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United States Government Accountability Office: 
GAO: 

Report to Congressional Requesters: 

January 2013: 

Water Quality: 

EPA Faces Challenges in Addressing Damage Caused by Airborne 
Pollutants: 

GAO-13-39: 

GAO Highlights: 

Highlights of GAO-13-39, a report to congressional requesters. 

Why GAO Did This Study: 

Atmospheric deposition, a process that transfers pollutants, including 
NOx, SO2, and mercury, from the air to the earth’s surface, can 
significantly impair the quality of the nation’s waters. EPA can 
potentially address atmospheric deposition through the CWA and the 
CAA, but concerns have been raised about its ability to do so. GAO was 
asked to examine EPA’s efforts to address atmospheric deposition of 
pollutants that impair waterbodies. This report examines (1) the 
extent to which atmospheric deposition of NOx, SO2, and mercury 
contributes to the impairment of the nation’s waters and identify the 
key sources of these pollutants; (2) the regulatory tools that EPA 
uses under the CWA to address the effects of atmospheric deposition, 
and the challenges, if any, that it faces in doing so; and (3) the 
regulatory tools that EPA uses under the CAA to address the effects of 
atmospheric deposition, and the challenges, if any, that it faces in 
doing so. To conduct this work, GAO reviewed EPA data, reports, and 
activities and interviewed agency officials and other experts. 

What GAO Found: 

Atmospheric deposition of nitrogen oxides (NOx), sulfur dioxide (SO2), 
and mercury contributes to the impairment of the nation’s waters, but 
the full extent is not known. For example, states provide EPA with 
data on the extent to which their waterbodies do not meet water 
quality standards, and some states have reported that some of their 
waterbodies are polluted because of atmospheric deposition. However, 
the states have not assessed all of their waterbodies and are not 
required to report on the sources of pollution. Similarly, federal 
studies show that atmospheric deposition of NOx, SO2, and mercury is 
polluting waterbodies but have data for only some waters. The main 
sources of NOx and SO2 are cars and other forms of transportation and 
coal-burning power plants. Power plants are also the largest U.S. 
source of mercury emissions, but international sources also contribute 
to the mercury deposited in U.S. waters. 

EPA has sought to address atmospheric deposition through Clean Water 
Act (CWA) programs but faces challenges in doing so. Specifically, 
states typically establish water quality standards—-considering EPA 
recommended criteria-—for each waterbody. If a waterbody does not meet 
standards, CWA generally requires the state to set a Total Maximum 
Daily Load (TMDL) that identifies the maximum amount of pollutant that 
can enter the waterbody and still meet standards. States are 
responsible for taking actions to ensure the TMDL is met. For point 
sources of pollution, such as a pipe from a sewer treatment plant, CWA 
requires new or renewed permits to be consistent with the TMDL. 
However, there is no similar statutory requirement for nonpoint 
sources of pollution, such as atmospheric deposition. States may take 
actions, such as providing technical or financial assistance to limit 
pollution from nonpoint sources, but face a challenge when atmospheric 
deposition pollution affecting their waters originates in emissions 
from a different state. 

EPA has also sought to address atmospheric deposition through Clean 
Air Act (CAA) regulations but faces challenges in doing so. EPA issued 
regulations that reduced emissions of NOx, SO2, and mercury and in 
turn the amount of pollution in waterbodies. Even with reduced 
emissions, NOx, SO2, and mercury continue to pollute the nation’s 
waterbodies. EPA’s recent attempt to address atmospheric deposition by 
establishing secondary National Ambient Air Quality Standards (NAAQS)—
standards to protect public welfare—targeting the effects of acid rain 
caused by NOx and SO2 on water bodies was not successful. EPA stated 
that uncertainty regarding atmospheric modeling and limitations in 
available data prevented determination of secondary NAAQS adequate to 
protect against the effects of acid rain, and the agency has not 
identified alternative strategies. EPA has begun a 5-year pilot 
program to gather additional scientific data, but it is unclear 
whether or when the agency will be able to address scientific 
uncertainties to enable adoption of a protective secondary NAAQS. EPA 
also did not set secondary NAAQS to address nutrient over-enrichment 
in aquatic ecosystems caused by NOx because of the limited available 
scientific data. Many sources of nitrogen can contribute to nutrient 
over-enrichment in a waterbody, including sources of nitrogen 
unrelated to atmospheric deposition. EPA recently announced an effort 
that is to lead to the development of an integrated nitrogen research 
strategy that includes approaches to reducing atmospheric deposition 
of NOx into waters impaired because of nutrient over-enrichment by 
nitrogen. 

What GAO Recommends: 

GAO recommends that EPA determine whether EPA can obtain in a timely 
manner the data it needs to establish secondary NAAQS adequate to 
protect against the effects of acid rain and, if not, identify 
alternative strategies to do so. EPA agreed with GAO’s recommendation. 

View [hyperlink, http://www.gao.gov/products/GAO-13-39]. For more 
information, contact David C. Trimble at (202) 512-3841 or 
trimbled@gao.gov. 

[End of section] 

Contents: 

Letter: 

Background: 

Air Pollution Affects U.S. Waterbodies and Transportation and Power 
Plants Are Its Key Sources: 

EPA Faces Challenges Using Clean Water Act Regulations to Address 
Atmospheric Deposition: 

Clean Air Act Regulations Address Atmospheric Deposition, but EPA 
Faces Challenges Addressing Effects on Waterbodies: 

Conclusions: 

Recommendation for Executive Action: 

Agency Comments and Our Evaluation: 

Appendix I: Scope and Methodology: 

Appendix II: Comments from the Environmental Protection Agency: 

Appendix III: GAO Contact and Staff Acknowledgments: 

Table: 

Table 1: Waterbodies Reported by States as Impaired by Atmospheric 
Deposition by Waterbody Type: 

Figures: 

Figure 1: Atmospheric Deposition: 

Figure 2: Location of Acid-Sensitive Ecosystems in the United States: 

Figure 3: Mercury-related Fish Consumption Advisories, 2010: 

Figure 4: Estimated Contributions of NOx Emissions from Domestic 
Sources in 2008: 

Figure 5: Estimated Contributions of Sulfur Dioxide Emissions from 
Domestic Sources in 2008: 

Figure 6: Estimated Contributions of Mercury Emissions from Domestic 
Sources in 2008: 

Figure 7: Estimated Contributions of Global Mercury Emissions from 
Human-Caused Sources in 2005: 

Figure 8: Sources of Atmospheric Deposition of Mercury in the 
Northeast Region, 2002: 

Figure 9: Location of New York State Forest Preserve Lands in the 
Adirondacks: 

Figure 10: Location of NOx Airshed and Chesapeake Bay Watershed: 

Figure 11: NOx and SO2 Emissions in 1990, 2000, and 2008 by Source: 

Figure 12: Mercury Emissions in 1990 and 2008 by Source: 

Abbreviations: 

ATTAINS: Assessment, TMDL Tracking, and Implementation System: 

EPA: Environmental Protection Agency: 

CAA: Clean Air Act: 

CENRS: Committee on Environment, Natural Resources, and Sustainability: 

CWA: Clean Water Act: 

MACT: maximum achievable control technology: 

MATS: Mercury and Air Toxics Standards: 

NAAQS: National Ambient Air Quality Standards: 

NADP: National Atmospheric Deposition Program: 

NAPAP: National Acid Precipitation Assessment Program: 

NEIWPCC: New England Interstate Water Pollution Control Commission: 

NESCAUM: Northeast States for Coordinated Air Use Management: 

NLFA: National Listing of Fish Advisories: 

NOAA: National Oceanic and Atmospheric Administration: 

NOX: nitrogen oxides: 

NPDES: National Pollution Discharge Elimination System: 

NSTC: National Science and Technology Council: 

NYSDEC: New York State Department of Environmental Conservation: 

PCBs: polychlorinated biphenyls: 

SO2: sulfur dioxide: 

SO4: sulfate: 

TMDL: Total Maximum Daily Load: 

UNEP: United Nations Environment Programme: 

[End of section] 

United States Government Accountability Office: 
Washington, DC 20548: 

January 24, 2013: 

The Honorable Timothy H. Bishop:
Ranking Member:
Subcommittee on Water Resources and the Environment:
Committee on Transportation and Infrastructure:
House of Representatives: 

The Honorable Eddie Bernice Johnson:
House of Representatives: 

Atmospheric deposition--a process that transfers pollutants from the 
air to the earth's surface--can significantly impair water quality in 
the nation's rivers, lakes, bays, and estuaries, and harm human health 
and aquatic ecosystems. In particular, airborne pollutants can fall to 
the ground in precipitation or as a gas or particle and be deposited 
either directly onto the surface of a waterbody or onto land and then 
transported into a waterbody through runoff. Three airborne pollutants 
commonly associated with atmospheric deposition are nitrogen oxides 
(NOx), sulfur dioxide (SO2), and mercury. NOx and SO2--which cause 
acid rain--threaten certain aquatic ecosystems, especially in lakes 
and streams in higher altitudes. In addition, NOx contributes to 
nutrient over-enrichment in estuaries, which can result in oxygen 
depleted areas known as "dead zones," where fish and other organisms 
cannot survive. Once in water, mercury becomes concentrated in fish 
and can harm the health of individuals who consume these fish, 
particularly children. 

Because these pollutants are transferred from air to water, the 
Environmental Protection Agency (EPA), whose mission is to protect 
human health and the environment, can potentially address atmospheric 
deposition through the Clean Water Act (CWA) and the Clean Air Act 
(CAA).[Footnote 1] The CWA provides EPA with responsibilities aimed at 
maintaining and improving the quality of the nation's waters, 
including the responsibility to establish recommended water quality 
criteria for specific pollutants. Authorized states, territories, and 
tribes--collectively referred to as states in this report--then use 
these recommended criteria along with designated uses and 
antidegradation provisions to establish state-specific water quality 
standards. States then apply these standards to individual 
waterbodies.[Footnote 2] If a waterbody does not meet water quality 
standards, these entities are to establish a pollutant "budget"--or a 
total maximum daily load (TMDL)--at a level that ensures that 
applicable water quality standards can be met and maintained.[Footnote 
3] Once a TMDL is established, states are generally responsible for 
implementation, while EPA and states share responsibility to ensure 
that permitted discharges to a waterbody do not exceed the level 
established by the TMDL. Analogous to the CWA, the CAA provides EPA 
with tools for regulating the emissions of pollutants into the air. 
Among the pollutants that EPA regulates in certain emissions are NOx, 
SO2, and mercury. 

EPA and others have recognized atmospheric deposition as a 
contributing factor to degraded water quality for several decades. 
However, congressional concerns have been raised about EPA's ability 
to address atmospheric deposition. In this context, you asked us to 
review EPA's efforts to address the atmospheric deposition of 
pollutants that impair the water quality and aquatic ecosystems of the 
nation's rivers, lakes, and bays. Specifically, our objectives were to 
examine (1) the extent to which atmospheric deposition of NOx, SO2, 
and mercury contributes to the impairment of the nation's waters and 
identify the key sources of these pollutants; (2) the regulatory tools 
that EPA uses under CWA to address the effects of atmospheric 
deposition, and the challenges, if any, that it faces in doing so; and 
(3) the regulatory tools that EPA uses under the CAA to address the 
effects of atmospheric deposition, and the challenges, if any, that it 
faces in doing so. 

To examine the extent to which atmospheric deposition of NOx, SO2, and 
mercury impairs waters, we analyzed the most recently available data 
from EPA's Assessment, TMDL Tracking, and ImplementatioN System 
(ATTAINS), a database containing state-reported information on the 
extent to which waterbodies meet water quality standards. To assess 
the reliability of its data, we interviewed knowledgeable agency 
officials and conducted electronic testing of the data. We determined 
that although the data have limitations, they were sufficiently 
reliable to present reported atmospheric deposition impairment on 
assessed waters for selected categories of water bodies. We also 
reviewed studies from EPA and others to further examine the extent to 
which atmospheric deposition is contributing to the impairment of the 
nation's waters. We assessed the reliability and methodology of these 
studies, including limitations and assumptions, and determined that 
they were sufficiently reliable for the purposes of examining the 
extent to which atmospheric deposition is contributing to the 
impairment of the nation's waters. To identify key sources of NOx, 
SO2, and mercury emissions, we used estimates from EPA's 2008 National 
Emissions Inventory (version 2)--a national database of sources and 
amounts of pollution emissions. We also interviewed officials and 
scientists from EPA and the Department of Commerce's National Oceanic 
and Atmospheric Administration (NOAA); and academic, state, and 
private sector scientists. 

To examine tools EPA uses under the CWA to address atmospheric 
deposition and the challenges it faces, we reviewed the law, relevant 
EPA regulations and guidance, and other pertinent documents and 
interviewed relevant EPA and state officials. We chose to examine a 
nonprobability sample of three TMDLs in greater detail--the Northeast 
Regional TMDL for mercury atmospheric deposition, a New York State 
TMDL for acid rain in the Adirondacks, and the Chesapeake Bay TMDL for 
nitrogen atmospheric deposition--to illustrate challenges.[Footnote 4] 
We selected each TMDL to represent a different effect caused by the 
atmospheric deposition of the three pollutants on which we are 
focusing. The three effects are acid rain, nutrient over-enrichment, 
and mercury contamination. We examined key documents about these TMDLs 
and interviewed EPA and state and regional officials responsible for 
developing and implementing them. 

