Which soil type is most favorable at removing viruses?

• A. Clay
• B. Gravel
• C. Fine, loamy sand
• D. Silt

Answer: (C)

Viruses are removed from water as it moves through soil mainly by sticking to soil particles (adsorption) and by being physically filtered by the soil’s pore spaces. The effectiveness depends on having enough surface area and contact time without overly restricting flow. Gravel has large pore spaces, so little contact occurs and viruses pass through easily. Clay offers a lot of surface area and adsorption sites, but its extremely fine texture can slow or clog flow, making it impractical for treating larger volumes. Silt is finer and can provide more contact, but its flow can still be slower and less predictable than a soil that blends fines with larger grains. Fine, loamy sand provides a favorable balance: it has enough fines to supply adsorption surfaces while maintaining better permeability than clay, giving viruses adequate contact time with soil surfaces without excessive clogging. This combination enhances removal efficiency compared with the other options.