Page 50 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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1.6 Groundwater 13
also be collected from the driftways and slopes of mines, galleries driven into moun-
tainsides specifically for this purpose, or abandoned mines. Some infiltration galleries
yield as much as a 1 MGD/1,000 ft (12.4 MLD/1,000 m) of gallery. They are particu-
larly useful in tapping aquifers of shallow depth or where deep saline waters are to be
excluded.

1.6.4 Recharging Devices
As outlined earlier, the yield of groundwater works can be augmented or maintained at
high level by water spreading or diffusion. The necessary structures are built close to the
collecting works within the groundwater shed. Charging ditches or basins are filled with
river or lake water by gravity or pumping. In the flooding method, water diverted from
streams by check dams is led onto a suitable area of pervious soils. The applied waters
soak into the ground and increase its natural flows. The incentive is either augmentation of
a dwindling or inadequate supply or taking advantage of natural filtration as a means of
water purification. Gathering a more uniformly cool water is also a consideration. Badly
polluted surface water may be partially purified before it is introduced into the charging
structure. Some diffusion galleries and wells return waters abstracted earlier from the
ground for cooling and other purposes.
Groundwater collection works usually include pumps. To them water flows from all or
much of the well field either by gravity through deep-lying conduits or under negative
pressure through suction mains. Individual pumping units are often used instead, espe-
cially when the water table lies at considerable depths.
Most natural groundwaters are clean, palatable, and cool. However, passage through
some soils may make them unpalatable, unattractive, corrosive, or hard (soap consuming).
Their treatment must be varied according to needs.
To determine the yield of groundwater areas, the engineer must know the geology
as well as the hydrology of the region. He can learn much from existing supplies in
nearby areas, but his ultimate judgment must generally rest on the behavior of test
wells.

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 (U.S. Customary System):
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1. 20 ft 500 ft 2 3 ft/d (7.5 gal/ft )>(1,440 min/d) 3l0 gpm.
3
2. 20 ft 1,000 ft 2 3 ft/d (7.5 gal/ft )>(1,000,000) 0.90 MGD.

Solution 2 (SI System):
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1. (6.10 m)(305 m)(0.91 m)(1000 L/m )(1/1,440 min) 1,176 L/min.
3
2. 2(6.10 m)(305 m)(0.91 m)/d 3397 m /d 3,396,500 L/d 3.4 MLD.