Page 31 - Water Engineering Hydraulics, Distribution and Treatment

EXAMPLE 1.3 DETERMINATION OF AQUIFER YIELD
Make a rough estimate of the yield of an aquifer 20 ft (6.10 m) deep through which water moves at a rate of 3 ft (0.91 m) a day (1) if
all of the groundwater laterally within 500 ft (152 m) of the well comes fully within its influence and (2) if a gallery 1,000 ft (305 m)
long collects water from both sides.
Solution 1 (US Customary System):
3
1. 20 ft × 500 ft × 2 × 3ft∕d × (7.5gal∕ft )∕(1,440 min∕d) = 310 gpm.
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2. 20 ft × 1,000 ft × 2 × 3ft∕d × (7.5gal∕ft )∕(1,000,000) = 0.90 MGD.
Solution 2 (SI System):
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1. (6.10 m)(305 m)(0.91 m)(1,000 L∕m )(1∕1,440 min) = 1176 L∕min. 1.7 Purification Works 9
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2. 2(6.10 m)(305 m)(0.91 m)∕d = 3,397 m ∕d = 3,396,500 L∕d = 3.4MLD.
and trenches are similar in conception. They are, in essence, To determine the yield of groundwater areas, the engi-
large, or long, shallow, open wells. Filter cribs built into neer must know the geology as well as the hydrology of the
alluvial deposits of streams intercept the underflow. Ground- region. He can learn much from existing supplies in nearby
water can also be collected from the driftways and slopes areas, but his ultimate judgment must generally rest on the
of mines, galleries driven into mountainsides specifically behavior of test wells.
for this purpose, or abandoned mines. Some infiltration gal-
leries yield as much as 1 MGD/1,000 ft (12.4 MLD/1,000 m)
of gallery. They are particularly useful in tapping aquifers 1.7 PURIFICATION WORKS
of shallow depth or where deep saline waters are to be
The quality of some waters from surface or ground sources
excluded.
is naturally satisfactory for all common uses. Disinfection
may be the only required safeguard. Other waters contain
objectionable substances that must be removed, reduced to
1.6.4 Recharging Devices
tolerable limits, destroyed, or otherwise altered in charac-
As outlined earlier, the yield of groundwater works can be ter before the water is sent to the consumer. Impurities are
augmented or maintained at high level by water spreading or acquired in the passage of water through the atmosphere, over
diffusion. The necessary structures are built close to the col- the earth’s surface, or through the pores of the earth. Their
lecting works within the groundwater shed. Charging ditches pollution is associated with man’s activities, in particular,
or basins are filled with river or lake water by gravity or pump- with his own use of water in household and industry and the
ing. In the flooding method, water diverted from streams by return of spent water to natural water courses. Some of the
check dams is led onto a suitable area of pervious soils. The heavy metals (lead, copper, zinc, and iron) come from the cor-
applied waters soak into the ground and increase its natural rosion of metallic water pipes. Contamination of distribution
flows. The incentive is either augmentation of a dwindling or systems through cross-connections with impure water sup-
inadequate supply or taking advantage of natural filtration as plies and through backflow in plumbing systems is another
a means of water purification. Gathering a more uniformly hazard. (Backflow permits water drawn into a fixture, tank,
cool water is also a consideration. Badly polluted surface or similar device to flow back into the supply line by gravity
water may be partially purified before it is introduced into or by siphonage.)
the charging structure. Some diffusion galleries and wells How to treat a given supply depends on its inherent
return waters abstracted earlier from the ground for cooling traits and on accepted water quality standards. Municipal
and other purposes. works must deliver water that is (a) hygienically safe, (b)
Groundwater collection works usually include pumps. aesthetically attractive and palatable, and (c) economically
To them water flows from all or much of the well field either satisfactory for its intended uses. The most common classes
by gravity through deep-lying conduits or under negative of municipal water purification works and their principal
pressure through suction mains. Individual pumping units functions are as follows:
are often used instead, especially when the water table lies at
considerable depths. 1. Filtration plants remove objectionable color, turbid-
Most natural groundwaters are clean, palatable, and ity, and bacteria as well as other potentially harmful
cool. However, passage through some soils may make them organisms by filtration through sand or other granular
unpalatable, unattractive, corrosive, or hard (soap consum- substances after necessary preparation of the water by
ing). Their treatment must be varied according to needs. coagulation and sedimentation (Fig. 1.4a).