Which of the following is NOT a category of bottom liners?

The correct answer identifies a category that does not belong among the common types of bottom liners used in landfill operations. The other three choices—geosynthetic clay liners (GCL), natural liners, and geomembranes or flexible membrane liners (FML)—are all recognized categories.

Geosynthetic clay liners are used for their ability to provide low permeability and are often made from sodium bentonite clay. Natural liners involve the use of native soils or clay compaction to create barriers. Geomembranes or flexible membrane liners refer to synthetic materials used to prevent leachate from contaminating groundwater and are critical in landfill design.

Geocomposite liners, however, while they are a form of liner system that combines different materials, are not generally categorized separately in the same way as the primary types listed. Instead, they are often considered combinations of other liner materials, rather than a distinct category on their own. This distinctive nature is why recognizing that geocomposite liners do not fit within the standard categories helps clarify the understanding of landfill liner systems.