What is an aquifer?

• A. A type of rainfall underground
• B. A river channel
• C. A water-bearing rock and sediment that stores groundwater
• D. A cloud formation

Answer: (C)

An aquifer is a layer of rock or sediment that stores groundwater and allows it to move through it. The spaces in porous materials like sandstone, gravel, or fractured rock hold water, and the rock’s permeability lets that water flow slowly under gravity and pressure. Groundwater is replenished (recharged) when rainfall infiltrates the surface and percolates down to fill these spaces. The upper surface of the saturated zone in an unconfined aquifer is called the water table, and confined aquifers are trapped between impermeable layers, often storing water under pressure.

These choices describe surface or atmospheric phenomena: rainfall underground isn’t a real underground process, a river channel is surface water flowing in a riverbed, and a cloud formation is in the atmosphere.