Permeable layers transmit groundwater through the subsurface in an aquifer. The permeability of these layers is dictated by the pore space available for water transmission through the geologic material they are composed of, such as porous rock, sediment or soil. Coarse material typically has a higher water transmission capability.
The pore space available in a specific geologic material is measured as porosity, or the ratio of open space to material that transmits water poorly because of its density. The entire permeability of all the layers in an aquifer is measured by its transmissivity. Transmissivity is defined as the rate of flow in gallons per minute of water through an aquifer. This flow rate, along with the saturated thickness of the aquifer, is often used to determine the yield of a well.
Confining layers that do not allow the transmission of water, or do so at a very slow rate, are called impermeable layers. These layers are composed of a very dense material, such as solid rock or clay. When ground water in an aquifer is bounded by impermeable layers, and tapped by a well, it can be forced up above the ground. The effect is called an artesian well, and can expel water with significant force.