This set of Irrigation Engineering Questions and Answers for Freshers focuses on “Aquifers and Their Type – 2”.
1. The line joining the static water levels in several wells excavated through a confined aquifer is known as the _____________
a) cone of depression
b) piezometric surface
c) perched water-table
d) hypsometric curve
Explanation: Perched water table is the top surface of the water held in the perched aquifer. Cone of depression is the surface of the water table surrounding the well which takes up a curved shape. The piezometric surface is an imaginary surface that defines the level to which water will rise representing the total head in an aquifer.
2. The permeability of an aquifer (m/day) will ____________
a) increase with an increase in temperature of water flowing through the aquifer
b) decrease with an increase in temperature of water flowing through the aquifer
c) not get affected by the change in temperature of water flowing through the aquifer
d) increase up to 20°C and then decreases with the increase in the temperature of water flowing through the aquifer
Explanation: The permeability is inversely proportional to the viscosity of water and viscosity decreases with increase in temperature. Hence, permeability will increase with an increase in temperature.
3. The discharge per unit drawdown at a well is known as ___________
a) specific yield
b) specific retention
c) specific capacity
d) specific storage
Explanation: Specific capacity of a well can be defined as the well-yield per unit of drawdown. The value is not constant but decreases as the discharge increases. The value is not the same for all drawdowns, it should be determined for the fall of the first meter.
4. Coefficient of storage (A) has the dimensions of _____
Explanation: Storage coefficient can be defined as the volume of water released from an aquifer of 1 m2 area and full depth per meter fall of the piezometric head. Its unit will be m3/m2/m i.e. dimensionless.
5. The units of specific capacity of a well are _____
d) no units
Explanation: Specific capacity is given by –
Specific capacity = discharge of the well / Drawdown = Q / (C1Q + C2Q2) = m3/sec/m = m2/sec. This equation also shows that the specific capacity of the well is not constant but decreases as the discharge increases.
6. Specific yield for an unconfined aquifer is ________________
a) greater than porosity
b) less than porosity
c) equal to porosity
d) unrelated to porosity
Explanation: The specific yield is expressed as the ratio of the volume of the water obtained by gravity drainage to the total volume of the material drained. Since, Specific yield + Specific retention = Porosity. Evidently, the specific yield is less than the porosity.
7. A perched aquifer is found within ____________
a) unconfined aquifer
b) confined aquifer
d) both confined and unconfined aquifer
Explanation: Sometimes an impervious medium carrying saturated soil-mass exists over the water table which can be used as a source of water. This body of saturated material is a kind of aquifer termed as a perched aquifer. It is a special case that is found to occur within an unconfined aquifer.
8. The specific capacity of a confined well under equilibrium condition and within the working limits of drawdown ______________
a) is constant at all drawdowns
b) decreases as the drawdown are increased
c) increases as the drawdown are increased
d) may increase or decrease
Explanation: The following equation shows that specific capacity decreases as Q increases and Q increase only when the drawdown is increased by heavier pumping and hence, specific capacity decreases as drawdown is increased.
Specific capacity = Q/(C1.Q + C2.Q2). i.e. Discharge/ Drawdown (Well loss + Aquifer loss).
9. The standard specific capacity of a well is the well-discharge per unit of drawdown. This value of discharge should be obtained at ________
a) any drawdown
b) a particular drawdown of 3 m
c) the first 1 m of drawdown
d) the first 2 m of drawdown
Explanation: Specific capacity is the rate of flow from a well per unit of a drawdown. The value is not the same for all drawdowns, it should be determined for the fall of the first meter.
10. An example of aquitard stratum in the ground profile is a layer of ___________
a) granite rock
c) sandy clay
Explanation: The geological formations which are porous and contain a good amount of water but does not yield water freely to wells due to its lesser permeability is called aquitard. The water yield from such a formation is insignificant. Sandy clay is an example of the aquitard.
11. An aquifer which is confined at its bottom but not at the top is called ___________
a) semi-confined aquifer
b) confined aquifer
c) unconfined aquifer
d) artesian aquifer
Explanation: Unconfined aquifer is the one in which the topmost water-bearing stratum having no confined impermeable overburden lying over it. Aquifer water serves as an upper zone of saturation and it is subjected to hydrostatic pressure only.
12. The clogging of well screens and consequent reduction in pump efficiency is indicated by _____________
a) the high value of the well-loss
b) the low value of the well-loss
c) variable value of the well-loss
d) both high and low value of the well-loss
Explanation: When the screen size is compatible to the surrounding porous media, the well loss is caused by the axial movement inside the well up to the pump intake (since the well loss through the screen is very small). When clogging of the screen increases, the well loss increases which adversely affects the pump efficiency.
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