Hydrogeology Questions and Answers

Answer: B

This water is called Runoff.

Runoff (stream flow) is the water that reaches and is discharged by streams. It is made up of surface or overland flow, interflow through the soil, and baseflow from groundwater, which keeps streams flowing between rains.

Precipitation is the water falling from the atmosphere, transpiration is water released by plants through their leaves, and evaporation is the change of surface water to vapour. These supply or remove water but are not the water discharged through streams.

Answer: C

The process that is not part of the hydrologic cycle is calcium carbonate dissolving in soil water and ground water.

The hydrologic cycle is the movement of water itself between the atmosphere, the land surface, the ground and the oceans: evaporation, condensation, precipitation, infiltration, runoff and groundwater flow. Dissolving calcium carbonate is a chemical weathering reaction, part of the carbon and rock cycles; the water involved moves anyway, but the dissolution is not a step in the water cycle.

Infiltration into soil and bedrock, evaporation from a lake, and water draining into creeks and streams after rain are all stages of the hydrologic cycle.

Answer: C

It is connate water.

Connate water (from Latin for 'born together') is water trapped in the pores of a sediment when it was deposited and sealed in as the rock formed. It is usually old and saline, and occurs, for example, alongside oil and gas in reservoir rocks.

Meteoric water comes from rain and snow and enters rocks from the surface. Juvenile water is new water released from magma, reaching the surface for the first time. Pellicular water is the thin film held on grains by molecular attraction in the unsaturated zone.

Answer: C

It is the zone of aeration.

Below the ground, the zone between the surface and the water table has pores holding both air and water. This is the zone of aeration (the vadose or unsaturated zone). Below the water table every pore space is filled with water.

That lower zone is the zone of saturation, which has no air in its pores. The zones of accumulation and wastage are terms from glaciers, the areas where ice is gained and lost, not from groundwater.

Answer: C

A hydrograph shows the change in discharge with time.

A stream hydrograph plots the discharge of a river (volume of water per unit time, in m\(^{3}\)/s) at a gauging station against time. A storm hydrograph shows the rising limb, the peak discharge, the lag time after the rainfall and the recession limb, and is used in flood studies.

Water level (stage) is plotted on a stage hydrograph and converted to discharge with a rating curve, but the standard hydrograph is of discharge. Volume and velocity alone do not describe flow past a point over time.

Answer: D

The key's answer is a semiconfined aquifer.

A semiconfined (leaky) aquifer is bounded above and below by semi-pervious layers, aquitards, through which water leaks slowly to or from the neighbouring aquifers. The paper describes the system as an aquifer between other water-bearing layers, which is how the key reads it: water can move between them, so the middle aquifer is neither fully confined nor free.

A confined aquifer is sealed by impermeable layers (aquicludes). A phreatic (unconfined) aquifer has the water table as its upper surface, and a perched aquifer is a local saturated zone held above the main water table by a lens of impermeable material.

Answer: C

This belt is the capillary fringe.

Just above the water table, water is drawn up from the saturated zone into the small pores of the soil or rock by capillary attraction and held there against gravity. The resulting moist belt is the capillary fringe; its thickness depends on pore size, from a few centimetres in coarse sand to a metre or more in fine-grained material.

Specific retention is the volume of water a rock holds against gravity after drainage, a property rather than a zone. Static water level is the level of water in a well at rest, and the potentiometric surface is the level to which water in a confined aquifer would rise.

Answer: A

(I) is true, (II) is false.

The height of capillary rise is inversely proportional to the radius of the pore or tube:

\[ h = \frac{2\gamma \cos\theta}{\rho g r} \]

so narrower pores raise water higher, as statement I says.

It follows that statement II is false. Clay-grade soils have very fine pores, so water rises much higher in them and their capillary fringe is thicker (up to a metre or more), while coarse sand, with large pores, has a thin fringe of only a few centimetres.

Answer: D

All four statements are correct, so the answer is (a), (b), (c), (d) are correct.

1. (a) A bare soil evaporates at nearly the rate of open water only while its surface stays wet; once the surface dries, the rate falls sharply because water must move up from below.
2. (b) Field capacity is the water a soil holds after free drainage under gravity has stopped.
3. (c) The wilting point is the moisture content below which plants cannot extract water, so the water still held is unavailable to them.
4. (d) The available water capacity is the difference between field capacity and the wilting point.

Each of the narrower options leaves out one of these true statements.