What does specific storage measure in hydrogeology?

• A. The volume of water a unit volume releases from or takes into storage from a unit change in pressure head
• B. The rate of water movement through a rock
• C. The total porosity of the rock
• D. The amount of water stored per unit area

Answer: (A)

Specific storage describes how much water is released from or stored in a unit volume of aquifer for a unit change in hydraulic head. It incorporates the slight compression of both the water and the solid skeleton under pressure, so a small head change causes a small amount of water to enter or leave storage. The units are inverse length (per meter). This is different from porosity, which is just the fraction of void space, and from hydraulic conductivity, which is about how fast water can move. It’s also distinct from storage per area, which depends on thickness and head change; specific storage uses volume and head change per unit volume of aquifer. For context, it’s typically small and is a key parameter in transient aquifer tests and well drawdown analyses.