Likewise, to examine the tools EPA uses under the CAA to address 
atmospheric deposition and the challenges it faces, we reviewed the 
law, relevant EPA regulations and guidance, and other pertinent 
documents and interviewed relevant EPA officials. We used data from 
the 2008 National Emissions Inventory (version 2), a 2011 peer-
reviewed EPA study that estimates the benefits and costs of the CAA 
amendments of 1990,[Footnote 5] and other information to identify 
reductions in NOx, SO2, and mercury that EPA has attributed to the CAA 
and the associated beneficial effects on water quality. We assessed 
the reliability and methodology of these sources of information, 
including limitations and assumptions, and determined that they were 
sufficiently reliable for the purposes of attributing estimates of 
some reductions in NOx, SO2, and mercury to the CAA. To understand the 
challenges that EPA faced in trying to develop an air quality standard 
to protect ecosystems sensitive to the effects of acid rain, we 
reviewed documentation developed as part of the rule making process 
and interviewed knowledgeable officials responsible for the 
rulemaking. To understand EPA's integrated nitrogen research strategy, 
we reviewed pertinent documents and interviewed relevant EPA 
officials. Appendix I presents a more detailed description of our 
scope and methodology. 

We conducted this performance audit from September 2011 to January 
2013 in accordance with generally accepted government auditing 
standards. Those standards require that we plan and perform the audit 
to obtain sufficient, appropriate evidence to provide a reasonable 
basis for our findings and conclusions based on our audit objectives. 
We believe that the evidence obtained provides a reasonable basis for 
our findings and conclusions based on our audit objectives. 

Background: 

This section discusses atmospheric deposition, its effect on 
waterbodies, and how the CWA and the CAA address it.[Footnote 6] 

Atmospheric Deposition Is the Transfer of Air Pollution to the: 

Earth's Surface: 

Air pollution can be carried through the atmosphere and deposited on 
land, water, or man-made structures in a process known as atmospheric 
deposition. The pollutants can get into waterbodies through either 
direct deposition or indirect deposition as follows: 

* Direct deposition occurs when air pollutants are deposited directly 
onto a waterbody; and: 

* indirect deposition occurs when air pollutants are deposited onto 
the watershed--the area of land surrounding the waterbody--and are 
transported to the waterbody through runoff, rivers, streams, and 
groundwater. 

Additionally, atmospheric deposition can be either wet or dry as 
follows: 

* Wet deposition occurs when pollutants are removed from the air and 
deposited via rain, snow, fog, or mist; and: 

* dry deposition occurs when air pollutants are deposited in the 
absence of rain or snow as falling particles or as the gas form of the 
pollutant. 

The pollution deposited through atmospheric deposition can come from 
human-caused sources, such as power plants, or natural sources, such 
as volcanoes. Figure 1 depicts this process. 

Figure 1: Atmospheric Deposition: 

[Refer to PDF for image: illustration] 

Depicted on the illustration: 

Source of pollutants: 
Human sources; 
Natural sources; 
Indirect deposition; 
Dry deposition; 
Wet deposition; 
Direct deposition. 

Source: EPA's 2001 Frequently Asked Questions About Atmospheric 
Deposition: Handbook for Watershed Managers. 

[End of figure] 

Three airborne pollutants commonly associated with atmospheric 
deposition are NOx, SO2, and mercury but there are other pollutants. 
Examples include ammonia, which contributes to nutrient over-
enrichment; and lead, polychlorinated biphenyls (PCBs), and other 
toxic substances, which can harm human health. 

Effects of Atmospheric Deposition on Waterbodies: 

NOx and SO2 cause acid rain, and NOx contributes to nutrient over-
enrichment of waterbodies. Once in water, mercury becomes concentrated 
in fish and can harm individuals who consume contaminated fish. 

NOx and SO2: 

NOx and SO2 cause acid rain, which can threaten certain aquatic 
ecosystems.[Footnote 7] NOx reacts chemically in the atmosphere to 
form nitric acid. SO2 reacts chemically in the atmosphere to form 
sulfuric acid.[Footnote 8] When these chemicals fall to the ground, 
they are known as acid rain. Some aquatic ecosystems are sensitive to 
acid rain. They are often at higher elevations and have underlying 
geologic conditions that limit their ability to neutralize or buffer 
acid. Acid rain in these ecosystems can cause a range of harmful 
effects. For example, acidification of lakes and streams can reduce or 
eliminate fish populations and decrease the numbers and types of 
aquatic plants and animals. 

NOx can also contribute to the nutrient over-enrichment of 
waterbodies, which can harm or kill aquatic species. NOx is one of 
several sources of nitrogen that can contribute to nutrient over-
enrichment. Other sources of nitrogen, such as runoff from 
agricultural and urban landscapes, do not enter water bodies through 
atmospheric deposition. Bays and estuaries are susceptible to the 
overabundance of nutrients, including an overabundance of nitrogen, 
because the growth of algae and plankton in these waters is generally 
limited by the availability of nitrogen. As nitrogen levels in the 
water increase, the growth of algae and plankton increases. When these 
organisms die, they are decomposed by bacteria that use the dissolved 
oxygen in the water. Dead zones characterized by low levels of 
dissolved oxygen in the water may result, harming or killing fish, 
crabs, shrimp, and other organisms. Benthic (bottom-dwelling) 
organisms such as oysters that are unable to move to more oxygenated 
waters are particularly susceptible. 

Mercury: 

Mercury is toxic to humans and animals. Exposure to mercury can affect 
neurological development, especially in children and developing 
fetuses, which are especially susceptible to mercury exposure. Mercury 
exposure at high levels can also harm the brain, heart, kidneys, 
lungs, and immune systems of all ages. Eating contaminated fish and 
shellfish is the primary way humans are exposed to mercury. In mercury 
contaminated waterbodies, bacteria in soils and sediments convert 
mercury into methylmercury, a highly toxic form of mercury. It can 
then be taken up by tiny aquatic plants and animals. Fish that eat 
these plants and animals build up methylmercury in their tissue 
through a process known as bioaccumulation. According to EPA, large 
predatory fish may have mercury concentrations many times higher than 
the concentrations in the waterbody in which they live. Thus, a 
relatively small concentration of mercury in a waterbody can harm 
humans who eat fish from these waterbodies. 

Atmospheric deposition is the primary pathway for mercury to reach 
many waterbodies, although nonatmospheric sources of mercury, such as 
legacy gold mining, can be a significant source of mercury to some 
waterbodies. Once airborne, mercury can travel long distances or be 
deposited close to its source--depending in part on its chemical form. 
Elemental mercury is the most prevalent form of mercury in the 
atmosphere. Because it does not readily react with other chemicals in 
the atmosphere, elemental mercury generally stays aloft for longer 
periods of time and can travel around the globe as part of the "global 
pool" of mercury. Other forms of mercury more readily react with 
chemicals in the atmosphere and travel shorter distances before they 
deposit. Unlike NOx and SO2, mercury persists in the environment 
indefinitely. Mercury that has been deposited on land, water, and 
other surfaces can repeatedly be reemitted into the atmosphere. 

The Clean Water Act and the Clean Air Act Address Water and Air 
Pollution: 

The CWA establishes the basic structure for establishing water quality 
standards for surface waters of the United States and for regulating 
discharges of pollutants into those waters. Among other things, CWA 
requires EPA to establish criteria that are then to be used by states 
to establish water quality standards for individual waterbodies. These 
standards consist of three key components: the designated uses that a 
given waterbody is intended to serve, such as recreation, water 
supply, or industrial; the qualitative (narrative) or numeric 
standards in order for a waterbody to be suitable for its designated 
uses; and antidegradation requirements. Where water quality standards 
are not being met, the waterbody is identified as impaired. States are 
required to identify and report a list of waterbodies that do not meet 
water quality standards, and for which pollution controls are not 
stringent enough to meet such standards.[Footnote 9] In such cases, 
the CWA generally requires states to establish a TMDL for each 
pollutant causing a given waterbody to be impaired. The TMDL must 
identify allocations, i.e., the amount of the pollutant that will come 
from point sources (e.g., a pipe from sewage treatment plant) and 
nonpoint sources (e.g., atmospheric deposition or agricultural and 
urban runoff). The total amount of pollution from both types of 
sources must be at a level that ensures that applicable water quality 
standards for that waterbody can be met and maintained. States are 
generally responsible for developing and implementing TMDLs, but their 
TMDLs must be approved by EPA. As of August 2012, EPA has approved 
more than 48,000 TMDLs ranging in size from a small segment of a river 
to a multistate region. Additionally, the CWA authorizes EPA or, if 
approved by EPA, the states to issue National Pollution Discharge 
Elimination System (NPDES) permits to a facility that discharges a 
pollutant from a point source into U.S. waters.[Footnote 10] 
Generally, the discharge restrictions established by NPDES permits 
must be consistent with the allocations in applicable TMDLs. The CAA 
establishes the basic structure for regulating the emissions of 
pollutants into the air. It requires that EPA develop National Ambient 
Air Quality Standards (NAAQS) for certain pollutants, including NOx 
and SO2.[Footnote 11] These standards include primary standards 
intended to protect public health, including the health of sensitive 
populations such as asthmatics, children, and the elderly, and 
secondary standards intended to protect public welfare from adverse 
effects such as decreased visibility and damage to animals, crops, 
vegetation, and buildings. EPA has developed primary and secondary 
standards for NOx and SO2. Through its regulatory process, EPA 
designed the current secondary NOx and SO2 standards to protect land-
based ecosystems, not water quality or aquatic ecosystems. EPA is 
required, at 5-year intervals, to review current NAAQS and make 
revisions and promulgate new standards as appropriate. In addition, 
the CAA authorizes EPA to set certain limits on emissions of "air 
toxics"--such as mercury--which are certain pollutants that may 
present, through inhalation or other exposures, a threat to human 
health, such as cancer and birth defects, or cause adverse 
environmental effects. EPA has issued regulations that limit mercury 
emissions from certain industries. 

Air Pollution Affects U.S. Waterbodies and Transportation and Power 
Plants Are Its Key Sources: 

Atmospheric deposition associated with NOx, SO2, and mercury 
contributes to the impairment of many waters in the United States, but 
the full extent is not completely known. Emissions from transportation 
and domestic power plants are key sources of NOx, SO2, and mercury, 
but international sources also contribute to mercury deposition in the 
United States. 

Atmospheric Deposition Pollutes Many U.S. Waterbodies, but the Full 
Extent is Not Known: 

Atmospheric deposition, including from NOx, SO2, and mercury, is a 
contributing source of pollution to many waterbodies that the states 
have identified as impaired--that is, do not meet CWA water quality 
standards--according to our analysis of EPA's ATTAINS database. 
[Footnote 12] For example, about 53,300 square miles of the Great 
Lakes are reported to be impaired, in part, because of atmospheric 
deposition.[Footnote 13] Table 1 shows for select waterbody types the 
percentage of waterbodies assessed, the percentage of assessed 
waterbodies that are reported as impaired, and the percentage of 
impaired waterbodies for which atmospheric deposition, including from 
NOx, SO2, and mercury, is identified as a source of pollution. 
However, the ATTAINS database likely underestimates the extent of the 
problem because the states have not assessed all of their waterbodies. 
[Footnote 14] The percentage of waterbodies assessed varies by 
waterbody type and ranges from about 1 percent of wetlands to about 88 
percent of the Great Lakes. Additionally, states may, but are not 
required, to report on the sources of a pollutant to a waterbody, 
including whether atmospheric deposition is a reason why water quality 
standards are not being met or the extent to which atmospheric 
deposition is contributing to the problem. 

Table 1: Waterbodies Reported by States as Impaired by Atmospheric 
Deposition by Waterbody Type: 

(Continued From Previous Page) 

Type of waterbody: Rivers and streams; 
Percentage of waterbodies assessed: 28%; 
Percentage of assessed waterbodies that are reported as impaired: 53%; 
Percentage of impaired waterbodies for which atmospheric deposition 
was identified as a contributing source: 20%. 

Type of waterbody: Lakes and reservoirs; 
Percentage of waterbodies assessed: 46%; 
Percentage of assessed waterbodies that are reported as impaired: 69%; 
Percentage of impaired waterbodies for which atmospheric deposition 
was identified as a contributing source: 37%. 

Type of waterbody: Bays and estuaries; 
Percentage of waterbodies assessed: 37%; 
Percentage of assessed waterbodies that are reported as impaired: 66%; 
Percentage of impaired waterbodies for which atmospheric deposition 
was identified as a contributing source: 36%. 

Type of waterbody: The Great Lakes; 
Percentage of waterbodies assessed: 88%; 
Percentage of assessed waterbodies that are reported as impaired: 100%; 
Percentage of impaired waterbodies for which atmospheric deposition 
was identified as a contributing source: 100%. 

Source: GAO analysis of ATTAINS database. 

Notes: GAO's analysis of ATTAINS database includes data on the 50 
states, the District of Columbia, American Samoa, the Commonwealth of 
Northern Mariana Islands, Guam, Puerto Rico, and the Virgin Islands. 
The most recently available state data are the most current year that 
each state reported. These years varied from 2002 to 2010 because some 
states reported more recently than others. 

[End of table] 

The table includes the percentage of assessed waterbodies that are 
reported as impaired by any pollutant, including NOx, SO2, and mercury 
and the percentage of impaired waterbodies for which atmospheric 
deposition of any pollutant, including NOx, SO2, and mercury, was 
identified as a contributing source. 

Federal studies also show that atmospheric deposition associated with 
NOx, SO2, and mercury is polluting waters in the United States. For 
example, a 2011 report from the National Acid Precipitation Assessment 
Program (NAPAP)--a federal interagency program that coordinates acid 
rain research--summarized the most recent studies on acid rain and 
identified waters in several acid-sensitive ecosystems that are most 
vulnerable and that continue to receive harmful levels of acid rain. 
[Footnote 15] According to the report, from 2006 through 2008, over 
550 lakes in the Adirondack Mountains and approximately 21,000 miles 
of streams in the Central Appalachian Mountains continued to 
experience levels of acid rain that exceeded their ability to 
neutralize the acid. However, according to an EPA document, there is 
relatively sparse data coverage in mountainous western areas where a 
number of sensitive ecosystems are located.[Footnote 16] Figure 2 
shows acid-sensitive ecosystems in the United States. 

