The pollution deposited through atmospheric deposition can come from  human-caused sources, such as power plants, or natural sources, such  as volcanoes. EPA has issued regulations that limit  mercury emissions from certain industries. Air Pollution Affects U.S. Waterbodies and Transportation and Power Plants Are Its Key Sources

Atmospheric deposition associated with NO, SO, SO, and mercury, but  international sources also contribute to mercury deposition in the United  States. Atmospheric Deposition Pollutes Many U.S. Waterbodies, but the Full Extent is Not Known

Atmospheric deposition, including from NO, SO For  example, about 53,300 square miles of the Great Lakes are reported to  be impaired, in part, because of atmospheric deposition. However, the ATTAINS database likely underestimates the  extent of the problem because the states have not assessed all of their  waterbodies. 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. EPA Faces Challenges Using Clean Water Act Regulations to Address Atmospheric Deposition

Listing impaired waters and TMDLs are the primary regulatory tools EPA  uses under the CWA to identify water pollution resulting from atmospheric  deposition, but EPA faces challenges in using these tools, as well as  other nonregulatory tools, to address the effects of atmospheric  deposition. 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. 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. Clean Air Act Regulations Address Atmospheric Deposition, but EPA Faces Challenges Addressing Effects on Waterbodies

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. 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. 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. 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. 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. The CAA provides EPA  with regulatory tools to reduce airborne emissions—and this has reduced  atmospheric deposition. 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. Recommendation for Executive Action

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. Appendix I: Scope and Methodology

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. 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.
Why GAO Did This Study

Atmospheric deposition, a process that transfers pollutants, including NOx, SO2, and mercury, from the air to the earths surface, can significantly impair the quality of the nations 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 EPAs 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 nations waters and identify the key sources of these pollutants; (2) the regulatory tools that EPA uses under the CWA to address the effects of atmospheric deposition, and the challenges, if any, that it faces in doing so; and (3) the regulatory tools that EPA uses under the CAA to address the effects of atmospheric deposition, and the challenges, if any, that it faces in doing so. To conduct this work, GAO reviewed EPA data, reports, and activities and interviewed agency officials and other experts.

What GAO Found

Atmospheric deposition of nitrogen oxides (NOx), sulfur dioxide (SO2), and mercury contributes to the impairment of the nation's waters, but the full extent is not known. For example, states provide EPA with data on the extent to which their waterbodies do not meet water quality standards, and some states have reported that some of their waterbodies are polluted because of atmospheric deposition. However, the states have not assessed all of their waterbodies and are not required to report on the sources of pollution. Similarly, federal studies show that atmospheric deposition of NOx, SO2, and mercury is polluting waterbodies but have data for only some waters. The main sources of NOx and SO2 are cars and other forms of transportation and coal-burning power plants. Power plants are also the largest U.S. source of mercury emissions, but international sources also contribute to the mercury deposited in U.S. waters.
EPA has sought to address atmospheric deposition through Clean Water Act (CWA) programs but faces challenges in doing so. Specifically, states typically establish water quality standards--considering EPA recommended criteria--for each waterbody. If a waterbody does not meet standards, CWA generally requires the state to set a Total Maximum Daily Load (TMDL) that identifies the maximum amount of pollutant that can enter the waterbody and still meet standards. States are responsible for taking actions to ensure the TMDL is met. For point sources of pollution, such as a pipe from a sewer treatment plant, CWA requires new or renewed permits to be consistent with the TMDL. However, there is no similar statutory requirement for nonpoint sources of pollution, such as atmospheric deposition. States may take actions, such as providing technical or financial assistance to limit pollution from nonpoint sources, but face a challenge when atmospheric deposition pollution affecting their waters originates in emissions from a different state.
EPA has also sought to address atmospheric deposition through Clean Air Act (CAA) regulations but faces challenges in doing so. EPA issued regulations that reduced emissions of NOx, SO2, and mercury and in turn the amount of pollution in waterbodies. Even with reduced emissions, NOx, SO2, and mercury continue to pollute the nation's waterbodies. EPA's recent attempt to address atmospheric deposition by establishing secondary National Ambient Air Quality Standards (NAAQS)--standards to protect public welfare--targeting the effects of acid rain caused by NOx and SO2 on water bodies was not successful. EPA stated that uncertainty regarding atmospheric modeling and limitations in available data prevented determination of secondary NAAQS adequate to protect against the effects of acid rain, and the agency has not identified alternative strategies. EPA has begun a 5-year pilot program to gather additional scientific data, but it is unclear whether or when the agency will be able to address scientific uncertainties to enable adoption of a protective secondary NAAQS. EPA also did not set secondary NAAQS to address nutrient over-enrichment in aquatic ecosystems caused by NOx because of the limited available scientific data. Many sources of nitrogen can contribute to nutrient over-enrichment in a waterbody, including sources of nitrogen unrelated to atmospheric deposition. EPA recently announced an effort that is to lead to the development of an integrated nitrogen research strategy that includes approaches to reducing atmospheric deposition of NOx into waters impaired because of nutrient over-enrichment by nitrogen.

What GAO Recommends

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