E. Earth and Space Systems · Water Systems

Groundwater, Aquifers and the Water Table

E2.3 A1.2 A1.4 A3.1About 50 minutes

Learning goals

I can describe how water soaks into the ground and fills the saturated zone.
I can explain what the water table and an aquifer are, using porosity and permeability.
I can explain how natural phenomena such as drought, heavy rain and snowmelt raise or lower the water table.
I can explain how human activities such as pumping wells, paving and draining wetlands change the water table.

Words to know

water table
The top of the saturated zone: the level below which all the spaces in the soil and rock are filled with water.

unsaturated zone
The layer of soil and rock above the water table, where the spaces hold both air and water.

saturated zone
The layer below the water table, where all the spaces are filled with water.

aquifer
A layer of sand, gravel or cracked rock that holds groundwater and lets it flow easily, so it can supply a well or spring.

aquitard
A layer, such as clay, that water moves through very slowly.

porosity
How much open space (pores) a material has to hold water.

permeability
How easily water can flow through a material.

recharge
Water soaking down to refill groundwater.

cone of depression
A dip in the water table around a well that is pumping water.

Remember from Lesson 1

Groundwater holds about 30% of Earth's fresh water, far more than all the lakes and rivers combined. Millions of Canadians, especially in rural areas and small towns, drink water pumped from wells. Some cities, such as Guelph, rely mostly on groundwater.

Dig a hole at the beach

If you dig a hole in the sand near a lake, at first the sand is just damp. Keep digging, and water starts to seep in and fill the bottom of the hole. You have reached the water table. Underground there is not a hidden lake, but sand and rock with water filling all the tiny spaces, like a soaked sponge.

A hole dug in sand on a lake beach has clear water in the bottom, with a blue toy shovel beside it. — The water seeping into the hole shows where the sand is saturated. You have dug down to the water table. AI-generated image

How groundwater forms

Rain and melting snow infiltrate (soak into) the ground.
The water percolates down through the unsaturated zone, where the spaces hold both air and water.
It reaches the saturated zone, where every space is full of water. The top of this zone is the water table.
Groundwater flows slowly, often only centimetres to metres per day, downhill toward places where it comes out (discharges): springs, streams, wetlands and lakes. In dry weather, most of the water in a stream may come from groundwater.

The water table usually follows the shape of the land, but more gently. Where the water table meets the surface, you find springs, wetlands, streams and lakes.

A cross-section of the ground near a lake (not to scale). Above the dashed water table is the unsaturated zone; below it, the saturated zone (shaded) is an aquifer. A clay layer slows water from moving deeper. The well reaches below the water table. Groundwater flows slowly toward the lake (arrow), and the lake's surface is at the level of the water table.

Porosity and permeability: what makes a good aquifer?

Porosity = how much space a material has to hold water.
Permeability = how easily water can flow through it.

Sand and gravel have big, connected spaces: high permeability. They make great aquifers. Clay has lots of tiny spaces (it can hold a lot of water) but they are so small that water barely moves: low permeability, an aquitard. Solid bedrock holds water only in cracks.

An important example in Ontario is the Oak Ridges Moraine, a long ridge of sand and gravel north of Toronto left by glaciers. Rain soaks into it easily, so it is a recharge area that feeds aquifers and the headwaters of many streams. Ontario protects it with the Oak Ridges Moraine Conservation Act (2001).

Natural phenomena	Effect on the water table
Heavy rain, spring snowmelt	Rises (more recharge)
Drought, long dry spells	Falls (less recharge; streams and wells may dry up)
Hot summer weather	Usually falls (more evaporation and transpiration, less water soaks down)
Frozen ground in winter	Little recharge until the thaw
Beaver dams, new wetlands	Can rise nearby (water held on the land soaks in)
A changing climate	Can change patterns: e.g. more winter rain and less snowpack, or longer dry spells

In much of Ontario, the water table is usually highest in spring and lowest in late summer or early fall.

Human activity	Effect on the water table	Why
Pumping wells (cities, farms, bottled water, golf courses)	Falls, especially near the well	Water is removed faster than recharge; a cone of depression forms
Paving, roofs, compacted soil	Falls	Less infiltration; rain runs off into storm drains
Draining wetlands, clearing forests	Falls	Less water held on the land to soak in
Dewatering quarries, mines, construction sites	Falls nearby	Water is pumped out to keep pits dry
Irrigation, leaky pipes, reservoirs behind dams	Can rise	Extra water soaks into the ground
Rain gardens, permeable pavement, protected recharge areas	Helps keep it up	More water soaks in

A high-capacity well pulls the water table down around it into a cone of depression (not to scale). The dashed line shows where the water table used to be. The neighbour's shallow well now ends above the lowered water table, so it runs dry.

Worked example

Let's explain a dry well

In late August, after a hot, dry summer, a family's shallow well runs dry. A new subdivision with large paved areas and several high-capacity wells was built nearby the year before. Explain two possible causes.

