2009年12月大學(xué)英語四級考試模擬試題(十)

　　Where Water Goes

　　Only 2.5 percent of all water on Earth is freshwater, water suitable for drinking and growing food, says Sandra Postel, director of the Global Water Policy Project in Amherst, Mass. Two�瞭hirds of this freshwater is locked in glaciers and ice caps. In fact, only a tiny percentage of freshwater is part of the water cycle, in which water evaporates and rises into the atmosphere, then condenses and falls back to Earth as precipitation(rain or snow).

　　Some precipitation runs off land to lakes and oceans, and some becomes groundwater, water that seeps into the earth. Much of this renewable freshwater ends up in remote places like the Amazon river basin in Brazil, where few people live. In fact, the world's population has access to only 12,500 cubic kilometers of freshwater-about the amount of water in Lake Superior. And people use half of this amount already. "If water demand continues to climb rapidly," says Postel, "there will be severe shortages and damage to the aquatic environment."

　　Close to Home

　　Water woes may seem remote to people living in rich countries like the United States. But Americans could face serious water shortages, too especially in areas that rely on groundwater. Groundwater accumulates in aquifers, layers of sand and gravel that lie between soil and bedrock. (For every liter of surface water, more than 90 liters are hidden underground).Although the United States has large aquifers, farmers, ranchers, and cities are tapping many of them for water faster than nature can replenish it. In northwest Texas, for example, over pumping has shrunk groundwater supplies by 25 percent, according to Postel.

　　Americans may face even more urgent problems from pollution. Drinking water in the United States is generally safe and meets high standards. Nevertheless, one in five Americans every day unknowingly drinks tap water contaminated with bacteria and chemical wastes, according to the Environmental Protection Agency. In Milwaukee, 400,000 people fell ill in 1993 after drinking tap water tainted with cryptosporidium, a microbe that causes fever, diarrhea and vomiting.

　　The Source

　　Where so contaminants come from? In developing countries, people dump raw sewage into the same streams and rivers from which they draw water for drinking and cooking; about 250 million people a year get sick from water borne diseases.

　　In developed countries, manufacturers use 100,000 chemical compounds to make a wide range of products. Toxic chemicals pollute water when released untreated into rivers and lakes. (Certain compounds, such as polychlorinated biphenyls, or PCBs, have been banned in the United States.)