High School Earth Science/Ground Water

Although lakes and rivers are visible sources of water, did you know that there is water present underground at almost every spot on Earth? Though this may be surprising, water beneath the ground is commonplace. It bubbles to the surface at times through springs and geysers. We also use wells to bring underground water to the surface, so that we can use this important resource in places where fresh surface water is not readily available.

Lesson Objectives

Define groundwater.
Explain the location, use, and importance of aquifers.
Define springs and geysers.
Describe how wells work, and why they are important.

Groundwater

As you have learned, most of the Earth's water is found in the oceans, with smaller amounts in frozen ice caps, and still smaller amounts present in lakes and rivers. Some water is found in the atmosphere in the form of water vapor or clouds. However the most common place to find fresh liquid water is under the Earth's surface, in a form called groundwater (Figure 13.20). Water from the surface seeps downward into the ground through tiny spaces or pores in the rock. At some point, though, it hits a layer of rock that no longer has pores, which stops the water from traveling downward. This rock is called impermeable because the water can no longer pass through it. The upper surface of the groundwater is called the water table. The water table will fall when there has been little rain in an area for a long time. The water table will also rise when it rains steadily for a long time. It is important to know how deep beneath the surface the water table is for anyone who intends to dig into the surface or make a well. Because groundwater involves interaction between the Earth and the water, the study of groundwater is called hydro geology.

Figure 13.20: Groundwater is found beneath the solid surface. Notice that the water table roughly mirrors the slope of the land's surface. A well penetrates the water table.

Aquifers

Large collections of groundwater can be found in aquifers (Figure 13.21). Aquifers are large regions of sediment or rock that can hold significant amounts of groundwater. Aquifers can be large, sustainable water resources when water pumped out of aquifers is replenished by the water cycle. However, some aquifers are overused; people pump out more water than can be replaced. As the water is pumped out, the water table slowly falls, requiring people to spend more energy pumping out the water from greater depths. In addition, some wells may go completely dry if they are not deep enough to reach into the lowered water table. Draining aquifers can lead to the ground sinking, sometimes under houses and other structures. And when coastal aquifers are overused, salt water from the ocean may enter the aquifer, contaminating the aquifer and making it less useful for drinking and irrigation.

Figure 13.21: Agricultural irrigation often depends on water from aquifers.

Most land areas have some kind of aquifer beneath them. Aquifers can occur at different depths and different geographic locations. The closer aquifers are to the surface, the more likely they will be used by humans. However, closeness to the surface also increases the probability that the aquifers could be contaminated by surface pollution that seeps through the porous rock along with the water. Aquifers are usually not open spaces like caverns or swimming pools, but instead are porous rock and sediment. The spaces between the sediments or rock particles are filled in with water. Wet sand at the beach is a good model for the consistency of most aquifers.

The Ogallala Aquifer is one of the world's largest aquifers, and is a particularly important source of freshwater in the United States. It lies beneath eight United States states — South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Texas. It ranges from less than a meter deep to hundreds of meters deep and covers about 440,000 square kilometers! It is widely used by people for municipal and agricultural needs. However, its rate of replenishment is only about 10% of the rate at which it is being used. In other words, for every 100 liters of water that people withdraw from the aquifer, only 10 liters are being naturally replaced by precipitation. This overuse of the aquifer has created political controversies and disputes in those areas that depend on the aquifer.

Springs and Geysers

Whenever water beneath the ground meets the surface, a spring is created (Figure 13.22). This is a natural point where groundwater emerges on the Earth's surface. When water from a spring flows downhill, it can create a stream. If it does not move downhill, it may be termed a seep, and may create a pond or lake. Depending upon the source of water, the spring may be either constant, or may only flow at certain times of year.


Figure 13.22: Big Spring in Missouri lets out 12,000 liters of water per second (left). Other springs are just tiny outlets like this one (right).

