An aquifer is a layer or body of soil, sediment, or rock that can both hold groundwater and allow it to move. The material might be sand and gravel, porous rock, or fractured rock. An aquifer is not an empty underground tank: its water occupies spaces within the material.
Two properties help describe how an aquifer works. Porosity is the amount of open space in a material, where water may be stored. Permeability describes how well those spaces connect and how readily water can pass through them. A material can have many pores yet transmit water slowly if the pores are poorly connected.
An unconfined aquifer has an upper surface that is not sealed by a low-permeability layer. Water can enter it from above as it infiltrates the ground, and its water level can rise or fall as conditions change. The top of the saturated zone is called the water table.
A confined aquifer lies beneath a layer that restricts the movement of water. The confining layer does not necessarily stop all water, but it can slow its passage. Water in a confined aquifer is under pressure; when a well reaches it, water may rise within the well above the top of the aquifer.
These terms describe general conditions, not a guarantee of how much water is available or how quickly it can be reached. Aquifers vary widely in their materials, extent, and connection to nearby water. Their behavior depends on local geology and the ways water enters and leaves the ground.
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