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Acid Deposition and Its Effect on the Environment



While the northeastern United States enjoys a relative abundance of freshwater, the quality of our water has degraded over the past century as a consequence of human activities. The deposition of acids is an important cause of this decline. Acid deposition is related to the combustion of fossil fuels, such as gasoline, oil, and coal, which releases sulfur dioxide (SO2) and nitrogen oxides (NOX) into the atmosphere. These gases mix with water vapor in the air and react to form acidic compounds, which fall to earth as rain or snow, sometimes hundreds of miles from their source. Over time, acid deposition, which includes acid rain, snow, and other kinds of acidic inputs, affects lakes and rivers. Many bodies of water across the Northeast have become too acidic to support aquatic life.

The acidity of precipitation associated with industrial pollution was first measured for the entire U.S. in the late 1950s. At the time, there was growing concern that fossil fuel combustion was causing air pollution problems. Industrial sources of emissions responded by raising smokestack heights in order to disperse emissions. Because higher smokestacks allow pollutants to travel great distances on the wind before falling to earth, the local urban problem thus became a national and international one. Further, as prevailing winds in the northern U.S. blow from west to east, the Northeast receives acid deposition originating from coal-burning power plants in the Midwest.

In the United States, electric utilities are the leading source of SO2 emissions, while transportation sources generate the highest contribution of NOX. While the 1970 Clean Air Act empowered the U.S. Environmental Protection Agency (EPA) to regulate SO2 and NOX emissions, federal action aimed specifically at reducing acid rain was not taken until 1990. The 1990 enactment of Title IV of the Clean Air Act established the acid rain program, including a market-based pollution reduction regime to address SO2 emissions and the National Acid Precipitation and Assessment Program, which reports to Congress. Since its implementation in 1995, Title IV has led to a 40% reduction of SO2 emissions nationwide from electric utilities at only 25% of the projected cost. The picture for NOX emissions has been less encouraging, with only some sources and source categories showing reductions in emissions, while others have shown increasing contributions.

Environmental Impacts of Acid Deposition

After several decades of acid deposition, surface waters in New York and the Northeast have become more acidic, less productive, and higher in toxic metals such as aluminum and mercury. Soils have become more acidic and less fertile, and forests in many areas are showing signs of acidification-related stress. The acidification of surface waters can lead to declines in the fish population. Acidic water also affects aquatic plants and insects that are eaten by fish. As a result of these effects, the entire aquatic food chain can be "simplified," leaving a lake or stream less healthy, resilient, and productive. In addition, acid deposition has altered the chemistry of soils across large areas of New York and the Northeast by causing the depletion of calcium and other nutrients, increasing the accumulation of sulfur and nitrogen, and mobilizing inorganic aluminum, which enters soil waters and ultimately surface waters.

As emissions of sulfur have been decreasing, surface waters are beginning to show signs of improvement. Recovery from the effects of acidification, however, will take years or decades. Data from the Adirondack lakes show widespread improvement in surface-water sulfate, varied improvement in surface-water nitrate, improved acid-neutralizing capacity (ANC), and reduced levels of inorganic aluminum. The acidity of most lakes studied has decreased, while increased acidity has been measured in a few.

An Acid Deposition Primer [PDF], prepared by NYSERDA, provides a more detailed overview of the science behind acid deposition and its effects on human health and the environment, as well as a summary of research findings in New York and the Northeast.