Through which type of material does microbial pollution travel only a short distance?

The type of material that causes microbial pollution to travel only a short distance is smooth clay. This is due to clay's fine particle size and compact nature, which create a low permeability environment. The small pores in clay restrict the movement of water and contaminants, including microbes. This results in a limited ability for pollutants to migrate far from their source. Consequently, if microbial contaminants enter a clay layer, the travel distance is significantly curtailed compared to more permeable materials such as gravel or sandy soils, where water and pollutants can move more freely and quickly. The unique physical properties of smooth clay make it a barrier to the spread of microbes, which is vital for protecting groundwater resources from contamination.