Once the landfill closes, they cap the entire site with a plastic liner, soil and vegetation. Modern landfills also have a liner system below the refuse, consisting of layers of plastic sheeting, gravel, soil and drainage pipes. This system is intended to collect leachate and prevent it from coming in contact with underlying ground water. The engineering challenge is formidable, but Southern California landfills have the advantage of a relatively dry climate. Landfills here do not present the same problems as those in wetter parts of the country, such as the Southeast, where rainfall is heavy and water tables high.
Ground water is water that flows underground through permeable soil and rock, often under pressure.
The piezometric level is the height such water would reach if it were allowed to rise freely, such as when it is tapped by a well, or in a fracture in bedrock. State regulations require that a landfill be designed so that refuse is at least five feet above this highest-anticipated level of ground water. Gregory Canyon Ltd., however, is requesting an exemption from the requirement. To get it, the company must convince state officials that meeting it would be too costly, or that following the rule would increase the threat to ground water. Gregory Canyon Ltd.'s reasons are both profit and increased risk to ground water, according to documents prepared by its consultants.