Water infiltrates the Earth's surface and accumulates on top of impermeable geologic materials. Areas in which the pore spaces are completely filled with water are called the saturated zone. Above the saturated zone is an area in which the pore spaces are filled with air and water called the unsaturated zone.

Aquifers can be confined by impermeable layers such as clay. The water under confining layers typically is under pressure. If a well is drilled, the water level will rise above the saturated zone. The hydraulic head or piezometeric water level can be measured in wells of confined aquifers.