13.3 Groundwater

Lesson Objectives

• Explain how water enters an aquifer.
• Explain how water leaves an aquifer.
• Define aquifer, and give an example.
• Define springs and geysers.
• State the purpose of wells and how they work.

Vocabulary

• aquifer
• groundwater
• spring
• water table
• well

Introduction

Rivers and lakes hold a lot of Earth’s liquid freshwater. However, far more is hidden from sight. Where is it? It is stored under the ground. In fact, 20 times more of Earth’s liquid freshwater is found below the surface than on the surface.

Groundwater

Freshwater below Earth’s surface is called groundwater. The water infiltrates, or seeps down into, the ground from the surface. How does this happen? And where does the water go?

Porous and Impermeable Rock

Water infiltrates the ground because soil and rock are porous. Between the grains are pores, or tiny holes. Since water can move through this rock it is permeable. Eventually, the water reaches a layer of rock that is not porous and so is impermeable. Water stops moving downward when it reaches this layer of rock.

Look at the diagram in Figure below. It shows two layers of porous rock. The top layer is not saturated; it is not full of water. The next layer is saturated. The water in this layer has nowhere else to go. It cannot seep any deeper into the ground because the rock below it is impermeable.

Water seeps into the ground through permeable material and stops when it reaches an impermeable rock. Predict the purpose of the well in the diagram.

The Water Table

The top of the saturated rock layer in Figure above is called the water table. The water table isn’t like a real table. It doesn’t remain firmly in one place. Instead, it rises or falls, depending on how much water seeps down from the surface. The water table is higher when there is a lot of rain and lower when the weather is dry.

Aquifer

An underground layer of rock that is saturated with groundwater is called an aquifer. A diagram of an aquifer is shown in Figure below. Aquifers are generally found in porous rock, such as sandstone. Water infiltrates the aquifer from the surface. The water that enters the aquifer is called recharge.

An aquifer is a layer of saturated porous rock. It lies below the water table. An impermeable layer, such as clay, is below the aquifer.

Human Use of Aquifers

Most land areas have aquifers beneath them. Many aquifers are used by people for freshwater. The closer to the surface an aquifer is, the easier it is to get the water. However, an aquifer close to the surface is also more likely to become polluted. Pollutants can seep down through porous rock in recharge water.

An aquifer that is used by people may not be recharged as quickly as its water is removed. The water table may lower and the aquifer may even run dry. If this happens, the ground above the aquifer may sink. This is likely to damage any homes or other structures built above the aquifer.

The Ogallala Aquifer

One of the biggest aquifers in the world is the Ogallala aquifer. As you can see from Figure below, this aquifer lies beneath parts of eight U.S. states. It covers a total area of 451,000 square kilometers (174,000 square miles). In some places, it is less than a meter deep. In other places, it is hundreds of meters deep.

In this map, the area over the Ogallala aquifer is shaded in blue.

The Ogallala aquifer is an important source of freshwater in the American Midwest. This is a major farming area, and much of the water is used to irrigate crops. The water in this aquifer is being used up ten times faster than it is recharged. If this continues, what might happen to the Ogallala aquifer?

Springs and Geysers

The top of an aquifer may be high enough in some places to meet the surface of the ground. This often happens on a slope. The water flows out of the ground and creates a spring. A spring may be just a tiny trickle, or it may be a big gush of water. One of the largest springs in the world is Big Spring in Missouri, seen in Figure below.

Big Spring is named for its large size. It releases more than 12,000 liters of water per second!

Water flowing out of the ground at a spring may flow downhill and enter a stream. That’s what happens to the water that flows out of Big Spring in Missouri. If the water from a spring can’t flow downhill, it may spread out to form a pond or lake instead. Lake George in New York State, which is pictured in Figure below, is a spring-fed lake. The lake basin was carved by a glacier.

Lake George gets its water from a number of springs.

Mineral Springs and Hot Springs

Some springs have water that contains minerals. Groundwater dissolves minerals out of the rock as it seeps through the pores. The water in some springs is hot because it is heated by hot magma. Many hot springs are also mineral springs. That’s because hot water can dissolve more minerals than cold water. Grand Prismatic Spring, shown in Figure below, is a hot mineral spring. Dissolved minerals give its water a bright blue color. The edge of the spring is covered with thick orange mats of bacteria. The bacteria use the minerals in the hot water to make food.

Grand Prismatic Spring in the Yellowstone National Park is the largest hot spring in the U.S. How can you tell from the photo that the water in this spring is hot?

Geysers

Heated groundwater may become trapped in spaces within rocks. Pressure builds up as more water seeps into the spaces. When the pressure becomes great enough, the water bursts out of the ground at a crack or weak spot. This is called a geyser. When the water erupts from the ground, the pressure is released. Then more water collects and the pressure builds up again. This leads to another eruption.

Old Faithful is the best-known geyser in the world. You can see a picture of it in Figure below. The geyser erupts faithfully every 90 minutes, day after day. During each eruption, it may release as much as 30,000 liters of water!

Old Faithful in Yellowstone National Park is a geyser named for its regular cycle of eruptions.

Wells

Most groundwater does not flow out of an aquifer as a spring or geyser. So to use the water that's stored in an aquifer people must go after it. How? They dig a well. A well is a hole that is dug or drilled through the ground down to an aquifer. This is illustrated in Figure below.

A well runs from the surface to a point below the water table. Why must a well go lower than the water table?

People have depended on water from wells for thousands of years. To bring water to the surface takes energy because the force of gravity must be overcome. Today, many wells use electricity to pump water to the surface. However, in some places, water is still brought to the surface the old-fashioned way — with human labor. The well pictured in Figure below is an example of this type of well. A hand-cranked pulley is used to lift the bucket of water to the surface.

This old water well uses human muscle power to bring water to the surface.

Lesson Summary

• Groundwater is freshwater below Earth’s surface. It seeps down from the surface through pores in soil and rock. It keeps seeping downward until it reaches a layer of impermeable rock.
• An aquifer is an underground layer of rock that is saturated with groundwater. One of the biggest aquifers in the world is the Ogallala aquifer in the American Midwest.
• Water that flows out of the ground where an aquifer meets the surface is called a spring. Spring water may contain dissolved minerals. It may also be heated by magma in the crust. Heated groundwater that erupts from the ground under pressure is called a geyser.
• Many people get their freshwater from an aquifer. They obtain the water through a well. A well is a hole that is dug or drilled through the ground down to an aquifer.

Lesson Review Questions

Recall

1. Define groundwater.

2. Describe how water enters the ground.

3. What is the water table? What might cause it to rise or fall?

4. Define aquifer. Where does an aquifer get its water?

5. What is the purpose of a well?

Apply Concepts

6. Assume you live in a town that gets its water from an aquifer. The aquifer lies beneath the town. Apply lesson concepts to predict what may happen if water is pumped out of the aquifer faster than it is recharged. Then, write a letter to the editor of the town’s newspaper. State what you think may happen. Argue for the need to use water wisely.

Think Critically

7. Compare and contrast springs and geysers.

8. LaShawna and her family went to Yellowstone National Park. They saw a spring called Green Dragon Spring. Steam was rising off the water. When LaShawna saw the steam, she said that the water must contain a lot of minerals. Do you agree with LaShawna’s statement? Why or why not?

Points to Consider

Freshwater is needed by many living things on Earth. However, most of Earth’s water is not fresh. Instead, it is salt water in the oceans.

• What do you know about Earth’s oceans? For example, how deep are they? And why is their water salty?
• Ocean water moves in waves, tides, and currents. Do you know what causes these ocean water movements?