An extended period of dry weather may decrease recharge and cause the water table to fall. Aquifer is the name given to underground soil or rock through which ground water can easily move. The amount of ground water that can flow through soil or rock depends on the size of the spaces in the soil or rock and how well the spaces are connected. The amount of spaces is the porosity . Permeability is a measure of how well the spaces are connected.
Aquifers typically consist of gravel, sand, sandstone, or fractured rock such as limestone.
These types of materials are permeable because they have large connected spaces that allow water to flow through. The spaces in a gravel aquifer are called pores . The spaces in a fractured rock aquifer are called fractures . If a material contains pores that are not connected, ground water cannot move from one space to another. These materials are said to be impermeable .