Groundwater facts for kids

An illustration showing groundwater in aquifers (in blue) (1, 5 and 6) below the water table (4), and three different wells (7, 8 and 9) dug to reach it.

Groundwater is the water found underground. It fills tiny spaces in rocks and soil. Imagine a sponge full of water; that's a bit like how groundwater exists. About 30% of all the fresh water we can easily use on Earth is groundwater.

Special rock layers or soil that can hold and give up a lot of water are called aquifers. The top level where the ground is completely filled with water is known as the water table.

Groundwater gets refilled when rain or melted snow soaks into the ground. It can naturally come back to the surface as springs or seeps. Sometimes, it creates oases in deserts or wetlands. People also dig wells to bring groundwater to the surface for farms, cities, and factories.

Studying how groundwater moves and where it's found is called hydrogeology.

Groundwater is often a great source of water. It can be cleaner and safer from pollution than water in rivers or lakes. Many cities get their drinking water from underground sources. In fact, over 2 billion people worldwide rely on groundwater for their daily needs.

However, using too much groundwater can cause problems. For example, if groundwater gets polluted, it's harder to clean than surface water. Pollution often comes from waste dumped on land. This includes chemicals from homes and factories, too much fertilizer from farms, and waste from mines. Leaking underground tanks can also cause pollution.

In coastal areas, pumping too much groundwater can cause saltwater intrusion. This means salty ocean water moves into the freshwater supply. Also, if too much water is removed, the ground above can start to sink. This is called land subsidence. It has caused cities like Bangkok to lose elevation. These issues are made worse by sea level rise and effects of climate change. Did you know that human groundwater pumping has even shifted Earth's axial tilt slightly?

What is Groundwater?

Groundwater is simply fresh water found beneath the Earth's surface. It fills tiny spaces, called pores, in soil and rocks. This water also flows through underground layers called aquifers, below the water table. Sometimes, we talk about "fossil water". This is very old groundwater that soaked into the ground thousands of years ago.

Groundwater's Journey in the Water Cycle

The water cycle, showing how water moves between the surface and underground.

Think of groundwater as a hidden part of the water cycle. It has inputs, outputs, and storage, just like rivers and lakes. The main way groundwater gets refilled is when rain or snowmelt soaks into the ground. This process is called infiltration. The water then slowly moves downwards, a process called percolation, until it reaches the groundwater.

Groundwater naturally flows out through springs or by seeping into oceans. It can stay underground for a very long time, from days to thousands of years. Deep groundwater, far from the surface, can take an incredibly long time to complete its journey through the water cycle.

Amazing Underground Rivers: The Great Artesian Basin

One of the biggest underground water systems is the Great Artesian Basin in Australia. It covers almost 2 million square kilometers! Scientists who study groundwater, called hydrogeologists, have found water in this basin that is over 1 million years old. They can tell the age of the water by studying tiny elements in it. The water is youngest where it enters the ground and gets older as it flows across the continent. This means the water travels very slowly, about 1 meter (3 feet) per year!

How Groundwater Gets Refilled

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Where Groundwater is Stored

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An aquifer is a special underground layer of rock or soil that holds water. Think of it like a giant sponge deep in the ground! This layer can be made of things like sand, gravel, clay, or even certain types of rock. The important thing is that water can easily flow into and through it.

Special Features of Groundwater

The Alapaha River in Florida flowing into a sinkhole, which leads to the underground Floridan Aquifer.

Groundwater's Steady Temperature

Groundwater stays at a fairly constant temperature. This is because water holds heat well, and the soil and rock above act like insulation. In some places, groundwater stays around 10°C (50°F). This steady temperature can be used to help heat or cool buildings. For example, cool groundwater can be pumped through a house in summer. In winter, its warmer temperature can help heat pumps work more efficiently.

How Much Groundwater is There?

Groundwater makes up about 30% of the world's fresh water supply. It is about 99% of all the liquid fresh water on Earth. The amount of groundwater stored globally is similar to all the freshwater in snow and ice, including the North and South Poles. This makes it a vital resource, especially during droughts when surface water is scarce.

Scientists can estimate how much water is in an aquifer. They measure water levels in wells and study rock samples from drilling. This helps them understand the size and depth of the water-filled layers. They also do pumping tests to see how water flows through the aquifer.

Aquifers are different depending on the type of rock and soil. The best aquifers for getting water are usually found in soft, sedimentary rocks. Harder, fractured rocks often hold less water.

