What is a common reason for the contamination of wells drilled through rock or clay?

The correct answer highlights an important aspect of well construction and maintenance. Proper sealing of the well casing is crucial to prevent contaminants from entering the well water supply. When a well casing is not sealed adequately, it creates a pathway for surface water and contaminants to flow directly into the well. This can be particularly concerning in areas where pollutants exist on the surface or where agricultural runoff might be prevalent. Effective sealing helps to isolate the groundwater from potential contamination sources.

The other options relate to various sources of contamination but do not directly address the construction integrity of the well. For instance, while seepage of pollution through the soil can lead to contamination, it is often a more gradual process. The porosity of the rock can influence how easily contaminants move through geological layers, but it does not directly cause contamination unless there is a failure in the well design or maintenance practices. Using inferior quality well casings may lead to other types of structural issues, but the focus of the question is on the immediate risk associated with improper sealing. Thus, the proper sealing of the well casing is essential for safeguarding drinking water supplies.