Figure 2: Location of Acid-Sensitive Ecosystems in the United States: 

[Refer to PDF for image: illustrated U.S. map] 

[End of figure] 

A 2007 study by NOAA assessed the extent to which 99 estuaries across 
the country experienced symptoms associated with nutrient over-
enrichment, such as algal blooms.[Footnote 17] The study found that 29 
of the estuaries experienced symptoms that affected at least 50 
percent of the estuary and/or were persistent and that another 35 
estuaries periodically experienced symptoms that affected a smaller 
portion of the estuary. The study identified atmospheric deposition as 
contributing source of nutrients for all coastal regions--except the 
Pacific coast, for which there were insufficient data--but did not 
quantify the relative amount of nitrogen, including NOx coming from 
atmospheric deposition. A 2011 NAPAP report did quantify the relative 
amount of nitrogen coming from atmospheric deposition.[Footnote 18] 
Summarizing other scientific studies, the report stated that the 
relative amount of nitrogen entering estuaries that comes from 
atmospheric deposition varied from about 15 percent to 25 percent in 
the Mississippi Delta region of the Gulf of Mexico, to 20 percent to 
50 percent from the Chesapeake Bay to Tampa Bay, and as much as 60 
percent to 80 percent in some coastal river estuaries of northern New 
England.[Footnote 19] The report did not provide any data about West 
Coast estuaries. 

Similarly, studies show that mercury is contaminating waters in the 
United States, but the extent of the contamination is less well 
understood. According to the National Atmospheric Deposition Program 
(NADP)--a public-private cooperative program that monitors atmospheric 
deposition of mercury, among other pollutants--atmospheric deposition 
is the primary pathway by which mercury reaches waters in many parts 
of the United States.[Footnote 20] Other studies report similar 
results. For example, a 2007 report to Congress by NOAA concludes that 
atmospheric deposition of mercury is a significant source of mercury 
in the Great Lakes.[Footnote 21] However, measuring mercury 
atmospheric deposition, especially dry deposition, is scientifically 
challenging, and representatives from NADP told us that there are 
significant data gaps on the amount of dry deposition across the 
country. Additionally, there are factors other than the amount of 
mercury in a waterbody that affect the amount of mercury in fish 
tissue. For example, the amount of mercury that has been converted to 
methylmercury--the form of mercury that bioaccumulates in fish--is 
influenced by various factors such as the chemical composition of the 
water. The amount of mercury in fish tissue also varies by the size 
and species of fish present in the waterbody. Nevertheless, according 
to EPA's 2010 National Listing of Fish Advisories, all 50 states 
reported mercury-related fish consumption advisories, and 25 states 
reported statewide freshwater advisories. Additionally, Alaska, 
Hawaii, all of the Gulf states, and most of the East Coast states, 
reported statewide coastal advisories. Figure 3 shows the extent of 
mercury-related fish consumption advisories in 2010. Statewide 
advisories are precautionary and warn the public of the potential 
contamination of certain fish types in specific waterbodies, resulting 
in a designated fish advisory for all rivers or lakes in the state. 

Figure 3: Mercury-related Fish Consumption Advisories, 2010: 

[Refer to PDF for image: illustrated U.S. map] 

Statewide fresh water advisory and additional advisories on specific 
waterbodies: 
Connecticut; 
Florida; 
Georgia; 
Idaho; 
Illinois; 
Indiana; 
Kentucky; 
Louisiana; 
Maryland; 
Massachusetts; 
Missouri; 
Montana; 
New Hampshire; 
New Jersey; 
North Carolina; 
North Dakota; 
Ohio; 
Oklahoma; 
Rhode Island; 
Vermont; 
Washington; 
West Virginia; 
Wisconsin; 
Wyoming. 

Statewide fresh water advisory: 
Maine. 

Advisories for specific waterbodies: 
Alabama; 
Alaska; 
Arizona; 
Arkansas; 
California; 
Colorado; 
Delaware; 
Iowa; 
Kansas; 
Mississippi; 
Nebraska; 
Nevada; 
New Mexico; 
New York; 
Oregon; 
Pennsylvania; 
South Carolina; 
South Dakota; 
Tennessee; 
Texas; 
Utah; 
Virginia. 

Statewide fresh water advisories for lakes only: 
Michigan; 
Minnesota. 

No mercury advisories: 
District of Columbia; 
Hawaii. 

Statewide coastal advisories: 
Entire East Coast; 
Entire Gulf Coast; 
Alaska; 
Hawaii. 

Sources: 2010 National Listing of Fish Advisories from EPA's National 
Fish and Wildlife Contamination Program; Map Resources (map). 

[End of figure] 

Domestic Transportation and Power Plants Are Sources of NOx and SO2, 
and Other Countries Contribute to Mercury Deposition: 

According to 2008 data, domestic transportation is the largest source 
of NOx, and domestic power plants are the primary source of SO2 and 
the largest source of U.S. mercury emissions, but international 
sources of mercury are a significant contributor to mercury 
atmospheric deposition in the United States. 

Domestic Transportation Is the Largest Source of NOx Emissions: 

Domestic transportation is the largest source of NOx emissions in the 
United States, according to EPA's 2008 National Emissions Inventory. 
[Footnote 22] Figure 4 shows that on-road vehicles, such as cars and 
trucks, and off-road transportation, such as commercial fishing 
vessels, passenger trains, and commercial aircraft, contributed 
approximately 63 percent of the total estimated 17.4 million tons of 
NOx emissions in the United States. Other sources of NOx emissions 
include fuel combustion, mainly from power plants and industrial 
boilers,[Footnote 23] which accounted for an estimated 28 percent of 
total estimated NOx emissions. According to an EPA official, the only 
international sources of NOx emissions of any note that reach the 
United States are in Canada and Mexico and, in general, emissions from 
these sources do not impact U.S. waters. 

Figure 4: Estimated Contributions of NOx Emissions from Domestic 
Sources in 2008: 

[Refer to PDF for image: horizontal bar graph] 

Source: Miscellaneous[A]; 
NOx emissions (tons in thousands): 262. 

Source: Transportation[B]; 
NOx emissions (tons in thousands): 
Off-road transportation: 3,763; 
On-road vehicles: 7,089; 

Source: Industrial processes; 
NOx emissions (tons in thousands): 1,081. 

Source: Fuel combustion; 
NOx emissions (tons in thousands): 
Power plants: 3,031; 
Industrial boilers: 1,391; 
Other[C]: 582. 

Source: 2008 National Emissions Inventory. 

[A] Miscellaneous includes sources such as gas stations, bulk gasoline 
terminals, and commercial cooking. 

[B] Some military aircraft are not included in the transportation 
category but are instead included in the fuel combustion category. 

[C] Other fuel combustion sources include residential, commercial, and 
institutional biomass and fossil-fuel burning units. 

[End of figure] 

Ammonia emissions, like NOx, can contribute to the atmospheric 
deposition of nitrogen. When nitrogen in fertilizers and animal waste 
breaks down, it is converted to ammonia gas and released into the air, 
where it can deposit on land and water. Agriculture accounted for 
approximately 83 percent of the total estimated 4.4 million tons of 
ammonia emissions, according to the 2008 National Emissions Inventory. 
Ammonia emissions can contribute to nutrient over-enrichment in 
waterbodies and, in some areas of the country, ammonia can be the 
dominant source of atmospheric nitrogen, especially those areas with 
abundant agricultural land, according to the 2011 NAPAP report. 

Domestic Power Plants Are the Primary Source of SO2 Emissions: 

Domestic power plants are the primary source of SO2 emissions in the 
United States, according to the 2008 National Emissions Inventory. 
Figure 5 shows that fuel combustion, mainly from power plants, 
contributed approximately 87 percent of the total estimated 10.4 
million tons of SO2 emissions in the United States.[Footnote 24] 
Industrial processes and transportation contributed approximately 8 
percent and 3 percent, respectively. According to an EPA official, the 
only international sources of SO2 emissions of any note that reach the 
United States are in Canada and Mexico and, in general, emissions from 
these sources do not impact U.S. waters. 

Figure 5: Estimated Contributions of Sulfur Dioxide Emissions from 
Domestic Sources in 2008: 

[Refer to PDF for image: horizontal bar graph] 

Source: Miscellaneous[A]; 
SO2 emissions (tons in thousands): 139. 

Source: Transportation; 
SO2 emissions (tons in thousands): 324. 

Source: Industrial processes; 
SO2 emissions (tons in thousands): 789. 

Source: Fuel combustion; 
SO2 emissions (tons in thousands): 
Power plants: 7,761; 
Industrial boilers: 1,055; 
Other[B]: 283. 

Source: 2008 National Emissions Inventory. 

Note: These power plants are primarily coal-fired. 

[A] Miscellaneous includes sources such as gas stations, bulk gasoline 
terminals and commercial cooking. 

[B] Other fuel combustion sources include residential, commercial and 
institutional biomass and fossil-fuel burning units. 

[End of figure] 

Domestic Power Plants Are the Largest Source of U.S. Mercury 
Emissions, but International Sources Contribute to Deposition: 

Domestic sources emitted about 61 tons of mercury, according to the 
2008 National Emissions Inventory. Figure 6 shows that fuel combustion 
from domestic power plants--mostly coal-burning power plants--
contributed approximately half of these emissions. The next three 
largest sources were electric arc furnaces, industrial, commercial, 
and institutional boilers and process heaters, and Portland cement 
nonhazardous waste.[Footnote 25] 

Figure 6: Estimated Contributions of Mercury Emissions from Domestic 
Sources in 2008: 

[Refer to PDF for image: pie-chart] 

Power plants: 48%; 
Other mercury sources[B]: 30%; 
Electric arc furnaces: 8%; 
Portland cement nonhazardous waste[A]: 7%; 
Industrial, commercial, institutional boilers, and process heaters: 7%. 

Source: 2008 National Emissions Inventory. 

[A] Portland cement is the most common type of cement produced around 
the world. Portland cement kilns emit mercury, among other pollutants. 

[B] Other sources of mercury emissions include gold mining, 
transportation, municipal waste combustors, medical waste 
incineration, and other types of incineration. 

[End of figure] 

While most of the NOx and SO2 that affects U.S. waters originates from 
human-caused domestic sources, this is not the case for mercury. Some 
of the mercury deposited in the United States comes from international 
and natural sources, such as volcanoes, and not all mercury emissions 
from domestic sources are deposited in the United States. According to 
a 2010 EPA report, human-caused mercury emissions from the United 
States, southern Canada, and northern Mexico contribute about 20 to 30 
percent of the total mercury atmospheric deposition in the continental 
United States.[Footnote 26] The remainder comes from other foreign 
sources and natural sources. However, according to a 2008 EPA report, 
local sources can be the dominant contributor of mercury atmospheric 
deposition in areas where there are sources that emit large amounts of 
mercury.[Footnote 27] A 2008 United Nations Environment Programme 
(UNEP) report estimated that human-caused mercury emissions from North 
America contributed about 8 percent of global human-caused mercury 
emissions in 2005 (see figure 7).[Footnote 28] Asia, on the other 
hand, contributed about 67 percent of global mercury emissions. 
According to the report, in 2005 about 46 percent of global mercury 
emissions came from burning fossil fuels, primarily coal, to produce 
electricity and heat. Another 24 percent of global mercury emissions 
came from gold production. Other global sources of mercury emissions 
include metal production (excluding gold) and cement production. 

Figure 7: Estimated Contributions of Global Mercury Emissions from 
Human-Caused Sources in 2005: 

[Refer to PDF for image: pie-chart] 

Asia: 67%; 
North America: 8%; 
Europe: 8%; 
South America: 7%; 
Africa: 5%; 
Russia: 4%; 
Oceania[A]: 2%. 

Source: United Nations Environmental Program Report. 

Note: Percentages do not add to 100 percent due to rounding. 

[A] The Oceania region includes the islands of the southern, western, 
and central Pacific Ocean, such as Australia, New Zealand, Melanesia, 
Micronesia, and Polynesia. 

[End of figure] 

EPA Faces Challenges Using Clean Water Act Regulations to Address 
Atmospheric Deposition: 

Listing impaired waters and TMDLs are the primary regulatory tools EPA 
uses under the CWA to identify water pollution resulting from 
atmospheric deposition, but EPA faces challenges in using these tools, 
as well as other nonregulatory tools, to address the effects of 
atmospheric deposition. Three TMDLs we reviewed illustrate the 
challenges associated with using these tools. 

Key Regulatory Tools Do Not Directly Regulate Atmospheric Deposition: 

EPA has regulatory and nonregulatory tools available under the CWA for 
addressing atmospheric deposition. 