How Humans Use Groundwater

Groundwater may be extracted through a water well.

People will rely on groundwater even more in the future. This is because the demand for water is growing, and effects of climate change are causing rainfall patterns to change. Groundwater is used for many things, including drinking, watering animals, and growing crops.

Drinking Water and Daily Life

Groundwater is the most common source of fresh water worldwide. It provides about half of the world's drinking water. It also supplies 40% of water for irrigation and one-third for industrial uses. Over 2.5 billion people depend only on groundwater for their basic daily water needs.

Globally, about one-third of all water taken from the Earth comes from underground. The rest comes from surface water. In 2017, people were taking about 3,880 cubic kilometers of fresh water each year. This amount has been increasing, especially for farming.

The Asia-Pacific region uses the most groundwater. Countries like India, China, and Pakistan account for about 60% of the world's total groundwater use.

Drinking Water Quality

Groundwater can be a safe source of water, but sometimes it has problems. Natural chemicals like arsenic and fluoride can be present. These can make the water unsafe to drink. The amount of salt in groundwater can also vary a lot. High salt levels make water undrinkable. This is often a problem in coastal areas, especially when too much groundwater is pumped out.

Scientists have found that groundwater is warming up due to climate change. For example, groundwater in Vienna, Austria, warmed by 1.4 degrees Celsius between 2011 and 2020. This warming could make groundwater too hot for drinking in some areas by the end of this century.

Water for Cities and Factories

Cities and industries get their water from large wells. Several wells together are called "wellfields." Groundwater from deep aquifers is often safer from pollution that might be on the surface. Some special wells, called "collector wells," are designed to draw in water from nearby rivers.

If groundwater is too salty, it can be treated using desalination. This process removes the salt, but then the salty leftover water needs to be safely disposed of.

Helping Farms Grow Food

Center-pivot irrigated fields in Kansas watered by the Ogallala Aquifer.

Irrigation, which is watering crops, uses a lot of water. About 20% of farmland is irrigated, producing 40% of our food. Aquifers are very important for agriculture. Deep aquifers in dry areas have long been used to water crops. About 70% of all groundwater taken out of the ground is used for farming. In India, 65% of irrigation comes from groundwater.

Sometimes, very old aquifers are used to supply water to cities and farms. In Libya, a project called the Great Manmade River pumps huge amounts of groundwater from under the Sahara Desert to coastal cities. This project saved money compared to removing salt from seawater. However, these aquifers might run dry in 60 to 100 years.

Water in Developing Countries

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Challenges for Groundwater

Groundwater faces several challenges. First, efforts to prevent floods can sometimes reduce how much groundwater gets refilled naturally. Second, using too much groundwater for a long time can cause the land to sink. This can damage buildings and roads. Third, in coastal areas, too much pumping can lead to saltwater mixing into freshwater. Fourth, draining certain soils can make water acidic and polluted.

Using Too Much Groundwater (Overdraft)

Over a long period, groundwater levels in California's Central Valley dropped. Short periods of recovery happened mostly during extreme floods.

Diagram showing the water balance of an aquifer.

Groundwater is a valuable resource, but it can be used up too quickly. This is called "overdraft" or "over-abstraction." When too much groundwater is pumped out, the water table drops. This means existing wells might no longer reach the water. People then have to drill deeper wells. In places like California, Texas, and India, the water table has dropped hundreds of feet. Satellites have shown that 21 of Earth's 37 major aquifers are being used up too fast. In the Punjab region of India, groundwater levels have dropped 10 meters (33 feet) since 1979, and it's getting worse.

A lowered water table can also cause other problems, like the ground sinking or saltwater mixing into freshwater. Using too much groundwater can also harm ecosystems. Groundwater helps feed rivers, wetlands, and lakes. Many plants depend on groundwater or the moisture it provides in the soil.

A study in 2021 looked at about 39 million groundwater wells. It found that 6–20% of these wells are at high risk of running dry if local groundwater levels drop by just a few meters.

Groundwater Pollution

Waterborne diseases can spread through a groundwater well contaminated by waste from pit latrines.

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Land Sinking (Subsidence)

Land subsidence happens when too much water is pumped from underground. This causes the empty spaces below the surface to collapse, making the ground above sink. Normally, the water pressure in the underground spaces helps support the weight of the soil and rocks above. When groundwater is removed, this pressure drops. The soil and rock layers then get compressed. This compression can cause the ground surface to drop.

