What If? Lab
What if a town used up its groundwater?
Under our feet, water hides in tiny spaces between rocks. Many towns pump it up to drink. What if a town used too much? Let's imagine it with Olly!
Below the ground, water fills the tiny gaps between grains of rock and sand. In the United States, this groundwater is the drinking water for about half of all people. Rain and snow refill it, but often slowly. Imagine a town pumping faster than nature refills it, and follow what might change and how people help.
USGS defines groundwater depletion as long-term water-level decline caused by sustained pumping, and compares an aquifer to a bank account: withdraw faster than deposits arrive and problems follow. This thought experiment follows one imagined town through its aquifer, its wells and the ways people manage water.
The big question
Some towns get their water from deep under the ground.
What if a town used the water faster than rain could put it back? Let's find out.
You might picture underground water as a hidden lake, but most of it sits in the tiny spaces between grains of sand and rock, like water in a sponge. People drill wells to pump it up.
What if a town pumped out its groundwater faster than rain and snow could refill it? In this thought experiment you can follow three paths: the water under our feet, wells that have to go deeper, and the ways people save water.
Groundwater is stored in aquifers, rock or sediment that holds water and transmits it readily to wells and springs. Precipitation recharges aquifers, but USGS stresses that the rate of recharge differs from aquifer to aquifer and must be considered when pumping.
This thought experiment imagines a town whose pumping steadily outpaces recharge. It reasons from documented effects of depletion in the United States; the town itself, and what happens to it, are imagined.
Explore the idea
Make a prediction
Where does the water under the ground come from?
What refills the water stored in aquifers underground?
According to USGS, which of these naturally recharges groundwater?
- Rain and snow soaking into the ground
- Seawater flowing under the land
- Rocks that make new water
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Established fact
Groundwater comes from rain and snow, and pumping too fast lowers it.
Rain and snow recharge aquifers, and pumping faster than recharge lowers the water table and can dry up wells (USGS).
Sustained pumping beyond recharge causes depletion, with dry wells, reduced streamflow, poorer quality, higher costs and subsidence (USGS).
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An idea scientists discuss
The thirsty town is pretend. We imagine what might happen.
The town in this experiment is imagined. What might happen to it is an idea built from what USGS has seen in real aquifers.
The imagined town combines effects USGS documents separately; its outcomes might differ greatly in any real aquifer.
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Established fact
Many people drink water that comes from under the ground.
In the United States, groundwater is the drinking water for about half of all people and nearly everyone in the countryside.
Groundwater is the source of drinking water for about half the U.S. population and nearly all the rural population (USGS).
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Established fact
There is lots of fresh water hidden under the ground.
There is about a hundred times more fresh water in the ground than in all the world's rivers and freshwater lakes.
USGS estimates roughly a hundred times more fresh water in the ground than in all rivers and freshwater lakes.
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Established fact
Some water under the ground is very old.
Shallow groundwater might be only a few hours old, while very deep groundwater can have been underground for several thousand years.
Shallow water may be hours old, moderate-depth water about 100 years, and deep or far-traveled water several thousand years or more (USGS).
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An idea scientists discuss
If a town used old water fast, it might take a long time to come back.
If a town pumped up water that took thousands of years to collect, the aquifer might take a very long time to refill.
Where recharge is slow relative to storage, depletion might be effectively irreversible on human timescales; the actual rate depends on the aquifer.
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Established fact
Pumping too fast can make a well give less water.
Pumping too much water too fast can make a well give less and less water, and even run dry.
Pumping too much water too fast draws down an aquifer and causes a well to yield less and less water and even run dry; it can dry a neighbor's well on the same aquifer (USGS).
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Established fact
In some places, the water under the ground has dropped a long way.
The High Plains aquifer lies under parts of eight states and is pumped heavily to water farms. In some areas its water level has dropped more than 100 feet.
The High Plains aquifer, including the Ogallala, underlies parts of eight states; since predevelopment, levels have fallen more than 100 feet in some areas and saturated thickness has halved in others (USGS).
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Established fact
Streams can get some of their water from under the ground.
Some, and often a great deal, of the water in rivers seeps in from groundwater, so pumping can leave streams and lakes with less water.
Groundwater contributes to streams in most settings; pumping can intercept that flow or draw water out of streams, lakes and wetlands (USGS).
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An idea scientists discuss
In the pretend town, wells might need to go deeper and streams might get smaller.
In the pretend town, wells might need deepening, pumping might cost more, and a nearby stream might carry less water.
The imagined town might face falling yields, rising costs and reduced streamflow together; which effect comes first might depend on its aquifer.
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Established fact
People can help water soak back into the ground.
