Acid Rain - Natural Factors That Affect Acid Rain Deposition
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Several factors contribute to the impact of acid rain on an area. Transport systems—primarily the movement of air—distribute acid emissions in definite patterns around the planet. The movement of air masses transports emitted pollutants many miles, during which the pollutants are transformed into sulfuric and nitric acid by mixing with clouds of water.
In the United States a typical transport pattern occurs from the Ohio River Valley to the northeastern United States and southeastern Canada, as prevailing winds tend to move from west to east and from south to north. About one-third of the total sulfur compounds deposited over the eastern United States originates from sources in the Midwest more than 300 miles away.
In drier climates, such as those of the western United States, windblown alkaline dust moves more freely through
FIGURE 7.3 Field measurements of pH values from the National Atmospheric Deposition Program/National Trends Network (2002)
the air and tends to neutralize atmospheric acidity. The effects of acid rain can be greatly reduced by the presence of basic (also called alkali) substances. Sodium, potassium, and calcium are examples of basic chemicals. When a basic and an acid chemical come into contact, they react chemically and neutralize each other. On the other hand, in more humid climates where there is less dust, such as along the eastern seaboard, precipitation is more acidic.
Areas most sensitive to acid rain contain hard, crystalline bedrock and very thin surface soils. When no alkaline-buffering particles are in the soil, runoff from rainfall directly affects surface waters, such as mountain streams. In contrast, a thick soil covering or soil with a high buffering capacity, such as flat land, neutralizes acid rain better. Lakes tend to be most susceptible to acid rain because of low alkaline content in lake beds. A lake's depth, its watershed (the area draining into the lake), and the amount of time the water has been in the lake are also factors.
The lakes and forests in and near the Adirondack Mountains in upstate New York are an example of what occurs in areas that do not have carbonate rock to quickly neutralize acid. Approximately half the lakes above the altitude of 2,000 feet have a pH of less than 5.0. Ninety percent of these lakes contain no aquatic life.
The states bordering and east of the Mississippi River contain approximately 17,000 lakes and 112,000 miles of streams. An estimated 25 percent of the land contains soil and bedrock that allow acidity to travel through underground water to these lakes and streams. Approximately half of these bodies of water have such a limited ability to neutralize acid that acid-laden pollutants will eventually cause acidification.
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