This article, written by the American Ground Water Trust was originally published in THE AMERICAN WELL OWNER, 2002, Number 4]


Water wells, if properly constructed, can provide a safe and reliable source of drinking water. An essential aspect of proper well construction is sealing the space between the well casing and the side of the drilled hole (called the well annulus). Well casing should also have a vermin-proof vented-cap and penetrate the ground at least 20 feet. If bedrock is present, the casing should be drilled into unweathered rock at least 5 feet. Because well construction regulations vary from state to state, homeowners should check with local authorities (e.g., State water well boards, health departments or state-level environmental protection agencies) to determine the specific regulations that apply to domestic drinking water wells.

If unsealed, the well annulus could provide a direct pathway from the surface to the ground water below at a significantly fast pace compared to infiltration through undisturbed soil. If the natural cleansing process incumbent during infiltration is short-circuited through the annular space, bacteria and other contaminants may be introduced to the ground water.

Properly sealing a well casing involves filling the annulus with a material that will stop water from flowing down the side of the casing. The process of filling the annulus is called grouting. Grouting typically involves pumping cement and/ or bentonite into the annular space starting at the bottom of the casing and filling back up to the surface. Starting the grouting process at the bottom of the hole reduces the possibility that air and water will be trapped in the space due to “bridging” and clogging conditions that commonly occur in the narrow portions of the annulus. To ensure that the annulus has enough space to accept grout without a high likelihood of clogging or having the casing touch the wall of the hole, the diameter of the drill hole should be 4 to 8 inches larger than the outside diameter of the well casing.

Bentonite is a clay that expands when mixed with water. When wet its volume increases by about 13 times compared to its volume when dry. This quality allows it to tightly fill small and irregular spaces between the casing and the hole wall. When mixed with cement the bentonite-cement-slurry creates a seal that is somewhat plastic so that it can absorb slight movement of the casing without cracking. The slurry is also more resistant than plain bentonite to washouts if high water content zones must be sealed. Cement by itself has a tendency to shrink as it cures and may pull away from the sides of the annular space. Shrinkage is especially likely if the cement is “mixed thin” with too much water. Also cement heats up significantly when it cures and so cement grout is not recommended for plastic or PVC well casing.

Drill cuttings (rock fragments released from the hole during drilling) are sometimes used as a backfill of the annular space. Although the drill cuttings as usually very small particles, they may not pack together well enough to provide the best seal for a well because of their composition, hard surfaces and shape. Grouting a drill hole may cost a little more initially, but it is a small price to pay compared to cleaning up a polluted well.

[© American Ground Water Trust. This article may be reprinted for non-commercial educational purposes provided it is used in its entirety and that reference is made to its source as an article in THE AMERICAN WELL OWNER, 2002, Number 4]