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Module 3 — Design and Construction of Water Wells
water to enter the well at a slower rate, causing a lower drop in pressure as the water moves
into the well. If the water flows too quickly, dissolved minerals in the water will precipitate
out of solution and create an incrustation build-up in restricting the flow of groundwater into
the well. The pore spaces in the aquifer immediately adjacent to the perforations may also
get plugged, restricting the flow even more.
Placement in the aquifer
The screen or perforations on the casing/liner must be placed adjacent to the aquifer. If
improperly placed, the well may produce fine sediment which will plug plumbing fixtures
and cause excessive wear on the pump. If the driller uses geophysical logging equipment to
accurately identify the boundaries of the aquifer, the exact placement will be easier.
Annular Seal
Sealing the well protects the well’s producing zone from contamination. The diameter
of the borehole is usually slightly larger than the casing being installed. The space between
the borehole and the casing is called the annulus of the well or the annular space. It must
be sealed to prevent any surface contamination from migrating downward and contaminating
the water supply. A properly sealed annulus also prevents any mixing of poor quality
water from upper aquifers with water from the producing aquifer of the well (see Figure 3,
Annular Seal).
Provincial regulations require the annulus be filled with impervious material such as
cement or bentonite. To isolate the producing zone of the well, the annulus should be filled
from immediately above the perforated zone to the ground surface.
Well Cap
A commercially manufactured, vermin-proof well cap is the only type of cap designed
to keep animals, insects and contaminants from entering your well. It comes equipped with
rubber gaskets and screened vents to ensure vermin stay out and air can circulate through.
Coverings for large diameter wells must be custom made because of their larger size.
Ideally they should be made of steel, or fiberglass or plastic that is stamped for potable water
use.
Figure 3 Annular Seal