What is the difference between confined and unconfined aquifers?

Confined aquifers are characterized by being surrounded by impermeable or semi-permeable layers of rock or sediment that prevent water from easily flowing in or out. This means that the water in a confined aquifer is under pressure, often resulting in the water rising above the level of the aquifer when tapped by a well, sometimes even flowing freely without the need for pumping.

In contrast, unconfined aquifers are directly recharged by surface water and are not bounded by impermeable layers above them. The top of an unconfined aquifer is known as the water table, which fluctuates based on the amount of recharge from precipitation and other sources. It is termed "open to the atmosphere" because it can freely interact with atmospheric pressure and does not have a confining layer above it.

This distinction in the configuration of confined and unconfined aquifers is critical for understanding groundwater movement, availability, and management. The difference also impacts aquifer characteristics such as porosity, permeability, and how they respond to extraction and recharge.