Unconfined Aquifer – Specific yield from pumped volume and water-table decline From an area of 5 km^2, a total of 3.0 × 10^6 m^3 of groundwater is pumped uniformly from an unconfined aquifer, lowering the water table from 102 m to 99 m. Compute the specific yield (dimensionless).

Introduction / Context:
Specific yield (S_y) of an unconfined aquifer is the volume of water that drains by gravity per unit area per unit decline in water table. It is central to water balance, storage calculations, and sustainable yield estimation for phreatic aquifers.

Given Data / Assumptions:

• Pumped volume V = 3.0 × 10^6 m^3.
• Area A = 5 km^2 = 5 × 10^6 m^2.
• Decline in water table Δh = 102 − 99 = 3 m.
• Drainage is gravity dominated; specific retention effects incorporated in S_y.

Concept / Approach:

For an unconfined aquifer with near-uniform decline, the drained water volume equals A * Δh * S_y. Rearranging gives S_y = V / (A * Δh). This relation assumes negligible compressibility effects and uniform decline across the stated area.

Step-by-Step Solution:

Verification / Alternative check:

Typical specific yield values: sands 0.20–0.30, silts 0.05–0.15, clays < 0.05. The computed 0.20 aligns with sandy aquifers.

Why Other Options Are Wrong:

0.30–0.50 would require larger pumped volumes or smaller area/decline; 0.10 would correspond to finer materials inconsistent with the given volume change.

Common Pitfalls:

Using saturated thickness instead of Δh, or forgetting to convert km^2 to m^2.

Final Answer:

0.20