What defines a confined aquifer?

A confined aquifer is characterized by the presence of water that is under pressure greater than atmospheric pressure. This pressure situation occurs because the aquifer is trapped between layers of impermeable rock or soil, which prevents water from escaping. As a result, when a well is drilled into a confined aquifer, groundwater will rise above the top of the aquifer or even flow to the surface without the need for pumping, depending on the pressure and the elevation of the well.

In contrast, an aquifer with water under atmospheric pressure, as mentioned in the first choice, is indicative of an unconfined aquifer, where the water table is directly influenced by atmospheric pressure and can fluctuate based on rainfall and water usage. The third choice regarding saltwater pertains to aquifers that might have saline groundwater but does not define the aquifer's confinement status specifically. The fourth choice regarding an aquifer that is always dry is also incorrect, as the defining feature of a confined aquifer is that it contains water, not that it is devoid of it. Thus, the definitive characteristic leading to the identification of a confined aquifer is the presence of water under pressure greater than atmospheric pressure.