Landfill leachate is liquid that moves through or drains from a landfill. This liquid may either exist already in the landfill, or it may be created after rainwater mixes with the chemical waste. Modern landfills are often designed to prevent liquid from leaching out and entering the environment; however, if not properly managed, the leachate is at risk for mixing with groundwater near the site, which can have dire effects.

The most common source of landfill leachate is rainwater filtering down through the landfill. Landfill leachate may be virtually harmless or dangerously toxic, depending upon the characteristics of the material in the landfill. Typically, landfill leachate has high concentrations of nitrogen, iron, organic carbon, manganese, chloride and phenols. Other chemicals including pesticides, solvents and heavy metals may also be present. In the past, this usually toxic soup was allowed to slowly leak away into the nearby environment, eventually mixing with the local groundwater system.

Groundwater is the source of drinking water for over 40% of the population, and up to 90% of the population in rural areas. Groundwater is the source that wells and springs tap into. It was formerly always assumed that this source of water was not subject to contamination, but recent studies have shown that this source of water can in fact become contaminated.

Modern landfill sites require that the landfill leachate be collected and treated. Since there is no method to ensure that rainwater cannot enter the landfill site, landfill sites must now have an impermeable layer at the bottom. The landfill leachate that collects at the bottom must be monitored and treated if required. This liquid can be treated in a similar manner to sewage, and the treated water can then be safely released into the environment.

Older landfill sites at which landfill leachate is a problem must be dug up, and either a new impermeable bottom must be installed, or the material must be relocated to another site. Frequently, the cost to dig up these old landfill sites is too high for the municipality to cover and nothing is done. Even when the site is dug up and either relocated or properly prepared, the damage has already been done and it may take years before the area can fully recover.