Past Year Questions: Sources of Water Supply, Distribution System & Well Hydraulics | Environmental Engineering - Civil Engineering (CE) PDF Download

Q. 1: Dupuit’s assumptions are valid for      [2018 : 1 Mark, Set-II]
(a) artesian aquifer
(b) confined aquifer
(c) leaky aquifer
(d) unconfined aquifer
Ans: (d)
Dupuit’s theory assumptions hold that groundwater flows horizontally in an unconfined aquifer and that ground water discharge is proportional to saturated aquifer thickness.

Q. 2: Water table of an aquifer drops by 100 cm over an area of 1000 km2. The porosity and specific retention of the aquifer material are 25% and 5%, respectively. The amount of water (expressed in km³ ) drained out from the area is__________..     [2016 : 1 Mark, Set-II]
Ans: 0.2km³

Q. 3: A tracer takes 100 days to travel from Well-1 to Well-2 which are 100 m apart. The elevation of water surface in Well-2 is 3 m below that in Well-1. Assuming porosity equal to 15%, the coefficient of permeability (expressed in m/day) is     [2016 : 2 Marks, Set-II]
(a) 0.30
(b) 0.45
(c) 1.00
(d) 5.00 
Ans: (d)
Seepage velocity

Q. 4: The hardness of a ground water sample was found to be 420 mg/L as CaC0₃. A softener containing ion exchange resins was installed to reduce the total hardness to 75 mg/L as CaC0₃ before supplying to 4 households. Each household gets treated water at a rate of 540L/day. If the efficiency of the softener is 100%, the bypass flow rate (expressed in L/day) is_______     [2016 :1 Mark, Set-II]
Ans:  385.7 l/day

Total water to be treated
= 540 x 4 = 2160 l/day
Resultant hardness required = 75 mg/l.
Let bypass rate be x l/day having hardness of 420 mg/l
Resultant hardness required = 75 mg/l

Q. 5:  The relationship between porosity (η), specific yield (S_y) and specific retention (S_r) of an unconfined aquifer is    [2015 : 1 Mark, Set-Il]
(a) S_y + S_r = η
(b) S_y + η = S_r
(c) S_r + η = S_y 
(d) S_y + S_r + η = 1 
Ans: (a)

Q. 6:  In an aquifer extending over 150 hectare, the water table was 20m below ground level. Over a period of time the water table dropped to 23m below the ground level. If the porosity of aquifer is 0.40 and the specific retention is 0.15, what is the change in ground water storage of the aquifer?    [2011 : 2 Marks]
(a) 67.5 ha-m
(b) 112.5ha-m
(c) 180.0 ha-m
(d) 450.0 ha-m
Ans: (b)
We know that,

The negative sign signifies the decrease in storage.

Change in volume of aquifer
= 150 x ( 23 - 20 ) = 450 ha-m
Specific yield is nothing but the actual volume of water that can be extracted by force of gravity from a unit volume of aquifer.

∴ Change in water storage
 = 450x0.25 = 112.5 ha-m

Q. 7  A well of diameter 20 cm fully penetrates a confined aquifer. After a long period of pumping at a rate of 2720 litres per minute, the observations of drawdown taken at 10m and 100m distances from the centre of the well are found to be 3m and 0.5 m respectively. The transmissivity of the aquifer is     [2010 : 2 Marks]
(a) 676m²/day
(b) 576m²/day 
(c) 526m²/day 
(d) 249m²/day
Ans:  576 m²/day