Old landfills were not much more than a hole in the ground. Modern landfill design is all about multiple barriers to permanently separate trash from the land and water around it.
An excellent example is the MSW landfill permitted by Ernest Kaufmann and his team in Meriwether, Georgia in 2012. The entire base of the new landfill is lined with 3 layers: 2 feet of low-permeable clay, followed by a tough 60-millimeter HDPE (high density polyethylene) impermeable barrier, topped by a thick layer of sand and gravel. Embedded in the sand and gravel layer are perforated pipes that collect any water that comes into contact with waste in the landfill, known as leachate, and transfer it to a qualified wastewater treatment plant. This way, leachate has no contact with surrounding land or water. Each site also contains a storm-water retention pond capable of handling the volume of two back-to-back "100-year" storm events, which is 10 times the state and federal requirements for landfill design. In addition to the back-to-back 100-year retention ponds, groundwater monitoring devices are placed around the perimeter of the landfill to monitor water quality levels.
Trash is deposited in “cells” which are covered every day by a layer of dirt to keep it contained and secure. When a section of the landfill reaches capacity, it is covered with a thick layer of soil, followed by clay, followed by another HDPE liner, followed by a geo-composite drainage system to send rainwater away from the buried trash. That section is then topped with new soil and grass to return the site to its natural state.
After the safe closure of a landfill is completed, there is a further 30 years of monitoring required by the operator. Assurance of closure and post-closure care of the landfill is a federal requirement that is paid for by the operator through funds set aside during the operation of any facility, thus guaranteeing safe closure and post closure care of the landfill.