The site must be 1 square mile. Time and money are important considerations. Even after the landfill is filled and sealed off, monitoring should continue for many years after. These materials are best because they act as natural seals between the landfill and the environment. Porous materials are rock and soils that have a significant amount of pore space between grains or contain alot of air.
Permeable materials are rocks and soils that liquids can move through.
Porous rocks are usually permeable. Almost all soils are permeable. Modern landfills usually start by digging a large pit into the ground and covering it with layers of trash until it is filled to the surface. The bottom and sides of the pit are lined with layers of dense clay or plastic to keep leachate - liquids from decaying waste, from leaking into the soil. Wells and pumps are installed in the landfills for monitoring.