Microbial pollution travels only a short distance through which material?

The correct choice is based on the physical characteristics of smooth clay. Smooth clay has very fine particles that create a compact structure, leading to reduced pore space and limited permeability. This means that water moves very slowly through smooth clay, which in turn limits the distance that microbes can travel. As a result, microbial pollutants tend to remain close to the source of contamination rather than migrating over long distances.

In contrast, materials like sandstone, fissured rock, and limestone typically have larger pore spaces or interconnected fractures, allowing for greater water flow and, subsequently, farther travel distances for microbial pollutants. For instance, sandstone often has more permeability due to its grain structure, while fissured rock contains fractures that can facilitate significant fluid movement. Limestone, being a soluble rock, can also create pathways through which water and microbes can travel more extensively. Thus, smooth clay is most effective at limiting the transport of microbial pollutants.