“Landfill leachate is liquid that moves through or drains from a dump or organized trash collection site. Some leachate exists on its own, usually a result of natural decomposition, liquids and chemicals that have been discarded also contribute. The biggest source of leachate is rainwater … when rain hits collected trash, it tends to pool up.”
– wiseGEEK, What is Landfill Leachate?
Lack of or improper management of leachate poses the risk of contaminating surface and groundwater near a landfill site. Many modern landfills are constructed with impermeable bottoms designed to contain leachate.