Leakage through the plastic liner is inevitable for a number of reasons:

“First, even the best liner and leachate collection systems will ultimately fail due to natural deterioration, and recent improvements in MSWLF [municipal solid waste landfill] containment technologies suggest that releases [of leachate] may be delayed by decades at some landfills." (Id., at p. 33345).

Second, human error may also contribute to leachate "releases due to design or operating errors (e.g., tearing of liners or disposing of wastes that are incompatible with the liner) and routine deterioration of liner." (Id., at p. 33344).

Industry argued during the development of the regulations that liners could contain landfill leachate forever. EPA disagreed:

“A liner is a barrier technology that prevents or greatly restricts migration of liquids into the ground. No liner, however, can keep all liquids out of the ground for all time. Eventually liners will either degrade, tear, or crack and will allow liquids to migrate out of the unit. Some have argued that liners are devices that provide a perpetual seal against any migration from a waste management unit. EPA has concluded that the more reasonable assumption, based on what is known about the pressures placed on liners over time, is that any liner will begin to leak eventually.”  47 Fed.Reg. (July 26, 1982), at pp. 32284-32285. See also  46 FR 11126, 11128 (1981); 53 FR 33314, 33344-33345 (1988).

Not only are landfill leaks inevitable, but dangerous chemicals present in modern consumer products make the toxicity of leachate produced by a solid waste landfill comparable to the leachate produced by a hazardous waste dump. According to EPA, "the concerns relating to failure of containment structures are the same for any landfill regardless of waste type." 53 FR at p. 33334.

This is not just a theoretical prediction.  In 1990 Virginia decided to invite commercial landfills into the state to enhance state revenues. In 1997, two of their seven new "state-of-the-art" dumps were found to have sprung leaks, according to The Washington Post: "In Amelia County, groundwater tests last year found elevated levels of lead, chromium and other substances, while another contaminant, antimony, showed up in groundwater near the Charles City landfill." Eric Lipton, "As Imported Garbage Piles Up, So Do Worries" (Wash. Post, 11/12/98).

In DuPage County, Illinois, the county government set aside $80 million to pay for post closure monitoring of a 2.2 million cubic yard municipal landfill, average in size by modern standards. Still, the landfill had to be declared a federal Superfund site to get the financing that ultimately was necessary to clean up hazardous leaks from the dump.