In terms of aquifer type, what does "confined" refer to?

A confined aquifer refers to a groundwater system that is encased between layers of impermeable or semi-permeable rock or soil that prevent water from easily flowing to or from the aquifer. This confinement creates pressure within the aquifer, meaning that when a well is drilled into it, the water level can rise above the top of the aquifer in the well, often causing the water to flow out naturally without the need for pumping. The presence of the surrounding impermeable materials is key to defining the nature of a confined aquifer, as these layers serve to protect the aquifer from contamination, as well as maintain its pressure and characteristics.

The other options do not accurately describe the nature of confined aquifers. Being located near sea level does not influence whether an aquifer is confined or unconfined; it relates more to geographical and hydrological factors. Exposure to the atmosphere is characteristic of unconfined aquifers, where the upper layer, known as the water table, is open to atmospheric pressure. High pollution levels are a concern for both confined and unconfined aquifers, but they do not define the type of aquifer itself. Thus, the description of being surrounded by impermeable materials clearly and accurately characterizes a confined aquifer