In environmental investigations it is, of course, important that the material that you are using meets all the requirements. Filter sand is delivered in bags with certain grain size. Bentonite is used to seal around the sampling tube. Bentonite is dried clay that swells on contact with water, creating a natural seal around your pipe.
Explanation from Wikipedia;
Bentonite slurry walls are used in construction, where the slurry wall is a trench filled with a thick colloidal mixture of Bentonite and water. A trench that would collapse due to the hydraulic pressure in the surrounding soil does not collapse as the slurry balances the hydraulic pressure. Forms for concrete, and rebar, can be assembled in a slurry filled trench, and then have concrete poured into the form. The liquid concrete is heavier than the bentonite slurry, and displaces it, and the bentonite slurry can subsequently be re-used in a new trench elsewhere on the construction site.
In addition, because the colloid is relatively impervious to water, a slurry wall can prevent the seepage of groundwater, which is useful in preventing the further spread of ground-water that has been contaminated by toxic material like industrial waste.