Page 98 - Water Engineering Hydraulics, Distribution and Treatment

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Chapter 3
Water Sources: Groundwater
Domestic systems commonly employ one of the follow-
ing pumps:
After a well is completed, it should be tested to deter-
mine its characteristics and productivity. Constant-rate and
1. For lifts under 25 ft (7.62 m), a small reciprocating
step-drawdown pumping tests are used for this purpose. The
or piston pump
test should be of sufficient duration; the specific capacity of a
2. For lifts up to 125 ft (38.10 m), a centrifugal pump
well based on a 1-hour test may be substantially higher than
to which water is lifted by recirculating part of the
that based on a 1-day test. Longer duration is also required to
discharge to a jet or ejector
detect the effect of hydraulic boundaries, if any. The extent
3. For lifts that cannot be managed by jet pumps, a
to which the specific yield would decrease depends on the
cylinder pump installed in the well and driven by
nature and the effectiveness of the boundaries.
pump rods through a jack mounted at the well head
Systems of choice normally incorporate pressure tanks 3.17.8 Testing
3.17.9 Sanitary Protection of Wells
for smooth pressure-switch operation. The well itself may
provide enough storage to care for differences between The design and construction of a well to supply drinking
demand rates in the house and flow rates from the aquifer. water should incorporate features to safeguard against con-
This is why domestic wells are seldom made less than 100 tamination from surface and subsurface sources. The protec-
ft (30 m) deep even though the water table may lie only a tive measures vary with the geologic formations penetrated
few feet below the ground surface. Deep wells and pump and the site conditions. The well should be located at such
settings maintain the supply when groundwater levels sink a distance from the possible sources of pollution (e.g., wells
during severe droughts or when nearby wells are drawn used for the disposal of liquid wastes or artificial recharge;
down steeply. seepage pits; and septic tanks) that there is no likelihood
Large-capacity systems are normally equipped with cen- of contaminated water reaching the well. The casing should
trifugal or turbine pumps driven by electric motors. A suf- be sufficiently long and watertight to seal off formations
ficient number of pump bowls are mounted one above the that have undesirable characteristics. Failure to seal off the
other to provide the pressure necessary to overcome static annular space between the casing and well hole has been
and dynamic heads at the lowest water levels. For moderate responsible for bacterial contamination in many instances.
quantities and lifts, submersible motors and pumps, assem- The casing should be sealed in place by filling the open space
bled into a single unit, are lowered into the well. The water around the casing with cement grout or other impermeable
being pumped cools the compact motors normally employed. material down to an adequate depth. This prevents seepage
Large-capacity wells should be equipped with suitable mea- of water vertically along the outside of the pipe. A properly
suring devices. Continuous records of water levels and rates cemented well is shown in Fig. 3.16. The well casing should
of withdrawal permit the operator to check the condition of extend above the ground. The top of the well should contain
the equipment and the behavior of the source of supply. This a watertight seal; the surface drainage should be away from
is essential information in the study and management of the it in all directions.
groundwater resource. An essential final step in well completion is the thorough
disinfection of the well, the pump, and the piping system.
Although the water in the aquifer itself may be of good san-
3.17.7 Development itary quality, contamination can be introduced into the well
Steps taken to open up or enlarge flow passages in the for- system during drilling operations and the installation of other
mation in the vicinity of the well are called development. elements of the system. Periodic disinfection of the well dur-
Thorough development of the completed well is essential ing the drilling is a good practice and should be encouraged.
regardless of the method of construction used to obtain higher In the case of an artificially gravel-packed well, all gravel-
specific capacities, to increase effective well radius, and to pack material should be sterilized before being placed in the
promote efficient operation over a longer period of time. well. Solution strengths of 50–200 mg/L chlorine are com-
This can be achieved in several ways. The method selected monly used for sterilizing wells. The effectiveness of disin-
depends on the drilling method used and on the formation fection should be checked after the completion of the work.
in which the well is located. The most common method Disinfection of the system is also necessary after repairs of
employed is overpumping, that is, pumping the well at a any part of the system.
higher capacity than the design yield. Temporary equipment
can provide the required pumping rates. Pumping is contin-
3.17.10 Maintenance
ued until no sand enters the well. Other methods used include
flushing, surging, high-velocity jetting, and backwashing. Good maintenance extends the life of a well. The mainte-
Various chemical treatments and explosives are used in spe- nance of the yield of a well depends on (a) the well con-
cial circumstances. struction, (b) the quality of water pumped (water may be