At the surface, all wells should have a sanitary seal, which prevents contamination from entering the:

• A. Well casing
• B. Aquifer
• C. Gravel pack
• D. Water table

Answer: (A)

The presence of a sanitary seal at the surface of all wells is crucial for maintaining water quality. This seal serves as a barrier that prevents contaminants from entering the well casing, which is the vertical structure installed to protect the well from surface influences.

A well casing is designed to facilitate the extraction of groundwater while shielding it from potential pollutants that could infiltrate from the surface. If contaminants were to breach the well casing due to an absence or failure of a sanitary seal, it could lead to contamination of the water supply, posing health risks to consumers.

In contrast, while the aquifer is the source of groundwater, the sanitary seal specifically protects the integrity of the well casing directly. Similarly, the gravel pack refers to a material used to filter out sediments and stabilize the well, and protecting it does not directly correlate with the function of the sanitary seal. The water table is essentially the level at which the ground becomes saturated with water and is influenced by various factors, but again, it is not the immediate focus of the sanitary seal's protective measure.