Why do sand and gravel allow contaminants to move more easily than clay?

Sand and gravel allow contaminants to move more easily than clay because of differences in their physical structure and permeability, which is the ability of a material to allow the transport of fluids and dissolved substances.1

• Sand and Gravel: These materials are characterized by larger particle sizes and larger pore spaces (macropores) between those particles. This high porosity and permeability facilitate the relatively free flow of water and any contaminants it carries through the soil.12
• Clay: In contrast, clay is virtually impermeable to water flow. This is primarily due to the small size of its particles and pores, as well as the platelike shape of the particles and the influence of molecular forces. These small pores restrict the passage of matter and bring contaminants into close proximity with chemical binding sites on the particle surfaces, which effectively slows or prevents their movement.12

Because of these properties, compacted clay is frequently used in sanitary landfills as an impermeable liner to contain leachate and prevent the contamination of groundwater.2