Page 161 - Water and wastewater engineering

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8. Screen slot size
9. Screen diameter

10. Screen entrance velocity
11. Pump power

Number of Wells
Redundancy Requirement. Many states require at least two wells. The maximum day demand
must be met with one well out of service. Thus, a complete backup system is available in case
one well fails. With the two-well system, it is better to alternate the operation of the redundant
and prime system than to operate just the prime and allow the redundant to remain idle.

Demand Requirement. When the demand is such that pumping from one well dewaters the
aquifer, then multiple wells must be operated at the same time (parallel operation). When wells
are operated in parallel or when the time sequencing of operation of nearby wells is not sufficient
to allow recovery of the cone of depression that results from drawdown, the overlap of the cones
of depression will result in an additional lowering of the piezometric surface ( Figure 4-8 ). The
wells must be far enough apart that the interference of the cones of depression is not excessive.

Location of Wells
An initial estimate in setting the location of wells relative to one another may be made using one
of the two following rules of thumb (Walton, 1970):
• For two wells, if the aquifer is 30 m or less in thickness, the distance between the two wells
should be at least 2 D where D is the thickness of the aquifer.
• For more than two wells, the distance between wells should be at least 75 m regardless of
the thickness of the aquifers.

Static water table
Discharge Discharge

Cone created by pumping Well A

Cone created by pumping
Wells A and B

Aquifer

(a) (b)
FIGURE 4-8
Effect of overlapping cones of depression. (U.S. EPA, 1973.)