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Read the text. Five sentences have been removed from it. Choose from sentences A-F the one that fits each gap (1-5). There is one sentence you do not need.

Water pollution, its causes and effects

W ater pollution is the contamination of water by sewage, toxic chemicals, metals, oils, or other substances. It can affect such surface waters as rivers, lakes, and oceans, as well as the water beneath the earth's surface, called ground water. Water pollution can harm many species of plants and animals. According to the World Health Organization, about 5 million people die every year from drinking polluted water. In a healthy water system, a cycle of natural processes turns wastes into useful or harmless substances. 1ٱٱٱ This digestion process releases nitrates, phosphates, and other nutrients (chemical substances that living things need for growth). Algae

and aquatic green plants absorb these nutrients. Microscopic animals called zooplankton eat the algae, and fish eat the zooplankton. 2ٱٱٱ These larger animals produce body wastes and eventually die. Bacteria break down dead animals and animal wastes, and the cycle begins again.

Water pollution occurs when people put so much waste into a water system that its natural cleansing processes cannot function properly. Some waste, such as oil, industrial acids, or farm pesticides, poisons aquatic plants and animals. 3 ٱٱٱ This pollution process is called eutrophication. The process begins when large amounts of nutrients flow into a water system.

4 ٱٱٱ As more algae grow, more also die. Bacteria in the water use up large amounts of oxygen consuming the excess dead algae. The oxygen level of the water then drops, causing many aquatic plants and animals to die.

Water pollution comes from businesses, farms, homes, industries, and other sources. It includes sewage, industrial chemicals, agricultural chemicals, and livestock wastes. Sewage consists of human wastes, garbage, and water that has been used for laundering or bathing. Water from rain or melted snow flows from farmland into streams, carrying chemical fertilizers and pesticides that farmers have used on the land. Animal wastes also can cause water pollution, particularly from feed lots with many animals. 5 ٱٱٱ Instead, much of their wastes run off into nearby streams. Water used for irrigation also may be polluted by salt, agricultural pesticides, and toxic chemicals on the soil surface before it flows back into the ground.

The effects of water pollution are appalling. Water polluted with human and animal wastes can spread typhoid fever, cholera, dysentery, and other diseases. The careless release of such toxic wastes, primarily into waste dumps, threatens ground water supplies. Scientists are concerned that drinking even small quantities of these substances over many years may have harmful effects.

Pollution prevents people from enjoying some bodies of water for recreation. For example, odors and floating debris make boating and swimming unpleasant, and the risk of disease makes polluted water unsafe. Oil spilled from ships or offshore wells may float to shore. It can kill water birds, shellfish, and other wildlife. Water pollution also affects commercial and sport fishing. Fish can be killed by oil or by a lack of oxygen in the water, or they may die because of a reduction in the quantity and quality of their food supply. Industrial wastes also harm fish.

Another form of water pollution is the clean but heated water discharged by power plants into waterways. This heated water, called thermal pollution, harms fish and aquatic plants by reducing the amount of oxygen in the water. Chemical and oil spills can also cause devastating water pollution that kills water birds, shellfish, and other wildlife.

Acid rain forms when water vapor in the air reacts with certain chemical compounds. These compounds, including sulphur dioxide and nitrogen oxides, come largely from the burning of coal, gasoline, and oil. Most automobiles, factories, and power plants burn such fuels for energy. Regions affected by acid rain include large parts of eastern North America, Scandinavia and central Europe, and parts of Asia. Acid rain harms thousands of lakes, rivers, and streams worldwide, killing fish and other wildlife. It also damages buildings, bridges, and statues. High concentrations of acid rain can harm forests and soil.

The best things in life are free. But because water is free we often take it for granted. A few years ago, people thought that the supply of clean water in the world was limitless. Today, many water supplies have been ruined by pollution and sewage. Clean water is now scarce, and we are at last beginning to respect this precious resource and to fight to protect what is left of the water supply.

A Other waste, such as phosphate detergents, chemical fertilizers, and animal manure, pollutes by supplying excess nutrients for aquatic life.

B Cattle, hogs, sheep, and poultry raised on feed lots do not distribute their wastes over widespread pastureland.

C The cycle begins when organisms called aerobic bacteria use the oxygen dissolved in water to digest wastes.

D One of the chemicals in smog is a toxic form of oxygen called ozone.

E These nutrients stimulate excessive growth of algae.

F The fish, in turn, may be eaten by larger fish, birds, or other animals.