4.5
The Water Table
The next time it rains, watch the raindrops hitting a window. Some of
them run down the glass while others seem to stay stuck to the window.
Two forces appear to act on the water: the force of gravity pulling it
downward and a force of attraction to the glass. Water shows a “stickiness,”
an attraction to many materials, because of its structure.
Water is composed of hydrogen and oxygen. The chemical formula for
water is H 2O, since there are two hydrogen atoms for every oxygen atom.
One part of the molecule has a slightly positive charge and the opposite
side has a slightly negative charge. These charges cause water molecules
to act like tiny magnets, with the positive and negative charges attracting
one another, as well as other types of molecules, like those in the window.
This also affects how water behaves underground. (See Figure 1.)
a Rain strikes the ground.
The attraction of water
molecules to soil particles
causes the water to spread
outward, moistening the
ground where it hits.
b Gravity causes the water to
sink into the ground,
dissolving salts and minerals
as it moves through the
spaces between the soil
particles. This process is
called percolation.
c The water eventually reaches an
aquitard, an impervious layer of
clay, silt, or rock that will not
allow the water to pass through
it fast enough to be used as a
water supply.
d As more rain falls, water completely
fills the spaces above the impervious
layer, causing the soil to become
saturated with water. This is
called the saturated zone.
Figure 1
a
e
b
f
d
g
c
e The upper level of the
saturated zone is
called the water table.
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Unit 4
f
Wells must reach the
saturated zone to be
good sources of water
for human use.
g Large accumulations of
underground water in soil or
rock are called aquifers.
Aquifers are excellent sources
of water, but some can be
depleted just like oil deposits.
The depth of the water table is directly
affected by what is happening locally in the
water cycle. As rain falls, the water table starts
to rise. During times of little rainfall, water
evaporates from the ground’s surface much
faster than it is replaced. As water evaporates,
capillary action draws more water up from
below ground due to the attraction between
the soil particles and the water. Considerable
water is also lost through evaporation from
ponds, rivers, and lakes. Water from the
saturated zone moves to replace some of this
lost water, and the water table drops. When a
well “runs dr y,” the water table has sunk lower
than the depth of the well.
Humans can affect water table levels.
Some wells, for example, supply entire towns
with water. If too much water is pumped from
a well, the water table in the ground around
the well can drop. Surrounding farms, with
shallower wells, may completely lose their
water supply.
Understanding Concepts
1. Explain how the water cycle affects water
table levels.
2. Explain why, as rain begins to fall, the water
doesn’t flow down to the saturated zone
immediately.
3. Describe how the following affect the
movement of water underground:
(a) aquitard
(b) capillary action
(c) percolation
Making Connections
4. During a dry season, a couple had 6000 L of
water pumped into their dry well. By that
evening, only 4000 L remained. They didn’t
have a swimming pool, they hadn’t yet taken
any baths, and they didn’t have livestock.
Explain how 2000 L of water could disappear in
less than a day.
Exploring
5. Research one of the following using print
and electronic resources. Prepare a report
8A for your classmates:
4A
(a) a comparison between dug wells and
drilled wells;
How might you use your knowledge
of water molecules’ attraction in the
design of an oil spill eliminator?
(b) how a well pump works; or
(c) the use of dugouts and sloughs for
watering farm animals.
Capillary Action in Soil
6
• Observe how high the water travels
up the tube.
• Try using different tube
diameters to see if the
distance changes.
Measure the diameters
and the distances.
• Use a chart to record
your results.
You can build a model to
demonstrate capillary action.
• Put on an apron. Roll a
sheet of clear plastic into
a tube and tape the seam.
• Place the tube upright in
an aluminum pie pan and
fill the tube with sand.
• Pour water into the pan
and let it stand for several
minutes. (See Figure 2.)
1. Does changing the type
of soil or type of liquid
affect your results?
Figure 2
SKILLS HANDBOOK: 4A Research Skills 8A Writing a Report 6 Observing and Recording Data
Water Systems
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