Groundwater Geology Questions and Answers

Answer: A

Correct answer: 750 m\( ^3 \)/day

The discharge through a width \( W \) of aquifer is

which is the Darcy law written for the whole saturated thickness. Note that the transmissivity has already absorbed the thickness, so the aquifer depth must not be brought in again.

Answer: B

Correct answer: 0.25

Porosity divides into what drains and what is held:

So a quarter of the aquifer’s volume is recoverable water and the remaining tenth stays held against gravity. It is the specific yield, not the porosity, that determines how much water an unconfined aquifer will actually give up.

Answer: C

Correct answer: 10 000 m\( ^3 \)

The volume released is the area times the thickness dewatered times the specific yield:

A hectare is 10,000 square metres. Using the porosity of 0.35 instead would give 14,000 and would overstate what can actually be drained, since the retained water does not come out.

Answer: D

Correct answer: 150 litres/min per metre

Specific capacity is yield over drawdown:

It is the standard measure of well performance, and recording it at commissioning gives the baseline against which later deterioration is judged - which is exactly what the next item does.

Answer: A

Correct answer: 75 litres/min per metre

Recomputing on the later figures,

The specific capacity has halved, and since the aquifer is unchanged the well itself must have deteriorated - the screen clogged by incrustation, bacterial growth or fine material. Noticing the trend early makes rehabilitation possible; left long enough, the well is lost.

Answer: B

Correct answer: 1-Q, 2-P, 3-R

The three terms divide formations by two independent properties, storage and transmission. An aquifer does both and can be pumped. An aquiclude stores water but will not transmit it - clay, which has high porosity and negligible permeability. An aquifuge does neither, having essentially no void space at all. The aquitard, transmitting slowly, sits between the first two.

Answer: C

Correct answer: 1-Q, 2-P, 3-R

Each spring type is a different geological reason for the water table to reach the surface. A contact spring emerges where a permeable bed rests on an impermeable one and the junction outcrops. A fault spring emerges where faulting has brought permeable rock against impermeable. A valley spring emerges where erosion has cut the valley floor below the water table.

Answer: D

Correct answer: 3, 4, 2, 1

From the surface downward: the soil water belt, where roots and evaporation dominate; the intermediate belt, through which water percolates; the capillary fringe, held above the water table by surface tension; and then the zone of saturation. The first three together make up the zone of aeration, and the water table is the boundary between the fringe and the saturated zone.

Answer: A

Correct answer: 1 and 2 only are correct

Statements 1 and 2 are correct. Statement 3 is the wrong way round - the seepage velocity is greater than the Darcy velocity, by the factor \( 1/n \), because the water moves only through the pore space while the Darcy velocity is computed over the whole cross-section. Getting this backwards would understate contaminant travel by several times.

Answer: B

Correct answer: increasing the aquifer’s transmissivity

Higher transmissivity means the aquifer delivers water to the well more readily, so the yield for a given drawdown rises. The other three all reduce it: a falling water table reduces the saturated thickness and hence the transmissivity, a neighbouring well’s cone of depression adds to the drawdown, and a clogged screen raises the well losses.