ICAR AICE-JRF/SRF Groundwater, Wells & Pumps: practice questions

1 · Aquifer types and properties: T, S and Sy · hard

A clay has a porosity of 0.45 while a medium sand has a porosity of 0.32, yet the sand is the far better aquifer. The best explanation is:

1. Clay has a higher specific yield than sand, but its lower porosity limits how much water it can store
2. Clay pores are larger than sand pores, so water drains out too quickly to be pumped from a well
3. Clay holds most pore water against gravity, so its specific yield and conductivity are very low
4. Porosity in clay is entirely secondary, formed by fractures, and is therefore unconnected to wells

2 · Darcy's law in aquifers · medium

The same sand column is tested with water at 10 °C and then at 30 °C. Compared with the first test, the second test gives:

1. Lower hydraulic conductivity, same intrinsic permeability
2. Same hydraulic conductivity, higher intrinsic permeability
3. Higher hydraulic conductivity and higher intrinsic permeability
4. Higher hydraulic conductivity, same intrinsic permeability

3 · Pumping tests · hard

A step-drawdown test yields s_w = BQ + CQ² with B = 1.2 × 10⁻³ day/m² and C = 2.0 × 10⁻⁷ day²/m⁵. At Q = 2,000 m³/day, the well efficiency is:

1. 25%
2. 300%
3. 75%
4. 100%

4 · Well interference · hard

On a Cooper-Jacob plot for an observation well, the slope of drawdown against log time doubles part way through a test. The most likely cause is:

1. A recharge boundary (such as a river)
2. Leakage from an aquitard
3. Onset of delayed yield from pore drainage
4. An impermeable (barrier) boundary within the cone of depression

5 · Well design: screen and gravel pack · hard

A well is to be completed in a fine, very uniform sand (Cu ≈ 1.5). The most appropriate design approach is:

1. Use a naturally developed well with a slot size passing 90% of the formation to maximise open area
2. Leave the hole open without a screen
3. Install an artificial pack of uniform, rounded siliceous grains, with slots retaining most of the pack
4. Use a slotted pipe with slots larger than the median aquifer grain so fines wash out freely during development

6 · Groundwater exploration: resistivity · hard

In interpreting a VES, a thin conductive layer between two resistive layers can be resolved only in terms of its longitudinal conductance. This ambiguity is known as:

1. The principle of suppression
2. Archie's law
3. The principle of equivalence
4. The Dupuit assumption

7 · Recharge estimation and artificial recharge · medium

Annual rainfall is 800 mm with chloride 2 mg/L (including dry deposition). Groundwater chloride is 40 mg/L and there are no other chloride sources. By chloride mass balance the recharge is:

1. 16,000 mm/year
2. 20 mm/year
3. 40 mm/year
4. 400 mm/year

8 · Specific speed and affinity laws · medium

Geometrically similar pumps operating at corresponding (similar) points have:

1. The same discharge
2. The same specific speed
3. The same shaft power
4. The same head

9 · NPSH and cavitation · hard

A pump with Thoma cavitation coefficient σ = 0.12 operates at 40 m head. Atmospheric minus vapour pressure head is 10.1 m and suction losses are 0.5 m. Ignoring any safety margin, the maximum static suction lift is:

1. 4.8 m
2. 9.6 m
3. 5.3 m
4. 5.8 m

10 · Solar pumping · hard

Researchers have raised concern that subsidised off-grid solar pumps may accelerate groundwater depletion. The underlying reason is that:

1. Solar pumps have a higher specific speed than grid-connected electric pumps, so they lift more water per unit
2. PV panels reduce aquifer recharge by shading fields and cutting infiltration beneath the arrays
3. Pumping has near-zero marginal cost once installed, so there is little incentive to limit extraction
4. Solar pumps cannot be switched off