Groundwater is water situated below the Earth's surface, filling void spaces in soil or geologic strata and is a critical component of the hydrologic cycle . Most groundwater originates from precipitation, infiltrating the soil until it reaches a saturation zone where all spaces are filled with water. Groundwater is constantly in motion, albeit slower than surface water, with its rate depending on the aquifer's transmissivity and storage capacity. Natural outflows occur through springs and riverbeds. Groundwater plays a crucial role in arid and semiarid regions, supporting agriculture and industries. However, it's vulnerable to chemical pollution from various sources, including fracking , agricultural chemicals, and landfills, which can compromise its usability.

How does groundwater form?

Groundwater primarily originates from precipitation . Rainwater infiltrates the soil, and when the soil is saturated, the water percolates downward to a zone of saturation, where all spaces are filled with water. This water continues to descend until it merges into dense rock. The process of replenishing groundwater is called recharge, which mainly happens during rainy seasons in tropical climates or winter in temperate ones. Typically, 10–20% of precipitation enters water-bearing strata known as aquifers . Some rocks allow considerable water storage and act as major water sources, while others restrict water flow.

Why is groundwater important for people and the environment?

Groundwater is essential to both people and the environment. It supports agriculture and industry, especially in arid regions, and serves as a primary water source for single homes and small towns. Because groundwater is often free of pathogens, it can be used domestically and industrially without purification. Groundwater sustains the base flow of streams, maintaining flow between storms, and contributes about 30 percent of total river runoff globally. However, over-extraction can deplete aquifers, leading to increased pumping costs, dry wells, and saline intrusion in coastal areas. Pollution from human activities also threatens groundwater quality, making it vital to monitor and conserve these freshwater systems.

An aquifer is a rock layer that holds water and releases it in significant amounts. The rock has interconnected, water-filled pore spaces that allow water to flow through it. Aquifers are also called water-bearing zones and are a key source of fresh water. Some rocks, called aquicludes, are impermeable, preventing water flow, while permeable rocks facilitate considerable water storage and supply. Aquifers can be unconfined or confined. Unconfined aquifers are underlain by permeable materials, whereas confined aquifers are overlain by an impermeable layer. The water table marks the boundary between saturated and unsaturated zones. The water in aquifers is replenished by drainage through the soil, known as groundwater recharge , which can be affected by rainfall and evapotranspiration rates.

How do wells help us access groundwater?

Wells are a primary way to access groundwater . They are constructed by drilling into aquifers, which are geologic formations that contain water. The depth and construction of a well depend on the aquifer's characteristics. Wells used for public water supplies usually go more than 30 meters deep and are lined with metal or casing to prevent contamination. A screen is attached to the bottom of the casing to filter out silt and sand. A submersible pump then brings the water to the surface. Some wells tap into artesian aquifers, where natural pressure pushes the water up.

What can cause groundwater pollution, and how can it be prevented?

Groundwater pollution is a growing concern, with human activities posing the greatest threat to clean drinking water. Industrial effluents, municipal discharges, chemicals from agricultural lands, mining, fracking, petroleum production, and improperly lined landfills can contaminate groundwater. In coastal regions, excessive groundwater withdrawal can lead to saltwater intrusion. To prevent groundwater contamination, sustainable agriculture practices, such as reduced-volume irrigation and the sparing application of fertilizers and pesticides, can minimize runoff. Proper waste management, including the use of impermeable liners and leachate collection systems in landfills, is also crucial. Additionally, monitoring groundwater and controlling methane emissions from landfills helps to protect this vital resource.

groundwater

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Images & Videos

A cross-sectional diagram of land shows the water table as the upper boundary of groundwater, with a stream and pond representing surface water where the water table meets the land surface. Unsaturated soil lies above the water table.

Searching for sustainable water supplies in Namibia

A cross-sectional diagram showing how a geyser and hot spring form above a magma chamber. Arrows illustrate heat rising from magma, groundwater moving through porous rock, and steam or hot water reaching the surface as a geyser or hot spring. Labels identify each layer and process.

