What is an aquifer?

• A. A geological formation capable of yielding a significant amount of water to a well or spring
• B. A geological formation capable of storing water but does not yield water
• C. A rocky layer with no water
• D. A man-made reservoir

Answer: (A)

An aquifer is a natural underground formation that can both store and transmit groundwater. It’s made of materials with enough porosity to hold water and enough permeability to let that water move so wells or springs can withdraw usable amounts. Water sits in the pore spaces and fractures, and the ability to yield water distinguishes an aquifer from underground layers that only store water but don’t supply it to wells (such as impermeable layers). The best description is a geological formation capable of yielding a significant amount of water to a well or spring. It’s also useful to note the difference between unconfined and confined aquifers, and to recognize that a man-made reservoir stores surface water, not groundwater underground.