Which type of soil is generally least effective in filtering out viruses?

Fine sandy soil is generally the least effective in filtering out viruses due to its smaller particle size combined with higher permeability. While fine sand can retain moisture and allows for water movement, its larger pore spaces compared to clay make it less effective at capturing and adsorbing pathogens, including viruses, as they can easily pass through these pores without being trapped.

In contrast, other soil types like clay have much smaller pore sizes, which create a denser structure promoting better adsorption and filtration of contaminants, including viruses. Loamy soil, which is a mix of sand, silt, and clay, balances drainage and retention, making it more effective at filtering compared to solely sandy soil. Coarse gravel, with its very large particles and gaps, also does not provide an effective medium for microbial filtration.