However, due to improper siting and other factors, many landfills which appeared to be properly operated were responsible for contamination of ground and surface waters. Landfills also have been responsible for fires and explosions on adjacent properties due to migration of landfill gas. Modern landfill design and operation includes the collection and treatment of leachate as well as the control of landfill gas. Below is a brief introduction to leachate, landfill gas, and settlement.