Once the ground sinks, it's usually permanent. The compressed aquifer also has less space to hold water in the future. The city of New Orleans, Louisiana, is actually below sea level partly because groundwater has been removed from beneath it. In California's San Joaquin Valley, some areas have sunk by up to 8.5 meters (28 feet) due to groundwater removal. Cities built on river deltas, like Venice, Italy, and Bangkok, Thailand, have also experienced sinking. Mexico City, built on a former lake bed, has sunk by as much as 40 centimeters (16 inches) per year.

For coastal cities, sinking land can make problems like sea level rise even worse. For example, by 2070, over 5 million people in Bangkok are expected to be at risk of coastal flooding due to these combined factors.

Saltwater Mixing In

Aquifers near the coast often have a layer of freshwater on top and denser saltwater underneath. This saltwater comes from the ocean. If too much freshwater is pumped out near the coast, the saltwater can move into the freshwater aquifers. This contaminates the drinking water supply. Many coastal aquifers, like the Biscayne Aquifer near Miami, face this problem due to overpumping and sea level rise.

Saltwater intrusion is a natural process, but human actions and climate change can make it worse. When seawater mixes into coastal groundwater, it can make the water unusable if it's more than 2–3% salt. Along about 15% of the US coastline, local groundwater levels are already below sea level.

Climate Change and Groundwater

A woman pumps water from a handpump in her village in Sindh, Pakistan.

Effects of climate change can greatly impact groundwater. One major effect is an increased demand for irrigation water due to hotter, drier conditions. This leads to more groundwater being used, especially in dry areas. Climate change also causes changes to the water cycle. This can lead to less groundwater storage, reduced refilling, and poorer water quality due to extreme weather. For example, intense rain and floods in tropical areas seem to increase groundwater refilling. However, severe droughts can dry out and compact soil, reducing how much water soaks into the ground.

In higher altitude regions, less snow and shorter snow seasons can mean less groundwater refilling in spring. The impact of melting glaciers on groundwater is still being studied.

Sea level rise due to climate change has caused seawater to intrude into coastal aquifers worldwide. This is especially true in low-lying areas and small islands. This threatens coastal ecosystems and people's livelihoods. Bangladesh and the Sundarbans mangrove forest are particularly vulnerable.

Groundwater pollution might also increase because of climate change. More frequent and intense storms can wash contaminants, like fertilizers or waste, into groundwater. Droughts can also reduce river flows and groundwater levels, increasing the risk of contamination.

Adapting to Changes

Using more groundwater, especially in Sub-Saharan Africa, is seen as a way to adapt to more intense or frequent droughts caused by climate change. Groundwater-based solutions can store seasonal or occasional water surpluses. They lose much less water to evaporation than surface dams. For example, in tropical Africa, pumping groundwater can help make water and food supplies more resilient to climate changes.

Reducing Climate Impact

Developing geothermal energy is important for reducing carbon emissions and helping to slow down climate change. Groundwater plays a role in storing, moving, and extracting this sustainable energy. In countries like the Netherlands and Sweden, groundwater is increasingly used in district heating and cooling systems. It acts as a seasonal source, sink, or thermal "buffer." Deep aquifers can also be used for carbon capture and sequestration. This is a process of storing carbon underground to reduce carbon dioxide in the atmosphere.

Managing Our Groundwater

Groundwater withdrawal rates from the Ogallala Aquifer in the Central United States.

Groundwater governance means the processes used to manage, plan, and set policies for groundwater. This happens at local, regional, and even international levels. Managing groundwater involves practical actions and daily operations. Because groundwater is often seen as a private resource, it can be hard to regulate. Governments need to ensure groundwater is protected for everyone's benefit.

Laws and rules help control access to groundwater and human activities that affect its quality. These laws also need to protect areas where groundwater is refilled and where it flows out. They also set limits on how much water can be taken out sustainably.

Groundwater Around the World

Groundwater is a very important water resource for drinking water, especially in dry countries. The Arab region is one of the most water-scarce areas globally. Groundwater is the main water source in at least 11 of its 22 countries. However, taking too much groundwater in many parts of this region has caused groundwater levels to drop, especially in cities and farming areas.

Images for kids

Dzherelo, a common source of drinking water in a Ukrainian village.
Families collecting water from a water well in Niger.