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If you live in a city in North America or Europe, you have probably never thought much about water. Whenever you need some, you turn on the tap and there it is. Millions of people in other parts of the world are not so lucky. They have trouble getting enough clean water for their basic needs. This situation may soon become common all around the world, scientists believe. In fact, they say that the lack of clean water may be one of the biggest issues in the twenty-first century. The reasons for this are clear. On the one hand. people are using more water than ever before. Over the last fifty years, the population of the world has more than doubled. On the other hand, many sources of surface water such as rivers, lakes, and streams are too polluted and unhealthy for use as drinking water. This has forced more and more people to drill wells so they can get water form underground. There are enormous amounts of water deep underground in lakes called aquifers. Until recently, scientists believed this underground water was safe from pollution. Then, in the 1980s. people in the United States began to find chemicals in their well water, and scientists took a closer look at what was happening. Weldon Spring, Missouri, for example, was the site of a bomb factory during World War Ⅱ. The factory was destroyed after the war. but poisonous chemicals remained on the ground. Very slowly, these chemicals dripped down through the ground and into the aquifer. Once they did, however, the water form that aquifer was no longer drinkable. | 