Diagram showing three stages of saltwater intrusion into a coastal aquifer: the first panel shows normal conditions with a stable interface between ocean saltwater and fresh groundwater; the second shows increased saltwater intrusion and a cone of depression when groundwater is pumped; the third shows how an injection well restores pressure and blocks saltwater intrusion. Labels identify ocean, aquifer, potentiometric surface, and water flow directions.

How stalactites and stalagmites form in caves

Can reed beds clean contaminated groundwater?

At a Glance

groundwater summary

Quick Facts

Related Topics:: cryoseism; groundwater recharge; artesian flow; phreatic zone; groundwater flow

On the Web:: Oklahoma State University Extension - Introduction to Groundwater Hydrology and Management (Nov. 01, 2026)

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groundwater

hydrology

Also known as: subsurface water

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Last updated

Nov. 1, 2026 •History

News •

Surprising new pattern of groundwater loss may signal a turning point for the Colorado River Basin • Oct. 22, 2026, 5:18 AM ET (Live Science)

Top Questions

What is groundwater?
Groundwater is water situated below the Earth's surface, filling void spaces in soil or geologic strata and is a critical component of the hydrologic cycle. Most groundwater originates from precipitation, infiltrating the soil until it reaches a saturation zone where all spaces are filled with water.

Groundwater is constantly in motion, albeit slower than surface water, with its rate depending on the aquifer's transmissivity and storage capacity. Natural outflows occur through springs and riverbeds.

Groundwater plays a crucial role in arid and semiarid regions, supporting agriculture and industries. However, it's vulnerable to chemical pollution from various sources, including fracking, agricultural chemicals, and landfills, which can compromise its usability.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How does groundwater form?
Groundwater primarily originates from precipitation. Rainwater infiltrates the soil, and when the soil is saturated, the water percolates downward to a zone of saturation, where all spaces are filled with water. This water continues to descend until it merges into dense rock. The process of replenishing groundwater is called recharge, which mainly happens during rainy seasons in tropical climates or winter in temperate ones. Typically, 10–20% of precipitation enters water-bearing strata known as aquifers. Some rocks allow considerable water storage and act as major water sources, while others restrict water flow.
6 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Why is groundwater important for people and the environment?
Groundwater is essential to both people and the environment. It supports agriculture and industry, especially in arid regions, and serves as a primary water source for single homes and small towns. Because groundwater is often free of pathogens, it can be used domestically and industrially without purification.

Groundwater sustains the base flow of streams, maintaining flow between storms, and contributes about 30 percent of total river runoff globally. However, over-extraction can deplete aquifers, leading to increased pumping costs, dry wells, and saline intrusion in coastal areas. Pollution from human activities also threatens groundwater quality, making it vital to monitor and conserve these freshwater systems.
10 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What is an aquifer?
An aquifer is a rock layer that holds water and releases it in significant amounts. The rock has interconnected, water-filled pore spaces that allow water to flow through it. Aquifers are also called water-bearing zones and are a key source of fresh water. Some rocks, called aquicludes, are impermeable, preventing water flow, while permeable rocks facilitate considerable water storage and supply.

Aquifers can be unconfined or confined. Unconfined aquifers are underlain by permeable materials, whereas confined aquifers are overlain by an impermeable layer. The water table marks the boundary between saturated and unsaturated zones. The water in aquifers is replenished by drainage through the soil, known as groundwater recharge, which can be affected by rainfall and evapotranspiration rates.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do wells help us access groundwater?
Wells are a primary way to access groundwater. They are constructed by drilling into aquifers, which are geologic formations that contain water. The depth and construction of a well depend on the aquifer's characteristics.

