What it shows
Groundwater is not an underground lake but water filling the spaces between grains of sand and gravel. A straw well draws it out, rain refills it from above, and anything spilled on the surface can reach the well, showing why wells go dry and why they get polluted.
The science
Rain soaks into the ground and fills the pores of permeable rock and sand; a layer that holds and yields water is an aquifer. The top of the saturated zone is the water table. A well is a hole that reaches below the water table, and pumping lowers the table around it in a cone. A clay layer is almost impermeable, so it can protect a deeper aquifer or trap a shallow one. Pollutants dissolved in surface water follow the same paths and reach the well.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A large clear jar or a cut 5 litre bottle
- Gravel, coarse sand and a layer of clay or modelling clay
- A straw or a transparent pipe as the well
- A syringe without a needle to pump
- A jug of water and a watering can for rain
- A drop of food colour as the pollutant
Safety
For this project: An adult should cut the bottle.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Put 3 centimetres of gravel in the jar, then 5 centimetres of sand, and press a thin clay layer across half the width.
Add another 4 centimetres of sand on top and push the straw down to the gravel on the side without clay.
Pour water slowly down one side and watch it fill the gravel and rise through the sand; mark the water table on the jar.
Pump with the syringe through the straw and watch the water table dip near the well.
Sprinkle "rain" from the watering can and watch the table rise again.
Keep pumping without rain until the well runs dry and note how far the table fell.
Put a drop of food colour on the surface above the well, sprinkle rain and watch the colour travel to the well.
Repeat the colour test above the clay layer and compare how it spreads.
What you should see
A visible water table that falls when pumped and rises with rain, a well that runs dry under over-pumping, and a colour plume that reaches the well in minutes but is held up by the clay.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is an aquifer?
A layer of permeable rock or sand that holds and yields groundwater.
2What is the water table?
The top of the zone where all the pores are full of water.
3Why do wells run dry?
Pumping faster than rain refills the aquifer lowers the water table below the well.
4How does a clay layer help?
It blocks water and pollutants from passing through, protecting the aquifer below.
Take it further
Add a second well on the clay side and show how a shallow and a deep aquifer behave differently.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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