Regulatory Tools: 

EPA's key regulatory tools for considering the effects of atmospheric 
deposition on water quality are listing of impaired waters and TMDLs, 
but the CWA does not directly regulate nonpoint sources of pollution, 
including pollution resulting from atmospheric deposition. When states 
identify waterbodies that are impaired--that is, that do not meet CWA 
water quality standards with existing controls--the CWA requires 
states to formally identify them as impaired, and generally to set a 
TMDL for each pollutant that does not meet the standards. TMDLs are 
not self-implementing and CWA does not expressly require they be 
implemented; states generally have the lead role for implementation. 
TMDL allocations for permitted point sources are typically implemented 
via NPDES permits. Relevant NPDES permits issued to facilities for 
point sources of that pollutant must be consistent with the 
allocations established by the TMDL, but there is no comparable 
permitting mechanism under the CWA directly limiting pollutants from 
nonpoint sources. Nevertheless, states must account for nonpoint 
sources of pollution in their TMDLs and may include estimated 
reductions from those nonpoint sources. Where a state provides EPA 
with "reasonable assurance" that the reductions from nonpoint sources 
of pollution identified in the TMDL will be achieved, EPA often can 
allow NPDES permits for point sources of pollution identified in the 
TMDL to be less stringent than they would have otherwise been. 
[Footnote 29] EPA makes "reasonable assurance" determinations based on 
factors such as whether practices to reduce the nonpoint source 
pollutant are technically feasible and likely to be implemented. While 
this creates an incentive for a state to reduce pollution from 
nonpoint sources, the CWA provides no direct requirement--such as 
permitting--that states implement such reductions. 

A further challenge in using TMDLs to address atmospheric deposition 
is that the pollution impairing a waterbody can originate outside of 
the state responsible for implementing the TMDL. For example, 
pollutants travel in the atmosphere and harm air quality and the 
environment in downwind communities. To help address this challenge, 
EPA allows states some flexibility in addressing these situations. 
Specifically, EPA allows states with a waterbody impaired primarily 
from atmospheric sources of mercury to defer developing a TMDL for 
that waterbody if the state is carrying out a comprehensive mercury 
reduction program and demonstrates progress in reducing in-state 
mercury sources. This approach provides a public accounting of the 
status of mercury impaired waters and a way for states to demonstrate 
progress in reducing in-state mercury sources. According to an EPA 
document describing this approach, it may also be useful in situations 
where other pollutants are primarily from atmospheric deposition. 
However, according to EPA officials, this approach is intended to 
encourage states to address in-state sources of mercury, and EPA has 
not encouraged the use of this approach for other pollutants such as 
NOx or SO2. 

According to EPA officials, despite these limitations of the CWA, 
TMDLs may play an important role in understanding the role of 
atmospheric deposition to impairment of a particular waterbody, such 
as identifying the contributions from air sources to water and the 
reductions needed in air sources in order to meet water quality 
standards. In addition, according to EPA officials, TMDLs may parse 
the contributions from various air sources to waterbodies, including 
separating the contributions from local and global sources, which may 
help states identify whether there are air sources that can be 
controlled at the state level. 

Nonregulatory Tools: 

EPA also has nonregulatory tools available under the CWA to provide 
incentives and assistance to the states to address nonpoint source 
pollution and improve water quality. However their use in addressing 
atmospheric deposition presents challenges. The most notable of these 
tools is the nonpoint source management program known as the Section 
319 Program. Under this program, EPA offers technical and financial 
assistance to states and local agencies to develop projects to control 
nonpoint sources of water pollution.[Footnote 30] According to an EPA 
official, only a small portion of projects that have been funded in 
this program have addressed atmospheric deposition. According to the 
official, projects typically involve source control--slowing down 
stormwater flow to prevent erosion or building fences to keep 
livestock out of streams--but the source of atmospheric deposition can 
be diffuse, as is the case with cars, or originate in other countries, 
as is the case with mercury. According to the official, a local or 
state agency that deals with water quality issues does not have 
authority or tools to control these types of sources and would find it 
difficult to develop a project to control atmospheric deposition. 

Another CWA tool relevant to atmospheric deposition is the interstate 
management conference process,[Footnote 31] whereby states or regions 
can petition the EPA Administrator to convene a management conference 
of all states that contribute nonpoint source pollution to the state's 
or region's waterbodies. The purpose of the conference is to forge an 
agreement to reduce the sources of nonpoint source pollution from the 
states or regions contributing to the pollution. However, even if a 
conference is held, there is neither a requirement that an agreement 
to address the problem be reached nor is there a requirement that any 
agreement that is reached be binding. EPA granted the petition for one 
such conference on the atmospheric deposition of mercury, which took 
place in 2010. However, as discussed below, participants in this 
conference did not reach an agreement on steps to reduce atmospheric 
deposition of mercury. 

Three TMDLs Highlight Challenges in Addressing Atmospheric Deposition 
under the Clean Water Act: 

The three TMDLs that we reviewed illustrate the challenges that states 
face in mitigating sources of atmospheric deposition when these 
sources originate from outside of the state or region. These three 
TMDLs are the Northeast Regional TMDL for mercury, a New York State 
TMDL for acid rain in the Adirondacks, and the Chesapeake Bay TMDL for 
nitrogen.[Footnote 32] 

Northeast Regional TMDL for Mercury: 

In the Northeast Region, more than 10,000 lakes, ponds, and 
reservoirs, and more than 46,000 river miles are listed as impaired 
for fish consumption primarily due to the presence of mercury--mostly 
from atmospheric deposition.[Footnote 33] A committee of 
representatives of each of the Northeast states and the New England 
Interstate Water Pollution Control Commission (NEIWPCC) developed a 
TMDL for mercury and an implementing strategy to reduce the mercury 
concentration in fish in Northeast Region waterbodies.[Footnote 34] 
EPA approved the TMDL in 2007. However, the TMDL acknowledged that 
given the magnitude of the reductions required to implement the TMDL, 
the Northeast Region cannot reduce in-region sources further to 
compensate for insufficient reductions from out-of-region sources. 
According to the TMDL, prior mercury emission limits implemented by 
the Northeast Region had contributed to reducing in-region mercury 
deposition by approximately 74 percent from 1998 to 2002. By 2002, 
most of the remaining mercury deposited in the region was from out-of-
region sources, with roughly two-thirds from the global pool of 
mercury and the remaining from domestic sources, split roughly evenly 
between sources in the Northeast Region states and sources in 11 other 
states, according to our analysis of a 2008 study by the Northeast 
States for Coordinated Air Use Management (NESCAUM).[Footnote 35] 
Figure 8 depicts the out-of-region sources of atmospheric deposition 
in the Northeast Region as of 2002. 

Figure 8: Sources of Atmospheric Deposition of Mercury in the 
Northeast Region, 2002: 

[Refer to PDF for image: illustrated map] 

States with the most significant contributions of mercury air 
deposition to the Northeast Region: 
Illinois; 
Indiana; 
Kentucky; 
Maryland; 
Michigan; 
New Jersey; 
North Carolina; 
Ohio; 
Pennsylvania; 
Virginia; 
West Virginia. 

Northeast Region states: 
Connecticut; 
Maine; 
Massachusetts; 
New Hampshire; 
New York; 
Rhode Island; 
Vermont. 

Northeast Region states also receive Global contributions. 

Sources: GAO analysis of data from The Northeast States for 
Coordinated Air Use Management, Sources of Mercury Deposition in the 
Northeast United States (March 2008); Map Resources (map). 

[End of figure] 

To address domestic but out-of-region mercury sources, in 2008, 
Northeast Region officials used the CWA interstate management 
conference process discussed above and petitioned EPA to hold a 
conference with 11 states that, according to a 2008 NESCAUM study, 
were the states outside the region that contributed the most to the 
Northeast's mercury problem. The conference participants discussed 
developing a "principles document" that would outline next steps. 
However, according to a NEIWPCC representative, there was not enough 
agreement among the states to move forward with the principles 
document, and no formal agreement was reached to reduce the 
atmospheric deposition of mercury. In the end, according to the 
representative, all of the participating states have made some efforts 
to reduce mercury, but the level of commitment varies by state. 
According to EPA officials, the conference brought together senior air 
and water program officials from upwind and downwind states and 
allowed participants to learn about successful mercury programs in 
other states. 

Adirondacks Region Lakes TMDL for Acid Rain: 

Over the last several decades, acid rain has resulted in the 
significant loss of fish populations in the Adirondacks Region located 
in northeast New York State. In 2006, the New York State Department of 
Environmental Conservation (NYSDEC) developed a TMDL for an acid rain 
restoration plan to address 143 impaired lakes on state-owned forest 
preserve lands in the Adirondacks.[Footnote 36] Figure 9 shows the 
forest preserve lands within the Adirondacks. 

Figure 9: Location of New York State Forest Preserve Lands in the 
Adirondacks: 

[Refer to PDF for image: illustrated may of New York] 

Map depicts: 
Border of Adirondacks; 
Forest preserve land. 

Source: GAO analysis of NYSDEC data. 

[End of figure] 

According to state officials, over the last two decades the state has 
reduced in-state emissions of SO2 by about 90 percent and reduced in-
state emissions of NOx to a lesser extent.[Footnote 37] This in turn 
has reduced acid rain and improved water quality in the Adirondack 
lakes. However, state officials said that reducing acid rain to the 
levels called for in the TMDL remains a challenge because almost all 
of the NOx, SO2, and other sulfur oxides contributing to acid rain in 
the region can be attributed to out-of-state sources--mostly from the 
Southeast, and Midwest United States and Canada. Specifically, data 
compiled by the NYSDEC in the mid-1980s estimated that 83 to 87 
percent of acid rain came from emissions of NOx, SO2, and other sulfur 
oxides originating out-of-state, and NYSDEC officials stated that this 
percentage is now likely higher as a result of the in-state 
reductions. According to NYSDEC officials, meeting the TMDL goals for 
acid rain is dependent on emissions reductions in other states, but 
they said the department will continue to address its responsibilities 
for improving air and water quality within the state through 
implementation of federal programs under the CAA, as well as state air 
programs, such as its Acid Deposition Reduction Program. According to 
NYSDEC officials, the state has not used other nonregulatory tools 
available under the CWA to help address acid rain, such as petitioning 
for conferences with other states, but they said that this is an 
option that needs to be considered. 

Chesapeake Bay TMDL for Nitrogen: 

Most of the Chesapeake Bay and its tidal waters are impaired, in part 
because of the effects of excess nitrogen. According to TMDL 
documents, atmospheric deposition of NOx accounts for about 22 percent 
of the total nitrogen entering the bay. Other sources of nitrogen 
include agricultural (i.e., ammonia from chemical fertilizer and 
manure), municipal and industrial wastewater, and storm water runoff. 
[Footnote 38] The Chesapeake Bay watershed covers 64,000 square miles 
and includes parts of six states--Delaware, Maryland, New York, 
Pennsylvania, Virginia, and West Virginia--and the District of 
Columbia. 

In December 2010, EPA established the Chesapeake Bay TMDL for 
nitrogen. Among other things, for each of the seven jurisdictions in 
the watershed, the TMDL sets an allocation for total nitrogen entering 
the bay watershed, including from atmospheric deposition, needed to 
reduce nitrogen to meet water quality standards.[Footnote 39] However, 
as with the Northeast and Adirondack TMDLs discussed, the 
jurisdictions that are a party to this TMDL can only control those 
pollution sources inside their own jurisdictions. The bay's watershed 
is contained within the boundaries of the seven jurisdictions party to 
the TMDL, but the bay's NOx airshed extends beyond the seven 
jurisdictions' boundaries. According to EPA, about 50 percent of the 
NOx impacting the bay comes from sources outside of the seven 
jurisdictions. Figure 10 depicts the Bay's watershed and NOx airshed--
defined by the area from which 75 percent of the NOx emissions that 
contribute to the bay originate.[Footnote 40] 

Figure 10: Location of NOx Airshed and Chesapeake Bay Watershed: 

[Refer to PDF for image: illustrated map of the Eastern U.S.] 

Map depicts: 
Chesapeake Bay watershed; 
Chesapeake Bay airshed. 

Sources: GAO analysis of Chesapeake Bay Foundation data; Map Resources 
(map). 

[End of figure] 

Clean Air Act Regulations Address Atmospheric Deposition, but EPA 
Faces Challenges Addressing Effects on Waterbodies: 

Implementation of CAA regulations has reduced emissions of NOx, SO2, 
and mercury and lessened the impact of atmospheric deposition on water 
quality and aquatic ecosystems. However, atmospheric deposition 
continues to affect water quality and harm aquatic ecosystems, and EPA 
faces challenges in using the CAA to further lessen the impact. 

CAA Regulations Have Lessened the Effect of Atmospheric Deposition on 
Waterbodies, but Such Deposition Is Still Occurring: 

Implementation of EPA's CAA regulations has reduced emissions of NOx, 
SO2, and mercury, but most were designed to address the effect of 
these pollutants on air quality and related human health effects, not 
water quality and aquatic ecosystems. While not designed specifically 
to address water quality, reductions in emissions that have occurred 
as a result of the implementation of CAA regulations have had a 
beneficial effect on water quality. Even with these reductions in 
pollutants, atmospheric deposition continues to affect water quality 
and harm aquatic ecosystems. 

NOx and SO2 Emissions: 

According to EPA's 2008 National Emissions Inventory, domestic 
emissions of NOx declined from about 26 million tons in 1990 to about 
17 million tons in 2008--about 74 percent of this reduction came from 
reduced emissions from power plants and on-road vehicles. Also 
according to the inventory, emissions of SO2 declined from 23 million 
tons in 1990 to 10 million tons in 2008--about 64 percent of this 
reduction came from reduced emissions from power plants. Figure 11 
shows the decline in NOx and SO2 emissions. 

Figure 11: NOx and SO2 Emissions in 1990, 2000, and 2008 by Source: 

[Refer to PDF for image: 2 stacked line graphs] 

Tons in thousands: 

NOx: 

Year: 1990; 
All other sources: 9,272; 
On-road vehicles: 9,592; 
Power plants: 6,663. 

Year: 2000; 
All other sources: 8,875; 
On-road vehicles: 8,394; 
Power plants: 5,330. 

Year: 2008; 
All other sources: 7,079; 
On-road vehicles: 7,089; 
Power plants: 3,031. 

SO2: 

Year: 1990; 
All other sources: 7,168; 
Power plants: 15,909. 

