What defines a confined aquifer?

A confined aquifer is characterized by being a saturated geological formation that is located between layers of material with low permeability, such as clay or certain types of rock. This configuration creates pressure within the aquifer, allowing the water to be stored under pressure. When a well taps into a confined aquifer, the pressure may be sufficient to allow groundwater to flow to the surface naturally without the need for pumping, leading to what is known as an artesian well.

The distinction of being sandwiched between impermeable layers is critical because it limits the flow of water in and out of the aquifer, making it different from an unconfined aquifer, where the water table is directly influenced by surface conditions and can rise and fall with rainfall and other environmental factors. The other choices reflect different concepts and do not correctly define a confined aquifer. For instance, fluctuations based on rainfall pertain to unconfined aquifers, and the mention of surface water for irrigation or a well that does not require a pump points to different hydrological dynamics altogether.