What defines a confined aquifer?

A confined aquifer is characterized as an underground layer where groundwater is stored and is often separated from surface water by an impermeable layer, such as clay or rock. This definition highlights the essential features of a confined aquifer, including the fact that the water within it is trapped under pressure between two impermeable layers. This pressure can influence the flow of water when accessed, allowing it to rise in a well or borehole.

The distinction from surface water is critical as it underscores the aquifer's role in groundwater storage, where the quality and purity of the water are often protected from direct contamination by surface activities. This context is crucial for understanding groundwater management and the impact of activities, such as those associated with underground storage tanks, on aquifer integrity.

In contrast to this definition, the other answer choices do not accurately describe a confined aquifer's properties or significance. For instance, exposure to surface water pertains more to unconfined aquifers, which lie closer to the surface and are not protected by impermeable layers. Similarly, defining a surface water body or drilling accessibility does not relate to the characteristics that define confined aquifers. Thus, the choice emphasizing groundwater storage and separation from surface water correctly captures essential attributes of a confined aquifer.