Artificial recharge adds water to an aquifer on purpose, through canals, ponds and basins, or by sending water down special wells.
Artificial recharge uses canals, infiltration basins, ponds, irrigation furrows or injection wells; aquifer storage and recovery stores available water through wells and retrieves it from the same wells in dry periods (USGS).
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Established fact
Fixing drips at home saves lots of water.
EPA's WaterSense program says household leaks can waste 180 gallons a week in an average family, and turning off the tap while brushing your teeth can save 8 gallons a day.
WaterSense reports average home use of 82 gallons a day per American (USGS, 2015), leaks of 180 gallons a week in an average family, and 8 gallons a day saved by turning off the tap while brushing (EPA).
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An idea scientists discuss
If the town pumps less and refills more, its water might slowly come back.
If the pretend town pumped less and helped more water soak in, its water table might slowly rise again, depending on how fast its aquifer refills.
Reduced withdrawals plus managed recharge might stabilize or raise the imagined water table; the timescale might range from years to far longer, depending on the aquifer.
In our world
Dig a hole at the beach, and water seeps in. That water was already in the sand!
Water hides under the ground almost everywhere. Rain and snow slowly soak down to refill it.
USGS has a neat way to see groundwater: dig a hole at the beach and watch it partly fill with water from the sand. The top of that wet zone is called the water table, and the soaked ground below is an aquifer.
There is about a hundred times more fresh water in the ground than in all the world's rivers and freshwater lakes. In the United States, groundwater is the drinking water for about half of all people and nearly everyone in the countryside. Rain and snow that soak into the ground refill it, and that is called recharge.
Below the water table, water fills the pores between rock particles and the fractures in rock. USGS estimates about a hundred times more fresh water in the ground than in all rivers and freshwater lakes. In the United States, groundwater supplies drinking water for about half the population and nearly all the rural population, and over 50 billion gallons per day for agriculture.
Groundwater can be young or very old: water at shallow depth may be only a few hours old, at moderate depth about 100 years, and at great depth several thousand years or more.
Imagine a thirsty town
Let's pretend a town pumps up lots and lots of water, every day.
The water under the town might sink lower. Wells might need to go deeper. These are only ideas.
Now imagine a town that pumps more water every year than rain and snow can put back. The water table under the town might sink lower and lower, the way a bank account shrinks if you spend more than you save.
Wells might give less water, and some might need to be drilled deeper. Nearby streams fed by groundwater might carry less water. These are ideas, but each one is something USGS has seen happen in real places where pumping outpaced recharge.
In the imagined town, sustained pumping beyond recharge might lower the water table year after year. Each well might draw a deeper cone of depression, yields might fall, and pumping might cost more because water must be lifted farther.
Streams and wetlands that receive groundwater might shrink, and in some aquifers the ground might slowly compact and sink. Near a coast, salt water might move inland. These are reasoned ideas for one imagined town; USGS documents each effect separately in real aquifers.
Why groundwater refills slowly
Rain and snow soak down into the ground. Under the ground, water can move very slowly through soil and rock.
Some water under the ground is very, very old.
After rain soaks in, groundwater can move very slowly. USGS says that in some rocks it travels several meters in a day, but in others only a few centimeters in a hundred years. Clay and shale let water through very slowly.
That is why pumping matters. USGS explains that pumping too much water too fast draws down the water in an aquifer, and a well can give less and less water. Pumping one well hard can even make a neighbor's well run dry if both share the same aquifer.
Flow through an aquifer depends on permeability: several meters a day in some materials, a few centimeters a century in others, and very slow through clay and shale. Confining layers can divert water sideways rather than letting it sink.
A pumping well creates a cone of depression around it. If withdrawal exceeds replenishment from infiltration or streams, the water table falls, briefly or for decades, by a little or by hundreds of feet. USGS lists the results: dry wells, less water in streams and lakes, poorer water quality, higher pumping costs and land subsidence.
How people take care of groundwater
People watch the water under the ground very carefully.
Some towns help water soak back into the ground. Families save water by fixing drips.
People are working on this. USGS measures water levels in wells to see how aquifers are changing. Some places use artificial recharge: they guide water into ponds and basins where it can soak down, or send it underground through special wells. Others store spare water underground through wells and pump it back out in dry times.
Towns can find other sources too: USGS notes that Tampa, Florida, built a plant that turns seawater into drinking water to depend less on groundwater. At home, EPA's WaterSense program says families can save water by fixing leaks and turning off the tap while brushing their teeth.
USGS monitors wells to evaluate depletion and recharge. Artificial recharge adds water to aquifers through canals, infiltration basins, ponds, irrigation furrows or injection wells; aquifer storage and recovery stores water through wells and retrieves it in dry periods. USGS has studied artificial recharge since 1905.
