Which of the following is used to properly seal a drilled well to prevent contamination of the water supply?

Grouting is the correct choice for sealing a drilled well to prevent contamination of the water supply. This method involves filling the annular space between the well casing and the surrounding soil or rock with a material, typically a cement-based grout, that creates a watertight seal. This sealing technique is essential for preventing surface water and contaminants from entering the well and affecting the quality of the drinking water.

By using grouting, the integrity of the well is maintained, ensuring that any potential contaminants from the surface or shallow ground can be blocked effectively. It is a standard practice in well construction and is required by many regulations to ensure public health safety regarding water supply sources.

Other options, while they may have specific applications in construction or sealing, do not provide the same level of efficacy or compliance with regulations for well sealing. For instance, epoxy and rubber seals can be useful in various contexts, but they are not specifically designed for addressing the challenges associated with well sealing against contamination. Bentonite clay, while sometimes used as a sealing material in certain contexts, is generally not as effective as grouting in maintaining a long-term seal needed for protection against contaminants infiltrating the well.