Natural cause: a hot, dry summer means little recharge, and plants and evaporation use the water that does fall. The water table naturally drops in late summer.
Human cause 1: the new subdivision's paving and roofs send rain into storm drains instead of letting it infiltrate, so less water recharges the aquifer.
Human cause 2: the nearby high-capacity wells create a cone of depression that can lower the water table under the family's property.
A shallow well is affected first, because its bottom is closest to the water table.
To find out which cause matters most, you would need evidence: water-table measurements over several years, rainfall records and pumping records.

The well likely went dry because the water table dropped below the bottom of the well: drought lowered recharge, paving reduced infiltration, and nearby pumping drew the water table down.

Just for fun

If the water table were a real table, it would be the wobbliest furniture ever: high in spring, low in August, with a cone-shaped dent wherever someone pumps a well. Nobody would trust it with a glass of milk.

Hands-on activity

Model an aquifer in a cup (A1.2, A1.4)

You need: a clear plastic cup or jar, aquarium gravel or small pebbles, sand, a piece of plastic wrap, a drinking straw, a small cup of water, and a turkey baster or a clean medicine syringe (optional).

Put about 4 cm of gravel in the cup, then about 3 cm of sand on top. This is the ground.
Stand the straw upright along the side, pushed down to the bottom: your well.
Slowly pour water onto the sand ("rain"). Watch it infiltrate and fill the bottom. Stop when the water level is halfway up the gravel. Mark the water table on the outside of the cup.
Pump the well: use the baster or syringe in the straw to remove water. What happens to the water table? (You can also blow gently into the straw to see water move, but don't drink it.)
Pave it: cover half the sand with plastic wrap and "rain" again with the same amount of water. Does all the water still soak in?
Drought: leave the cup for two days. What happens to the water table?
Record your observations in a table: action, prediction, what happened, why.

What to look for: water fills the spaces from the bottom up; pumping lowers the level; plastic wrap makes water run off to the edge instead of soaking in evenly; the level drops slowly with evaporation.

Safety (A1.4): do this over a tray, wash your hands after handling wet sand and gravel, and never drink the model water.

Careers connection (A3.1)

Well technicians (who drill and build wells), hydrogeologists (scientists who study groundwater), environmental technicians and plumbers (a skilled trade) all work with groundwater. Well technicians in Ontario must be licensed by the province, and wells must be built to rules that keep surface water and pollution from running down the outside of the well casing.

Your turn

What is the water table?

Describe the difference between the unsaturated and saturated zones.

What makes sand and gravel good aquifers?

Name two natural events that raise the water table.

Name two human activities that lower the water table.

What is a cone of depression?

Why does a stream keep flowing in a dry summer even when it hasn't rained for weeks?

More practice: warm-up

What is recharge?

Is clay an aquifer or an aquitard?

Where does groundwater come out at the surface? Name two places.

In what season is Ontario's water table usually highest?

What is the name of the sand and gravel ridge north of Toronto that recharges aquifers?

More practice: on your own

Explain the difference between porosity and permeability, using clay as an example.

Why does a city's stormwater system lower the water table if it sends rain straight into pipes?

Why should a gas station not be built on top of a recharge area?

A farmer wants to irrigate crops using a new high-capacity well. What could happen to neighbours' wells, and what could reduce the problem?

Draining a wetland to build a parking lot affects the water table in two ways. Explain both.

Challenge

The water table under a town dropped 2 m over 10 years. Rainfall was normal. Suggest two human causes and one way to collect evidence for each.

Why can groundwater pollution last much longer than pollution in a river?

Explain why a deeper well is less likely to go dry in a drought than a shallow one.

Check your understanding

Quiz

The water table is...

Which material makes the best aquifer?

Which of these would most likely RAISE the water table?

A dip in the water table around a pumping well is called a...

Why does paving lower the water table?

Permeability describes...

For parents and teachers

Big idea (E2.3): groundwater fills the saturated zone below the water table; aquifers are permeable layers that store and transmit it. Natural phenomena (precipitation, drought, seasons, snowmelt) and human activities (pumping, paving, draining wetlands, dewatering, irrigation) raise or lower the water table. The cup model is a hands-on investigation (A1.2) with simple safety routines (A1.4); the careers tip connects to skilled trades (A3.1).

How to help: if you have a well, talk about how deep it is and whether it was ever affected by drought. Visit a spring, creek or wetland in late summer and ask where the water comes from when it hasn't rained.

Common difficulties:

Picturing groundwater as underground lakes or rivers. It mostly fills spaces between grains, like a soaked sponge.
Confusing porosity and permeability. Holding versus flowing.
Forgetting time: recharge and groundwater flow are slow.

Facts used: Ontario requires a Permit to Take Water for most takings over 50 000 litres per day; the Oak Ridges Moraine Conservation Act was passed in 2001; well construction in Ontario is regulated and well technicians must be licensed.