Leachate is contaminated liquid that has percolated through hazardous waste and leaked into the environment. Because this runoff contains a variety of toxic chemicals, it typically causes an impact on the environment and human health, especially when it originates from solid waste in landfills and migrates into the groundwater, rivers and streams, and the ocean.
Landfills are the most widely used method for the disposal of solid waste around the world. When water from rain or melting snow percolates through the decaying organic and inorganic waste of these landfills, it becomes polluted. All landfills generate this form of pollution, even landfills that have been closed and abandoned for decades. Typically, one ton of landfill waste generates about 53 gallons of highly contaminated wastewater leachate.
Landfills are different from other groundwater pollution sources because after wastes are buried, a series of physical, chemical and biological reactions take place that amplify the toxic concentration of the waste that runs off as leachate. In rare cases, whole new compounds are created. The common leachate from a landfill has high concentrations of ammoniacal nitrogen and moderately high recalcitrant compounds, which include halogenated hydrocarbons such as carbon tetrachloride and methylene chloride, and complex polymers which contain heavy metals that are not easily degradable. Of these toxic pollutants, the decomposition of organic nitrogen into ammoniacal nitrogen is not only as a long-lived pollutant, but also highly toxic to most aquatic species. Over the last 100 years, arsenic has played a major industrial role in the manufacture of lead batteries, metal alloys, semiconductors, wood preservatives and glass, as well as animal feed, herbicides and pesticides. All these material find their way into landfills. The Agency for Toxic Substances and Disease Registry and the United States Environmental Protection Agency list arsenic as number 1 of 275 substances most commonly found at superfund sites on the National Priorities List, and so pose the potentially greatest threat of human exposure.
Leachate contaminants that seep from landfills are a serious threat to the underlying groundwater, a major source of drinking water worldwide. This risk has become an important concern in developing countries where most landfills are built without any preventive environmental engineering such as liners or leachate collection systems. Once the leachate enters groundwater, it is difficult and expensive to clean it up and restore the drinking water source to a usable state.
Hazardous leachate leaking into sewers is a common problem. Public wastewater treatment plants generally don’t accept leachate due to its toxic intensity. For those treatment plants that do accept leachate untreated, both physical and health threats exist. The presence of dissolved methane can be a physical hazard. In inadequately ventilated sewer systems, this methane can cause violent explosions. The high concentrations of ammoniacal nitrogen in leachate can also pose a health threat to sewer maintenance workers.