Module 9 — Plugging Abandoned Wells
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Materials
Materials that are used to plug a well must be uncontaminated and impervious.
They must prevent any movement of water. See the chart below for acceptable and
unacceptable materials.
Acceptable Materials
Unacceptable Materials
•
grout - neat cement (cement mixed with water)
-
sand cement (cement, sand and water)
•
concrete (cement, sand and aggregate mixed with water)
•
manufactured high yield bentonite products
•
clean, uncontaminated clay (for large diameter wells)
sand
gravel
drilling mud or fluid
High yield bentonite is a special type of clay that swells when wet to provide a very
effective impervious seal. It comes in a powder that when mixed with water produces a slurry
that can be pumped into the well. It is also manufactured in pellet or granular form that is
designed to pour into the well. This type of bentonite when mixed with water will actually
swell to about eight times its original size and will form a water-tight plug.
It is important to understand that bentonite cannot be used as a plugging material in
some situations. When the chloride level in the well water is greater than 4000 mg/L, or the
calcium level is greater than 700 mg/L, bentonite will not swell properly, so then it is best to
use a cement grout.
Large diameter or bored wells pose special problems because of their size and the volume
of material required to fill them. A lower cost alternative for the plugging material is clean,
uncontaminated clay that can be shovelled into the well until it is filled. This must be done
carefully, however, to ensure the clay reaches the bottom of the well and seals off all empty
space. The cribbing must be cut off below ground surface and the well should be topped up
with high yield bentonite to make a water-tight seal.
Cement grout and concrete may shrink
after setting so may not create as good a
seal as bentonite.
Sand and gravel are not acceptable
materials. They are not impervious
materials because water can easily move
through them.