Year: 2000; 
All other sources: 4,951; 
Power plants: 11,396. 

Year: 2008; 
All other sources: 2,591; 
Power plants: 7,761. 

Source: 2008 National Emissions Inventory. 

Note: Regarding the figure depicting SO2 emissions, the category "All 
other sources" includes on-road vehicles. According to the National 
Emissions Inventory, on-road vehicles emitted 503 tons of SO2 in 1990 
and 117 tons in 2008. 

[End of figure] 

EPA has identified the following three key regulations or programs as 
contributing to these reductions (1) CAA Title II emission standards 
for moving sources (i.e., vehicle emission standards), (2) actions 
designed to meet primary NAAQS,[Footnote 41] and (3) the Acid Rain 
Program. Neither the vehicle emissions standards nor the actions to 
meet primary NAAQS are designed to address the effect of NOx and SO2 
on waterbodies.[Footnote 42] Vehicle emission standards limit certain 
vehicles' emissions of pollutants, including NOx, to the greatest 
extent achievable through the application of available technology, 
giving appropriate consideration to cost, energy, and safety factors. 
Even with these reductions in pollutants, atmospheric deposition 
continues to affect water quality and harm aquatic ecosystems. EPA 
established primary NAAQS to provide public health protection, 
including protecting the health of "sensitive" populations such as 
asthmatics, children, and the elderly, for six principal pollutants, 
which are called "criteria" pollutants to protect public health based 
on scientific information on their effects.[Footnote 43] In contrast, 
the Acid Rain Program was designed, in part, to address the effect of 
NOx and SO2 on waterbodies. The program implements CAA requirements 
for significant decreases in the emissions of NOx and SO2 from power 
plants to improve air quality and protect ecosystems that have 
suffered damage from acid rain, including aquatic ecosystems. 
According to the 2011 NAPAP report, the mandated emission reductions 
have been achieved, and thus no additional reductions in emissions can 
be expected from this program.[Footnote 44] 

However, two broad trends--recent environmental regulations and 
changing market conditions--could affect emissions from power plants. 
Regarding environmental regulations, as we have previously reported, 
beginning in June 2010, EPA proposed or finalized two key regulations 
that would, among other things, reduce emissions of NOx and SO2 from 
coal-burning power plants, but the requirements and deadlines these 
regulations may establish for power plants are uncertain.[Footnote 45] 
In particular, both regulations have been challenged in federal court. 
For example, EPA finalized the Cross-State Air Pollution Rule in 
August 2011. The regulation would have required reductions of certain 
emissions of air pollutants, including NOx and SO2, in 28 states. The 
U.S. Court of Appeals for the D.C. Circuit recently issued an opinion 
that would strike down the regulation, and EPA has asked the full 
court to rehear the case, creating uncertainty as to what may be 
required from power plants in the future to address such 
emissions.[Footnote 46] Regarding broader market conditions, important 
market drivers have been weighing on the viability of coal-burning 
power plants. Key among these has been the recent decrease in the 
price of natural gas, which has made it more attractive for power 
companies to use natural gas instead of coal to generate electricity. 
Power plants that use natural gas instead of coal to produce 
electricity generally emit less NOx and SO2 per unit of electricity 
generated. Nevertheless, coal is likely to continue to be a key fuel 
source for electricity generation in the United States, although its 
share as a source of electricity is expected to decline.[Footnote 47] 

Additionally, there have recently been key mobile source standards, 
like the 2010 Emissions Control Area and the 2011 Heavy Duty Vehicle 
GHG standards, which provide large reductions in both NOx and SOx from 
ocean-going ships and heavy-duty trucks, respectively. In addition, 
according to EPA officials, the upcoming planned Tier 3 vehicle 
standards, which are scheduled to be proposed in 2013, will provide 
significant NOx reductions in the future. 

The reduced domestic emissions of NOx and SO2 into the atmosphere are 
having a beneficial impact on water quality and aquatic ecosystems, 
according to EPA's 2011 report on the benefits and costs of the CAA. 
[Footnote 48] The report analyzed how reductions in NOx and SO2 
emissions attributable to the CAA are affecting the levels of acid 
rain and the amount of nitrogen contributing to nutrient over-
enrichment in bays and estuaries, among other things. The analysis 
compared two scenarios for the time frame 1990 through 2020. The first 
scenario assumes that federal, state, and local air regulations 
promulgated from 1990 through September 2005 are implemented through 
2020. The second scenario freezes federal, state, and local air 
pollution controls at the levels of stringency and effectiveness that 
prevailed in 1990. The analysis assumes that the geographic 
distributions of population and economic activity remain the same 
between the two scenarios. According to the report, air regulations 
limit levels of acid rain, and the greatest benefits for areas 
sensitive to acid rain occur in the Blue Ridge Mountains, the mid-
Appalachian Region, western Virginia, and central Pennsylvania. The 
report also notes that air regulations limit nitrogen deposition, and 
the greatest benefits for estuaries occur along the southeastern 
coast, Gulf Coast, and Southern California coast. 

Mercury Emissions: 

Mercury emissions from human-caused domestic sources declined from 
about 246 tons in 1990 to about 61 tons in 2008, according to the 2008 
National Emissions Inventory. More than half of this decline can be 
attributed to reduced emissions from municipal waste combustors and 
medical waste incineration.[Footnote 49] According to the inventory, 
in 1990, these two sources emitted about 108 tons of mercury; in 2008, 
they emitted less than 2 tons of mercury. Mercury emissions from power 
plants also declined from about 59 tons of mercury in 1990 to about 30 
tons of mercury in 2008. Figure 12 shows the decline in human-caused 
domestic mercury emissions. 

Figure 12: Mercury Emissions in 1990 and 2008 by Source: 

[Refer to PDF for image: stacked line graph] 

Tons: 

Year: 1990; 
All other sources: 79.4; 
Power plants: 58.8; 
Municipal waste combusters: 57.2; 
Medical waste incinerators: 51. 

Year: 2008; 
All other sources: 30.2; 
Power plants: 29.5; 
Municipal waste combusters: 1.3; 
Medical waste incinerators: 0.1. 

Source: 2008 National Emissions Inventory. 

Note: all other sources include electric arc furnaces; Portland cement 
nonhazardous waste; and industrial, commercial, and institutional 
boilers, and process heaters. 

[End of figure] 

EPA issued regulations in 1995 and 1997 under the CAA limiting mercury 
emissions from municipal waste combustors and medical infectious waste 
incinerators.[Footnote 50] They were not designed to address the 
effect of mercury on waterbodies but were instead technology-based 
standards, as directed by CAA.[Footnote 51] According to EPA, power 
plant emissions of mercury also declined. Specifically, according to 
EPA, power plant emissions declined primarily because of state rules 
targeting mercury emissions and CAA regulations targeting emissions of 
other pollutants that incidentally also reduced mercury emissions. 
Additionally, in February 2012, EPA promulgated the Mercury and Air 
Toxics Standards (MATS),[Footnote 52] establishing for the first time 
emission limitations on mercury and other toxic pollutants from 
existing and new power plants.[Footnote 53] The rule was challenged in 
federal court. Regarding the provisions for existing units at power 
plants, the legal challenges are pending before the U.S. Court of 
Appeals for the D.C. Circuit. Regarding the provisions applicable to 
certain new units at power plants, EPA granted a petition for 
reconsideration; the court proceedings are on hold, and the provisions 
will not take effect before EPA completes that process, expected in 
March 2013. Similar to the regulations limiting mercury emissions from 
municipal waste combustors and medical waste incinerators, MATS are 
technology-based standards, as directed by CAA, and thus are not 
designed or established with the specific purpose of addressing the 
effect of mercury emissions on waterbodies. EPA projects that MATS 
will reduce future mercury emissions from power plants to about 9 tons 
by 2016, a 70 percent reduction from 2008.[Footnote 54] According to 
an EPA official, the reduced emissions of mercury have lessened the 
amount of mercury deposition into water because most mercury in water 
comes from atmospheric deposition, but estimates of the amount of 
mercury that has been prevented from depositing in water are difficult 
to determine. Even with these reductions in mercury, atmospheric 
deposition continues to affect water quality and harm aquatic 
ecosystems. 

EPA Faces Challenges in Addressing Atmospheric Deposition with Clean 
Air Act: 

EPA faces challenges in using air regulations to further address the 
effects of atmospheric deposition from NOx, SO2, and mercury. 
According to the agency, its recent efforts to establish secondary 
NAAQS to address acid rain and nutrient over-enrichment were not 
successful because of insufficient scientific data. Air regulations 
have limited and, are expected by EPA to further limit, domestic 
mercury emissions, but mercury emissions originating outside of the 
United States are also contaminating U.S. waters. 

NOx and SO2 Emissions: 

As part of its most recent 5-year review of secondary NAAQS, EPA 
concluded in the 2011 notice for its proposed rule on secondary NAAQS 
that current levels of NOx and SO2 are sufficient to cause 
acidification of aquatic ecosystems through acid rain and contribute 
to nutrient over-enrichment in estuaries, among other effects, and 
that existing secondary NAAQS for NOx and SO2 do not provide adequate 
protection from these effects.[Footnote 55] Ultimately, however, the 
agency did not establish any additional secondary NAAQS to protect 
against these effects for ecosystems, citing challenges in available 
sufficient scientific data to set standards.[Footnote 56] 

To address the effects of acid rain, EPA developed an innovative 
approach to establish a secondary NAAQS but faced challenges including 
insufficient scientific data to determine the degree of protectiveness 
that would likely be provided by such standards. Specifically, the CAA 
requires that NAAQS be both national in scope and provide the 
requisite degree of protection--that is, that the standards are 
neither more nor less stringent than necessary. Because many areas in 
the United States are naturally resistant to the effects of acid rain 
due to underlying geological conditions, EPA's approach was to 
establish a secondary NAAQS that would apply nationwide but would 
account for differences in the sensitivity to acid rain of ecosystems 
across the country.[Footnote 57] As part of this approach, EPA 
identified 84 regions, referred to as ecoregions, based on ecological 
factors relevant to acid rain in aquatic ecosystems. The 84 ecoregions 
cover the contiguous United States. However, in the notice for its 
April 2012 final rule on secondary NAAQS, EPA concluded that it could 
not determine with an acceptable degree of scientific certainty that 
the secondary NAAQS under this approach would provide the requisite 
degree of protection for each ecoregion because of uncertainties 
related to ecological and atmospheric modeling and limitations in 
available field data. An important piece of field data needed to 
determine the requisite degree of protection is "critical load"--the 
amount of acid rain that an area can experience before ecological 
damage occurs. The number of critical load estimates for the 84 
ecoregions ranged from about 700 estimates for one ecoregion to less 
than five estimates for several ecoregions. 

To address the uncertainty related to modeling and field data, EPA has 
begun a 5-year field pilot program to enhance its understanding of the 
degree of protectiveness that would likely be afforded by a secondary 
NAAQS using its new approach. EPA described the pilot program in its 
April 2012 final rule notice, stating that the program would serve to 
inform future reviews of NAAQS to address acid rain. However, 
according to an EPA official, the pilot program is a "proof of 
concept" project and not designed specifically to obtain sufficient 
scientific data to meet applicable legal standards for establishing 
secondary NAAQS. The official said that three sites have been chosen 
from which to gather additional data, but that resource constraints 
prevent EPA from moving forward with two of those areas. It is unclear 
if the agency will be able to address the scientific uncertainties 
related to ecological and atmospheric modeling and limitations in 
available field data in time for the next 5-year interval for 
reviewing NAAQS and establishing a secondary NAAQS under its 
innovative approach. According to agency officials, EPA has not 
identified alternative strategies to address the acidification of 
aquatic ecosystems if it cannot resolve the scientific uncertainties. 

EPA also faces challenges in addressing the contribution of NOx to 
nutrient over-enrichment under the CAA. EPA announced in its 2011 
notice for its proposed rule on secondary NAAQS that it would not 
consider establishing an additional secondary NAAQS to address other 
effects from atmospheric deposition associated with NOx and SO2, 
including the contributions of NOx to nutrient over-enrichment in 
aquatic ecosystems, because of the limited quantity and quality of 
available scientific data relative to that available for acid rain. 
According to an EPA official, establishing a secondary NAAQS for NOx 
to protect against nutrient over-enrichment in a waterbody would be 
difficult because there can be many sources of nitrogen unrelated to 
atmospheric deposition, and these sources can be the dominant 
contributor of nitrogen to the waterbody. 

Furthermore, in a 2011 report, the EPA Science Advisory Board made 
recommendations regarding the adverse effects of nitrogen from all 
sources on water quality and aquatic ecosystems, as well as the 
effects of nitrogen on biodiversity, forests, and human health, among 
other effects.[Footnote 58] In response, EPA announced in 2012 that it 
will develop a "Nitrogen Roadmap" that identifies how it will foster 
research, program implementation, and policy integration for nitrogen 
and other pollutants across the agency. According to EPA officials, 
the roadmap is a scoping document that will precede more concrete 
steps to develop an integrated nitrogen research strategy to address 
acidification and nutrient over-enrichment. The goals of the strategy 
are to ensure coordination among research areas within EPA, and among 
EPA scientists and policy analysts. An EPA official also noted that 
developing partnerships with other federal agencies, state and local 
governments, and nongovernmental organizations, will be an important 
step that will help foster nonregulatory solutions to address nutrient 
over-enrichment. According to the official, the roadmap is scheduled 
to be released in 2013, but the effort to address nutrient over-
enrichment will likely be long-term. This effort may lead to more 
suitable approaches to address nutrient over-enrichment than 
establishing secondary NAAQS by themselves. 

