Which subsurface formation is associated with the highest potential for groundwater contamination?

Limestone formations have a unique structure characterized by significant porosity and the presence of solution channels formed by chemical weathering. This porosity can facilitate the movement of water and contaminants, making limestone a highly productive aquifer. However, this same characteristic also means that when contaminants enter the groundwater system, they can spread more easily and extensively than in less permeable formations.

In addition, limestone is often more susceptible to a phenomenon known as karstification, where the dissolution of the rock creates sinkholes and caves. These features can serve as direct pathways for surface contaminants to reach the groundwater quickly, increasing the risk of contamination.

Granite, clay, and sandstone formations each have properties that can limit groundwater movement and contamination to varying degrees. Granite is typically a very hard rock with low permeability, making it less prone to contamination. Clay has a high ability to retain water and thus usually acts as a barrier to contaminants. Sandstone may have moderate permeability, but it does not generally facilitate contamination to the degree that limestone does. This makes limestone the formation associated with the highest potential for groundwater contamination.