Which of the following soils classified by the Unified Soil Classification System would be the best aquitard?

• A. CL
• B. CH
• C. SM
• D. ML

Answer: (A)

An aquitard is a layer with very low hydraulic conductivity that slows or stops vertical groundwater flow. Fine-grained, cohesive soils like clays generally provide much lower permeability than silts or sands, making them better barriers to flow.

Among the options, the silty sand would allow more water to pass, so it’s not the best aquitard. A silt (low plasticity) can be finer than sand but still typically has higher permeability than clays, so it’s not as effective either. Both clays are strong aquitard candidates, but their behavior differs.

Lean clay tends to form a tighter, denser matrix with very low permeability, making it a reliable barrier to vertical flow. High-plasticity clay, while also very cohesive and low in permeability, can swell and develop microcracks under certain conditions, potentially creating pathways that undermine a consistent low-flow barrier. Because lean clay combines very low permeability with greater structural stability in typical conditions, it serves as the best aquitard among the choices.