Resource Library

ENCYCLOPEDIC ENTRY

ENCYCLOPEDIC ENTRY

Groundwater

Groundwater

Groundwater is water that has infiltrated the ground to fill the spaces between sediments and cracks in rock. Groundwater is fed by precipitation and can resurface to replenish streams, rivers, and lakes.

Grades

5 - 8

Subjects

Biology, Chemistry, Earth Science, Ecology

Image

Well

An old well located in the Moroccan desert. Groundwater has been an extremely important source of water for many years, especially in arid climates.

Photograph by Tatsiana Volskaya

Water that has travelled down from the soil surface and collected in the spaces between sediments and the cracks within rock is called groundwater. Groundwater fills in all the empty spaces underground, in what is called the saturated zone, until it reaches an impenetrable layer of rock. Groundwater is contained and flows through bodies of rock and sediment called aquifers. The amount of time that groundwater remains in aquifers is called its residence time, which can vary widely, from a few days or weeks to 10 thousand years or more.

The top of the saturated zone is called the water table, and sitting above the water table is the unsaturated zone, where the spaces in between rocks and sediments are filled with both water and air. Water found in this zone is called soil moisture, and is distinct from groundwater.

Existing groundwater can be discharged through springs, lakes, rivers, streams, or manmade wells. It is recharged by precipitation, snowmelt, or water seepage from other sources, including irrigation and leaks from water supply systems.

To artificially discharge groundwater, a well must be drilled into an aquifer, and a well typically requires a pump to move water upward out of the aquifer. Artesian wells are drilled into aquifers that are bounded by an impenetrable rock layer from both above and below and water pressure from a recharging source located above the well outlet point will cause groundwater to be pushed upward through the artesian well, making the use of a pump unnecessary.

One important reason why groundwater is extracted through wells is to provide drinking water. In fact, groundwater provides drinking water for over 50 percent of the U.S. population, including almost 100 percent of the rural U.S. population. It is also used for domestic, industrial, and commercial purposes, though most groundwater is actually used for irrigation of farmland.

We must take care, however, we do not pump out too much groundwater at once. This can cause wells to dry up if water inputs from recharging cannot keep up with our rate of groundwater removal. This has already happened in East Porterville, California, where an extended drought has led people to drill deeper wells, which has resulted in decreased groundwater levels and caused wells to further dry up. Another threat to groundwater is pollution by fertilizers, pesticides, and waste from septic tanks, all of which can seep down into aquifers from the soil surface.

Groundwater is everywhere beneath the soil surface and can be ever-present in many places if allowed to recharge. Even in dry conditions, it maintains the flow of rivers and streams by replenishing them, providing a valuable substitute for precipitation.

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.

Director
Tyson Brown, National Geographic Society
Author
National Geographic Society, National Geographic Society
Production Managers
Gina Borgia, National Geographic Society
Jeanna Sullivan, National Geographic Society
Program Specialists
Sarah Appleton, National Geographic Society, National Geographic Society
Margot Willis, National Geographic Society
Specialist, Content Production
Clint Parks,
Producer
André Gabrielli, National Geographic Society
other
Last Updated

May 20, 2022

For information on user permissions, please read our Terms of Service. If you have questions about licensing content on this page, please contact ngimagecollection@natgeo.com for more information and to obtain a license. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher. She or he will best know the preferred format. When you reach out to him or her, you will need the page title, URL, and the date you accessed the resource.

Media

If a media asset is downloadable, a download button appears in the corner of the media viewer. If no button appears, you cannot download or save the media.

Text

Text on this page is printable and can be used according to our Terms of Service.

Interactives

Any interactives on this page can only be played while you are visiting our website. You cannot download interactives.

Related Resources