Which of the following best describes an aquifer?

An aquifer is best described as a rock formation or a geological structure that is capable of yielding significant quantities of water. This capacity is crucial for aquifers, as they serve as vital sources of groundwater for various uses, such as drinking water supply, agriculture, and industrial processes. Aquifers are typically composed of porous materials like sandstone, limestone, or gravel that allow for the infiltration and storage of water.

The term "economical quantities" implies that the water yield from the aquifer is sufficient to meet the demands of users, which is a key characteristic distinguishing aquifers from other geological formations. Understanding this definition is essential for groundwater management, hydrogeology, and water resource planning, as it highlights the importance of aquifers in providing sustainable water resources.

While other options refer to different geological contexts, they do not encompass the primary function of an aquifer as a source of water. A geological formation containing no water is not applicable to aquifers, nor is an underground cavity storing oil and gas relevant to groundwater resources. Similarly, an impermeable layer is not an aquifer; rather, it represents a confining layer that restricts the movement of water. Recognizing the specific attributes of an aquifer helps in grasping its role in hydrology