In which subsurface formation is groundwater contamination likely to travel the farthest?

Groundwater contamination is influenced heavily by the characteristics of the subsurface formation through which it travels. Limestone is often characterized by its porosity and permeability, allowing water—and thus potential contaminants—to move more freely and over greater distances compared to other formations.

In limestone formations, the presence of fractures and solution cavities can also facilitate the movement of groundwater. This means that not only does water flow readily through the material, but contaminants can likewise migrate more swiftly and extensively, leading to a higher likelihood of long-distance travel in comparison to less permeable formations.

Other types of formations, such as clay, granite, and gravel, have varying degrees of permeability that limit or funnel the movement of groundwater. Clay is particularly impermeable, trapping contaminants close to the source. Granite, being a hard rock with low porosity, provides very limited paths for groundwater movement, and therefore, any contamination usually remains localized. Gravel, while more permeable than clay and granite, does not typically have the extensive channels found in limestone that can enable contaminants to travel significant distances.

Thus, the characteristics of limestone make it the subsurface formation where groundwater contamination is likely to travel farthest.