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.
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.
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.
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.
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
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.