Any water well design should be aimed at the extraction of high quality ground water without contaminating it in the process. Locating and sealing the well properly, as well as limiting the sources of pollution should help keep the quality of ground water high.
Locating the Well
Locating a well too close, or downslope from, a pollution source
invites pollution of the well. Wells should be located on the highest ground
possible and far enough from the pollution sources to insure that
contaminated water flowing from the source will not intercept the well's
zone of contribution (zoc). Information concerning the general direction
of ground-water flow and the zoc of the well can be supplied by the driller
or can be determined from well pump tests. See (12) for more on determining
the direction of ground water movement.
Most states recommend that wells be sited at least from 100 to 150 feet
from a pollution source. Increase that distance in sandy soils.
For more specific recommendations, contact the state or county health
department.
Sealing the Well
The penetration of a water-bearing formation by a well provides two main
routes for possible contamination of ground water by both surface and
subsurface sources. These are the open, top end of the casing and
annular space between the casing and the borehole. Prevention of
contamination through these two routes is carried out both at the
upper terminal and lower terminal of the casing.
Upper Terminal
Well casing should extend at least 1 foot above the general
level of the surrounding land surface; or 1 foot above the highest
expected level of water during a one-hundred year flood where the
well is near a body of water. It should be surrounded at the ground
surface by a 4-inch thick slab extending at least 2 feet in all
directions. The upper surface of the slab and its immediate
surroundings should also be sloping so as to drain water away from
the well. It is also good practice to place a drain around the edge
of the concrete apron and extend it to a discharge point at some
distance from the well. Lastly, a sanitary seal should be
provided at the top of the well to prevent the entrance of contaminated
water or other objectionable material directly into the well.
Lower Terminal and Casing
For artesian aquifers, the water-tight casing should be
extended downwards into the impermeable formation which caps the
aquifer.
In water-table aquifers, the casing should be extended at least 5 feet below the lowest expected pumping level. This distance should be extended to 10 feet where the pumping level is less than 25 feet from the surface.
The above are general rules which should be applied with some flexibility where geologic conditions so require.
In addition, wells showing the presence of colliform bacteria
have not been properly sealed. Although every precaution may have
been applied on the ground surface and upper terminal, vertical seepage
can occur readily though the annular space between the well casing
and the borehole.
Grouting and Sealing the Casing
Grouting or cementing well casing involoves filling the
space around the pipe, usually between the pipe and the drilled hole,
with a suitable slurry of cement or clay. If pressure grouting is
used the cracks and spaces of the borehole are filled in too. In the
figure on the right, one side of
the well is properly grouted, while the other side is not allowing water
to flow from the surface and other water-bearing layers into the well
screen.
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