Question

If a confined aquifer has a specific yield of 0.40, what does this value indicate about the aquifer's ability to release groundwater?

• A. It can release 40% of its water under pumping
• B. It can release 40% of its water without pumping
• C. It can release 60% of its water under pumping
• D. It can release 60% of its water without pumping

Hint:

• Specific Yield (S$_y$) is mainly for unconfined aquifers: It's the volume of water drained by gravity per unit volume of aquifer when the water table drops.
• For confined aquifers, the term is Storativity (S): Water is released by compaction of the aquifer and expansion of water when pressure (head) is reduced by pumping. Storativity values for confined aquifers are typically very small (e.g., $10^{-3}$ to $10^{-5}$).
• A "specific yield" of 0.40 (or 40\%) is very high, typical of porous unconfined material like gravel.
• If we must apply the term "specific yield" as given, it means that fraction of the aquifer's total volume can be yielded as water, usually through pumping which lowers the head.

Answer 1

The Correct Option is A

The question asks about the meaning of specific yield in a confined aquifer.

1. Understanding the Concepts:

- Confined Aquifer: An aquifer that is bounded above and below by impermeable layers.
- Specific Yield (Sy): The ratio (expressed as a percentage) of the volume of water that a saturated rock or soil will *yield by gravity* to the total volume of the rock or soil. It represents the water that can be drained by gravity.
- Specific Retention (Sr): The amount of water retained in the formation against gravity. 
In confined aquifers, the water is released due to the compression of the aquifer and expansion of water when the pressure is reduced due to pumping, which is quantified by the Storage Coefficient (S), not the Specific Yield (Sy). Specific Yield is applicable to unconfined aquifers.

2. Analyzing the Options:

- It can release 40% of its water under pumping: This is partially correct in the sense that water is released under pumping but 'Specific Yield' is strictly defined for gravity drainage in unconfined aquifers, not confined aquifers.
- It can release 40% of its water without pumping: Incorrect, water isn't released without a pressure change induced by pumping or other means.
- It can release 60% of its water under pumping: Incorrect percentage and applies more to total porosity or specific retention if relating to percentages.
- It can release 60% of its water without pumping: Incorrect, as water isn't released without a pressure change.

3. Determining the correct answer with caveat:

Technically, the term "specific yield" is not applicable to confined aquifers. A specific yield of 0.40 (or 40%) means that for every unit volume of the aquifer, 40% of that volume can be drained by gravity.

Final Answer:

While "specific yield" is not strictly applicable to confined aquifers, the closest interpretation of a specific yield of 0.40 would be: It can release 40% of its water under pumping.