In order to protect aquifers near landfills material is placed under the soil to prevent pollution. Which property must the protective material possess?

The protective material must possess low permeability to effectively prevent the movement of contaminants from the landfill into the aquifer. This property ensures that water and pollutants cannot easily pass through the material, thereby safeguarding the groundwater resources below. Additionally, it should be chemically inert to avoid any reactions with potential pollutants.