Groundwater contamination potential is least likely in which geological formation?

Clay formations have very low permeability, which means that water, and any contaminants within it, has a difficult time passing through this type of soil. The compact and fine-grained nature of clay creates a barrier that limits the movement of water and contaminants, making it less likely for groundwater to become contaminated in these areas.

In contrast, sand and gravel have higher permeability and allow for easier movement of water, which can lead to a greater potential for contamination as pollutants can travel through these materials more readily. Limestone, being a soluble rock, can also contribute to groundwater contamination, especially in karst landscapes where water can flow through cracks and fissures. Thus, clay offers the greatest protection against groundwater contamination compared to the other materials listed due to its physical properties.