When rain soaks into the ground, it trickles down through tiny spaces between grains of soil and rock. Deep enough, every space is full of water. That water is groundwater, about 30% of Earth's fresh water, fed by the infiltration step of the water cycle.
- The unsaturated zone: near the surface, the spaces hold both air and water.
- The water table: the top of the zone where every space is filled with water.
- The saturated zone: below the water table, all the spaces are full.
- An aquifer: a layer of rock or sediment that holds water and lets it flow, such as sand, gravel or sandstone. Wells are drilled into aquifers, below the water table.
Key idea
Groundwater fills the tiny spaces in soil and rock. Porosity is how much open space a material has; permeability is how easily water flows through it. A good aquifer needs both.
Why it works
Porosity is a percent: pore volume ÷ total volume. Clay has high porosity, often around 50%, but its pores are so tiny that water barely moves through them: low permeability. Sand and gravel have less pore space, 25 to 40%, but the pores are large and connected, so water flows easily. That makes sand and gravel good aquifers and clay a barrier.
Worked example: Measuring porosity
A 500 cm³ jar of dry gravel can hold 150 cm³ of water poured into its spaces. What is its porosity?
- Porosity = pore volume ÷ total volume.
- $150 \div 500 = 0.30$, which is 30%.
Answer: 30%
Worked example: Where to drill
Layer 1 is clay; layer 2 below it is sand; the water table is 20 m down, in the sand. Where should a well end?
- The well must reach below the water table, where the spaces are full of water.
- It should end in the sand, which lets water flow in; clay would give very little.
Answer: In the sand, below 20 m
Common mistake
Picturing groundwater as underground lakes and rivers. Except in caves, groundwater mostly fills tiny spaces between grains. Also, porosity (how much space) is not permeability (how easily water flows): clay is very porous but holds its water tightly. Check: a good aquifer is both porous and permeable.