Encyclopedic Entry

The study of aquifers and the water flows in them is called hydrogeology.

Illustration by Tim Gunther

Great Artesian Basin
The world's largest known aquifer is the Great Artesian Basin, in Australia, at more than 1.7 million square kilometers (661,000 square miles).

An aquifer is an underground layer of water-bearing rock. Water-bearing rocks are permeable, meaning that they have openings that liquids and gases can pass through. Sedimentary rock such as sandstone, as well as sand and gravel, are examples of water-bearing rock. The top of the water level in an aquifer is called the water table.

An aquifer fills with water from rain or melted snow that drains into the ground. In some areas, the water passes through the soil on top of the aquifer; in others, it enters through joints and cracks in rocks. The water moves downward until it meets less permeable rock.

Aquifers act as reservoirs for groundwater. Water from aquifers sometimes flows out in springs. Wells drilled into aquifers provide water for drinking, agriculture, and industrial uses. Aquifers can dry up when people drain them faster than nature can refill them. Because aquifers fill with water that drains from the surface of the Earth, they can be contaminated by any chemical or toxic substance found on the surface.

There are two types of aquifers. An unconfined aquifer is covered by permeable rock and can receive water from the surface. The water table of an unconfined aquifer rises or falls depending on the amount of water entering and leaving the aquifer. It is only partly filled with water.

In contrast, a confined aquifer lies between two layers of less permeable rocks and is filled with water. Water trickles down through cracks in the upper layer of less permeable rock, a nearby water source, such as an underground river or lake, or a nearby unconfined aquifer.

An artesian well is a type of confined aquifer that flows upward to the Earth's surface without the need for pumping. The artesian well sits below the water table at the bottom of U-shaped aquifers. Pressure from water in the long sides of the aquifer pushes the water up the well shaft.

Vocabulary

Term Part of Speech Definition Encyclopedic Entry

agriculture

Noun

the art and science of cultivating the land for growing crops (farming) or raising livestock (ranching).

Encyclopedic Entry: agriculture

aquifer

Noun

an underground layer of rock or earth which holds groundwater.

Encyclopedic Entry: aquifer

artesian well

Noun

type of confined aquifer that flows to the Earth's surface without the need for pumping.

confined aquifer

Noun

layer of water-bearing rock between two layers of less permeable rock.

contaminate

Verb

to poison or make hazardous.

gravel

Noun

small stones or pebbles.

groundwater

Noun

water found in an aquifer.

Encyclopedic Entry: groundwater

industrial

Adjective

having to do with factories or mechanical production.

permeable

Adjective

allowing liquid and gases to pass through.

rain

Noun

liquid precipitation.

Encyclopedic Entry: rain

reservoir

Noun

natural or man-made lake where water is stored.

Encyclopedic Entry: reservoir

sand

Noun

small, loose grains of disintegrated rocks.

sandstone

Noun

rock formed by grains of sand.

sedimentary rock

Noun

rock formed from fragments of other rocks or the remains of plants or animals.

snow

Noun

precipitation made of ice crystals.

soil

Noun

top layer of the Earth's surface where plants can grow.

spring

Noun

small flow of water flowing naturally from an underground water source.

toxic

Adjective

poisonous.

unconfined aquifer

Noun

layer of water-bearing rock covered by permeable rock.

water-bearing rock

Noun

rock that can hold water in tiny pores.

water table

Noun

underground area where the Earth's surface is saturated with water. Also called water level.

Encyclopedic Entry: water table

well

Noun

a hole drilled in the Earth to obtain a liquid or gaseous substance.

Credits

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Writer

Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt

Illustrator

Tim Gunther
Mary Crooks, National Geographic Society

Editor

Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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