Boreholes are drilled on land for various purposes, including for measuring water flows, for landfill gas extraction and for gethermal installations.
Good practice dictates that water courses should be protected from contamination. Various forms of bentonite are used to achieve this, depending on the design of the borehole and its location.
In general terms :
Sealing at the surface requires either bentonite granules, bentonite powder, or a bentonite-cement powder blend.
Sealing at depth requires either bentonite pellets, or bentonite-cement pellets, which do not begin to swell before reaching their intended position, which is frequently underwater deep down in the borehole.