Groundwater is water that, after infiltrating and percolating through surface soils, flows into an aquifer, an underground water-bearing layer of porous rock. About one-third of the UK's drinking water is drawn from aquifers.
To permit economic development, an aquifer must be able to transmit large quantities of water from one point to another and therefore it must have a high permeability. The groundwater contained in aquifers is released from springs and can be responsible for the bulk of river flows.
Usually, aquifers are alluvial sands (sands which have been carried in suspension by rivers or floods) and gravels, the coarser sedimentary rocks such as sandstone, and rocks such as limestone in which chemical action has increased the water-bearing capacity. The strata of relatively impermeable rocks that lie either above or below confined aquifers (aquifers trapped between two impervious layers of rock) are called aquicludes.
The flow of groundwater takes place through the layer that is completely saturated. This layer, not surprisingly, is called the zone of saturation (as shown in Figure 5).