Tampa built a desalination plant to reduce its dependence on groundwater (USGS). Demand matters as well: EPA's WaterSense program reports an average of 82 gallons a day per American at home (citing USGS for 2015) and says household leaks can waste 180 gallons a week in an average family.
Amazing facts
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Some water under the ground has been there for thousands of years.
Shallow groundwater might be just a few hours old, but deep groundwater can have been underground for several thousand years or longer.
Groundwater ages range from hours near the surface to about 100 years at moderate depth and several thousand years or more at great depth (USGS).
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People have found fresh water more than a mile under the ground.
Rocks holding fresh water have been found more than 6,000 feet below the surface.
Freshwater-yielding rocks have been found at depths of more than 6,000 feet, and salty water has come from oil wells as deep as 30,000 feet (USGS).
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A few drops of food coloring can find a leaky toilet.
EPA suggests putting a few drops of food coloring in a toilet tank; if color shows up in the bowl after 10 minutes, the toilet is leaking.
EPA's leak test: food coloring in the tank that reaches the bowl within 10 minutes reveals a leak; flush right after to avoid staining.
Words to know
- groundwater
- water stored underground in the spaces between grains of soil and rock and in cracks in rock
- aquifer
- underground rock or sediment that holds water and lets it flow to wells and springs
- water table
- the top of the underground zone where every space is filled with water
- recharge
- water from rain and snow that soaks down and refills an aquifer
- depletion
- a long-term drop in groundwater levels caused by pumping more than is refilled
- well
- a hole drilled or dug down below the water table to bring up groundwater
Discovery challenge
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What is an aquifer?
What is an aquifer?
Which description fits an aquifer?
- Rock or sand underground that holds water and lets it flow to wells
- A machine that makes rain
- A lake at the top of a mountain
Show the answer
Rock or sand underground that holds water and lets it flow to wells
Rock or sand under the ground that holds water.
Underground rock or sediment that holds water and lets it flow to wells and springs.
A water-bearing rock that readily transmits water to wells and springs (USGS).
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How much fresh water is under the ground, compared with all the rivers and lakes?
Compared with all the world's rivers and freshwater lakes, how much fresh water is in the ground?
USGS estimates that the fresh water in the ground, compared with all rivers and freshwater lakes, is…
- About 100 times more
- About the same
- About half as much
Show the answer
About 100 times more
Much more under the ground: about 100 times more!
About a hundred times more fresh water is in the ground.
…about a hundred times more (USGS).
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If people pump water faster than rain puts it back, what happens to the water under the ground?
What happens to the water table when pumping is faster than recharge?
Sustained pumping beyond recharge makes the water table…
- It drops lower
- It rises higher
- It stays exactly the same
Show the answer
It drops lower
It sinks lower, so wells may need to go deeper.
It drops, and wells can give less water or even run dry.
…fall, possibly for decades and by hundreds of feet (USGS).
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How can you check a toilet for a leak?
Which simple test does EPA suggest for finding a toilet leak?
EPA's WaterSense toilet-leak test uses…
- A few drops of food coloring in the tank
- Turning on the hot water
- Leaving the lid open overnight
Show the answer
A few drops of food coloring in the tank
Food coloring! If the color reaches the bowl, there is a leak.
Food coloring in the tank: if color shows up in the bowl after 10 minutes, there is a leak.
…food coloring in the tank; color in the bowl after 10 minutes means a leak (EPA).
Experiments to try
- Watch Try it What If? Lab What if a habitat lost its water supply? Water makes plants grow, and plants feed animals. In our model, a dry spell cuts the plant food from 50 tonnes a year to 20, so the land can feed far fewer deer. Watch the herd shrink to fit, then find out how drought affects real wildlife and how people help.
- Watch What If? Lab What if the Sahara turned green? Thousands of years ago, the Sahara Desert was a land of grass, wetlands and lakes, where cave paintings show hippos in watering holes. A slow wobble of Earth on its axis brought stronger monsoon rains. Find out why the Sahara was green, why it dried, and what might change if it turned green again.
- Watch What If? Lab What if you found a meteorite? Meteorites are pieces of space rock that survived a fiery trip through Earth's air. Most come from asteroids, a few from the Moon or Mars, and many are older than any rock ever found on Earth. Imagine finding one: learn the clues scientists look for, what is inside, and why the right first step is to tell a grown-up and ask a museum or university.
- Watch What If? Lab What if the ground were made of limestone? Limestone is a rock that rainwater slowly dissolves. Over thousands of years, water seeping through it carves caves, opens sinkholes and bubbles back up as springs. About a fifth of the land in the United States is already this kind of landscape, called karst. Imagine the ground everywhere were limestone, and find out what would happen to the rain.
