Leachate is the liquid produced when water percolates through any permeable material. It can contain either dissolved or suspended material, or usually both. It is most commonly encountered in connection with landfills where it is a toxic product that can enter groundwater as a result of rain percolating through the waste.

The risks from waste leachate are due to pathogenic micro-organisms and toxic substances which might be present in it. These can be greatly mitiged by using sites which are geologically impermeable or more often by lining the landfill with an impermeable sheeting. The use of linings is now mandatory within both the United States and the European Union except where the waste is genuinely impermeable. In addition toxic materials such as cadmium and toluene cannot be disposed of in landfills.

In older landfills, the leachate was directed to the sewers, but this caused problems. One of these was the contamination by toxic metals which passed through the sewerage treatment plant and eventually entered the environment. Another problem was that if the landfill contained large amounts of organic material then methane was produced, some of which dissolved in the leachate. This could be released in poorly ventilated areas in the treatment plant and lead to an explosion. Finally, it can contain high concentrations of ammonia which can pose a health hazard to treatment plant workers, particularly in acidic leachite.

Waste leachate can also be produced from land which contained factories, mines or storage sites for toxic materials.

Links

• An introduction to leachate
• Landfill Leachate