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

The most favorable soil condition for the removal of viruses is fine loamy sand over coarse sand and gravel. This combination provides a balance of filtration and retention, which is crucial for effective virus removal.

Fine loamy sand has smaller particle sizes that enhance the soil's surface area and allow for better filtration of contaminants, including viruses. The loamy texture also retains moisture, promoting the adsorption of viruses to soil particles. When underlain by coarse sand and gravel, water movement can be managed effectively. This allows for adequate percolation while still trapping viruses due to the finer upper layer.

In contrast, soils characterized by larger particles, such as gravel or coarse sand, may allow viruses to migrate more easily through the soil structure. Coarse materials typically do not provide sufficient surface area for viruses to adhere to or be filtered out, leading to poorer retention and potential contamination of groundwater. Similarly, fine clay can create a barrier that impedes water flow, which can also adversely affect the natural filtering process but is not as advantageous for virus removal compared to the ideal combination of fine loamy sand.