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.