Water Quality: EPA Faces Challenges in Addressing Damage Caused by Airborne Pollutants
Published: 2013-01-24T00:00:00

Fast Facts:

Highlights:
section_title: Why GAO Did This Study
paragraphs: 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.
section_title: What GAO Found
paragraphs: 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.
section_title: What GAO Recommends
paragraphs: 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.

section_title: Letter
paragraphs: 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 (NO), sulfur  dioxide (SO), and mercury. NO and SO 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). 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. 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. 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 NO, SO, 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.
The water quality standard for a water body consists of three components: criteria,  designated uses, and antidegradation provisions. (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 NO, SO, 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. 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, and other information to identify reductions in NO, SO, SO, 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.
subsections: section_title: Background
paragraphs: This section discusses atmospheric deposition, its effect on waterbodies,  and how the CWA and the CAA address it.
subsections: section_title: Atmospheric Deposition Is the Transfer of Air Pollution to the Earth’s Surface
paragraphs: 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.
section_title: Effects of Atmospheric Deposition on Waterbodies
paragraphs: NO and SO cause acid rain, and NO contributes to nutrient over- enrichment of waterbodies. Once in water, mercury becomes  concentrated in fish and can harm individuals who consume  contaminated fish.   cause acid rain, which can threaten certain aquatic  ecosystems. NO reacts chemically in the atmosphere to form nitric acid.  SO 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.
NO can also contribute to the nutrient over-enrichment of waterbodies,  which can harm or kill aquatic species. NO 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. is a component of sulfur oxide, commonly referred to as “SO”. 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. ), are found in the atmosphere but at  concentrations much lower than SO. and SO and SO 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.
section_title: Air Pollution Affects U.S. Waterbodies and Transportation and Power Plants Are Its Key Sources
paragraphs: Atmospheric deposition associated with NO, SO, SO, and mercury, but  international sources also contribute to mercury deposition in the United  States.
subsections: section_title: Atmospheric Deposition Pollutes Many U.S. Waterbodies, but the Full Extent is Not Known
paragraphs: Atmospheric deposition, including from NO, SO For  example, about 53,300 square miles of the Great Lakes are reported to  be impaired, in part, because of atmospheric deposition. 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 NO, SO, 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. 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.
Federal studies also show that atmospheric deposition associated with  NO, SOvulnerable and that continue to receive harmful levels of acid rain.  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. Figure 2 shows acid-sensitive ecosystems in the United States.
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. 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 NO coming from atmospheric deposition. A 2011 NAPAP report  did quantify the relative amount of nitrogen coming from atmospheric  deposition. 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.  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. 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. 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.
Domestic transportation is the largest source of NO emissions in the  United States, according to EPA’s 2008 National Emissions Inventory.  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 NO emissions in the United  States. Other sources of NO emissions include fuel combustion, mainly  from power plants and industrial boilers, which accounted for an  estimated 28 percent of total estimated NO emissions. According to an  EPA official, the only international sources of NO 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.
Ammonia emissions, like NO, 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.
subsections: section_title: Domestic Power Plants Are the Largest Source of U.S. Mercury Emissions, but International Sources Contribute to Deposition
paragraphs: 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.
While most of the NO and SO 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. 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. 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  fig. 7). 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.
section_title: EPA Faces Challenges Using Clean Water Act Regulations to Address Atmospheric Deposition
paragraphs: 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.
subsections: section_title: Key Regulatory Tools Do Not Directly Regulate Atmospheric Deposition
paragraphs: EPA has regulatory and nonregulatory tools available under the CWA for  addressing atmospheric deposition.
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.  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 NO  or SO 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, 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.
section_title: Three TMDLs Highlight Challenges in Addressing Atmospheric Deposition under the Clean Water Act
paragraphs: 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.
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. 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. 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). Figure 8 depicts the out-of-region sources of atmospheric  deposition in the Northeast Region as of 2002.
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.
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. Figure 9 shows the forest preserve  lands within the Adirondacks.
According to state officials, over the last two decades the state has  reduced in-state emissions of SO by about 90 percent and reduced in- state emissions of NO to a lesser extent. 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 NO, SO, SO, 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.
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 NO 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. 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. 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 NO airshed  extends beyond the seven jurisdictions’ boundaries. According to EPA,  about 50 percent of the NO impacting the bay comes from sources  outside of the seven jurisdictions. Figure 10 depicts the Bay’s watershed  and NO airshed—defined by the area from which 75 percent of the NO  emissions that contribute to the bay originate.
section_title: Clean Air Act Regulations Address Atmospheric Deposition, but EPA Faces Challenges Addressing Effects on Waterbodies
paragraphs: Implementation of CAA regulations has reduced emissions of NOx, SO, 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.   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 NO and SOEPA 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, 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 NO and SO Vehicle  emission standards limit certain vehicles’ emissions of pollutants,  including NO, 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. In contrast, the Acid Rain Program was  designed, in part, to address the effect of NO and SO on waterbodies.  The program implements CAA requirements for significant decreases in  the emissions of NO and SO  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 NO and SO from  coal-burning power plants, but the requirements and deadlines these  regulations may establish for power plants are uncertain. 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 NO and SO, 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. 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 NO and SO  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 NO and SO 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.  The report analyzed how reductions in NO and SO 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.
EPA issued regulations in 1995 and 1997 under the CAA limiting mercury  emissions from municipal waste combustors and medical infectious waste  incinerators. They were not designed to address the effect of mercury  on waterbodies but were instead technology-based standards, as directed  by CAA. 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),  establishing for the first time emission limitations on mercury and other  toxic pollutants from existing and new power plants. 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. 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.
subsections: section_title: EPA Faces Challenges in Addressing Atmospheric Deposition with Clean Air Act
paragraphs: EPA faces challenges in using air regulations to further address the  effects of atmospheric deposition from NO, SO, 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.   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 NO and SO 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. emissions from power plants.
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. 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 NO 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 NO and SO 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. 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.
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. 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. 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.
section_title: Conclusions
paragraphs: 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 NO 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.
section_title: Recommendation for Executive Action
paragraphs: To help ensure that EPA can address atmospheric deposition of NO and  SO 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.
section_title: Agency Comments and Our Evaluation
paragraphs: 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 NO and SO, 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 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.
section_title: Appendix I: Scope and Methodology
paragraphs: Our objectives were to (1) examine the extent to which the atmospheric  deposition of nitrogen oxides (NO), sulfur dioxide (SO 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. 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 NO, SO and SO, 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 NO, SO, SO, 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 NO, SO. Finally, we chose the Chesapeake Bay TMDL because it  address nutrient over-enrichment, including from NO. 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.
section_title: Appendix II: Comments from the Environmental Protection Agency
section_title: Appendix III: GAO Contact and Staff Acknowledgments
subsections: section_title: GAO Contact
section_title: Staff Acknowledgments
paragraphs: 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.