Some minerals may become dissolved in groundwater, changing the water's flavor. Even carbon dioxide can dissolve in the groundwater, causing the water to be naturally carbonated. This water is sometimes sold as "mineral water".

Groundwater can be heated by magma below the Earth's surface. The heated water can create hot springs, springs with water that is naturally hot (Figure 13.23). Some hot springs are used as natural hot tubs and are considered therapeutic and spiritual by some people. However, hot springs can be dangerous, too. Their temperatures can be exceedingly hot, dissolved substances can be poisonous, and organisms like viruses and bacteria can spread disease. Be sure a hot spring is safe before entering one.

Figure 13.23: Green Dragon Spring is a hot spring found at Yellowstone National Park.

Figure 13.24: Old Faithful Geyser during an eruption.

When heated groundwater is trapped in narrow spaces, the pressure builds up and causes water to actually rocket upward. A geyser is the result of such a pressurized spring. Most geysers do not erupt constantly, but rather in periodic spurts, because pressure decreases during an eruption and then increases again after an eruption. Old Faithful, probably the most famous geyser in the world, got its name for erupting in regular cycles lasting 90 minutes (Figure 13.24). Its eruptions last for a couple of minutes and discharge 15,000 to 30,000 liters of water during each eruption.

Wells

A well is an artificial structure created by digging or drilling in order to reach groundwater present below the water table. In Figure 13.25, you can see how a well penetrates the groundwater. When the water table is close to the surface, wells can be a very convenient method for extracting water. You may have made a very simple well by digging a hole in the sand at the beach until you see a pool of water at the bottom. When the water table is far below the surface, digging wells can be quite a challenge. Most wells use motorized pumps to bring water to the surface, but many wells still require people to use a bucket to draw water up.

Figure 13.25: An old-fashioned well that uses a bucket drawn up by hand.

Wells have been an important source of water for humans through the ages. Obviously, in places that have little precipitation, wells are vital to life. Using groundwater at a faster rate than it can be replenished by the water cycle, will cause the water table in an aquifer to fall. A well using that groundwater might therefore go dry, as the water that supplies the well gets used up. It is important to use water at a rate at which it can be naturally replenished. In addition, humans must be careful not to pollute groundwater, since pollution can make water supplies unusable by humans.

Lesson Summary

Groundwater, water that infiltrates the ground, forms our largest source of readily available freshwater.
The water table forms the top of the zone of saturation, where pore spaces in sediment or rock are completely filled with water.
Aquifers are underground areas of sediment or rock that hold groundwater.
In steep areas, where groundwater intersects the ground surface, a spring or seep can form.
If groundwater is heated by magma, it can form hot springs and geysers.
In order to access groundwater supplies, humans drill wells and pump water from the ground.

Review Questions

What is groundwater?
What is the water table?
What are aquifers and why are they so important?
Replenishing an aquifer is important because it makes the aquifer a resource that can last a long time. What do you think are ways to keep the amount of water used and the amount of water replenished the same?
Earthquakes can often change the frequency of eruptions or the amount of water released by geysers. Why do you think this is so?
Why can hot springs be dangerous?
How does a well work?
Groundwater is invisible to people on the surface of the Earth. Explain one way that you might monitor how humans are affecting the amount of groundwater in an aquifer.

Vocabulary

groundwater: Water present under the ground, between the spaces in sediment or rock. Impermeable rock lies beneath the groundwater.
hot spring: A spring in which the water has been heated by magma.
hydrogeology: The study of groundwater.
impermeable: Something that water cannot penetrate.
seep: A point where a small amount of groundwater moves up onto the Earth's surface. Seeps do not produce enough water to create a stream, but they may create a small pond or wetland.
spring: A point on the Earth's surface, at which water groundwater bubbles up.
water table: The upper surface of the groundwater.

Points to Consider

Which fresh water source do you think would be cleaner: water from a river or water from a well? Why?
Why is pollution and overuse of our natural resources always a big concern?
What policies might people put in place to conserve water levels in lakes and aquifers?

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