Page 143 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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3.17 Well Construction 105
3.17.9 Sanitary Protection of Wells

The design and construction of a well to supply drinking water should incorporate fea-
tures to safeguard against contamination from surface and subsurface sources. The
protective measures vary with the geologic formations penetrated and the site condi-
tions. The well should be located at such a distance from the possible sources of pollu-
tion (for example, wells used for the disposal of liquid wastes or artificial recharge;
seepage pits; and septic tanks) that there is no likelihood of contaminated water reach-
ing the well. The casing should be sufficiently long and watertight to seal off forma-
tions that have undesirable characteristics. Failure to seal off the annular space be-
tween the casing and well hole has been responsible for bacterial contamination in
many instances. The casing should be sealed in place by filling the open space around
the casing with cement grout or other impermeable material down to an adequate
depth. This prevents seepage of water vertically along the outside of the pipe. A prop-
erly cemented well is shown in Fig. 3.16. The well casing should extend above the
ground. The top of the well should contain a watertight seal; the surface drainage
should be away from it in all directions.
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
sanitary quality, contamination can be introduced into the well system during drilling

Rock outcrop At least 6˝
Slope

Coment grout forced Top soil to be
up from bottom sealed off
At least 4˝ larger
diameter than casing
Drill hole

Shattered rock
to be sealed off

Well casing
Water-bearing stratum
to be sealed off Liner

Sound impervious rock
Not less than 20´ Cement grout forced
into sound rock
up from bottom

Drill hole
Water-bearing formation to be tapped

Figure 3.16 Drilled Well and Its Sanitary Protection. (After Iowa State Department
of Health). Conversion factor: 1 1 ft 0.3048 m; 1 1 in. 25.4 mm