Groundwater Geology Questions and Answers

Answer: A

Correct answer: turbulent, as in open fractures or coarse gravel

Darcy’s law assumes laminar flow, in which discharge is proportional to gradient, and that holds while the Reynolds number based on grain size stays below about one. In open fractures, karst conduits and very coarse gravel the flow becomes turbulent, the discharge grows more slowly than the gradient, and the linear law overestimates it. Very low gradients are the regime in which Darcy is at its most reliable.

Answer: B

Correct answer: only the pore space, not the whole cross-section

The Darcy velocity is the discharge divided by the whole area, grains included, but the water can only move through the voids, so the actual velocity in the pores is greater:

With a porosity of 0.25 the seepage velocity is four times the Darcy velocity. It is the seepage velocity that governs how quickly a contaminant travels, which is why the distinction matters.

Answer: C

Correct answer: the drawn-down water surface surrounding it

Pumping lowers the water level at the well, and the level rises with distance until it merges into the undisturbed water table, so the drawn-down surface takes the shape of an inverted cone centred on the well. Its depth and extent depend on the pumping rate and on the aquifer’s transmissivity and storage - which is precisely what a pumping test measures.

Answer: D

Correct answer: before pumping and during pumping

Drawdown is the lowering of the water level at a point caused by pumping, measured from the static level before pumping began. It is the quantity plotted against time and distance in a pumping test, and its relation to the discharge gives the specific capacity of the well and, through the Theis or Jacob analysis, the transmissivity and storage coefficient of the aquifer.

Answer: A

Correct answer: cones of depression of adjacent wells overlap

Where two wells are close enough for their cones to overlap, each adds to the drawdown at the other, so both must be pumped against a lower water level and each yields less than it would alone. The effect sets a minimum spacing for a wellfield, and where it is ignored the field’s total yield falls well short of the sum of the individual well yields.

Answer: B

Correct answer: drawdown

Specific capacity is

the discharge per unit of drawdown, in litres per minute per metre or an equivalent unit. It measures the performance of the well and aquifer together, and because it is easy to measure it is the standard indicator of how a well is behaving over its life.

Answer: C

Correct answer: clogging or corrosion of the well screen

If the aquifer is unchanged, a falling specific capacity means the well itself is deteriorating: the screen is being blocked by chemical incrustation, by bacterial growth, or by fine material drawn in, or it is corroding and collapsing. The remedy is rehabilitation - acidising, brushing or surging - and it is far cheaper than a new well, provided the decline is noticed early.

Answer: D

Correct answer: the water table intersects the ground surface

A spring is simply the water table emerging: wherever the ground surface cuts below it, water flows out. That is why springs are found on valley sides, at breaks of slope and at the foot of escarpments. The various spring types - contact, fault, valley, karst - are just different geological reasons for that intersection to occur where it does.

Answer: A

Correct answer: a permeable bed rests on an impermeable one

Water percolating through the permeable bed is stopped by the impermeable one beneath and travels laterally along the contact until that contact reaches the surface, where it emerges as a line of springs. It is one of the commonest spring types - the sandstone-on-clay junction of a scarp face - and the springs form a line along the outcrop of the boundary rather than a single point.

Answer: B

Displacement can juxtapose the two so that flow is dammed and forced up along the fault plane. The same fault elsewhere may instead conduct water along its crushed zone, so the effect is not predictable without mapping.