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— Acid Rain —

Rain is naturally slightly acidic due to chemical reactions with carbon dioxide in the atmosphere. The term "acid rain" is used to describe rain that has a pH lower than what is natural for a given area. pH is a measurement of how acidic or basic a material is and ranges from 0 to 14. Precipitation with a pH value less than 5 is considered acid rain.

How does acid rain form?

Enormous quantities of manmade and natural material are added to the air every day.  The burning of fossil fuels like coal and oil products by automobiles and power plants releases large amounts of sulfur dioxide and nitrogen oxides into the air. While being transported by winds, some of these particles get caught up in clouds. When sulfur dioxide and nitrogen oxides gases and particles come in contact with water droplets in clouds, chemical reactions can occur, resulting in acid rain. Additional processes called rainout and washout mix these acidic gases, liquids, and particles into rain drops and carry them to the ground.

Because of global winds and mixing in the atmosphere, every country's air pollution contributes to some degree to the earth's problem of acid rain. In 1993, the United States released approximately 90 billion pounds of sulfur dioxide and nitrogen oxides into the air. Rain and snow that falls in the Eastern U.S. typically has a much lower pH than precipitation in other parts of the country. The pH of rain and snow in the Eastern U.S. is much lower than in less populated areas.

How does acid rain affect water-quality?

The effects of acid rain are most clearly seen in the aquatic, or water, environments, such as streams, lakes, and marshes. Acid rain flows to streams, lakes, and marshes after falling on forests, fields, buildings, and roads. Acid rain also falls directly on aquatic habitats.

Most lakes and streams have a pH between 6 and 8. However, some lakes are naturally acidic even without the effects of acid rain. Lakes and streams become acidic (pH value goes down) when the water itself and its surrounding soil cannot buffer the acid rain enough to neutralize it. The adjacent chart  shows that not all fish, shellfish, or their food insects can tolerate the same amount of acid.

Generally, the young of most species are more sensitive than adults. Frogs may tolerate relatively high levels of acidity, but if they eat insects like the mayfly, they may be affected because part of their food supply may disappear. As lakes and streams become more acidic, the numbers and types of fish and other aquatic plants and animals that live in these waters decrease. Some types of plants and animals are able to tolerate acidic waters. Others, however, are acid-sensitive and will be lost as the pH declines. At pH 5, most fish eggs cannot hatch. At lower pH levels, some adult fish die.

Sources: Primer on Acid Rain, USGS; A Student's First Sourcebook. Acid Rain Program, U.S. EPA.