CEMP-E
TI 814-01
3 August 1998
- Pressure Gage
- Screened Casing Vent
- Sampling Tap
- Water Treatment Equipment (if required)
- Well Operating Records
If climatic or other conditions are such that a well house is not necessary, then the well should
be protected from vandals or unauthorized use by a security fence having a lockable gate.
(5) Pit construction. Pit construction is only acceptable under limited conditions such as
temporary or intermittent use installations where the well pump must be protected from the
elements when not in use. The design must allow for cleaning and disinfection. Underground
pitless construction for piping and wiring may be adequate for submersible pump installations.
These designs may be used only when approved by the responsible installation medical
authority.
i. Spacing and Location. The grouping of wells must be carefully considered because of
mutual interference between wells when their cones of depression overlap. Minimum well
spacing shall be 75 m (250 ft).
(1) Drawdown interference. The drawdown at a well or any other location on the water
table is a function of the following:
- number of wells being pumped
- distance from point of measurement to pumping wells
- volume of discharge at each well
- penetration of each well into aquifer.
For simple systems of 2 or 3 wells, the method of super position may be used. The procedure
is to calculate the drawdown at the point (well) of consideration and then to add the drawdown
for each well in the field. For multiple wells, the discharge must be recalculated for each
combination of wells, since multiple wells have the effect of changing the depth of water in
equations 5-1 and 5-2. For large systems the following conditions should be noted:
- boundary conditions may change
- change in recharge could occur
- recharge may change water temperature, an increase in water temperature increases
the coefficient of permeability
- computer analysis may be helpful to recalculate the combinations.
It is seldom practicable to eliminate interference entirely because of pipeline and other costs,
but it can be reduced to manageable proportions by careful well field design. When an aquifer
is recharged in roughly equal amounts from all directions, the cone of depression is nearly
symmetrical about the well and "R" is about the same in all directions. If, however,
substantially more recharge is obtained from one direction; e.g., a stream, then the surface
elevation of the water table is distorted, being considerable higher in the direction of the
stream. The surface of the cone of depression will be depressed in the direction of an
5-20
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