Mercury Emissions: 

EPA also faces challenges in addressing mercury contamination because 
most mercury atmospheric deposition within the continental United 
States comes from sources that are outside of the scope of EPA's 
authority. According to a 2010 National Research Council report, 
studies suggest that 10 percent to 80 percent of the mercury deposited 
in the contiguous United States is from human-caused domestic 
emissions--depending on location--with an average of about 30 percent 
for the country as a whole.[Footnote 59] According to the report, the 
remainder comes from natural sources, human-caused emissions from 
other countries, and the global pool of mercury. The United States is 
responsible for a relative small percentage of the global mercury 
emissions that contribute to the global pool of mercury. According to 
the 2008 UNEP report, total global emissions of mercury from human 
activity in 2005 ranged from 1,300 tons to 3,200 tons.[Footnote 60] 
According to data EPA used for the MATS rule, domestic mercury 
emissions from human activity totaled about 105 tons in 2005. Thus, 
even without reductions in mercury emissions beyond the 2005 level, 
over 90 percent of human-caused mercury emissions originate outside of 
the United States. 

EPA is involved in international negotiations to limit global mercury 
releases into the environment. In February 2009, the Governing Council 
of the UNEP agreed on the need to develop a global, legally binding 
agreement on mercury to reduce risks to human health and the 
environment, including reducing atmospheric emissions of mercury. The 
council outlined a negotiating process to reach an agreement. The 
first negotiation meeting was held in June 2010, and the fifth and 
final meeting is scheduled to be held in 2013. The third negotiating 
session (held in October and November 2011) produced a draft text of 
the agreement. The draft text includes language for consideration that 
would call on parties to the agreement that have significant aggregate 
mercury emissions from listed source categories to adopt national or 
numeric emission reduction goals that are, at a minimum, consistent 
with best available practices for reducing and, where feasible, 
eliminating atmospheric mercury emissions from sources such as coal-
burning power plants. Language for consideration in the draft text 
would also call on parties to (1) develop and maintain an inventory of 
these sources and reliable emissions estimates and (2) develop and 
implement an action plan that includes strategies and timetables for 
achieving emission reduction goals. 

Conclusions: 

EPA has long recognized atmospheric deposition as a problem and has 
sought to mitigate its effects; however, EPA's efforts are being 
hindered by limitations in the regulatory tools available to it. The 
CAA provides EPA with regulatory tools to reduce airborne emissions--
and this has reduced atmospheric deposition. Even with these 
reductions, atmospheric deposition continues to affect water quality 
and harm aquatic ecosystems. One CAA tool for airborne emissions is 
also currently available to EPA to directly address the effects of 
atmospheric deposition--secondary NAAQS--but EPA has not been 
successful in using secondary NAAQS to protect water quality and 
aquatic ecosystems from acid rain or nutrient over-enrichment. 

We acknowledge the innovative approach EPA developed for establishing 
secondary NAAQS to address acid rain. However, as of April 2012, EPA 
stated it did not have sufficient scientific data to determine with an 
acceptable degree of scientific certainty if its new approach would 
meet applicable legal standards to establish the secondary NAAQS. 
Further, the 5-year pilot program it has initiated to inform future 
reviews of NAAQS to address acid rain is not designed specifically to 
obtain these data. However, EPA has not identified alternative 
strategies to address the acidification of aquatic ecosystem if it 
cannot resolve the scientific uncertainties that prevented it from 
issuing the secondary NAAQS in April 2012. Furthermore, secondary 
NAAQS may not be well suited to address acid rain because EPA must 
satisfy the legal requirement that secondary NAAQS provide the 
requisite degree of protection--that is, that the standards are 
neither more nor less stringent than necessary--for all parts of the 
nation, and some areas of the nation are sensitive to the effects of 
acid rain, while others are naturally resistant to them. 

Additionally, EPA does not have the scientific data it needs to 
establish a secondary NAAQS to address the contribution of NOx to 
nutrient over-enrichment in aquatic ecosystems. Because there can be 
many sources of nitrogen that contribute to nutrient over-enrichment 
in a waterbody, including sources of nitrogen unrelated to atmospheric 
deposition, secondary NAAQS by themselves may not be well suited to 
address nutrient over-enrichment. EPA announced in 2012 that it will 
develop a "Nitrogen Roadmap" that identifies how it will foster 
research, program implementation, and policy integration for nitrogen 
and other pollutants across the agency. This roadmap is intended to be 
a scoping document that is to precede more concrete steps to develop 
an integrated nitrogen research strategy to address nutrient over-
enrichment. We encourage EPA to continue its effort to develop an 
integrated nitrogen research strategy, which may lead to more suitable 
approaches to address nutrient over-enrichment. 

Regarding mercury, because most emissions originate outside of the 
United States, EPA's regulatory tools are of limited applicability to 
the majority of emissions, but the agency is involved in the UNEP 
negotiations to, among other things, reduce atmospheric emissions of 
mercury on a global scale. This will be an important step toward 
reducing mercury levels in our nation's waters. 

Recommendation for Executive Action: 

To help ensure that EPA can address atmospheric deposition of NOx and 
SO2 impairing the nation's waters, we recommend that the EPA 
Administrator determine whether EPA can obtain in a timely manner the 
data it needs to establish secondary NAAQS adequate to protect against 
the effects of acid rain and, if not, identify alternative strategies 
to do so. 

Agency Comments and Our Evaluation: 

We provided a draft of this report to EPA for comment. In written 
comments, which are reproduced in appendix II, EPA agreed with the 
recommendation in our report. EPA stated that under the CAA, it is 
required every 5 years to complete a thorough review of the latest 
scientific knowledge, and as necessary, make revisions to the NAAQS or 
promulgate new NAAQS. For NOx and SOx, it stated that it will continue 
to review the science, as it develops and gather new data as resources 
permit. Additionally, EPA provided technical comments to provide 
additional clarity and completeness to the report. An overview of 
these technical comments is included in the written comments 
reproduced in appendix II. We incorporated the technical comments as 
appropriate. 

As agreed with your offices, unless you publicly announce the contents 
of this report earlier, we plan no further distribution until 30 days 
from the report date. At that time, we will send copies to the 
Administrator of the Environmental Protection Agency, the appropriate 
congressional committees, and other interested parties. In addition, 
the report will be available at no charge on the GAO website at 
[hyperlink, http://www.gao.gov]. 

If you or your staff members have any questions about this report, 
please contact me at (202) 512-3841 or trimbled@gao.gov. Contact 
points for our Offices of Congressional Relations and Public Affairs 
may be found on the last page of this report. GAO staff who made key 
contributions to this report are listed in appendix III. 

Signed by: 

David C. Trimble: 
Director: 
Natural Resources and Environment: 

[End of section] 

Appendix I: Scope and Methodology: 

Our objectives were to (1) examine the extent to which the atmospheric 
deposition of nitrogen oxides (NOx), sulfur dioxide (SO2), and mercury 
contributes to the impairment of the nation's waters and identify the 
key sources of these pollutants;(2) identify the regulatory tools that 
the Environmental Protection Administration (EPA) uses under the Clean 
Water Act (CWA) to address the effects of atmospheric deposition, and 
the challenges, if any, that it faces in doing so; and (3) identify 
the regulatory tools that EPA uses under the Clean Air Act (CAA) to 
address the effects of atmospheric deposition, and the challenges, if 
any, that it faces in doing so. 

To examine the extent to which atmospheric deposition of NOx, SO2 and 
mercury contributes to the impairment of the nation's waters, we 
analyzed EPA's Assessment, TMDL (Total Maximum Daily Load) Tracking, 
and Implementation System (ATTAINS) database. To assess the 
reliability of its data, we interviewed knowledgeable agency officials 
and conducted electronic testing of the data. We determined that, 
although the data have limitations, as specified below, they were 
sufficiently reliable to present reported atmospheric deposition 
impairment on assessed waters for selected categories of water bodies. 
The ATTAINS data we analyzed were the most recent available data from 
states and not the most current reporting-cycle year, 2012, because 
these 2012 data are undergoing review. Because some states are more 
current than others in reporting, the state data we analyzed for a 
given state could be from any reporting cycle year from 2002 to 2010, 
depending on the state.[Footnote 61] Our analysis also included data 
from the 50 states, the District of Columbia, Puerto Rico, Guam, the 
U.S. Virgin Islands, American Samoa, and the Commonwealth of Northern 
Mariana Islands. ATTAINS also included data from the Delaware River 
Basin Commission, but we excluded these data in our analysis because 
EPA does not include it in its reports or queries related to the 
quality of the nation's waters. In analyzing the data, we first 
identified how many miles, acres, or square miles of waters in each 
waterbody category in ATTAINS were assessed and classified as 
impaired. The ATTAINS database contained 10 different categories of 
waters. However, we did not report on coastal waters, ocean and near 
coastal waters, wetlands, Great Lakes shoreline, inland lake 
shoreline, and Great Lakes connecting channels because states have 
only assessed a small percentage of these waters, and there was 
insufficient data or, in the case of Great Lakes shoreline, because we 
were reporting on Great Lakes open water. We reported on impaired 
waters in the following four categories: 

* Streams, creeks and rivers; 

* Lake, reservoirs, and ponds; 

* Bays and estuaries; and: 

* Great Lakes open water. 

We then identified what pollution sources the states associated with 
these impaired miles, acres, or square miles of waters and considered 
only those waters whose impairment was attributed by states to 
atmospheric deposition. Our analysis of ATTAINS data represents only 
those waters that states have assessed and associated with atmospheric 
deposition as an impairment source. It is possible that additional 
impairment due to atmospheric deposition has occurred, but that states 
either did not report atmospheric deposition as an impairment source 
in ATTAINS or reported it as unknown or unspecified. In addition, our 
findings represent waters impaired by the atmospheric deposition of 
all reported air pollutants and are not specific to the atmospheric 
deposition of NOx, SO2, and mercury. We examined data on the reported 
causes and pollutants for waters with impairment that the states 
associated with atmospheric deposition and found that we could not 
reliably report on the causes or pollutants, such as NOx and SO2, 
because of data inconsistencies. 

For additional information on the extent to which atmospheric 
deposition of these three pollutants contribute to the impairment of 
the nation's waters, we reviewed studies from EPA and other federal 
agencies, the United Nations, and others. We assessed the reliability 
and methodology of these studies, including limitations and 
assumptions, and determined that they were sufficiently reliable for 
the purposes of examining the extent to which atmospheric deposition 
is contributing to the impairment of the nation's waters. In addition, 
we examined state reported-data in EPA's 2010 National Listing of Fish 
Advisories (NLFA) and assessed the reliability of the data. We 
determined that despite some NLFA limitations--such as states' 
different monitoring methods for sampling contaminants in fish tissues 
and limited waters monitoring - the data were sufficiently reliable 
for the purposes of reporting the number of states with mercury-
related fish advisories and the extent of mercury-related fish 
advisories for certain types of waters. We also interviewed officials 
and scientists from EPA's offices, including the Office of Air and 
Radiation and the Office of Water; the Department of Commerce's 
National Oceans and Atmospheric Administration; and academic, state, 
and private sector scientists. 

To identify the key sources of NOx, SO2, and mercury, we examined 
emission estimates from EPA's National Emissions Inventory (version 
2)--a national database of air emissions information with input from 
numerous state and local air agencies, tribes, and industry. For 
source estimates on NOx, SO2, and mercury emissions, we used 2008 
National Emissions Inventory estimates because EPA is currently 
collaborating with state, local, and tribal agencies in preparing the 
2011 inventory. We also talked with EPA officials and reviewed 
National Emissions Inventory-related documentation on quality 
assurance to assess the reliability of National Emissions Inventory 
estimates. We determined that despite some limitations--such as 
methodological and modeling differences used in estimating emission 
across various years and versions of the inventory--the 2008 National 
Emissions Inventory data were sufficiently reliable for the purposes 
of identifying in the national aggregate the top emission sources of 
NOx, SO2, and mercury, and for reporting emission estimates from the 
top emitting sources. In addition, we reviewed EPA documents related 
to mercury emissions and sources, as well as a 2008 United Nations 
Environment Programme (UNEP) assessment on global mercury sources, 
emissions, and transport. 

To examine the regulatory tools that EPA uses under the CWA to address 
the effects of atmospheric deposition, and the challenges, if any, it 
faces in doing so, we reviewed the law, relevant EPA regulations and 
guidance, and other pertinent documents, and interviewed relevant EPA 
and state officials. As a result, we identified listing impaired 
waters and the TMDL as the primary regulatory tools EPA uses to 
address the effects of atmospheric deposition. We also chose to 
examine a nonprobability sample of three TMDLs in greater detail to 
illustrate the challenges EPA faces in using CWA to address 
atmospheric deposition. We selected each TMDL because it addressed a 
different effect caused by the atmospheric deposition of the three 
pollutants on which we are focusing. Specifically, we chose the 
Northeast Regional TMDL because it is addressing mercury, including 
atmospheric deposition of mercury. We chose a New York State TMDL 
because it addresses acid rain caused by NOx and S02. Finally, we 
chose the Chesapeake Bay TMDL because it address nutrient over-
enrichment, including from NOx. To describe these TMDLs and the 
challenges they present, we examined key documents and interviewed 
EPA, state, and regional officials responsible for implementing the 
TMDLs. 

