单选题
Water Pollution
The demand for freshwater rises continuously as the world's population grows. From 1940 to 1990 ,withdrawal of fresh water from rivers, lakes, reservoirs, and other sources has increased fourfold. Of the water consumed each year,69 percent is used for agriculture,23 percent for industry, and 8 percent for domestic uses.
Sewage, industrial wastes, and agricultural chemicals such as fertilizers and pesticides are the main causes of water pollution. In 1995,the US Environmental Protection Agency(EPA). reported that about 37 percent of the country's lakes and estuaries, and 36 percent of its rivers, are too polluted for basic uses such as fishing or swimming, during all or part of the year. In developing nations, over 95 percent of urban sewage is discharged untreated into rivers and bays, creating a major human health hazard.
Water runoff carries fertilizing chemicals such as phosphates and nitrates from agricultural fields and yards into lakes, streams, and rivers. These combine with the phosphates and nitrates from sewage to speed the growth of algae, a type of aquatic plant. The water body may then become choked with decaying algae, which severely depletes the oxygen supply. This process can cause the death of fish and other aquatic life. Runoff also carries toxic pesticides and urban and industrial wastes into lakes and streams.
Erosion, the wearing away of topsoil by wind and rain, also contributes to water pollution. Soil and silt washed from logged hillsides, plowed fields, or construction sites, can clog waterways and kill aquatic vegetation. Even small amounts of silt can eliminate desirable fish species. For example, when logging removes the protective plant cover from hillsides, rain may wash soil and silt into streams, covering the gravel beds that trout or salmon use for spawning.
The marine fisheries supported by ocean ecosystems are an essential source of protein, particularly for people in developing countries; approximately 950 million people worldwide consume fish as their primary source of protein. Yet pollution in coastal bays, estuaries, and wetlands threatens fish stocks already depleted by over fishing. In 1989,260,000 barrels of oil was spilled from the oil tanker Exxon Valdez into Alaska's Prince William Sound, a pristine and rich fishing ground. In 1992 there were 8,790 reported spills in and around U.S. waters, involving 5.7 million liters(1.5 million gallons)of oil.