This section discusses atmospheric deposition, its effect on waterbodies, and how the CWA and the CAA address it. 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. 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. Atmospheric deposition associated with NO, SO, SO, and mercury, but international sources also contribute to mercury deposition in the United States. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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.

Summary: 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. 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. 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.