An aquifer has storativity 0.002, porosity 0.4, hydraulic conductivity 10 gpd/ft^2. Is this aquifer confined or unconfined?

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Multiple Choice

An aquifer has storativity 0.002, porosity 0.4, hydraulic conductivity 10 gpd/ft^2. Is this aquifer confined or unconfined?

Explanation:
Storativity measures how much water is released from an aquifer for a given drop in hydraulic head. In a confined aquifer, storativity is typically small because storage comes mainly from the compressibility of water and the aquifer system over the thickness, so S tends to be on the order of 10^-3 to 10^-2. In an unconfined aquifer, storage is governed by the specific yield, which is usually a larger fraction of the porosity and commonly around 0.01–0.3 for many materials. A storativity of 0.002 is small and fits the characteristic range for a confined aquifer, even though the porosity is 0.4. The porosity tells you how much pore space exists, but in a confined system the stored water is not simply the pore volume; it's controlled by compressibility and confinement. Hydraulic conductivity affects how fast water moves, not whether the system is confined. Therefore, the aquifer is confined.

Storativity measures how much water is released from an aquifer for a given drop in hydraulic head. In a confined aquifer, storativity is typically small because storage comes mainly from the compressibility of water and the aquifer system over the thickness, so S tends to be on the order of 10^-3 to 10^-2. In an unconfined aquifer, storage is governed by the specific yield, which is usually a larger fraction of the porosity and commonly around 0.01–0.3 for many materials.

A storativity of 0.002 is small and fits the characteristic range for a confined aquifer, even though the porosity is 0.4. The porosity tells you how much pore space exists, but in a confined system the stored water is not simply the pore volume; it's controlled by compressibility and confinement. Hydraulic conductivity affects how fast water moves, not whether the system is confined.

Therefore, the aquifer is confined.

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