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116 Chapter 3 Water Sources: Groundwater
PROBLEMS/QUESTIONS

3.1 Differentiate between porosity and permeability and be- in the well at 8 a.m. the next day when a new cycle of pumping
tween specific retention and specific yield. is to start? Assume no recharge or leakage. The transmissivity
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3.2 Differentiate among aquifers, aquicludes, and aquitards. of the artesian aquifer is 4.1 l0 gpd/ft (509.14 m /d/m).
3.3 Explain the classification of aquifers, and why the uncon- 3.10 Two wells with effective diameters of 36 in. (914 mm)
fined aquifers can be easily contaminated. are located 1,500 ft (457.2 m) apart in an artesian aquifer with
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T 2.5 10 gpd/ft (310.45 m /d/m) and S 4 l0 .
3.4 Demonstrate the relatively slow rate of groundwater
Compute the drawdown at each well when the two wells are
movement by using Eq. 3.2 to determine the rate of movement 3
pumped at 500 gpm (2,725 m /d) for 20 days.
through an aquifer and a confining bed. The following data are
3.11 Suppose it is desired to restrict the drawdown in each of
assumed:
the wells to 40 ft in Problem 3.10. What will be the correspon-
• Aquifer composed of coarse sand: K 60 m/d, dh/dl
ding discharges for individual wells?
I 1 m/l,000 m and 0.20.
3.12 A gravel-packed well with an effective diameter of 36 in.
• Confining bed composed of clay: K 0.0001 m/d,
(914 mm) pumps water from an artesian aquifer having T 4.2
dh/dl I 1 m/10 m and 0.50. 4 3 5
l0 gpd/ft (521.56 m /d/m) and S 4.4 l0 . The well lies at a
3.5 The observed data from a pumping test were plotted in a
distance of 2,000 ft (610 m) from a stream that can supply water
figure similar to Fig. 3.4 along with a Theis type curve, as if the fast enough to maintain a constant head. Find the drawdown in the
transparency of the observed data had been moved into place well after 20 days of pumping at 550 gpm (2,725 m /d).
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over the type curve. The observation well represented by the
data is 400 ft (121.92 m) from a pumping well where the rate of 3.13 The interrelationship of pumping rate, drawdown, and
3 specific yield serves as a basis for the selection of optimal de-
discharge is 275 gpm (1,498.75 m /d). Calculate the formation
sign capacity. List the factors on which the obtainable yield
constants T and S using these match-point coordinates:
from a well depends.
W(u) 3.0 3.14 A well having an effective diameter of 36 in. (914 mm)
s 3.7 ft (1.13 m) is to be located in a relatively homogeneous artesian aquifer
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2 with a transmissivity of 15,000 gpd/ft (186.27 m /d/m) and a
u 2.3 10 4
2 7 6 storage coefficient of 3 l0 . The initial piezometric sur-
r >t 1.4 l0 (SI 1.3 10 )
face level is 30 ft (9.14 m) below the land surface. The depth
3.6 A time-drawdown curve for an observation well at a dis- to the top of the aquifer is 140 ft (42.67 m) and the thickness
tance of 400 ft (121.92 m) from a pumping well discharging at of the aquifer is 60 ft (18.28 m). The well is to be finished with
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a constant rate of 275 gpm (1,498.75 m /d) is shown in Fig. 3.5. a screen length of 30 ft (9.14 m). Compute the specific capac-
Determine the transmissivity and storage coefficient of the ity of the well and its maximum yield after 10 days of pump-
aquifer. ing. Neglect well losses.
3.7 In the aquifer represented by the pumping test in Problem 3.15 Discuss the sanitary protection of wells constructed for
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3.5 (T 2.6 l0 gpd/ft 322.87 m /d/m and S 2.3 10 ), supplying drinking water.
a gravel-packed well with an effective diameter of 36 in. 3.16 Visit the U.S. Environmental Protection Agency website
(914 mm) is to be constructed. The design flow of the well is (www.epa.org) and find the Groundwater Rule (GWR). Explain
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1,000 gpm (5,450 m /d). Calculate the drawdown at the well with the background of the GWR and outline briefly the most recent
total withdrawals from storage (that is, with no recharge or leak- final GWR final requirements.
ages) after (a) 1 min, (b) 1 h, (c) 1 day, (d) 1 month, and (e) 1 year
3.17 About 25 % of Earth’s freshwater supply is stored be-
of continuous pumping, at design capacity.
neath the surface of the land, where it remains for thousands
3.8 Determine the drawdown of a quasi-steady state for a of years. Only a relatively small proportion of this groundwa-
proposed 36-in. (914 mm) effective diameter well pumping ter is located in soil formations (aquifers) from which it can be
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continuously at 350 gpm (1,907 m /d) in an elastic artesian withdrawn in significant amounts. Explain the following
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aquifer having a transmissivity of 5,000 gpd/ft (62 m /d/m) briefly:
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and a storage coefficient of 4 10 . Assume that the dis- (a) The reasons why groundwater is the principal source of
charge and recharge conditions are such that the drawdown water for small systems
will be stabilized after 180 days. (b) The U.S. EPA’s Groundwater Rule promulgation
3.9 A well was pumped at a constant rate of 500 gpm (c) The meaning of “groundwater under the direct influence of
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(2,725 m /d) between 8 a.m. and 11 a.m., 2 p.m. and 5 p.m., and surface water” (GWUDI) and the water monitoring and treat-
remained idle the rest of the time. What will be the drawdown ment requirements of GWUDI