This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Aquifer Properties”.
1. The porosity of an aquifer is given as x%. What does it signify?
a) x% of volume of aquifer are voids
b) x% of volume of aquifer are air voids
c) x% of volume of aquifer voids are filled with air
d) x% of volume of aquifer voids are filled with water
View Answer
Explanation: Porosity is the property of an aquifer which is defined as the ratio of the volume of voids to the total volume of the aquifer medium. It depends on the size shape and distribution of particles in the medium.
2. A saturated formation in a soil has a porosity of 23%. Which of the following is true about the formation?
a) High water holding capacity
b) Moderate water holding capacity
c) Very low water holding capacity
d) Insufficient data
View Answer
Explanation: The aquifers maybe classified depending on their water retaining capacity based on the material porosity. A porosity above 20% is considered high, and such aquifers are adequate water aquifers.
3. An aquifer with porosity of 7% is classified as a low water capacity aquifer?
a) True
b) False
View Answer
Explanation: A porosity between 5 and 20% is considered medium, and such aquifers are moderate water aquifers. However, aquifers with medium of capacity less than 5% are considered as low water aquifers.
4. If A is the specific yield, B is the specific retention and C is the porosity of an aquifer, which of the following is the correct relationship between them?
a) A + B + C = 1
b) \(\frac{A}{B}\)=C
c) A + B – C = 0
d) \(\frac{A}{B}=\frac{1}{C}\)
View Answer
Explanation: The sum of specific yield and specific retention is equal to the porosity of the aquifer. Thus,
A+B=C ⇒A+B-C=0
5. Which of the following statements is true?
a) All the pore water present in aquifer is available for extraction
b) Half the pore water present in aquifer is unavailable for extraction
c) None of the pore water in aquifer is available for extraction
d) Some of the porewater present in aquifer is unavailable for extraction
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Explanation: All the water is an aquifer is held within the pores of its medium and is signified by its porosity. However, some of the water is held back due to attraction and surface tension. Hence, all the water stored in the aquifer cannot be extracted.
6. Which of the following is the best definition of specific yield of an aquifer?
a) Volume of water that can be extracted by gravity
b) Volume of water that can be extracted by gravity to the volume of aquifer
c) Volume of water that can be extracted by gravity to the total volume of water
d) Volume of water that can be extracted by gravity to the volume of water retained
View Answer
Explanation: All the water in the aquifer cannot be extracted by gravity as some is held within the pores. The volume of water that can be extracted to the volume of aquifer medium is called as specific yield of the aquifer. It is expressed as a percentage.
7. If the specific yield and specific retention of an aquifer are equal, what is the porosity of the aquifer?
a) 10%
b) 25%
c) 50%
d) Data insufficient
View Answer
Explanation: The specific yield (Sy), specific retention (Sr) and porosity (n) are related as, Sy+Sr=n.
Given that specific retention ad yield are same. Taking them as S, we have
S+S=n ⇒2S=n
Therefore, without knowing the value of either specific yield or specific retention, the porosity cannot be computed.
8. The intrinsic permeability depends on which of the following?
a) Porous medium
b) Fluid properties
c) Both medium and fluid properties
d) Neither medium nor fluid properties
View Answer
Explanation: The intrinsic permeability of a medium is generally expressed as,
K0=\(CD_m^2\).
Where, C = factor depending on porosity, grain size distribution and packing of medium Dm = mean particle size of medium. Hence, intrinsic permeability is a function of the medium only.
9. Three aquifers X, Y and Z of clay, sand and gravel respectively are present in identical soil masses. Which of the following statement is incorrect regarding the properties of these aquifers?
a) Porosity of X is greater than Z
b) Porosity of Y is lesser than X
c) Specific yield of X is greater than Y
d) Specific yield of Z is greater than X
View Answer
Explanation: Clayey aquifers have the highest porosity in the range of 45-55%, followed by sandy and gravelly aquifers. However, clayey formations have a very low specific yield compared to sand and gravel aquifers.
10. The coefficient of permeability has units of which of the following?
a) Time
b) Velocity
c) Head
d) Unitless
View Answer
Explanation: The coefficient of permeability is used as the proportionating constant in Darcy’s law, which relates the flow velocity and hydraulic gradient. It is the value that indicates the permeability of the porous medium and has units of velocity.
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