Which soil condition is most favorable for the removal of viruses?

The most favorable soil condition for the removal of viruses is represented by fine loamy sand over coarse sand and gravel. This is due to the combination of particle size and soil structure that influences retention, filtration, and adsorption processes.

Fine loamy sand provides a larger surface area as it comprises smaller particles that help to trap viruses more effectively. The loamy texture also allows for better moisture retention, which is crucial for the survival and potential inactivation of viruses. Moreover, the finer particles can create a sort of "filter" effect, which is essential for holding back contaminants, including viruses.

The presence of coarse sand and gravel beneath ensures that water can still drain efficiently while providing sufficient space for viruses to interact with the fine particles above. This stratification enhances the soil's overall capacity to reduce the concentration of viruses as they move through the soil layers.

In contrast, other soil combinations may not create the optimal environment for virus removal. For instance, sand over gravel might allow for rapid drainage without sufficient filtration, while gravel over semi-porous clay could lead to poor virus retention. Coarse sand and gravel over fine sand or clay may also facilitate very quick drainage, limiting the interaction time necessary for effective virus removal.