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Chapter 4: Main Idea |
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Acid deposition develops when sulfur dioxide and nitrogen oxides combine with water and hydrogen in the atmosphere to produce sulfuric and nitric acids. These acids reach earth as wet deposition in the form of acidic rain, snow, and fog and as dry deposition in the form of particulate matter and gases. The acidification of lakes and streams kills fish, crustaceans, and insects. Acid precipitation promotes forest decline by increasing the acidity of poorly buffered soils, nutrient depletion, and aluminum toxicity in the soil, and by inhibiting the activity of soil fungi and bacteria. Acid deposition can so weaken trees that they succumb to other stresses. Acid pollution also causes crop losses and ruins roads, bridges, buildings, and monuments.
Heavy metals, toxic to life, follow their own biogeochemical cycles. Some cycles are local; others, such as lead, can become regional and even global. Lead is one of the most pervasive heavy metals. Mining, smelting, lead-consuming industries, combustion of coal, burning of leaded fuel, and decay of lead-painted surfaces release small particles of lead into the atmosphere. These particles settle on soil and vegetation and pass through the food chain. Lead may remain in upper levels of soil for millennia.
Of serious consequence globally is the insecticidal use of chlorinated hydrocarbons. Following biogeochemical cycles, these pesticides have contaminated global ecosystems. Because they become concentrated at higher trophic levels, chlorinated hydrocarbons affect predaceous animals most, interfering with their production. Such insecticides kill more than the targeted pest species, reducing species diversity. They endanger human health.
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