Aquifer structure

Groundwater sits in the pore spaces and fractures of rock, not in underground lakes. An aquifer is a rock formation that holds and transmits usable quantities of water.

Unconfined aquifer: Open to the atmosphere through the soil above. Recharges quickly, and is correspondingly vulnerable to surface pollution.
Confined aquifer: Capped by an impermeable layer. Recharges only where it outcrops, often far away, and can be under enough pressure to flow without pumping.
Porosity and permeability: Porosity is how much water a rock holds; permeability is how easily it flows. Clay has high porosity but almost no permeability, which is why it makes a confining layer rather than an aquifer.

Vertical scale is exaggerated. Real aquifers extend for tens or hundreds of kilometres horizontally and are often only metres to tens of metres thick.

Text description

A cross-section through the ground. Below the surface is the unsaturated zone, where pores contain both air and water. Below the water table is the saturated zone, where all pores are full. An unconfined aquifer sits directly beneath the water table. Lower down, a confined aquifer is sandwiched between two impermeable layers, so water in it is under pressure and rises in a well above the top of the aquifer.