Groundwater Geology Questions and Answers

Answer: C

Holding a great deal of water yet passing almost none is precisely what makes a formation an aquiclude. An aquifuge such as unfractured granite neither holds nor transmits, which is a materially different thing in a water balance.

Answer: D

Correct answer: neither storing nor transmitting water

The two terms mark different failures. An aquiclude, such as clay, is porous and stores a great deal of water but will not transmit it. An aquifuge, such as unweathered unfractured granite or massive quartzite, has essentially no void space at all, so it neither holds water nor passes it. Both act as barriers to flow; only the aquiclude contains water while doing so.

Answer: A

Correct answer: slowly, enough to leak between aquifers

An aquitard is intermediate: too impermeable to be pumped, but permeable enough that water leaks through it over time and over large areas. Where an aquitard rather than a true aquiclude confines an aquifer, the system is described as leaky, and the leakage supplies a significant part of the water pumped - which the well test analysis must allow for.

Answer: B

Correct answer: a confining layer of much lower permeability

A confined aquifer is sealed above by a bed of much lower permeability, so the water within it is under pressure greater than atmospheric and there is no water table inside it. Puncturing the confining layer lets the water rise up the borehole to the piezometric surface, and where that surface stands above the ground the well flows without pumping.

Answer: C

Correct answer: the imaginary surface to which its water would rise

The piezometric or potentiometric surface is the level to which water rises in wells tapping a confined aquifer - a surface of hydraulic head rather than a physical water surface, since the aquifer itself is full throughout. It may lie above ground, giving artesian flow, and it is drawn from the levels measured in a network of wells much as a water table is.

Answer: D

Correct answer: rises above ground level without pumping

An artesian well taps a confined aquifer whose piezometric surface lies above the ground at that point, so the water flows at the surface under its own pressure. The requirements are a confined aquifer, an intake area at higher elevation, and a well site below the piezometric surface. Where that surface lies below ground the well is confined but sub-artesian, and pumping is still needed.

Answer: A

Correct answer: held above the main water table on a local impermeable bed

A perched aquifer sits on a lens of clay or other impermeable material within the zone of aeration, holding a local body of water with unsaturated ground beneath it. It is separated from the regional water table below by that unsaturated zone. Perched water bodies are typically small, seasonal and easily exhausted, and they are a common cause of a shallow well that dries up.

Answer: B

The constant of proportionality is the hydraulic conductivity, which lumps the properties of both the medium and the fluid. The linear relation holds only for laminar flow, so very coarse gravel and open fractures depart from it.

Answer: C

Correct answer: velocity

From Darcy’s law \( v = Ki \), and since the hydraulic gradient is dimensionless, the conductivity has the same dimensions as velocity - metres per day being the usual unit in groundwater work. Note that it is a conductivity and not an actual velocity: it is the discharge per unit area of the whole cross-section, grains included, under a unit gradient.

Answer: D

It expresses how much water the whole thickness will transmit under a unit gradient, which is what matters to a well rather than the conductivity alone. A thin highly permeable bed and a thick poor one can transmit equally.