The water on the earth is recycled constantly in a process known as the hydrologic cycle. First, the water in the oceans evaporates. It changes into vapour and forms clouds in the sky. Water accumulates in clouds and returns to the surface of the earth in some form of precipitation, which can be either rain, snow, or ice. When the water reaches the earth’s surface, it runs off into streams, rivers, lakes, and at last, into the oceans, where the cycle begins again. The water on the surface of the eartli and in the atmosphere is known as the hydrosphere. Not all precipitation goes into rivers. Some of it seeps into the ground by a process called infiltration. This water collects under the earth’s surface and is groundwater. Groundwater is important for two reasons. First, 95 per cent of the earth’s water is in the oceans. It is salty and useless for plants, animals, or humans. Fresh water, which people can use for drinking or for agriculture, is either on the earth’s surface in lakes and rivers or underground. Surface water is .05 per cent of the earth’s water while underground water is 4 per cent of the earth’s water. Consequently, groundwater provides 95 per cent of the available fresh water on the earth. Second, groundwater is important not only because of the size of the supply, but also because of its dependability. It is always available since it does not depend on seasonal precipitation.
Today, there seems to be a problem with groundwater. Until recently, groundwater was clean. It was not necessary to purify it before people drank it. However, for many years, people have been burying garbage and poisonous wastes underground. These poisons have polluted the groundwater in many places. Therefore, it is unsafe for human use unless the dirty and harmful substances are removed first.
Aquifers are geologic formations that allow groundwater to accumulate and move through them. Although they are often called underground rivers, these formations are not like surface rivers. The water accumulates in one area underground. The amount of water an aquifer contains is enough to be easily pumped out for use. People have been using groundwater for many years. With an increasing population, the need for water has also increased. Some cities depend only on groundwater for their water supply. They are using underground water very quickly. In some places the water supply may soon be used up, and there will be no water for a large population. One example of this is Tucson, Arizona, which is located in the Sonora desert in southwestern United States. It is on a very large aquifer which supplies water for the area at the present. The aquifer provides water for an increasing population in the city and for agriculture throughout southern Arizona. At the present time, the city is using 225,000 acre feet of water per year, 75,000 acre feet are being returned to the aquifer through the natural processes of the hydrologic cycle. Therefore, people are using about three times more water than nature is supplying. The water table, which is the level of the water in the aquifer, is dropping lower every year. Some wells have already gone dry and have either been closed or drilled deeper. Scientists predict that the supply of water in the aquifer will run out in twenty to eighty years.
Aquifers contain a generous supply of water. They are large, easily available, and mostly clean. Still, people who depend only on aquifers for their water supply must use their water carefully. Their lives and 60 their children’s lives depend on conserving the water they have.