Glofelty reported in June 2024 Water Well Journal that the well annular seal is typically composed of cement or bentonite clay, and both these materials have advantages and disadvantages depending on the site-specific conditions of the well.
The surface annular seal of a water well is installed to meet regulatory requirements, provide wellhead protection, and to stabilize the work area around the drilling rig during well construction. The rationale for the design and installation of the well annular seal is more focused on functionality of the well rather than on land surface conditions or regulatory issues.
Bentonite annular seals, as with other construction materials for water wells, should have National Sanitation Foundation and American National Standards Institute (NSF/ANSI 61) Certification. Various sources report that bentonite will swell 8 to 15 times its dry volume when hydrated. Bentonite's swelling and sealing properties along with its inert chemistry make it a good option for use as an annular seal in water wells. However, bentonite is sometimes considered difficult to install since it can sometimes swell prematurely and clog tremie pipes or bridge within the annulus above the desired seal depth.
Three common types of bentonite annular seals used in water wells are high-solids bentonite grout slurry, bentonite chips, and bentonite pellets (which are also called bentonite tablets and can be either coated or uncoated). High-solids bentonite grout is a slurry mixture of powdered bentonite and water that generally contains about 20% solids by weight. Since high-solids grout contains 20% solids, it is also 80% water. This means the bentonite grout slurry can be readily pumped into place, but if the high-solids grout is not kept hydrated, the slurry volume will be greatly reduced, and the sealing characteristics will be compromised.
Bentonite chips are quarried granules of naturally occurring bentonite that do not contain additives. Bentonite chips are commonly provided in ¼- to ½-inch or ½- to ¾-inch granule sizes. This bentonite seal material (at least in my area) is probably the most used for water well seals, although installation of the chips to significant depths can sometimes be challenging.
Bentonite pellets (tablets) are a good alternative to bentonite chips in some cases. Bentonite pellets are made with high-quality bentonite that is first field dried, and then further kiln dried and compressed into pellets of uniform shape and size, typically ranging from ¼-inches to ½-inches. Since the pellets are rounder and denser than bentonite chips, they tend to be easier to install through a tremie pipe.
[Note: read article for details]
[For 1975 Issue of WWJ on Water Well Cementing (here).]
[Also, here full WWJ issue (here)]
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