To examine the regulatory tools that EPA uses under CAA to address the 
effects of atmospheric deposition and the challenges that it faces in 
doing so, we reviewed the law, relevant EPA regulations and guidance, 
and other pertinent documents and studies, and interviewed relevant 
EPA officials. We used data from the 2008 National Emissions Inventory 
and 2011 EPA study on the benefits and costs of the CAA amendments of 
1990 to identify the reductions in NOx, SO2, and mercury, key 
regulations that contributed to the reductions, and the associated 
beneficial effects on water quality and aquatic ecosystems. We 
assessed the reliability and methodology of these sources of 
information, including limitations and assumptions, and determined 
that they were sufficiently reliable for the purposes of attributing 
estimates of some reductions in NOx, SO2, and mercury to CAA. We 
reviewed the CAA and pertinent regulations to determine the design of 
the regulations. To understand the challenges that EPA faced in trying 
to develop a secondary National Ambient Air Quality Standard (NAAQS) 
to reduce emissions of NOx and SO2 in order to protect ecosystems 
sensitive to the effects of acid rain, we reviewed documentation 
developed as part of the rulemaking process and interviewed 
knowledgeable officials responsible for the rulemaking. To understand 
EPA's integrated nitrogen research strategy, we reviewed pertinent 
documents and interviewed relevant EPA officials. For challenges 
related to mercury, we reviewed a UNEP study on global mercury 
emissions, sources, and transport, and other UNEP mercury-related 
documents on global efforts to reduce mercury emissions and 
interviewed knowledgeable EPA officials. 

We conducted this performance audit from September 2011 to January 
2013 in accordance with generally accepted auditing standards. Those 
standards require that we plan and perform the audit to obtain 
sufficient and appropriate evidence to provide a reasonable basis for 
our findings and conclusions based on our audit objectives. We believe 
that the evidence obtained provides a reasonable basis for our 
findings and conclusions based on our audit objectives. 

[End of section] 

Appendix II: Comments from the Environmental Protection Agency: 

United States Environmental Protection Agency: 
Office of Air and Radiation: 
Washington, DC 20460: 

January 7, 2013: 

Mr. David C. Trimble: 
Director: 
Natural Resources and Environment: 
U.S. General Accountability Office: 
Washington, D.C. 20548: 

Dear Mr. Trimble: 

Thank you for the opportunity to review and comment on the General 
Accountability Office's (GAO) draft report "Water Quality: EPA Faces 
Challenges in Addressing Damage Caused by Airborne Pollutants." As 
stated in the draft report, the objectives in this review were to: (1) 
examine the extent to which atmospheric deposition of NOx, SOx and 
mercury contribute to the impairment of the nation's waters and 
identify the key sources of these pollutants; (2) assess the 
regulatory tools that the EPA uses under the Clean Water Act (CWA) to 
address the effects of atmospheric deposition, and the challenges, if 
any, that it faces in doing so; and (3) assess the regulatory tools 
that the EPA uses under the Clean Air Act (CAA) to address the effects 
of atmospheric deposition, and the challenges, if any, that it faces 
in doing so. 

I am responding on behalf of the EPA offices that participated in this 
review — the Office of Air and Radiation and the Office of Water. 
Below, I am providing our comments on the report's recommendation, as 
well as an overview of a few important clarifications on the technical 
findings. Under separate cover, EPA will be submitting suggested 
modifications to the technical findings that could provide more 
clarity and completeness to the report. 

Recommendation: To help ensure that the EPA can address atmospheric 
deposition of NOx and SO2 impairing the nation's waters, we recommend 
that the EPA Administrator determine whether the EPA can obtain in a 
timely manner the data it needs to establish secondary NAAQS adequate 
to protect against the effects of acid rain and, if not, identify 
alternative strategies to protect these areas. 

Response: The EPA accepts the GAO's recommendation. We are committed 
to protecting human health and the environment from the detrimental 
effects of atmospheric deposition, including the protection of our 
nation's waterways. As the report notes, efforts were made by the EPA 
to develop secondary national ambient air quality standards (NAAQS) 
for NOx and SOx to protect waters from acidic deposition. These 
efforts made major progress in the scientific understanding of the 
effects of atmospheric deposition on waterways, but the EPA concluded 
that the uncertainties in translating the available information into 
an appropriate NAAQS were too great to allow the Administrator to set 
a standard for NOx and SOx deposition. Under the CAA, the EPA is 
required every five years to complete a thorough review of the latest 
scientific knowledge, and as necessary, make revisions in the NAAQS or 
promulgate new NAAQS. For NOx and SOx, the EPA will continue to review 
the science, as it develops, and gather new data, as resources permit. 

The EPA appreciates the discussion regarding the limitations of the 
CWA in addressing pollutants from air deposition. However, despite 
these limitations, Total Maximum Daily Loads (TMDLs) can play an 
important role in identifying the contributions from air sources to 
water. In particular, TMDLs can parse the contributions from various 
air sources to waterbodies, including separating the contributions 
from local and global sources. This, in turn, can help states identify 
whether there are air sources amenable to controls at the state level. 
The EPA has provided some suggestions in the technical comments on 
where the report may be able to include this important point. In 
addition, the EPA would like to note some of the positive aspects of 
the CWA Section 319(g) management conference on mercury held in 2010 
between the Northeast states and eleven upwind states. While no formal 
agreement or consensus was reached among the participating states, the 
conference was the first to bring together air and water managers from 
both upwind and downwind states to share lessons learned from their 
respective mercury programs. 

The EPA appreciates the GAO effort to summarize some of the CAA rules 
relevant to water quality. However, a few more rules are important to 
mention. Since 2005, there have been key mobile source standards, such 
as the 2010 Emissions Control Area and the 2011 Heavy Duty Vehicle 
standards, which provide large reductions in both NOx and SOx from 
ocean-going ships and heavy-duty trucks, respectively. In addition, 
the planned Tier 3 vehicle standards will provide significant NOx 
reductions in the future. The 2012 adjustments to the air toxics 
standards for industrial boilers and certain incinerators will reduce 
SO2 emissions by 572,000 tons per year. The Mercury Air Toxics 
Standards (MATS) are noted in the report for their impact on mercury 
emissions, but the report fails to mention the large expected 
reductions in SO2 emissions (1.5 million tons). The report also notes 
that the EPA's MATS rule substantially reduces mercury emissions from 
power plants (the largest source in the U.S.), but does not adequately 
recognize that those emissions are a significant contributor to 
mercury deposition in many freshwater lakes and streams in the U.S. 

In addition to the Acid Rain Program, NOx emissions reductions under 
the NOx Budget Program and NOx and SO2 emissions reductions under the 
Clean Air Interstate Rule (CAIR) have led to improved air quality, 
decreased atmospheric deposition and some improvements to aquatic and 
terrestrial ecosystems. While the EPA is appealing the decision by the 
U.S. Court of Appeals for the D.C. Circuit in August 2012 that struck 
down the Cross-State Air Pollution Rule (CSAPR), CAIR remains in 
effect and will remain in effect until replaced by another rule 
requiring SO2 and NOx emission reductions. Annual 2012 SO2 emissions 
levels from power plants within the CSAPR region are on track to be 
23% below what CSAPR would have required in 2012. Similarly, annual 
NOx and ozone season NOx emissions in the CSAPR region are projected 
to be 12% and 5% below what CSAPR required for 2012. 

In closing, the EPA remains committed to protecting public health and 
the environment, including our nation's waterways. As the GAO found, 
utilizing the tools of the CWA and the CAA can be complicated, but the 
current rules achieve much of the reductions that are needed. Thank 
you again for the opportunity to review and respond to the draft GAO 
report, If you have any questions, please contact Randy Waite at (919) 
541-5447. 

Sincerely, 

Signed by: 

Gina McCarthy: 
Assistant Administrator: 

[End of section] 

Appendix III: GAO Contact and Staff Acknowledgments: 

GAO Contact: 

David C. Trimble, (202) 512-3841 or trimbled@gao.gov: 

Staff Acknowledgments: 

In addition to the individual named above, Barbara Patterson, 
Assistant Director; Elizabeth Beardsley; Greg Carroll; Bernice Hill 
Dawson; Charles Egan; Michelle Fejfar; Michael Hix; Catherine Hurley; 
Alison O'Neill; Madhav Panwar; Carla Rojas Paz; and Carol Herrnstadt 
Shulman made key contributions to this report. 

[End of section] 

Footnotes: 

[1] The Federal Water Pollution Control Act Amendments of 1972, Pub. 
L. No. 92-500, § 2, 86 Stat. 816 (amending the Act of June 30, 1948, 
ch. 758, 62 Stat. 1155) (codified as further amended at 33 U.S.C. ch. 
26 §§ 1251-1387 (2012) and commonly referred to as the Clean Water 
Act); Clean Air Act Amendments of 1970, Pub. L. No. 91-604, 84 Stat. 
1676 (1970) (codified as amended at 42 U.S.C. §§ 7401-7671q (2011) 
(commonly referred to as the Clean Air Act). 

[2] The water quality standard for a water body consists of three 
components: criteria, designated uses, and antidegradation provisions. 

[3] TMDL is a calculation of the maximum amount of a pollutant that a 
body of water can receive and still meet water quality standards. 
TMDLs allocate or apportion the "budget" among the various sources, 
including permitted dischargers as well as nonpoint sources of 
pollution, or polluted runoff. 

[4] Because we selected a nonprobability sample, our analysis is not 
generalizeable to all TMDLs but can illustrate the challenges faced by 
EPA in using this tool. 

[5] U.S. Environmental Protection Agency, Office of Air and Radiation, 
The Benefits and Costs of the Clean Air Act from 1990 to 2020, Final 
Report (Washington, D.C.: March 2011). 

[6] This report focuses on the atmospheric deposition of pollutants 
and their effects on waterbodies, but atmospheric deposition of 
pollutants can also harm land-based ecosystems such as forests. 

[7] Acid rain is also known as acidic deposition. 

[8] SO2 is a component of sulfur oxide, commonly referred to as "SOx". 

[9] Pollution controls include technology-based and water quality-
based effluent limitations contained in discharge permits, as well as 
any required best management practices. 

[10] According to EPA, the agency has authorized 46 states to issue 
and enforce NPDES permit requirements under their state laws. For 
those states that are not so authorized (Idaho, Massachusetts, New 
Hampshire, New Mexico, the District of Columbia, American Samoa, the 
Commonwealth of Northern Mariana Islands, Guam, and Puerto Rico), EPA 
retains the authority to issue permits and enforce permit 
requirements. EPA may also issue a compliance order or bring a civil 
suit in U.S. district court against persons who violate the terms of a 
permit. Those responsible for point sources are required to maintain 
records and to carry out effluent monitoring activities. Permits are 
effective for up to 5 years by which time permittees must apply for 
permit renewal. 

[11] EPA has set NAAQS for six pollutants, termed "criteria" 
pollutants: carbon monoxide, lead, NOx, ozone, particulate matter, and 
sulfur dioxides. EPA's NAAQS for SO2 is designed to protect against 
exposure to all the components of sulfur oxide. Other components of 
sulfur oxide, such as sulfate (SO4), are found in the atmosphere but 
at concentrations much lower than SO2. 

[12] As described earlier, ATTAINS is an EPA database containing state 
reported information on the extent to which waterbodies meet water 
quality standards, among other things. 

[13] This reference is to Great Lakes open water. ATTAINS collects 
separate data on Great Lakes shoreline, inland lake shoreline, and 
connecting channels. According to ATTAINS, there are about 60,500 
square miles of Great Lakes open water. 

[14] States are required to identify those waters that are impaired--
that is, waters for which pollution controls are insufficient to meet 
water quality standards--and report this list to EPA every 2 years. 
According to EPA, states have not assessed all their waterbodies 
because of limited resources, but many states target their limited 
resources to waterbodies that they suspect are impaired. 

[15] National Science and Technology Council, National Acid 
Precipitation Assessment Program Report to Congress 2011: An 
Integrated Assessment (Washington, D.C.: 2011). The CAA Amendments of 
1990 authorized NAPAP to, among other things, periodically assess the 
effectiveness of the Acid Rain Program and report its findings to 
Congress. NAPAP operates under the auspices of the National Science 
and Technology Council (NSTC) Committee on Environment, Natural 
Resources, and Sustainability (CENRS). The NAPAP member agencies are 
the EPA, the U.S. Department of Energy, the U.S. Department of 
Interior/U.S. Geological Survey, the U.S. Department of 
Interior/National Park Service, the National Aeronautics and Space 
Administration, and NOAA. 

[16] See EPA, Secondary National Ambient Air Quality Standards for 
Oxides of Nitrogen and Sulfur; Proposed Rule, 76 Fed. Reg. 46,084, 
46,134 (Aug. 1, 2011). 

[17] Bricker, S., B. Longstaff, W. Dennison, A. Jones, K. Boicourt, C. 
Wicks, and J. Woerner. Effects of Nutrient Enrichment In the Nation's 
Estuaries: A Decade of Change. NOAA Coastal Ocean Program Decision 
Analysis Series No. 26. National Centers for Coastal Ocean Science 
(Silver Spring, MD: 2007). The study initially chose 141 estuaries to 
assess but did not have sufficient data to assess 42 of them. 

[18] National Science and Technology Council, National Acid 
Precipitation Assessment Program Report to Congress 2011: An 
Integrated Assessment (Washington, D.C.: 2011). 

