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8.8: Introduction to Groundwater

Difficulty Level: At Grade Created by: CK-12
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Is there always water flowing beneath the land surface?

Although this may seem surprising, water beneath the ground is commonplace, moving slowly and silently through an aquifer and then bubbling to the surface at a spring. Groundwater is an extremely important source of water in many parts of the world where development and agriculture outmatch the amount of water available from rainfall and streams.


Groundwater resides in aquifers , porous rock and sediment with water in between. Water is attracted to the soil particles, and capillary action , which describes how water moves through porous media, moves water from wet soil to dry areas.

Aquifers are found at different depths. Some are just below the surface and some are found much deeper below the land surface. A region may have more than one aquifer beneath it and even most deserts are above aquifers. The source region for an aquifer beneath a desert is likely to be far away, perhaps in a mountainous area.


The amount of water that is available to enter groundwater in a region, called recharge , is influenced by the local climate, the slope of the land, the type of rock found at the surface, the vegetation cover, land use in the area, and water retention, which is the amount of water that remains in the ground. More water goes into the ground where there is a lot of rain, flat land, porous rock, exposed soil, and where water is not already filling the soil and rock.

Fossil Water

The residence time of water in a groundwater aquifer can be from minutes to thousands of years. Groundwater is often called “fossil water” because it has remained in the ground for so long, often since the end of the ice ages.

Diagram of an aquifer and groundwater flow

A diagram of groundwater flow through aquifers showing residence times. Deeper aquifers typically contain older "fossil water."


  • Groundwater is in aquifers, a porous and permeable rock layer.
  • Groundwater recharges in wet regions.
  • Much groundwater is from the end of the ice ages, so it is called fossil water.

Explore More

Use this resource to answer the questions that follow.

  1. What is surface water?
  2. What is groundwater?
  3. What is groundwater stored in?
  4. What is the largest aquifer in the United States?
  5. What is the top of the aquifer called?
  6. What is the area above and the area below the water table called?
  7. How does water end up in groundwater?
  8. How long can water stay in the aquifer?
  9. How does groundwater reach the surface naturally? '
  10. How does surface water become groundwater?
  11. What do humans do with groundwater?
  12. Under what conditions could humans drain aquifers?


  1. How does water move through an aquifer?
  2. Where does groundwater come from in a region that has very little rainfall?
  3. If groundwater is used, how will there be more? Is there always the same amount of water in an aquifer.




A layer of rock, sand, or gravel that holds large amounts of ground water.
capillary action

capillary action

Water moves from wet to dry regions in soil.


Water that moves down from the surface into the groundwater.

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Difficulty Level:

At Grade


Date Created:

Feb 24, 2012

Last Modified:

Jan 07, 2015
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