Under what circumstance is groundwater least likely to become contaminated?

Groundwater is least likely to become contaminated when it is protected by an impermeable layer, such as clay. This is because impermeable layers act as a barrier that prevents contaminants from reaching the groundwater. When a protective layer is present, even if there are sources of potential contamination above, the impermeability restricts the movement of pollutants downward into the aquifer, effectively safeguarding the groundwater from being compromised.

In situations involving heavy rainfall, there's a higher chance for surface runoff to carry contaminants into the soil and potentially reach the groundwater. Similarly, the presence of a permeable layer would allow contaminants to migrate through it, increasing the risk of groundwater contamination. Lastly, a high water table can exacerbate the situation during events like flooding, as it can lead to saturation of the soil and facilitate the movement of pollutants into the groundwater. Thus, the presence of an impermeable layer plays a crucial role in protecting groundwater from contamination.