*TM 5-813-1/AFM-88-10, Vol. 1
from vandals or unauthorized use by a security fence
The surface of the cone of depression will be depressed
having a lockable gate.
in the direction of an impermeable boundary because
(5)
Pit construction. Pit construction is
little or no recharge is obtained from the direction of the
only acceptable under limited conditions such as
impermeable boundary.
temporary or intermittent use installations where the well
(2)
Location.
Where a source of
pump must be protected from the elements when not in
recharge, such as a stream, exists near the proposed
use.
The design must allow for cleaning and
well field, the best location for the wells is spaced out
disinfection. Underground pitless construction for piping
along a line as close as practicable to and roughly
and wiring may be adequate for submersible pump
parallel to the stream. On the other hand, multiple water
installations. These designs may be used only when
supply wells should be located parallel to and as far as
approved by the responsible installation medical
possible from an impermeable boundary. Where the
authority.
field is located over a buried valley, the wells should be
i.
Spacing and location. The grouping of wells
located along and as close to the valley's center as
must be carefully considered because of mutual
possible. In hard rock country, wells are best located
interference between wells when their cones of
along fault zones and lineaments in the landscape where
depression overlap. Minimum well spacing shall be 250
recharge is greatest. These are often visible using aerial
feet.
photographs. Special care should be exercised to avoid
(1)
Drawdown
interference.
The
contamination in these terrains since natural filtration is
drawdown at a well or any other location on the water
limited.
j.
Pumps. Many types of well pumps are on
table is a function of the following:
the market to suit the wide variety of capacity
-number of wells being pumped
requirements, depth to water and power source. Electric
-distance from point of measurement to pumping
power is used for the majority of pumping installations.
wells
Where power failure would be serious, the design should
-volume of discharge at each well
permit at least one pump to be driven by an auxiliary
-penetration of each well into aquifer.
engine, usually gasoline, diesel or propane. The most
For simple systems of 2 or 3 wells, the method of super
appropriate type is dictated by many factors for each
position may be used. The procedure is to calculate the
specific well. Factors that should be considered for
drawdown at the point (well) of consideration and then to
installation are:
add the drawdown for each well in the field. For multiple
-capacity of well
wells, the discharge must be recalculated for each
-capacity of system
combination of wells, since multiple wells have the effect
-size of well
of changing the depth of water in equations 5-1 and 5-2.
-depth of water
For large systems the following conditions should be
-power source
noted:
-standby equipment
-boundary conditions may change
-well drawdown
-change in recharge could occur
-total dynamic head
-recharge may change water temperature,
-type of well
an increase in water temperature increases the
(1)
Type. There are several types of well
coefficient of permeability
pumps.
The most common are lineshaft turbine,
-computer analysis may be helpful to recalculate
submersible turbine, or jet pumps. The first two operate
the combinations.
on exactly the same principal. The difference being
It is seldom practicable to eliminate interference entirely
where the motor is located. Line-shaft turbine pumps
because of pipeline and other costs, but it can be
have the motor mounted above the waterline of the well
reduced to manageable proportions by careful well field
and submersible turbine pumps have the motor mounted
design. When an aquifer is recharged in roughly equal
below the water line of the well. Jet pumps operate on
amounts from all directions, the cone of depression is
the principal of suction lift. A vacuum is created
nearly symmetrical about the well and "R" is about the
sufficient to "pull" water from the well. This type of pump
same in all directions. If, however, substantially more
is limited to wells where the water line is generally no
recharge is obtained from one direction; e.g., a stream,
more than 25 feet below the pump suction. It also has
then the surface elevation of the water table is distorted,
small capacity capability.
being considerable higher in the direction of the stream.
5-16
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