What distinguishes a confined aquifer from an unconfined aquifer?

A confined aquifer is characterized by being overlain by a confining layer, also known as an aquitard or aquiclude, which restricts water movement between the aquifer and the surface. This confining layer prevents water in the confined aquifer from easily interacting with the surface environment or unconfined aquifers above it. As a result, water in a confined aquifer is typically under pressure, leading to the potential for artesian conditions where water can rise above the top of the aquifer when tapped into by a well.

In contrast, an unconfined aquifer does not have a confining layer on top and is directly influenced by surface conditions. Water levels in unconfined aquifers fluctuate with changes in precipitation and surface water, while confined aquifers maintain more stable water levels due to their protective layers. This fundamental difference in structure and pressure explains why being overlain by a confining layer is a key distinguishing feature of confined aquifers.