Leachate has long been an issue for garbage collection sites because of typically high levels of toxic contaminants and alarmingly high concentrations of ammonia. Liquids, whether originating from biological decay, or from natural precipitation invariably pick up particles and liquids as they pass through the layers of waste, much as they do when passing through a layer of fresh ground coffee. In this case, though, the leachate often carries dissolved organic compounds, inorganic macro compounds, heavy metals such as copper, lead, cadmium, chromium and nickel, while pathogenic microorganisms such as disease-causing viruses and bacteria are particularly common in fresh leachate. These pollutants from landfill leachate have accumulative and detrimental effects on ecology and food chains, leading in turn to carcinogenic effects, acute toxicity, and genotoxicity among human beings. When natural water systems are exposed to highly toxic landfill leachate, chronic toxin accumulation in soils and highly polluted drinking water sources can be found even in far distant locations.
In addition, landfills that handle organic material (most municipal facilities and consumer collection points) will almost always produce methane. While most of the gaseous methane will be captured and removed using common methane collection systems, some will dissolve in the leachate. This methane does not flow like the gaseous form, so it isn’t diverted and removed. Instead, it may inadvertently be released from its dissolved state and subsequently collect in air pockets, sometimes at such concentrations that the potential for explosion and fire is significant.
Still, leachate can be a desirable part of the decomposition process, as it feeds beneficial bacteria and fungus which accelerate decomposition. The problem arises when the organic decomposition accelerates to the point where available oxygen is consumed and anoxic processes take over. As the conditions transform to a more acidic state, further powered by abundant ammonia, some metals may dissolve that would normally remain, including fire ash, and cement-based and gypsum-based building materials. If enough gypsum materials are present, the leachate may react with it to generate large amounts of hydrogen sulfide, a poisonous, corrosive and flammable gas.
Specific treatment strategies are dependent on the seasonal composition of the leachate, flow rate, and volume, so the strategies cannot be immutable. Instead, management of leachate requires careful, regular monitoring and a constantly adjusted treatment regimen.
Interestingly, this kind of toxic leachate is not only found in modern landfills but can be a problem in any kind of contaminated land, say, from previous industrial activity or old, unlined landfills. Ironically, the original sanitary landfill in the US, the Fresno Municipal Sanitary Landfill is both a national historic landmark and a National Superfund site, largely thanks to the large amount of toxic waste it contains and the highly toxic leachate it produces.