- Watch What If? Lab What if a drought lasted for years? A drought is a long stretch with much less rain or snow than normal, long enough that water runs short. Droughts can last a few weeks, a few months or several years. Imagine a land where the dry years keep coming, and follow its rivers, the ways people save water, and what it takes for a drought to end.
- Watch What If? Lab What if there were no wetlands? Marshes, swamps and bogs are wetlands: places where water covers the soil or sits just below it. EPA compares them to natural sponges that soak up rain and floodwater and let it go slowly. They also clean water and shelter many birds and young fish. Imagine a world with no wetlands, and follow what might change.
Explore next
- Watch Try it What If? Lab What if a habitat lost its water supply? Water makes plants grow, and plants feed animals. In our model, a dry spell cuts the plant food from 50 tonnes a year to 20, so the land can feed far fewer deer. Watch the herd shrink to fit, then find out how drought affects real wildlife and how people help.
- Watch What If? Lab What if the Sahara turned green? Thousands of years ago, the Sahara Desert was a land of grass, wetlands and lakes, where cave paintings show hippos in watering holes. A slow wobble of Earth on its axis brought stronger monsoon rains. Find out why the Sahara was green, why it dried, and what might change if it turned green again.
- Watch What If? Lab What if you found a meteorite? Meteorites are pieces of space rock that survived a fiery trip through Earth's air. Most come from asteroids, a few from the Moon or Mars, and many are older than any rock ever found on Earth. Imagine finding one: learn the clues scientists look for, what is inside, and why the right first step is to tell a grown-up and ask a museum or university.
- Watch What If? Lab What if the ground were made of limestone? Limestone is a rock that rainwater slowly dissolves. Over thousands of years, water seeping through it carves caves, opens sinkholes and bubbles back up as springs. About a fifth of the land in the United States is already this kind of landscape, called karst. Imagine the ground everywhere were limestone, and find out what would happen to the rain.
- Watch What If? Lab What if a drought lasted for years? A drought is a long stretch with much less rain or snow than normal, long enough that water runs short. Droughts can last a few weeks, a few months or several years. Imagine a land where the dry years keep coming, and follow its rivers, the ways people save water, and what it takes for a drought to end.
- Watch Geography Where Earth's water is About 71 percent of Earth's surface is covered by water, and the oceans hold about 96.5 percent of all of it. Only about 2.5 percent of Earth's water is fresh, and most of that is frozen in ice or hidden underground. The lakes and rivers we use every day hold only a tiny part.
- Watch What If? Lab What if there were no wetlands? Marshes, swamps and bogs are wetlands: places where water covers the soil or sits just below it. EPA compares them to natural sponges that soak up rain and floodwater and let it go slowly. They also clean water and shelter many birds and young fish. Imagine a world with no wetlands, and follow what might change.
- Watch Try it What If? Lab What if Earth had no atmosphere? Earth's air works like a jacket: it keeps heat in and carries it around. Take the air away in our model of a sandy patch of ground on the equator, and the coldest part of the night drops from about 13 °C to about 2 °C.
For parents & teachers
What children will learn
- Groundwater is stored in the spaces between grains of rock and sediment, not mostly in underground lakes.
- Rain and snow recharge aquifers, often slowly.
- Pumping faster than recharge lowers the water table and affects wells, streams and land.
- People manage groundwater with monitoring, artificial recharge, new sources and saving water.
Recommended ages: 9 to 17 · About 15 minutes
Talk about it
- Why might two neighbors who share one aquifer need to talk about how much water they pump?
- Where does the water in your home come from? How could you find out?
- Which ways of saving water at home could your family try this week?
Try at home or in class
- Try the two-sponge model from USGS's Water Science School: stack two sponges, pour water on top and watch it soak down; then put plastic wrap between them and see the water flow out sideways, as it does above a confining rock layer.
- With a grown-up, do EPA's toilet leak test: add a few drops of food coloring to the tank, wait 10 minutes without flushing and look for color in the bowl; flush straight afterwards.
Key vocabulary: groundwater, aquifer, water table, recharge, depletion, well
Where these facts come from
- Groundwater Decline and Depletion , USGS Water Science School
- Aquifers and Groundwater , USGS Water Science School
- Groundwater: What is groundwater? , USGS Water Science School
- Land Subsidence , USGS Water Science School
- Artificial Groundwater Recharge , USGS Water Resources Mission Area
- Statistics and Facts , U.S. Environmental Protection Agency, WaterSense
- Fix a Leak Week , U.S. Environmental Protection Agency, WaterSense
Checked, not yet human-reviewed. Last updated October 27, 2026. Spotted a mistake? Tell us.