Groundwater flows underground within cracks in rocks or through pores in gravel and sands. The layers of rocks or sediment that contain groundwater in sufficient quantities for use are called aquifers.
Aquifers can be seen as natural storage tanks that are recharged by rain or river flow. Because water can be banked between rainfall events, it is a more reliable supply than rivers.
Groundwater stored in aquifers is accessed by drilling a well down to the water-bearing layer. The annual volume of water yielded from an aquifer is usually only a small fraction of its total storage capacity.
The two main types of aquifers which we draw our water from have either an unconfined or a confined structure.
In an unconfined aquifer, groundwater only partly fills the aquifer and the upper surface of the water table is free to rise and fall. There are no overlying 'confining beds' of low permeability materials to physically isolate the groundwater system from the surface or atmosphere.
A confined aquifer is capped by a layer of less permeable material, such as clay, which pressurises the aquifer, a bit like forcing water into a balloon. If a well is drilled into a confined aquifer, the natural pressure can force groundwater up the well to the surface, just like piercing a balloon. This type of free-flowing well is called an artesian well. Springs are created by the upwelling of water from aquifers in low-lying areas or water that has been forced through cracks in the capping layer of a confined aquifer.