Rain falling on the top of the landfill is the main contributor to the generation of leachate, and is by far the largest contributor for modern sanitary landfills which do not accept liquid waste.
The decomposition of carbonaceous material produces some additional water, and a wide range of other materials including methane, carbon dioxide and a complex mixture of organic acids, aldehydes, alcohols and simple sugars, which dissolve in the leachate cocktail.
The precipitation percolates through the waste and takes in dissolved and suspended components from the biodegrading waste, through physical and chemical reactions.
Most landfills are designed to minimise the amount of leachate they create during their lifetimes.
The environmental risks of leachate generation arise from it escaping into the environment around landfills, particularly to watercourses and groundwater. These risks can be mitigated by properly designed and engineered landfill sites. Such sites are those that are constructed on geologically impermeable materials or sites that use impermeable liners made of geotextiles or engineered clay . The use of linings is now mandatory within both the United States and the European Union, except where the waste closely controlled and genuinely inert.
Most toxic and difficult materials are now specifically excluded from landfill. However, despite much stricter statutory controls the leachates from modern sites are currently stronger than ever. They also contain a huge range of contaminants. In fact, anything soluble in the waste disposed will enter the leachate. Within the lists of substaces present in leachate are very low concentrations of “trace contaminants” which can have quite strongly contaminating effects. These are nowadays most often derived from materials in household and domestic retail products which enter the waste stream perfectly legally.
Unfortunately, the leachate draining from most landfills will continue to reflect the contaminants of past years, when regulatory controls were less.
These substances include extremely low concentrations of heavy metals (for example from batteries), herbicides and pesticides (as used in gardens), etc.