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considerably. In fact, as a result of more stringent manufacturing practices, better resins, improved shipping, storage and handling procedures, more effective designs, and better construction quality assurance, composite liner systems constructed today have the potential of providing much greater protection of groundwater quality than those of even a decade ago. Recent research by Bonaparte (1995) "Long -Term Performance of Landfills" in Proceedings of the ASCE Specialty Conference Geoenvironment 2000, Vol. 1, New Orleans, February, concluded that "the service life for an HDPE geomembrane used as a component of a liner system in an appropriately designed and constructed MSW landfill should be in excess of the time period for leachate and gas production; the available information suggests that service life of this material will be measured in terms of hundreds of years". Geomembrane durability and resistance to chemical attack has been studied by nearly all HDPE manufacturers. When subjected to a mixture of chemicals at concentrations similar to or greater than those observed in landfill leachates, HDPE geomembrane liners show little if any degradation, even over prolonged periods of time (Koerner, 1999; Colorado Lining International, 2000). Therefore, geomembrane failure as a result of chemical degradation is considered very unlikely. In addition to the manufacturing process, extensive quality assurance observation and testing is performed during construction to minimize the potential for construction related defects to be introduced into the system. During construction, thorough inspection of the installed geomembrane is conducted on a daily basis by the installation contractor and by qualified third -party construction quality assurance personnel. If any defects, abrasions, or holes are identified they are repaired immediately. Seams and welds are also tested for strength and permeability. Any materials or welds that fail quality assurance tests are rejected, removed from the site, replaced with acceptable materials, and retested for conformance with project specifications. 2.3 Compacted Clay Liner The soil component of the liner system below the geomembrane is composed of stable geological materials that are highly resistant to degradation and that are expected to function indefinitely in the chemical and physical environment of a MSW landfill liner system. Two leading authorities on the long-term properties of clay liners are Professor James K Mitchell of the Virginia Polytechnic Institute, and State University, and Professor David Daniel of the University of Illinois. In Mitchell and Jaber (1990) "Factors Controlling the Long -Term Properties of Clay Liners, " Waste Containment Systems: Construction, Regulation, and Performance, ASCE Geotechnical Special Publication No. 26, it states that `By their very nature most clay soils are quite stable materials in their natural state, because they are towards the end point of the degradation phase of the weathering and rock -forming cycle. Thus, if a naturally occurring clay soil is compacted to high density, thereby producing material with a very low hydraulic conductivity, and it is maintained within the same ranges of temperature, pressure, and chemical and biological environment, it would be expected to function as a seepage barrier indefinitely." In Daniel and Liljestrand (1984) "Effects of Landfill Leachates on 10 C:12000-0 MForwsrd Composite Uner VJ Aoc109/19102 Geologic Associates