Ground water

The hydrologic cycle continues as water seeps down into soil and rock beneath the earth's surface. Water flowing below the surface is called groundwater. Gravity, the force that prevents us from floating up into the air, pulls water downward through air spaces and cracks in rock and soil. This area is called the zone of aeration because of its many air spaces, or pores.

Water seeps into the ground at varying rates depending on the soil. Sandy soil has many little pores between its individual grains of sand. Gravel has larger pores between its rock pieces. The larger the pores and passageways, the easier it is for water to trickle through. If a bucket of water is poured onto a pebbly driveway, the water disappears faster than it would in sand because water flows more rapidly through the pebbles' larger pore spaces. Other soil and rocks have smaller air spaces between their grains. Clay, for example, has very tiny grains and pores; water does not flow as rapidly through the tiny air passageways. That's one of the reasons people make spots and bowls out of clay.

Rock that has many air spaces is called porous rock. Water often flows inside this kind of rock. When water moves easily through porous rock, the rock is said to be permeable.

Usually, groundwater that flows inside porous rock does not move very quickly, especially if the pores and pathways are tiny. Most groundwater flows an average of an inch or less per day, but water can flow as quickly as tens of yards per day in permeable rock. Groundwater follows the easiest, route, usually to a lake or stream, but may take thousands of years to reach the land's surface. To find out where groundwater travels and how quickly it is moving, scientists sometimes inject dye into groundwater. They measure how long the coloring takes to reach a nearby well.

Below the zone of aeration, downward-moving water mixes with groundwater that has already seeped beneath the earth's surface. In this area, all the air spaces are already filled with water. Because the passageways between soil and rock grains are saturated, or completely filled with water, this area the zone of saturation. The top of the zone of the saturation is referred to as the water table. The water table can be far blow the ground or very near the surface. Streams and lakes mark the water table at the land's surface. A shallow hole dug fairly close to a lake's or ocean's shoreline quickly fills with water because the water table is near the surface. In other places, the water table may be many feet beneath the surface. During periods of drought, groundwater drains from the soil into streams, causing the water table to drop even farther below the surface. Sometimes the land surface ends abruptly, perhaps at a valley wall, cliff, or along cracks in rocks. If the water table crosses these places, groundwater flows out. This flowing groundwater is called a spring.

Groundwater is the main source of drinking water for many cities. When people drill wells looking for water, they try to find a aquifer. An aquifer is a layer of permeable underground rock saturated with groundwater that can flow easily into wells. Aquifers can be found under more than half the land areas of the United States.

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