Groundwater Geology Questions and Answers

Answer: C

Correct answer: very fine grain size with poorly connected pores

Fine, poorly connected pores are what make clay a poor aquifer despite its high porosity: the viscous resistance is enormous and much of the water is held to the mineral surfaces. The other three are all favourable - high interconnected porosity provides both storage and flow paths, substantial saturated thickness gives transmissivity, and in hard rock a well-developed fracture network supplies the permeability the intact rock lacks.

Answer: D

Crystalline rock has negligible pore space, so the water is entirely in the fractures and a borehole either strikes them or does not. It is why hard rock yields vary so sharply over short distances and why siting matters.

Answer: A

Correct answer: over-abstraction drawing the saline interface inland

Fresh groundwater floats on denser sea water, and the interface stands in equilibrium with the seaward flow of fresh water. Pumping more than the aquifer receives reduces that outflow and lets the wedge advance inland and rise beneath the well - and by the Ghyben-Herzberg relation, a fall of one metre in the water table raises the interface by about forty. Recovery is slow, so the remedy is to reduce abstraction and to recharge artificially before it occurs.

Answer: B

Correct answer: both the assertion and the reason are true and the reason explains the assertion

Both are true and the reason accounts for both halves of the assertion. High porosity means clay contains a great deal of water; very low permeability means it will not give it up, so it cannot serve as an aquifer - and the same low permeability is exactly what makes it an effective confining bed above one. One property, two consequences of opposite sign.

Answer: C

Correct answer: both the assertion and the reason are false, the flow being turbulent

Both statements fail. Darcy’s law assumes laminar flow, and in a karst conduit the velocities are high enough for the flow to be turbulent, so discharge is no longer proportional to gradient and the law does not apply. The reason states that proportionality as though it were universal, which is precisely the assumption karst violates - along with fractured rock and very coarse gravel.

Answer: D

Subtracting the specific retention leaves the fraction gravity will actually release, which is what a well can obtain. In a clay that difference is nearly the whole of the porosity, which is why porosity alone misleads.

Answer: A

Correct answer: the presence of an overlying bed of much lower permeability

Confinement is defined by what lies above the aquifer, not by the aquifer itself. The same sand may be unconfined where it outcrops and confined where a clay covers it downdip. Porosity, grain size and the depth of the wells drilled into it are all properties of the aquifer or of how it is exploited, and none of them determines whether it is confined.