A major disadvantage to burying trash in landfills is the potential to pollute surrounding soil and groundwater with toxins and leachate. When trash decomposes, it releases methane and other greenhouse gases that contribute to climate change and may also cause health problems to humans and animals.
Trash normally contains a huge amount of toxic chemicals, such as mercury, arsenic, lead and PVC. These toxins build up in the landfill over time and have the potential to pollute and cause severe damage to the surrounding area if they are not contained. Another potentially major source of pollution is leachate, which is the highly toxic liquid that forms as trash breaks down.
Landfill gas, which refers to the huge amounts of carbon dioxide, methane and other harmful greenhouse gases produced during decomposition, is another problem. In addition to harming the environment, methane is incredibly flammable, and there have been cases of it seeping into buildings near landfills and leading to explosions.
All of these problems are worsened by the fact that it takes hundreds to potentially thousands of years for most of the trash in landfills to decompose. This means that all of these problems could continue and even worsen over time.