Microbial pollution travels only a short distance through which of the following?

Microbial pollution tends to have limited mobility in materials that have a dense structure and lower permeability due to their particles' arrangement. Sandy loam or clay, particularly when dry and compact, can restrict the movement of water and, consequently, the travel of microbial contaminants. The small pore spaces within clay tend to hold water tightly and create conditions that limit the migration of microbes.

In contrast, solution channels in limestone and fissured rock allow for more significant water flow and microbial movement due to the spaces created by natural processes. Therefore, microbial pollution could travel longer distances through these materials. Dried out, cracked clay may also let microbes travel along fissures, especially if wet conditions return. However, sandy loam or clay provide the most resistance to movement, leading to short-distance travel for microbial pollutants. Thus, this context clarifies why the selected answer effectively reflects the behavior of microbial pollution in these various contexts.