A confined aquifer is a water-bearing layer held between two layers of low-permeability material, so its water is under pressure.
A confined aquifer lies between an upper and a lower layer that let very little water through, such as clay or unfractured rock. Because the water is trapped, it is under pressure greater than atmospheric pressure. When a well reaches the aquifer, the water rises inside the well above the top of the aquifer. The level it rises to is the piezometric surface. If that surface is above the ground, the well flows without a pump and is called a flowing artesian well. Confined aquifers are usually recharged where the layer comes to the surface, which can be far from where the water is pumped, or by slow leakage through the confining layers. Water can take decades to thousands of years to move through them, so a confined aquifer can be depleted long before recharge makes up for it. They store and release water differently from unconfined aquifers. The pores stay full during pumping. Water comes out only because the water expands slightly and the aquifer skeleton compresses as pressure falls. Storativity is therefore very small, typically 0.00005 to 0.005. This is why a pressure drop from pumping spreads quickly and over long distances. The overlying layer does give good protection from surface pollution.