[19] The report summarized the following studies: McIsaac, G.F., M.B. 
David, G.Z. Gertner, and D.A. Goolsby, "Eutrophication: Nitrate flux 
in the Mississippi River," Nature 414 (2001):166-167; Boyer, E.W., 
C.L. Goodale, N.A. Jaworski, and R.W. Howarth, "Anthropogenic nitrogen 
sources and relationships to riverine nitrogen export in the 
northeastern U.S.A." Biogeochemistry 57/58(2002):137-169.; Poor, N., 
C. Pollman, P. Tate, M. Begum, M. Evans, and S. Campbell, "Nature and 
magnitude of total inorganic nitrogen and other inorganic species to 
the Tampa Bay watershed, FL, USA," Water, Air, & Soil Pollution 
170(2006):267-83; Whitall, D., S. Bricker, J. Ferreira, A.M. Nobre, T. 
Simas, and M. Silva, "Assessment of eutrophication in estuaries: 
Pressure-state-response and nitrogen source apportionment," 
Environmental Management 40(2007):678-690; Alexander, R.B., R.A. 
Smith, G.E. Schwarz, E.W. Boyer, J.V. Nolan, and J.W. Brakebill, 
"Differences in phosphorus and nitrogen delivery to the Gulf of Mexico 
from the Mississippi River," Environmental Science & Technology 
42(2008):822-830; and Howarth, R.W., "Estimating atmospheric nitrogen 
deposition in the northeastern United States: Relevance to 
Narragansett Bay," ch. 3, in A. Desbonnet and B. A. Costa-Pierce, eds. 
Science for Ecosystem based Management: Narragansett Bay in the 21st 
Century, Springer Series on Environmental Management (New York: 
Springer) 47-65. 

[20] NADP is a cooperative effort among groups--including federal, 
state, tribal, and local governmental agencies, educational 
institutions, private companies, and nongovernmental agencies--that 
monitor atmospheric deposition of mercury, among other pollutants. The 
federal agencies involved in NADP include the Agricultural Research 
Service and the Forest Service within the Department of Agriculture; 
the Bureau of Land Management, the Fish and Wildlife Service, the 
Geological Survey, and the National Park Service within the Department 
of the Interior; the National Aeronautics and Space Administration; 
the National Science Foundation; NOAA; EPA; and the Tennessee Valley 
Authority. 

[21] Cohen, M., R. Artz, and R. Draxler. Report to Congress: Mercury 
Contamination in the Great Lakes. NOAA Air Resources Laboratory 
(Silver Spring, MD: 2007). 

[22] The 2008 National Emissions Inventory (version 2) provides the 
most complete collection of emissions data. EPA is currently 
collaborating with state, local, and tribal agencies in preparing the 
2011 inventory. 

[23] Industrial boilers are a type of electrical generating unit that 
burns fossil fuels. 

[24] Power plants contributed approximately 7.8 million tons of SO2, 
or about 75 percent of the total estimated 10.4 million tons of SO2 
emissions. 

[25] Electric arc furnaces are heating devices that produce highly 
luminous and intensely hot electrical currents that jump between 
electrical conductors in the form of an arc and are used for melting 
metals such as steel. Portland cement nonhazardous waste is waste 
produced from cement kilns that produce the most common cement around 
the world known as Portland cement. Industrial, commercial, and 
institutional boilers and process heaters are types of electrical 
generating units that burn fossil fuels. 

[26] The Environmental Protection Agency, Effects of Air Pollutants on 
Ecological Resources: Literature Review and Case Studies, Draft Report 
(Washington, D.C.: February 2010). In its literature review, EPA cited 
two studies: (1) Seigneur, C., K. Vijayaraghavan, K. Lohman, P. 
Karamchandani, and C. Scott, "Global Source Attribution for Mercury 
Deposition in the United States" Environmental, Science and 
Technology, 38 (2004), 555-569, and (2) Selin, N.E., D.J. Jacob, R.J. 
Park, R.M. Yantosca, S. Strode, L. Jaegle, and D. Jaffe, "Chemical 
cycling of atmospheric mercury," Journal of Geophysical Research 
(January 2007). Reemissions of mercury also contribute to mercury 
deposition in the United States, but the two studies that EPA reviewed 
either did not cite reemission contributions or included reemission 
contributions in the emission contributions from natural or human-
caused sources. 

[27] The Environmental Protection Agency, Model-Based Analysis and 
Tracking of Airborne Mercury Emissions to Assist in Watershed Planning 
(Washington, D.C.: August 2008). 

[28] The United Nations Environment Programme, The Global Atmospheric 
Mercury Assessment: Sources, Emissions and Transport (Geneva, 
Switzerland: December 2008). 

[29] EPA requires reasonable assurance to allow EPA to determine that 
the TMDL has been established at a level necessary to implement water 
quality standards. Under EPA guidance, reasonable assurance is only 
needed for TMDLs that include allocations for both point and nonpoint 
sources. A TMDL developed for waters impaired solely by point sources 
does not have to demonstrate reasonable assurance. Also, the NPDES 
permit, and the requirements associated with it, constitutes the 
reasonable assurance that a TMDL's allocation for point sources will 
result in the attainment of water quality standards. 

[30] For more information, see GAO, Nonpoint Source Water Pollution: 
Greater Oversight and Additional Data Needed for Key EPA Water 
Program, [hyperlink, http://www.gao.gov/products/GAO-12-335] 
(Washington, D.C.: May 31, 2012). 

[31] See 33 U.S.C. § 1329(g) (2012). When a state petitions EPA for a 
conference, the provision does not give EPA specific responsibilities 
or authority beyond holding the conference. 

[32] In addition to nitrogen, the TMDL addresses other pollutants, but 
the atmospheric deposition of these other pollutants is not a 
significant source to the bay. 

[33] The Northeast Region, as referred to here, includes the New 
England states (Connecticut, Maine, Massachusetts, New Hampshire, 
Rhode Island, and Vermont) and New York State. According to TMDL 
documents, about 98 percent of the mercury comes from atmospheric 
deposition. 

[34] The NEIWPCC is a not-for-profit interstate agency, established by 
an act of Congress, which serves its member states individually and 
collectively by providing coordination, public education, research, 
training, and leadership in water management and protection in the New 
England region and New York State. 

[35] The Northeast States for Coordinated Air Use Management, Sources 
of Mercury Deposition in the Northeast United States (March 2008). 
This study presents numeric estimates of the amount of mercury 
deposited in the Northeast Region states and the sources of that 
mercury. These estimates were obtained from a report prepared for EPA 
in 2006, based on 2002 data (ICF International, Model-Based Analysis 
and Tracking of Airborne Mercury Emissions to Assist in Watershed 
Planning (San Rafael, CA: Nov. 30, 2006). According to the authors of 
the ICF report, the numeric estimates result from a model that 
includes some uncertainty, and the estimates should be used to show 
the relative amounts of mercury originating from, and depositing in, 
different states or originating from global sources. The most recent 
year for which this data is available is 2002. 

[36] These lands are regulated by the New York State Constitution as 
being "forever wild." 

[37] NOx emissions decreased from 30 percent to 90 percent depending 
on the emission source but there is not reliable data on the overall 
reduction. 

[38] Atmospheric deposition of ammonia accounts for about 11 percent 
of total nitrogen impacting the bay. 

[39] The total limit for the jurisdictions was set at about 186 
million pounds, or about a 25 percent reduction from previous limits. 

[40] According to EPA, 25 percent of NOx emissions that contribute to 
the bay come from outside the seven jurisdictions but within the NOx 
airshed, and 25 percent come from outside of the airshed. 

[41] Actions designed to meet primary NAAQS include, for example, 
emissions reductions under the Clean Air Interstate Rule, as well as 
other state-led programs such as the NOx Budget Program. 

[42] While EPA has not promulgated such regulations explicitly 
designed to address atmospheric deposition, EPA officials pointed out 
that EPA's mobile source standards do in fact address impacts on 
public welfare, which would include impacts of air pollutants on 
waterbodies. These officials noted that mobile source standards are 
"applicable to the emission of any air pollutant…which may reasonably 
be anticipated to endanger public health or welfare." The officials 
stated that they had addressed the mobile source standards' impact on 
public welfare in their rulemakings. 

[43] EPA has also established secondary NAAQS. Secondary NAAQS can be 
specifically designed to address a pollutant's impact on water quality 
and aquatic ecosystems, but the current secondary NAAQS for NOx and 
SO2 are designed to protect land-based ecosystems. 

[44] National Science and Technology Council, National Acid 
Precipitation Assessment Program Report to Congress 2011: An 
Integrated Assessment (Washington, D.C.: 2011). 

[45] See GAO, EPA Regulations and Electricity: Better Monitoring by 
Agencies Could Strengthen Efforts to Address Potential Challenges, 
[hyperlink, http://www.gao.gov/products/GAO-12-635] (Washington, D.C.: 
July 17, 2012). 

[46] As of December 2012, the court has not yet ruled on the petitions 
for rehearing. The Cross-State Air Pollution Rule is not in effect 
pending resolution of the case; rather, a predecessor rule, the Clean 
Air Interstate Rule, remains in effect. Specifically, in 2008 the U.S. 
Court of Appeals for the D.C. Circuit remanded the Clean Air 
Interstate Rule, but did not vacate it, finding that allowing the rule 
to remain in effect until it is replaced by a rule consistent with the 
opinion would at least temporarily preserve environmental values. The 
Cross-State Air Pollution Rule was EPA's response to the court's 
remand of the Clean Air Interstate Rule. 

[47] See Electricity: Significant Changes Are Expected in Coal-Fueled 
Generation, but Coal is Likely to Remain a Key Fuel Source, 
[hyperlink, http://www.gao.gov/products/GAO-13-72] (Washington, D. C.: 
Oct. 29, 2012). 

[48] U.S. Environmental Protection Agency, Office of Air and 
Radiation, The Benefits and Cost of the Clean Air Act from 1990 to 
2020, Final Report (Washington, D.C.: March 2011). Section 812 of the 
1990 Clean Air Act Amendments requires EPA to develop biannual reports 
that estimate the benefits and costs of the Clean Air Act. This report 
is the third in the Section 812 series. The previous two reports 
covered different time frames. 

[49] Municipal waste combustors are incinerators or facilities that 
generate energy from garbage. 

[50] See EPA, Standards of Performance for New Stationary Sources and 
Emission Guidelines for Existing Sources: Municipal Waste Combustors, 
60 Fed. Reg. 65,387 (Dec. 19, 1995) and EPA, Standards of Performance 
for New Stationary Sources and Emission Guidelines for Existing 
Sources: Hospital/Medical/Infectious Waste Incinerators, 62 Fed. Reg. 
48,348 (Sept. 15, 1997). 

[51] Each regulation sets maximum achievable control technology (MACT) 
standards. A MACT standard is a technical standard applying to a 
particular industry that releases hazardous air pollutants, such as 
mercury. MACT standards are designed to reduce emissions of hazardous 
air pollutants to a maximum achievable degree, taking into 
consideration the cost of reductions and other factors. 

[52] CAA required EPA to study the public health hazards from 
electricity generating units' emissions of mercury and other hazardous 
air pollutants and to regulate those emissions under section 112 if it 
finds that such regulation is "appropriate and necessary." EPA made 
such a finding regarding certain electricity generating units in 2000 
but did not issue a regulation under section 112. In 2005, EPA 
reversed this finding and finalized a regulation under section 111 of 
the CAA regulating mercury emissions from certain electricity 
generating units, which a federal court later struck down. Pursuant to 
a settlement agreement to resolve a lawsuit for failing to meet the 
statutory deadline for issuing a section 112 regulation, EPA published 
the final MATS regulations in February 2012. 

[53] See EPA, National Emission Standards for Hazardous Air Pollutants 
From Coal-and Oil-Fired Electric Utility Steam Generating Units and 
Standards of Performance for Fossil-Fuel-Fired Electric Utility, 
Industrial-Commercial-Institutional, and Small Industrial-Commercial-
Institutional Steam Generating Units; Final Rule, 77 Fed. Reg. 9304 
(Feb. 16, 2012). In addition to the new standards for emission of 
mercury and other air toxics, the rulemaking also included amendments 
to new source performance standards for several criteria pollutants. 

[54] EPA also projects that MATS will reduce SO2 emissions from power 
plants. 

[55] See EPA, Secondary National Ambient Air Quality Standards for 
Oxides of Nitrogen and Sulfur; Proposed Rule, 76 Fed. Reg. 46,084 
(Aug. 1, 2011). According to EPA, it based its proposed conclusions on 
the following assessments: U.S. EPA. Integrated Science Assessment for 
Oxides of Nitrogen and Sulfur Ecological Criteria, U.S. Environmental 
Protection Agency, Washington, DC, EPA/600/R-08/082F, 2008 and U.S. 
EPA, Risk and Exposure Assessment for Review of the Secondary National 
Ambient Air Quality Standards for Oxides of Nitrogen and Oxides of 
Sulfur, U.S. Environmental Protection Agency, Washington, D.C., 
EPA/452/R-09-008a, 2009. 

[56] EPA, Secondary National Ambient Air Quality Standards for Oxides 
of Nitrogen and Sulfur; Final Rule, 77 Fed. Reg. 20,218 (April 3, 
2012). 

[57] EPA describes its new approach for developing NAAQS for acid rain 
at length in its 2011 proposed rule. See 76 Fed. Reg. 46,084 (Aug. 1, 
2011). 

[58] The EPA Science Advisory Board is a federal advisory committee 
established by Congress in 1978 with a broad mandate to advise the 
agency on technical matters. The board's recommendations are found in 
its report: Reactive Nitrogen in the United States: An Analysis of 
Inputs, Flows, Consequences, and Management Options - A Report of the 
EPA Science Advisory Board, EPA-SAB-11-013, August 2011. 

[59] National Research Council, Global Sources of Local Pollution: An 
Assessment of Long-Range Transport of Key Air Pollutants to and from 
the United States. (National Academies Press, Washington, D.C.: 2010). 

[60] The United Nations Environment Programme, The Global Atmospheric 
Mercury Assessment: Sources, Emissions and Transport Mercury 
Assessment (Geneva, Switzerland: December 2008). 

[61] One state had data from reporting cycle 2002; one state had data 
from 2004; six states had data from 2006; six other states had data 
from 2008, and the rest of the states had data from reporting cycle 
2010. 

[End of section] 

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