In hydrogeology, storativity is a measure of how much water an aquifer releases from storage per unit decline in head. A very small storativity indicates which type of aquifer?

• A. Unconfined aquifer with high storage
• B. Confined aquifer with limited storage
• C. Aquifer with high porosity
• D. Perched aquifer

Answer: (B)

Storativity is about how much water a unit volume of an aquifer will release for a given drop in hydraulic head. When storativity is very small, the aquifer releases only a tiny amount of water as the head falls. This happens most clearly in confined aquifers, where the water is pressed between impermeable layers. In these systems, the water and the aquifer compress only modestly under pressure, so the amount of water that becomes available with a head decline is limited. That’s why a very small storativity points to a confined aquifer with limited storage.

If the aquifer were unconfined, most of the storage change comes from drainage of the pore water as the water table drops, which leads to a much larger water release per unit head change, i.e., a higher storativity. Porosity by itself doesn’t determine storativity in the same way, and perched aquifers aren’t characterized by the same low-storativity behavior as confined systems.