Rainfall is the main contributor to the generation of leachate in landfills, and is by far the largest contributor for modern sanitary landfills which do not accept liquid waste. Additional leachate volume is produced during waste decomposition, and if surface water runs onto waste from its surroundings.
Leachate is characterized by high values of COD, pH, ammonia nitrogen and heavy metals. These factors make leachate treatment difficult and complicated. The environmental risks of leachate generation arise from it escaping into the environment around landfills, particularly to waterways and groundwater. It has high organic contaminant concentrations and much higher levels of ammonia than found in any other organic effluent, along with other pathogenic microorganisms and toxic substances.