What type of material is typically used on monitor wells for a seal above the filter packs?

• A. Portland cement grout
• B. Bentonite pellets
• C. Gravel
• D. Fine silica sand

Answer: (B)

A monitor well seal above the filter pack uses a material that expands when wet to form a tight, low-permeability barrier around the outside of the casing. Bentonite pellets are placed in this zone because they hydrate and swell, filling gaps and creating an effective seal that prevents vertical movement of groundwater and contaminants into or along the well annulus. This flexible, self-sealing property is advantageous in monitoring wells where soil conditions and occasional movement could crack a rigid seal. Cement grout, while sometimes used in other sealing applications, can crack and shrink, potentially creating pathways for leakage. Gravel and fine silica sand are used for the filter pack itself and are too permeable to serve as an adequate seal above the pack. Thus, bentonite pellets are the typical choice for this application.