Question

The water table over an area of 1 km² was lowered by 4 m. If the porosity of rock is 30% and the specific retention is 10%, the change in the groundwater storage is ______ × 10³ m³. (In integer)

Answer 1

Correct Answer: 800

Given:
Area of aquifer: \(A = 1 \text{ km}^2 = 1 \times 10^6 \text{ m}^2\) 
Lowering of water table: \(\Delta h = 4 \text{ m}\)
Porosity: \(n = 0.30\)
Specific retention: \(S_r = 0.10\)
Step 1 — Compute the specific yield:
For unconfined aquifer, the change in groundwater storage per unit area is \[ \Delta S = \Delta h \cdot (n - S_r) \] \[ n - S_r = 0.30 - 0.10 = 0.20 \] 
Step 2 — Compute the total change in storage:
\[ \Delta V = A \cdot \Delta S = 1 \times 10^6 \cdot 4 \cdot 0.20 = 800,000 \text{ m}^3 \] \[ \Delta V = 800 \times 10^3 \text{ m}^3 \] Answer: \(\boxed{800 \times 10^3 \text{ m}^3}\)