Groundwater Geology Questions and Answers

Answer: C

Fresh water floats on the denser sea water, and lowering its head lets the interface move inland and upward. Once salt reaches a well the damage is slow to reverse, so prevention by controlled abstraction is essential.

Answer: D

Correct answer: the weathered mantle and the fracture network

Fresh granite and gneiss have almost no primary porosity, so the groundwater of hard rock terrain is held in the weathered and fractured zone - the saprolite mantle, which stores it, and the joints and fractures beneath, which transmit it. Yields are therefore modest and highly variable from one borehole to the next, and siting depends on locating fracture zones by lineament mapping and geophysics.

Answer: A

Correct answer: thick permeable sand and gravel with interbedded clay

The Indo-Gangetic plain is filled to great depth with alluvium laid down by the Himalayan rivers: permeable sands and gravels forming aquifers, separated by clay lenses that confine them locally. The combination of great thickness, high permeability and continuing recharge makes it one of the most productive aquifer systems in the world - and one of the most heavily over-exploited.

Answer: B

Correct answer: concentrated in conduits and often turbulent

Solution has enlarged the joints of a karst limestone into a network of conduits, so most of the flow is concentrated in a few large channels rather than distributed through the rock mass. Velocities are high, the flow is often turbulent, and Darcy’s law does not apply. A karst aquifer therefore responds within hours to rainfall, transmits contamination rapidly and almost unfiltered, and cannot be characterised by a single hydraulic conductivity.

Answer: C

Correct answer: replenish an over-exploited aquifer

Artificial recharge directs surface water into the ground - through spreading basins, percolation tanks, check dams or injection wells - to restore an aquifer being pumped faster than it is naturally replenished. It also stores monsoon runoff for the dry season and, on the coast, holds back saline intrusion. It cannot alter the porosity of the rock, which is a property of the material.

Answer: D

Removing water transfers load to the soil skeleton and the compressible clay interbeds consolidate, much of it irreversibly. Cities founded on thick alluvium have subsided metres this way, damaging wells and drainage alike.

Answer: A

Correct answer: 0.08 m/day

Darcy’s law gives

Eight centimetres a day is a typical groundwater rate, and it is worth noticing how slow it is: a contaminant travelling at this speed would take three years to move a hundred metres. This is the Darcy velocity through the whole cross-section, not the velocity in the pores.

Answer: B

Correct answer: 0.32 m/day

The water moves only through the voids, so the actual velocity is the Darcy velocity divided by the porosity:

Four times the Darcy figure, because only a quarter of the cross-section is open to flow. It is this seepage velocity that governs contaminant travel times, and using the Darcy velocity instead would overestimate the time available to respond by a factor of four.

Answer: C

Correct answer: 750 m\( ^2 \)/day

Transmissivity is conductivity times saturated thickness:

It expresses the rate at which the aquifer transmits water through its full thickness under a unit gradient, and it is the quantity a pumping test measures directly - conductivity being obtained afterwards by dividing by the thickness.

Answer: D

Correct answer: 0.002

The gradient is the head difference over the distance, in consistent units:

The kilometres must be converted to metres first - using 3 gives 2.0, which would be a gradient steeper than any natural groundwater flow by three orders of magnitude.