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Module 9 — Plugging Abandoned Wells
Method
Aside from choosing the appropriate plugging material, the method of placing material
into the well is most critical. Regulation requires that the plugging material must be
introduced from the bottom of the well and placed progressively upward to ground surface.
If the plugging material is cement grout, concrete or bentonite slurry, special equipment
is needed. The material must be placed into the well through a tremie pipe that is usually
about 3 in. in diameter. At all times this pipe must be kept below the surface of the plugging
material to prevent it from diluting or separating. It is recommended that you hire a licensed
water well contractor when a slurry is chosen as the plugging material because they will
have the proper equipment and experience to do the job correctly.
When bentonite pellets are chosen for the plugging material, they can be poured into the
well from the ground surface. These pellets have a weight material added to help them sink
to the bottom of the hole. They are also coated to prevent immediate swelling on contact with
water. When poured slowly, they should reach the bottom of the well before swelling.
If you are not careful, however, these pellets will bridge off down hole and the well will be
only partially plugged (see Figure 3, Bridging).
Before you pour in the pellets, you can determine how many feet of well casing can be
filled with the size of pellets you have chosen. As the well is being filled, measure the depth
to the top of the plugging material quite frequently. Then you will know if the plug is rising
faster than expected indicating a bridge has formed. If this happens, be sure to break it up
before adding more material to the well.
Figure 3 Bridging