Groundwater Geology Questions and Answers

Answer: A

Correct answer: zone of saturation

The water table is the surface at which the pore water pressure equals atmospheric, and it separates the zone of saturation below, where every void is water-filled, from the zone of aeration above, where the voids hold both air and water. It is not a fixed level: it rises after recharge and falls in drought, and it broadly follows the topography in a subdued form.

Answer: B

It comprises the soil water belt, the intermediate belt and the capillary fringe, and its pores hold air as well as water. Water in it is held above atmospheric pressure only by capillarity and cannot be pumped from a well.

Answer: C

Correct answer: drawn up above the water table by surface tension

Water is drawn above the water table into the fine pores by capillary action, so the fringe is fully or nearly saturated yet stands at a pressure below atmospheric. Its height depends on the pore size - a few centimetres in gravel, several metres in clay - and because that water is held in tension it will not flow into a well, so the fringe cannot be pumped.

Answer: D

Correct answer: voids

Porosity is

the fraction of the total volume that is void, whether those voids are connected or not and whether they contain water or air. It says nothing about the ability to transmit water, which depends on how the voids connect and how large they are - the distinction that makes clay porous and yet a poor aquifer.

Answer: A

Transmission requires the voids to be interconnected and large enough for flow, which is why clay is porous but nearly impermeable. It is permeability rather than porosity that decides whether a formation is an aquifer.

Answer: B

Correct answer: numerous but extremely small and poorly connected

Clay may have a porosity of 50 per cent, higher than most sands, yet its permeability is many orders lower. The pores are of colloidal size, so viscous resistance is enormous and much of the water is held to the mineral surfaces by molecular attraction and cannot move at all. High porosity with low permeability is exactly the combination that makes clay useless as an aquifer and excellent as a confining bed.

Answer: C

Granite and basalt have negligible primary porosity, so all their yield comes from the fracture network and, in limestone, from dissolved channels. It follows that a well in hard rock succeeds only if it strikes fractures.

Answer: D

Together with specific retention it makes up the total porosity, and it is the figure that determines how much water an unconfined aquifer will actually give. A clay's specific yield is very small despite a high porosity.

Answer: A

Correct answer: remains held against gravity after drainage

Specific retention is the water clay-sized surfaces and fine pores hold by molecular attraction and capillarity after gravity has drained what it can. It and specific yield together make up the porosity:

In gravel the retention is negligible and almost all the water is recoverable; in clay it is nearly the whole of the porosity, which is why a clay holds so much water and yields none.

Answer: B

Correct answer: stores and transmits water in useful quantity

Both properties are required. A formation that stores water without transmitting it is an aquiclude - clay is the type example - and one that does neither is an aquifuge. An aquifer must have interconnected voids large enough to yield water to a well at a rate worth pumping, which in practice means sand, gravel, sandstone, or fractured or weathered hard rock.