In simple terms
An aquifer is a layer of rock, sand or gravel below the ground that is saturated with water and can give that water up to wells and springs.
It is not an underground lake. The water sits in the spaces between grains and in the cracks running through rock, much as water sits in a sponge.
How it works
There are two kinds. In an unconfined aquifer, also called a water table aquifer, the top of the saturated zone is at atmospheric pressure. The US Geological Survey describes that surface as free to rise and fall.
In a confined aquifer, layers of poorly permeable material such as clay or shale lie above and below. The water is held under pressure, so when a well is drilled into it, the water rises above the top of the aquifer by itself.
The difference is practical, not academic. Unconfined aquifers usually sit closer to the surface, and the USGS notes that they are affected by drought sooner than confined aquifers are.
Water enters an aquifer where rain and surface water soak down into it, and leaves through springs, rivers and pumped wells. When more is taken out than goes in, the water level falls.
Why it matters
Groundwater is almost all of the fresh water there is. The UN World Water Development Report 2022 puts it at approximately 99 percent of all liquid fresh water on Earth. Aquifers are where that water is kept.
It is also in daily use. The same report says groundwater supplies half the volume of water withdrawn for domestic use worldwide, including drinking water for most rural people without a piped supply, and around 25 percent of all water withdrawn for irrigation.
Because the store is invisible, it is easy to draw down without noticing until wells run dry.
Where you’ll see it
- Well drilling and household water supply
- Irrigation planning and farm water rights
- Drought reporting on falling water levels
- Groundwater contamination and cleanup work
Example
A village well is drilled into an unconfined aquifer, and the depth to water in that well rises and falls with the seasons and the rain.
Often confused with
An aquifer is not the water table. The aquifer is the water-bearing rock or sediment itself. The water table is the upper surface of the saturated zone inside an unconfined aquifer.
Key facts
- An aquifer is a body below the land surface that is saturated with water.1
- In a confined aquifer, layers of impermeable material above and below put the water under pressure, so it rises above the top of the aquifer when a well penetrates it.1
- In an unconfined, or water table, aquifer the upper water surface is at atmospheric pressure and can rise and fall, and these aquifers are affected by drought sooner.1
- Groundwater accounts for approximately 99 percent of all liquid fresh water on Earth.2
- Groundwater provides half the volume of water withdrawn for domestic use worldwide and around 25 percent of water withdrawn for irrigation.2
Related concepts
Quick checkWhat is the difference between a confined and an unconfined aquifer?Show answer
A confined aquifer is sealed above and below by impermeable layers, so its water is under pressure. An unconfined aquifer has a free upper surface at atmospheric pressure.
Sources
- U.S. Geological Survey. What is the difference between a confined and an unconfined (water table) aquifer?. Undated (accessed 17 September 2026)
- UNESCO World Water Assessment Programme. The United Nations World Water Development Report 2022: Groundwater, Making the Invisible Visible. 24 March 2022 (accessed 17 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 17, 2026Researched and drafted with AI assistanceReport an issue