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(苏必利尔湖). 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 (水的) environment.”
CLOSE TO HOME
Water woes(灾难) 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 (地下蓄水层)，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(补充) it. In northwest Texas, for example, overpumping 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 (隐孢子虫)，a microbe (微生物) that causes fever, diarrhea (腹泻) and vomiting.
THE SOURCE
Where so contaminants come from? In developing countries, people dump raw (未经处理的) sewage(污水) 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 (饮水传染的) 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 (多氯化联二苯)，or PCBs, have been banned in the United States.)
But almost everyone contributes to water pollution. People often pour household cleaners, car antifreeze, and paint thinners (稀释剂) down the drain; all of these contain hazardous chemicals. Scientists studying water in the San Francisco Bay reported in 1996 that 70 percent of the pollutants could be traced to household waste.
Farmers have been criticized for overusing herbicides and pesticides, chemicals that kill weeds and insects but insects but that pollute water as well. Farmers also use nitrates, nitrogen--rich fertilizer that helps plants grow but that can wreak havoc (大破坏) on the environment. Nitrates are swept away by surface runoff to lakes and seas. Too many nitrates “overenrich” these bodies of water, encouraging the buildup of algae, or microscopic plants that live on the surface of the water. Algae deprive the water of oxygen that fish need to survive, at times choking off life in an entire body of water.