Wells used for public water supplies usually go more than 30 meters deep and are lined with metal or casing to prevent contamination. A screen is attached to the bottom of the casing to filter out silt and sand. A submersible pump then brings the water to the surface. Some wells tap into artesian aquifers, where natural pressure pushes the water up.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What can cause groundwater pollution, and how can it be prevented?
Groundwater pollution is a growing concern, with human activities posing the greatest threat to clean drinking water. Industrial effluents, municipal discharges, chemicals from agricultural lands, mining, fracking, petroleum production, and improperly lined landfills can contaminate groundwater. In coastal regions, excessive groundwater withdrawal can lead to saltwater intrusion.

To prevent groundwater contamination, sustainable agriculture practices, such as reduced-volume irrigation and the sparing application of fertilizers and pesticides, can minimize runoff. Proper waste management, including the use of impermeable liners and leachate collection systems in landfills, is also crucial. Additionally, monitoring groundwater and controlling methane emissions from landfills helps to protect this vital resource.
6 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

groundwater, water that occurs below the surface of Earth, where it occupies all or part of the void spaces in soils or geologic strata. It is also called subsurface water to distinguish it from surface water, which is found in large bodies like the oceans or lakes or which flows overland in streams. Both surface and subsurface water are related through the hydrologic cycle (the continuous circulation of water in the Earth-atmosphere system).

A brief treatment of groundwater follows. For full treatment, see hydrosphere: Groundwaters.

Most groundwater comes from precipitation. Precipitation infiltrates below the ground surface into the soil zone. When the soil zone becomes saturated, water percolates downward. A zone of saturation occurs where all the interstices are filled with water. There is also a zone of aeration where the interstices are occupied partially by water and partially by air. Groundwater continues to descend until, at some depth, it merges into a zone of dense rock. Water is contained in the pores of such rocks, but the pores are not connected and water will not migrate. The process of precipitation replenishing the groundwater supply is known as recharge. In general, recharge occurs only during the rainy season in tropical climates or during winter in temperate climates. Typically, 10 to 20 percent of the precipitation that falls to the Earth enters water-bearing strata, which are known as aquifers.

Cave pool thermal spring in Grjotagja Cave, near Myvatn lake in Iceland. Groundwater. Lava cave

Britannica Quiz

Groundwater Quiz

Groundwater is constantly in motion. Compared to surface water, it moves very slowly, the actual rate dependent on the transmissivity and storage capacity of the aquifer. Natural outflows of groundwater take place through springs and riverbeds when the groundwater pressure is higher than atmospheric pressure in the vicinity of the ground surface. Internal circulation is not easily determined, but near the water table the average cycling time of water may be a year or less, while in deep aquifers it may be as long as thousands of years.

Groundwater plays a vital role in the development of arid and semiarid zones, sometimes supporting vast agricultural and industrial enterprises that could not otherwise exist. It is particularly fortunate that aquifers antedating the formation of deserts remain unaffected by increases in aridity with the passage of time. Withdrawal, however, will deplete even the largest of groundwater basins so that development based on the existence of aquifers can be only temporary at best.

A vast amount of groundwater is distributed throughout the world, and a large number of groundwater reservoirs are still underdeveloped or uninvestigated. Scientists estimate that some 5.97 quintillion gallons (22.6 million cubic km [5.4 million cubic miles]) of groundwater reside in the upper 2 km (1.2 miles) of Earth’s surface. The most frequently investigated or exploited groundwater reservoirs are of the unconsolidated clastic (mainly sand and gravel) or carbonate hardrock type found in alluvial valleys and coastal plains under temperate or arid conditions.

Though some groundwater dissolves substances from rocks and may contain traces of old seawater, most groundwater is free of pathogenic organisms, and purification for domestic or industrial use is not necessary. Furthermore, groundwater supplies are not seriously affected by short droughts and are available in many areas that do not have dependable surface water supplies. However, aquifers and other groundwater supplies are at risk of chemical pollution from fracking, agricultural chemicals, leaking or unfit landfills and septic tanks, and other point and nonpoint sources of pollution. Such contamination can render groundwater unfit for use and is expensive and difficult to clean up.

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