated the potential for allowance
trading. For this evaluation, EPA grouped units
into three categories:
* UNITS CONTROLLED BY 2010: Units with
emission rates in 2010 of less than 0.8 Ibs
SO2 per million Btu before consideration of
an additional 50 percent reduction fall into
this category. EPA assumed that because:
(1) these units would already be tightly
controlled, and (2) the allowance cap
would be reduced by 50 percent and addi-
tional SO2 reduction requirements would
only drive up allowance prices, that emis-
sions from these units would not change
(i.e., they would not pursue any additional
allowance trading).
* UNITS APPLYING ADDITIONAL CONTROL: The
units identified as applying retrofit scrub-
bers in order to achieve the additional 50
percent SO2 reduction fall into this cate-
gory. Even though EPA did not consider
how allowance allocations would be modi-
fied under a 50 percent reduction in the al-
lowance cap, it is reasonable to assumed
that these units may have excess allow-
ances available.
• UNITS NOT AS TIGHTLY CONTROLLED: After
assuming an additional 50 percent SO2
emission reduction, only a small set of
units remain that have SO2 emissions
greater than 0.8 Ibs. SO2 per million Btu.
Because EPA assumed that the allowance
cap would be reduced by 50 percent and
SO2 allowance prices could only increase,
EPA assumed these units would not in-
crease their emissions as a result of trading.
Therefore, EPA assumed no change in
emissions from these units.
The commenter is correct to point out that the
2010 scenario (with trading) forecasts that about
0.5 million tons of previously banked allowances
are used in 2010. As a result, total utility SO2
emissions in 2010 are forecast to be about 9.41
million tons instead of 8.95 million tons—the long
term SO2 allowance cap. This use of banked al-
lowances in 2010 was forecast by the sector
model that EPA used to evaluate the impacts of al-
lowance trading. This sector model is not de-
signed to forecast impacts beyond 2010. There-
fore, EPA had little choice but to rely on this 2010
forecast. It should be noted that because of the
model's assumption that banked allowances will
be used in 2010, utility SO2 emissions are only
about 5 percent greater than the 8.95 million ton
cap—well within the range of uncertainty of the
scenarios evaluated in the study.
EPA disagrees with the commenter regarding the
appropriateness of using average cost per ton SO2
removed as a measure of cost-effectiveness. The
commenter believes that marginal cost per ton SO2
removed would be a better measure. Both average
cost and marginal cost can be appropriate
measures of cost-effectiveness. Which measure is
more appropriate depends on what the cost-effec-
tiveness measure is to be used for. If EPA were at-
tempting to assess the impact of alternative addi-
tional reduction scenarios on the allowance mar-
ket price, marginal cost-effectiveness would be
appropriate because it would indicate the underly-
ing market value of allowances. This was not
EPA's goal in this report, however. In the case of
the study, EPA wanted to provide a meaningful
metric that would facilitate a broad comparison of
the overall cost-effectiveness of each alternative
additional reduction scenario. For this purpose,
EPA believes that average cost-effectiveness will
be more meaningful to members of Congress than
marginal cost-effectiveness.
Any additional reduction scenario will have differ-
ing impacts on local economies. Until detailed
regulatory proposals are evaluated, it is not possi-
ble to determine the specific geographic distribu-
tion of emissions reductions.
EPA agrees with the commenter that approaches
provide flexibility in compliance by using allow-
ance trading will provide greater impetus for tech-
nological innovation. Note, however, that EPA
believes that a regionally targeted approach could
be developed that would continue to rely on al-
lowance -trading. Under a regionally targeted ap-
proach, regional allowance markets could de-
velop.
D-25
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ACID DEPOSITION STANDARD FEASIBILITY STUDY
EXHIBIT D-2. LISJ OF COMMFNTFRS
1. John M. Barnes, Department of Agriculture, Washington, DC. Letter of March 8, 1995.
2. Kathy A. Tonnessen, National Biological Service, Denver, Colorado. Letter of April 1, 1995.
3. Robert J. Devlin, Forest Service, Portland, Oregon. Letter of March 24, 1995.
4. Gary Spielman, New York State Department of Environmental Conservation, Albany, New York.
Letters of March 13 and May 3, 1995.
5. Bernard Melewski, The Adirondack Council, Elizabethtown, New York. Letter of March 31, 1995.
6. Robert Glennon, Adirondack Park Agency, Ray Brook, New York. Letter of March 31, 1995.
7. David Gibson and Daniel R. Plurnley, Association for the Protection of The Adirondacks, Schenec-
tady, New York. Letter of March 30, 1995.
8. Myron J. Mitchell, State University of New York, Syracuse New York. Letter of March 30, 1995.
9. Rick Webb, University of Virginia, Charlottesville, Virginia. Letter of March 31, 1995.
10. Lisa J. Thorvig, Minnesota Pollution Control Agency, St. Paul, Minnesota. Letter of March 31, 1995.
11. Donald Theiler, Wisconsin Department of Natural Resources, Madison, Wisconsin. Letter of
March 31, 1995.
12. Samuel A. Leonard, General Motors Corporation, Detroit, Michigan. Letter of April 1, 1995.
13. Jon M. Loney, Tennessee Valley Authority, Knoxville, Tennessee. Letter of March 31, 1995.
14. Roger Morris, Department of Energy, Washington, DC. Letter of March 31, 1995.
15. Quinlan j. Shea and Michael L. Teague, Hunton & Williams, Washington, DC. Letter of March 31,
1995. Representing the Utility Air Regulatory Group and the National Mining Association.
16. Mary Ann Allan, Electric Power Research Institute, Palo Alto, California. Letter of March 30, 1995.
17. H.A. Clarke, Environment Canada, Ottawa, Ontario. Letter of April 3, 1995.
D-26
• U.S. GOVERNMENT PRINTING OFFICE: 1995-715-635/82458
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