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Earth & Space Science · Foundations

Groundwater

9 min read 5 practice questions
10 key termsSourcesReport an error
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On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Quick check
  8. Study tools
  9. Sources & references

In 30 seconds

is water stored underground in the open spaces of rocks and sediment. Its upper surface, the , rises and falls with the landscape and with the seasons. Whether the ground can hold and deliver water depends on and . Aquifers supply wells and springs, water moves through them slowly, and they are renewed by from rain. Overuse can lower water tables and even make the land sink, and contaminants that enter the ground are hard to remove. In soluble bedrock, groundwater dissolves caves and sinkholes.

Why this matters

A large portion of Earth's fresh water, roughly 30 percent, sits underground, which makes groundwater central to drinking water, irrigation, and industry. Groundwater also shapes the surface: it feeds springs and streams and, in limestone regions, dissolves caves and sinkholes. Understanding how water moves underground explains why wells run dry, why the ground can slowly sink, and why pollution that enters the soil is so slow to disappear. The same concepts, porosity and permeability, recharge and withdrawal, appear in water-resource decisions in nearly every region.

The college version

Water under the surface: the water table

Groundwater is water that occupies the open spaces within rocks and unconsolidated sediments below the land surface. Those openings include pore spaces between grains, fractures, and, in some rocks, larger cavities. Near the surface, soil and rock hold both water and air in their openings; this is the unsaturated zone. Deeper down, every available opening is filled with water, a region called the . The water table is the upper surface of that saturated zone, and it is not a fixed underground line. In areas with relief, the water table generally follows the land surface, standing higher beneath hills and coming closer to the surface in valleys, where it can intersect streams, lakes, or springs. The water table also moves through time: it rises when rain soaks in and falls during dry seasons and droughts. Anyone who has dug a hole on a beach and watched it fill with water has seen the water table, at that moment and in that material.

Porosity, permeability, and aquifers

Whether rock or sediment can hold groundwater depends on porosity, the percentage of its volume that is open space. Loose, well-sorted sediments can be very porous; fine-grained silt and clay can reach about 70 percent porosity, while many sedimentary rocks fall between about 10 and 30 percent. Whether stored water can be removed depends on permeability, which describes how well the openings connect and how easily water moves from one to the next. A material can be porous yet nearly impermeable, as clay often is, because its tiny pores are poorly connected for flow. An is a water-bearing body of rock or sediment that stores and transmits enough water to supply wells and springs; an is a low-permeability layer, such as clay or shale, that slows movement. An unconfined aquifer has the water table as its upper boundary. A lies between aquitards, and the weight of overlying material keeps its water under pressure. When a well taps such a pressurized aquifer, water can rise above the top of the aquifer, and in artesian conditions it may reach the surface without pumping.

Wells, springs, and how groundwater moves

A is simply a place where the water table meets the ground surface and groundwater flows out. Elsewhere, wells are drilled below the water table, deep enough to stay submerged as the water level changes with seasons and pumping. Pumping removes water from the well first, lowering the water level inside it; water then flows from the surrounding aquifer toward the well, and the water table slopes inward in a shape called a cone of depression. Groundwater moves because water flows from places where the water table stands high to places where it stands low, losing energy to friction as it travels through pores. The movement is slow: in permeable material with a strong gradient, water may advance centimeters per day, and in many places only centimeters per year, whereas a surface river can move kilometers in a day. With rare exceptions such as karst caves, groundwater does not flow in underground rivers or lakes; it creeps through connected openings.

A hidden reservoir: recharge, overuse, and contamination

Groundwater is a small slice of all of Earth's water, roughly 0.7 percent, but about 30 percent of Earth's fresh water occurs as groundwater, making it a major usable store. Aquifers are renewed by recharge, the infiltration of precipitation and surface water through the ground. Recharge rates vary with climate, geology, and land cover; pavements and buildings shrink the area where rain can soak in. Where wells withdraw water faster than recharge restores it, water tables fall and wells can go dry, sometimes lowering the water table across a wide area rather than at only one well. In aquifers made of fine-grained sediments, heavy long-term pumping can compact the material, so the land surface subsides: in the United States, more than 17,000 square miles in 45 states have been affected, and more than 80 percent of identified subsidence has been linked to groundwater exploitation. Groundwater often looks clean because the ground filters particles, but dissolved and liquid contaminants, such as fertilizers, pesticides, and leaking tanks and spills, can reach the water table and travel with the slow flow. Because movement is slow and contact with aquifer material is long, contaminated groundwater is very difficult to clean up.

Karst: where groundwater dissolves the rock

In some regions the bedrock itself is soluble. Where limestone, other carbonate rocks, or salt beds lie underground, circulating groundwater slowly dissolves the rock, a form of chemical weathering, enlarging fractures into caves and caverns. This kind of landscape is called karst. As subsurface openings grow, the ground above can lose support, and sinkholes form when the surface collapses into the space below. Sinkholes range widely in size, from a few feet across to hundreds of acres, and from shallow depressions to more than 100 feet (about 30 meters) deep. Karst also changes how groundwater behaves: instead of creeping evenly through pores, water can move through open cave networks far more rapidly. The chemical weathering that carves karst is treated in its own lesson; here the point is that groundwater is not only a storage reservoir but also an active agent that shapes the ground it moves through.

Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Dig down far enough in most places and the ground gets wet: every tiny space between grains and inside cracks is filled with water. That underground water is groundwater, and the top of the wet zone is the water table. The water table is not flat like a tabletop; it rises and falls with the hills, and it moves up and down with the seasons and with pumping. Water enters when rain soaks into the ground, creeps slowly through the pore spaces, and leaves at springs, streams, and wells. In some places the bedrock itself slowly dissolves, opening caves and sinkholes. Use more water than the ground can soak back up, and the water table drops; pump hard for years in soft sediments, and the land can even settle downward.

Picture it like this

Picture a giant sponge lying beneath a landscape, its top surface shaped like the hills. Pour water on it and the water soaks in, spreading slowly through the sponge's holes. The wet line inside the sponge is like the water table. Squeeze one spot and keep squeezing: you pull out water, the wet line sinks nearby, and the sponge gradually settles and thins where you pressed it. Water poured on one end slowly works its way through the whole sponge, the way groundwater creeps from high ground toward streams.

Where the picture stops working

A sponge has one continuous set of holes, but real ground is built of many layers with different pore sizes and connections; some layers, like clay, barely let water through at all. Squeezing is instant, while real groundwater moves at most centimeters to meters per day and often only centimeters per year. And the wet line in a sponge stays put, whereas a real water table rises and falls with seasons, weather, and decades of pumping.

Worked example

A small farming community draws irrigation water from wells in an unconfined sandy aquifer. In spring, rain soaks into the fields and the water table sits a few meters down; by late summer, heavy pumping has lowered the water table several meters beneath the wells, and the well nearest the irrigation center develops a deep cone of depression. A neighbor's shallower well begins to run dry, not because the aquifer lost its porosity but because pumping lowered the water level below the well's intake. Observation wells across the valley show the decline, and paved lanes that now cover former recharge areas reduce the amount of rain that reaches the aquifer. The community responds by spreading pumping across more wells and letting some fields rest so recharge can catch up.

Key takeaway

Groundwater is stored in the pore spaces of rocks and sediment and moves slowly beneath a water table that follows the landscape. Porosity and permeability decide where water can be stored and pumped, while recharge, overuse, and contamination shape how long it remains usable.

Quick check

3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.

Question 1 of 3foundational

What is the water table?

Choose an answer, then check it.
Question 2 of 3intermediate

In hilly terrain, how does the water table usually sit relative to the land surface?

Choose an answer, then check it.
Question 3 of 3intermediate

A well pumps very little water from a layer that holds plenty of water. Which pair of properties best explains the result?

Choose an answer, then check it.
Practice all 5

Keep learning

Ready to build on this? Continue to the next lesson.

Practice this lesson
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related

You’ll learn to

  • Define groundwater, the water table, porosity, permeability, aquifer, and aquitard.
  • Explain why the water table generally follows the shape of the land surface.
  • Distinguish unconfined and confined aquifers, and describe artesian conditions.
  • Explain why groundwater moves slowly through pore spaces compared with surface rivers.
  • Analyze how pumping, recharge, and contamination affect groundwater as a resource.
  • Describe how groundwater dissolves soluble bedrock to form caves and sinkholes in karst regions.

Common mistakes

  • Imagining groundwater as an underground lake or river.

    With rare exceptions such as caves in karst regions, groundwater fills connected pore spaces and moves slowly; it does not flow in open channels.

  • Confusing porosity with permeability.

    Porosity is storage capacity; permeability is how easily water moves. Clay can be highly porous yet almost impermeable.

  • Assuming the water table is flat or fixed.

    It generally follows the land surface, rises and falls with seasons and weather, and drops around pumping wells.

  • Thinking pumping affects only the well that pumps.

    A cone of depression can lower the water table around neighboring wells, and sustained overdraft can compact aquifers and make the land subside.

Easily confused

Groundwater vs. Surface river water

Groundwater is stored in and moves through connected pore spaces at speeds of centimeters per year to centimeters per day; rivers flow rapidly in open channels.

Porosity vs. Permeability

Porosity measures how much water a material can hold; permeability measures how easily it transmits that water.

Unconfined aquifer vs. Confined aquifer

An unconfined aquifer has the water table as its upper boundary; a confined aquifer sits between low-permeability layers, and its water can be pressurized enough to rise or flow from a well without pumping.

Key vocabulary

Groundwater
Water stored underground in the open spaces within rocks, sediments, and fractures.
Water table
The upper surface of the saturated zone, below which all open spaces are filled with water.
Zone of saturation
The underground region in which every pore and crack is completely filled with water.
Porosity
The percentage of a material's total volume that consists of open space such as pores or cracks.
Permeability
How easily a rock or sediment transmits water, set by the size and interconnection of its openings.
Aquifer
A water-bearing body of rock or sediment that stores and transmits enough water to supply wells and springs.
Aquitard
A layer of low-permeability material, such as clay or shale, that greatly slows groundwater movement.
Confined aquifer
An aquifer bounded above and below by low-permeability layers, often holding water under pressure.
Recharge
The addition of water to an aquifer, mainly by infiltration of precipitation or surface water.
Spring
A place where the water table meets the ground surface and groundwater flows out on its own.

Sources & references

  1. Aquifers and Groundwater — U.S. Geological Survey, Water Science School
  2. Groundwater Storage and the Water Cycle — U.S. Geological Survey, Water Science School
  3. Contamination of Groundwater — U.S. Geological Survey, Water Science School
  4. Sinkholes — U.S. Geological Survey, Water Science School
  5. Land Subsidence — U.S. Geological Survey, Water Science School
  6. Physical Geology, Chapter 14: Groundwater — BCcampus Open Education / Open Textbook BC

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Researched 2026-08-21

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