thumb|300px|A leachate evaporation pond in a landfill site located in [[Cancún, Mexico]]
A leachate is any liquid that passes through matter and extracts soluble or suspended solids, or any other component of the material it passed through.
Leachate is a widely used term in the environmental sciences where it has the specific meaning of a liquid that contains environmentally-harmful substances that can enter the environment. It is commonly used in the context of land-filling of decomposable or industrial waste.
In the narrow environmental context, leachate is any liquid that drains from land or stockpiled material and contains significantly-elevated concentrations of undesirable material from what it passed through.
Landfill leachate
Use in mining
Heap leaching
In situ leach
Other types
Leachate can also be produced from land that was contaminated by chemicals or toxic materials used in industrial activities such as factories, mines or storage sites.
Composting sites in areas and/or times of high rainfall also produce leachate. Due to the makeup of compost, which often consists of materials that degrade relatively easily compared to the makeup other waste collection sites, prolonged moisture exposure and occurrence of anaerobic digestion, which can occur due to the collapse of compost piles. The critical portion then involves exposure to precipitation, which causes the decomposed matter to flow, becoming runoff and therefore leachate.
Leachate is associated with stockpiled coal and with waste materials from metal ore mining and other rock extraction processes, especially those in which sulfide containing materials are exposed to air producing sulfuric acid, often with elevated metal concentrations.
In the context of civil engineering (more specifically reinforced concrete design), leachate refers to the effluent of pavement wash-off (that may include melting snow and ice with salt) that permeates through the cement paste onto the surface of the steel reinforcement, thereby catalyzing its oxidation and degradation. Leachates can be genotoxic in nature.
A possible risk for the aquatic environment due to the occurrence of organic micropollutants in raw or treated landfill leachates has also been reported in recent studies.