Acid rain is a mixture of wet and dry material from the atmosphere that contains elevated amounts of sulfur and nitrogen compounds. When acid rain is deposited on the ground as rain, snow, fog, or dust particles, it enters soils and surface waters and raises acid levels. This can cause drastic ecological changes to occur. Entire sections of forest may die off and fish populations vanish from lakes and streams.

Acidity is measured using the pH scale, whose readings reflect the concentration of hydrogen ions in a substance. The lower the pH, the more acidic a substance is. Pure water has a pH of 7.0. Normal rain, which reacts with carbon dioxide in the atmosphere to create a weak form of acid called carbonic acid, has a pH of 5.6. Generally speaking, low pH levels in lakes, streams, or forest soils create conditions inhospitable for most living inhabitants.

Chemical elements are continuously released to the atmosphere through natural processes. For instance, active volcanoes and decaying vegetation emit sulfur and carbon compounds that react with water, oxygen, and other chemicals in the atmosphere to form various acidic compounds. But acid rain is largely a man-made problem. Electric utilities and motor vehicles emit sulfur dioxide and nitrogen oxides in large quantities as they burn fossil fuels for energy.

The effects of acid rain on an ecosystem depend on two variables: how capable local water and soils are in buffering (or neutralizing) acidic compounds, and how sensitive animal and plant life are to increased acidity. For instance, although certain soils naturally contain higher quantities of calcium carbonate (lime), which reacts with acid so that no change in soil pH occurs, other soils have low buffering capacity and therefore cannot neutralize excess acids. The increased acidity of soils can cause metals such as aluminum and mercury, which are normally locked in clay particles, minerals, and rocks, to become soluble and to leach into lakes and streams. These metals are highly toxic to most aquatic life.

Snails are among the animal species most sensitive to acidity and its effects. Because the buffering process robs calcium from the soil, and because snails depend on calcium to form their shells, they begin to die when the soil's pH level nears 6.0. As pH dips further, frogs, crayfish, and mayflies die. Few, if any, fish survive in water with a pH reading below 4.0. In forests, the normally green leaves and needles of trees and other plant life turn brown and fall off when soils become too acidic.

The effects of this environmental problem are neither quickly nor easily remedied. Acid rain penetrates deeply into the fabric of an ecosystem, changing the chemistry of the soil and streams. It can take many years for ecosystems to recover, even after emissions are reduced and rain pH is restored to normal.

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